Changeset cdbab55 for doc/papers
- Timestamp:
- Aug 21, 2018, 2:24:29 PM (8 years ago)
- Branches:
- ADT, aaron-thesis, arm-eh, ast-experimental, cleanup-dtors, deferred_resn, enum, forall-pointer-decay, jacob/cs343-translation, jenkins-sandbox, master, new-ast, new-ast-unique-expr, no_list, persistent-indexer, pthread-emulation, qualifiedEnum, stuck-waitfor-destruct
- Children:
- 2a6292d
- Parents:
- 2b79a70 (diff), efa8b6a (diff)
Note: this is a merge changeset, the changes displayed below correspond to the merge itself.
Use the(diff)links above to see all the changes relative to each parent. - Location:
- doc/papers
- Files:
-
- 3 edited
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AMA/AMA-stix/ama/WileyNJD-v2.cls (modified) (3 diffs)
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general/Makefile (modified) (1 diff)
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general/Paper.tex (modified) (124 diffs)
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doc/papers/AMA/AMA-stix/ama/WileyNJD-v2.cls
r2b79a70 rcdbab55 1854 1854 \vspace*{8.5\p@}% 1855 1855 \rightskip0pt\raggedright\hspace*{7\p@}\hbox{\reset@font\abstractfont{\absheadfont#1}}\par\vskip3pt% LN20feb2016 1856 {\abstractfont\baselineskip15pt\ifFWabstract\hsize\textwidth\fi #2\par\vspace*{0\p@}}%1856 {\abstractfont\baselineskip15pt\ifFWabstract\hsize\textwidth\fi\hsize0.68\textwidth#2\par\vspace*{0\p@}}% 1857 1857 \addcontentsline{toc}{section}{\abstractname}% 1858 1858 }}%\abstract{}% … … 1882 1882 }% 1883 1883 % 1884 \def\fundinginfohead#1{\gdef\@fundinginfo@head{#1}}\fundinginfohead{Funding Information}%1884 \def\fundinginfohead#1{\gdef\@fundinginfo@head{#1}}\fundinginfohead{Funding information}% 1885 1885 \def\fundinginfoheadtext#1{\gdef\@fundinginfo@head@text{#1}}\fundinginfoheadtext{}% 1886 1886 \gdef\@fundinginfo{{% … … 2319 2319 %% Keywords %% 2320 2320 2321 \def\keywords#1{\def\@keywords{{\keywordsheadfont\textbf{KEYWORDS :}\par\removelastskip\nointerlineskip\vskip6pt \keywordsfont#1\par}}}\def\@keywords{}%2321 \def\keywords#1{\def\@keywords{{\keywordsheadfont\textbf{KEYWORDS}\par\removelastskip\nointerlineskip\vskip6pt \keywordsfont#1\par}}}\def\@keywords{}% 2322 2322 2323 2323 \def\@fnsymbol#1{\ifcase#1\or \dagger\or \ddagger\or -
doc/papers/general/Makefile
r2b79a70 rcdbab55 60 60 dvips ${Build}/$< -o $@ 61 61 62 ${BASE}.dvi : Makefile ${BASE}.out.ps WileyNJD-AMA.bst ${GRAPHS} ${PROGRAMS} ${PICTURES} ${FIGURES} ${SOURCES} \62 ${BASE}.dvi : Makefile ${BASE}.out.ps ${Macros}/WileyNJD-v2.cls WileyNJD-AMA.bst ${GRAPHS} ${PROGRAMS} ${PICTURES} ${FIGURES} ${SOURCES} \ 63 63 ../../bibliography/pl.bib | ${Build} 64 64 # Must have *.aux file containing citations for bibtex -
doc/papers/general/Paper.tex
r2b79a70 rcdbab55 1 1 \documentclass[AMA,STIX1COL]{WileyNJD-v2} 2 \setlength\typewidth{170mm} 3 \setlength\textwidth{170mm} 2 4 3 5 \articletype{RESEARCH ARTICLE}% 4 6 5 \received{26 April 2016} 6 \revised{6 June 2016} 7 \accepted{6 June 2016} 8 7 \received{12 March 2018} 8 \revised{8 May 2018} 9 \accepted{28 June 2018} 10 11 \setlength\typewidth{168mm} 12 \setlength\textwidth{168mm} 9 13 \raggedbottom 10 14 … … 187 191 } 188 192 189 \title{\texorpdfstring{\protect\CFA : Adding Modern Programming Language Features to C}{Cforall : Adding Modern Programming Language Features to C}}193 \title{\texorpdfstring{\protect\CFA : Adding modern programming language features to C}{Cforall : Adding modern programming language features to C}} 190 194 191 195 \author[1]{Aaron Moss} 192 196 \author[1]{Robert Schluntz} 193 \author[1]{Peter A. Buhr *}197 \author[1]{Peter A. Buhr} 194 198 \authormark{MOSS \textsc{et al}} 195 199 196 \address[1]{\orgdiv{Cheriton School of Computer Science}, \orgname{University of Waterloo}, \orgaddress{\state{Waterloo, O N}, \country{Canada}}}197 198 \corres{ *Peter A. Buhr, Cheriton School of Computer Science, University of Waterloo, 200 University Avenue West, Waterloo, ON,N2L 3G1, Canada. \email{pabuhr{\char`\@}uwaterloo.ca}}200 \address[1]{\orgdiv{Cheriton School of Computer Science}, \orgname{University of Waterloo}, \orgaddress{\state{Waterloo, Ontario}, \country{Canada}}} 201 202 \corres{Peter A. Buhr, Cheriton School of Computer Science, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada. \email{pabuhr{\char`\@}uwaterloo.ca}} 199 203 200 204 \fundingInfo{Natural Sciences and Engineering Research Council of Canada} 201 205 202 206 \abstract[Summary]{ 203 The C programming language is a foundational technology for modern computing with millions of lines of code implementing everything from hobby projects to commercial operating-systems. 204 This installation base and the programmers producing it represent a massive software-engineering investment spanning decades and likely to continue for decades more. 205 Nevertheless, C, first standardized almost thirty years ago, lacks many features that make programming in more modern languages safer and more productive. 206 207 The goal of the \CFA project (pronounced ``C-for-all'') is to create an extension of C that provides modern safety and productivity features while still ensuring strong backwards compatibility with C and its programmers. 208 Prior projects have attempted similar goals but failed to honour C programming-style; 209 for instance, adding object-oriented or functional programming with garbage collection is a non-starter for many C developers. 210 Specifically, \CFA is designed to have an orthogonal feature-set based closely on the C programming paradigm, so that \CFA features can be added \emph{incrementally} to existing C code-bases, and C programmers can learn \CFA extensions on an as-needed basis, preserving investment in existing code and programmers. 211 This paper presents a quick tour of \CFA features showing how their design avoids shortcomings of similar features in C and other C-like languages. 207 The C programming language is a foundational technology for modern computing with millions of lines of code implementing everything from hobby projects to commercial operating systems. 208 This installation base and the programmers producing it represent a massive software engineering investment spanning decades and likely to continue for decades more. 209 Nevertheless, C, which was first standardized almost 30 years ago, lacks many features that make programming in more modern languages safer and more productive. 210 The goal of the \CFA project (pronounced ``C for all'') is to create an extension of C that provides modern safety and productivity features while still ensuring strong backward compatibility with C and its programmers. 211 Prior projects have attempted similar goals but failed to honor the C programming style; 212 for instance, adding object-oriented or functional programming with garbage collection is a nonstarter for many C developers. 213 Specifically, \CFA is designed to have an orthogonal feature set based closely on the C programming paradigm, so that \CFA features can be added \emph{incrementally} to existing C code bases, and C programmers can learn \CFA extensions on an as-needed basis, preserving investment in existing code and programmers. 214 This paper presents a quick tour of \CFA features, showing how their design avoids shortcomings of similar features in C and other C-like languages. 212 215 Experimental results are presented to validate several of the new features. 213 216 }% 214 217 215 \keywords{ generic types, tuple types, variadic types, polymorphic functions, C, Cforall}218 \keywords{C, Cforall, generic types, polymorphic functions, tuple types, variadic types} 216 219 217 220 218 221 \begin{document} 219 \linenumbers % comment out to turn off line numbering222 %\linenumbers % comment out to turn off line numbering 220 223 221 224 \maketitle 222 225 223 226 227 \vspace*{-10pt} 224 228 \section{Introduction} 225 229 226 The C programming language is a foundational technology for modern computing with millions of lines of code implementing everything from hobby projects to commercial operating -systems.227 This installation base and the programmers producing it represent a massive software -engineering investment spanning decades and likely to continue for decades more.228 The TIOBE index~\cite{TIOBE} ranks the top 5 most \emph{popular} programming languages as: Java 15\%, \Textbf{C 12\%}, \Textbf{\CC 5.5\%}, Python 5\%, \Csharp 4.5\% = 42\%, where the next 50 languages are less than 4\% each,with a long tail.229 The top 3 rankings over the past 30 years are:230 The C programming language is a foundational technology for modern computing with millions of lines of code implementing everything from hobby projects to commercial operating systems. 231 This installation base and the programmers producing it represent a massive software engineering investment spanning decades and likely to continue for decades more. 232 The TIOBE index~\cite{TIOBE} ranks the top five most \emph{popular} programming languages as Java 15\%, \Textbf{C 12\%}, \Textbf{\CC 5.5\%}, and Python 5\%, \Csharp 4.5\% = 42\%, where the next 50 languages are less than 4\% each with a long tail. 233 The top three rankings over the past 30 years are as follows. 230 234 \begin{center} 231 235 \setlength{\tabcolsep}{10pt} 232 \lstDeleteShortInline@% 233 \begin{tabular}{@{}rccccccc@{}} 234 & 2018 & 2013 & 2008 & 2003 & 1998 & 1993 & 1988 \\ \hline 235 Java & 1 & 2 & 1 & 1 & 18 & - & - \\ 236 \fontsize{9bp}{11bp}\selectfont 237 \lstDeleteShortInline@% 238 \begin{tabular}{@{}cccccccc@{}} 239 & 2018 & 2013 & 2008 & 2003 & 1998 & 1993 & 1988 \\ 240 Java & 1 & 2 & 1 & 1 & 18 & -- & -- \\ 236 241 \Textbf{C}& \Textbf{2} & \Textbf{1} & \Textbf{2} & \Textbf{2} & \Textbf{1} & \Textbf{1} & \Textbf{1} \\ 237 242 \CC & 3 & 4 & 3 & 3 & 2 & 2 & 5 \\ … … 241 246 Love it or hate it, C is extremely popular, highly used, and one of the few systems languages. 242 247 In many cases, \CC is often used solely as a better C. 243 Nevertheless, C, first standardized almost fortyyears ago~\cite{ANSI89:C}, lacks many features that make programming in more modern languages safer and more productive.244 245 \CFA (pronounced ``C -for-all'', and written \CFA or Cforall) is an evolutionary extension of the C programming language that adds modern language-features to C, while maintaining source and runtime compatibility in the familiar C programming model.246 The four key design goals for \CFA~\cite{Bilson03} are :247 (1) The behaviour of standard C code must remain the same when translated by a \CFA compiler as when translated by a C compiler;248 (2) Standard C code must be as fast and as small when translated by a \CFA compiler as when translated by a C compiler;249 (3) \CFA code must be at least as portable as standard C code;250 (4) Extensions introduced by \CFA must be translated in the most efficient way possible.251 These goals ensure existing C code-bases can be convertedto \CFA incrementally with minimal effort, and C programmers can productively generate \CFA code without training beyond the features being used.252 \CC is used similarly , but has the disadvantages of multiple legacy design-choices that cannot be updated,and active divergence of the language model from C, requiring significant effort and training to incrementally add \CC to a C-based project.253 254 All language sfeatures discussed in this paper are working, except some advanced exception-handling features.255 Not discussed in this paper are the integrated concurrency -constructs and user-level threading-library~\cite{Delisle18}.248 Nevertheless, C, which was first standardized almost 30 years ago~\cite{ANSI89:C}, lacks many features that make programming in more modern languages safer and more productive. 249 250 \CFA (pronounced ``C for all'' and written \CFA or Cforall) is an evolutionary extension of the C programming language that adds modern language features to C, while maintaining source and runtime compatibility in the familiar C programming model. 251 The four key design goals for \CFA~\cite{Bilson03} are as follows: 252 (1) the behavior of standard C code must remain the same when translated by a \CFA compiler as when translated by a C compiler; 253 (2) the standard C code must be as fast and as small when translated by a \CFA compiler as when translated by a C compiler; 254 (3) the \CFA code must be at least as portable as standard C code; 255 (4) extensions introduced by \CFA must be translated in the most efficient way possible. 256 These goals ensure that the existing C code bases can be converted into \CFA incrementally with minimal effort, and C programmers can productively generate \CFA code without training beyond the features being used. 257 \CC is used similarly but has the disadvantages of multiple legacy design choices that cannot be updated and active divergence of the language model from C, requiring significant effort and training to incrementally add \CC to a C-based project. 258 259 All language features discussed in this paper are working, except some advanced exception-handling features. 260 Not discussed in this paper are the integrated concurrency constructs and user-level threading library~\cite{Delisle18}. 256 261 \CFA is an \emph{open-source} project implemented as a source-to-source translator from \CFA to the gcc-dialect of C~\cite{GCCExtensions}, allowing it to leverage the portability and code optimizations provided by gcc, meeting goals (1)--(3). 257 262 % @plg2[9]% cd cfa-cc/src; cloc ArgTweak CodeGen CodeTools Common Concurrency ControlStruct Designators GenPoly InitTweak MakeLibCfa.cc MakeLibCfa.h Parser ResolvExpr SymTab SynTree Tuples driver prelude main.cc … … 269 274 % SUM: 223 8203 8263 46479 270 275 % ------------------------------------------------------------------------------- 271 The \CFA translator is 200+ files and 46 ,000+ lines of code written in C/\CC.272 A translator versus a compiler makes it easier and faster to generate and debug C object-code rather than intermediate, assembleror machine code;276 The \CFA translator is 200+ files and 46\,000+ lines of code written in C/\CC. 277 A translator versus a compiler makes it easier and faster to generate and debug the C object code rather than the intermediate, assembler, or machine code; 273 278 ultimately, a compiler is necessary for advanced features and optimal performance. 274 279 % The translator design is based on the \emph{visitor pattern}, allowing multiple passes over the abstract code-tree, which works well for incrementally adding new feature through additional visitor passes. 275 Two key translator components are expression analysis, determining expression validity and what operations are required for its implementation, and code generation, dealing with multiple forms of overloading, polymorphism, and multiple return values by converting them into C code for a C compiler that supports none of these features.276 Details of these components are available in Bilson~\cite{Bilson03} Chapters 2 and 3,and form the base for the current \CFA translator.280 Two key translator components are expression analysis, determining expression validity and what operations are required for its implementation, and code generation, dealing with multiple forms of overloading, polymorphism, and multiple return values by converting them into the C code for a C compiler that supports none of these features. 281 Details of these components are available in chapters 2 and 3 in the work of Bilson~\cite{Bilson03} and form the base for the current \CFA translator. 277 282 % @plg2[8]% cd cfa-cc/src; cloc libcfa 278 283 % ------------------------------------------------------------------------------- … … 289 294 % SUM: 100 1895 2785 11763 290 295 % ------------------------------------------------------------------------------- 291 The \CFA runtime system is 100+ files and 11 ,000+ lines of code, written in \CFA.296 The \CFA runtime system is 100+ files and 11\,000+ lines of code, written in \CFA. 292 297 Currently, the \CFA runtime is the largest \emph{user} of \CFA providing a vehicle to test the language features and implementation. 293 298 % @plg2[6]% cd cfa-cc/src; cloc tests examples benchmark … … 316 321 317 322 323 \vspace*{-6pt} 318 324 \section{Polymorphic Functions} 319 325 320 \CFA introduces both ad -hoc and parametric polymorphism to C, with a design originally formalized by Ditchfield~\cite{Ditchfield92},and first implemented by Bilson~\cite{Bilson03}.321 Shortcomings are identified in existing approaches to generic and variadic data types in C-like languages and how these shortcomings are avoided in \CFA.322 Specifically, the solution is both reusable and type -checked, as well as conforming to the design goals of \CFA with ergonomic use of existing C abstractions.326 \CFA introduces both ad hoc and parametric polymorphism to C, with a design originally formalized by Ditchfield~\cite{Ditchfield92} and first implemented by Bilson~\cite{Bilson03}. 327 Shortcomings are identified in the existing approaches to generic and variadic data types in C-like languages and how these shortcomings are avoided in \CFA. 328 Specifically, the solution is both reusable and type checked, as well as conforming to the design goals of \CFA with ergonomic use of existing C abstractions. 323 329 The new constructs are empirically compared with C and \CC approaches via performance experiments in Section~\ref{sec:eval}. 324 330 325 331 326 \subsection{Name Overloading} 332 \vspace*{-6pt} 333 \subsection{Name overloading} 327 334 \label{s:NameOverloading} 328 335 329 336 \begin{quote} 330 There are only two hard things in Computer Science: cache invalidation and \emph{naming things} --Phil Karlton337 ``There are only two hard things in Computer Science: cache invalidation and \emph{naming things}.''---Phil Karlton 331 338 \end{quote} 332 339 \vspace{-9pt} 333 C already has a limited form of ad -hoc polymorphism in its basic arithmetic operators, which apply to a variety of different types using identical syntax.340 C already has a limited form of ad hoc polymorphism in its basic arithmetic operators, which apply to a variety of different types using identical syntax. 334 341 \CFA extends the built-in operator overloading by allowing users to define overloads for any function, not just operators, and even any variable; 335 342 Section~\ref{sec:libraries} includes a number of examples of how this overloading simplifies \CFA programming relative to C. 336 343 Code generation for these overloaded functions and variables is implemented by the usual approach of mangling the identifier names to include a representation of their type, while \CFA decides which overload to apply based on the same ``usual arithmetic conversions'' used in C to disambiguate operator overloads. 337 As an example: 344 345 \newpage 338 346 \begin{cfa} 339 347 int max = 2147483647; $\C[4in]{// (1)}$ … … 341 349 int max( int a, int b ) { return a < b ? b : a; } $\C{// (3)}$ 342 350 double max( double a, double b ) { return a < b ? b : a; } $\C{// (4)}\CRT$ 343 max( 7, -max ); $\C {// uses (3) and (1), by matching int from constant 7}$351 max( 7, -max ); $\C[3in]{// uses (3) and (1), by matching int from constant 7}$ 344 352 max( max, 3.14 ); $\C{// uses (4) and (2), by matching double from constant 3.14}$ 345 353 max( max, -max ); $\C{// ERROR, ambiguous}$ 346 int m = max( max, -max ); $\C{// uses (3) and (1) twice, by matching return type} $354 int m = max( max, -max ); $\C{// uses (3) and (1) twice, by matching return type}\CRT$ 347 355 \end{cfa} 348 356 … … 350 358 In some cases, hundreds of names can be reduced to tens, resulting in a significant cognitive reduction. 351 359 In the above, the name @max@ has a consistent meaning, and a programmer only needs to remember the single concept: maximum. 352 To prevent significant ambiguities, \CFA uses the return type in selecting overloads, \eg in the assignment to @m@, the compiler use @m@'s type to unambiguously select the most appropriate call to function @max@ (as does Ada).360 To prevent significant ambiguities, \CFA uses the return type in selecting overloads, \eg in the assignment to @m@, the compiler uses @m@'s type to unambiguously select the most appropriate call to function @max@ (as does Ada). 353 361 As is shown later, there are a number of situations where \CFA takes advantage of available type information to disambiguate, where other programming languages generate ambiguities. 354 362 355 \Celeven added @_Generic@ expressions ~\cite[\S~6.5.1.1]{C11}, which is used with preprocessor macros to provide ad-hoc polymorphism;363 \Celeven added @_Generic@ expressions (see section~6.5.1.1 of the ISO/IEC 9899~\cite{C11}), which is used with preprocessor macros to provide ad hoc polymorphism; 356 364 however, this polymorphism is both functionally and ergonomically inferior to \CFA name overloading. 357 The macro wrapping the generic expression imposes some limitations; 358 \eg, it cannot implement the example above, because the variables @max@ are ambiguous with the functions @max@. 365 The macro wrapping the generic expression imposes some limitations, for instance, it cannot implement the example above, because the variables @max@ are ambiguous with the functions @max@. 359 366 Ergonomic limitations of @_Generic@ include the necessity to put a fixed list of supported types in a single place and manually dispatch to appropriate overloads, as well as possible namespace pollution from the dispatch functions, which must all have distinct names. 360 \CFA supports @_Generic@ expressions for backward s compatibility, but it is an unnecessary mechanism. \TODO{actually implement that}367 \CFA supports @_Generic@ expressions for backward compatibility, but it is an unnecessary mechanism. 361 368 362 369 % http://fanf.livejournal.com/144696.html … … 365 372 366 373 367 \subsection{\texorpdfstring{\protect\lstinline{forall} Functions}{forall Functions}} 374 \vspace*{-10pt} 375 \subsection{\texorpdfstring{\protect\lstinline{forall} functions}{forall functions}} 368 376 \label{sec:poly-fns} 369 377 370 The signature feature of \CFA is parametric-polymorphic functions~\cite{forceone:impl,Cormack90,Duggan96} with functions generalized using a @forall@ clause (giving the language its name) :378 The signature feature of \CFA is parametric-polymorphic functions~\cite{forceone:impl,Cormack90,Duggan96} with functions generalized using a @forall@ clause (giving the language its name). 371 379 \begin{cfa} 372 380 `forall( otype T )` T identity( T val ) { return val; } … … 375 383 This @identity@ function can be applied to any complete \newterm{object type} (or @otype@). 376 384 The type variable @T@ is transformed into a set of additional implicit parameters encoding sufficient information about @T@ to create and return a variable of that type. 377 The \CFA implementation passes the size and alignment of the type represented by an @otype@ parameter, as well as an assignment operator, constructor, copy constructor and destructor.378 If this extra information is not needed, \egfor a pointer, the type parameter can be declared as a \newterm{data type} (or @dtype@).379 380 In \CFA, the polymorphic runtime -cost is spread over each polymorphic call, because more arguments are passed to polymorphic functions;381 the experiments in Section~\ref{sec:eval} show this overhead is similar to \CC virtual -function calls.382 A design advantage is that, unlike \CC template -functions, \CFA polymorphic-functions are compatible with C \emph{separate compilation}, preventing compilation and code bloat.383 384 Since bare polymorphic -types provide a restricted set of available operations, \CFA provides a \newterm{type assertion}~\cite[pp.~37-44]{Alphard} mechanism to provide further type information, where type assertions may be variable or function declarations that depend on a polymorphic type-variable.385 For example, the function @twice@ can be defined using the \CFA syntax for operator overloading :385 The \CFA implementation passes the size and alignment of the type represented by an @otype@ parameter, as well as an assignment operator, constructor, copy constructor, and destructor. 386 If this extra information is not needed, for instance, for a pointer, the type parameter can be declared as a \newterm{data type} (or @dtype@). 387 388 In \CFA, the polymorphic runtime cost is spread over each polymorphic call, because more arguments are passed to polymorphic functions; 389 the experiments in Section~\ref{sec:eval} show this overhead is similar to \CC virtual function calls. 390 A design advantage is that, unlike \CC template functions, \CFA polymorphic functions are compatible with C \emph{separate compilation}, preventing compilation and code bloat. 391 392 Since bare polymorphic types provide a restricted set of available operations, \CFA provides a \newterm{type assertion}~\cite[pp.~37-44]{Alphard} mechanism to provide further type information, where type assertions may be variable or function declarations that depend on a polymorphic type variable. 393 For example, the function @twice@ can be defined using the \CFA syntax for operator overloading. 386 394 \begin{cfa} 387 395 forall( otype T `| { T ?+?(T, T); }` ) T twice( T x ) { return x `+` x; } $\C{// ? denotes operands}$ 388 396 int val = twice( twice( 3.7 ) ); $\C{// val == 14}$ 389 397 \end{cfa} 390 which works for any type @T@ with a matching addition operator. 391 The polymorphism is achieved by creating a wrapper function for calling @+@ with @T@ bound to @double@, then passing this function to the first call of @twice@. 392 There is now the option of using the same @twice@ and converting the result to @int@ on assignment, or creating another @twice@ with type parameter @T@ bound to @int@ because \CFA uses the return type~\cite{Cormack81,Baker82,Ada} in its type analysis. 393 The first approach has a late conversion from @double@ to @int@ on the final assignment, while the second has an early conversion to @int@. 394 \CFA minimizes the number of conversions and their potential to lose information, so it selects the first approach, which corresponds with C-programmer intuition. 398 This works for any type @T@ with a matching addition operator. 399 The polymorphism is achieved by creating a wrapper function for calling @+@ with the @T@ bound to @double@ and then passing this function to the first call of @twice@. 400 There is now the option of using the same @twice@ and converting the result into @int@ on assignment or creating another @twice@ with the type parameter @T@ bound to @int@ because \CFA uses the return type~\cite{Cormack81,Baker82,Ada} in its type analysis. 401 The first approach has a late conversion from @double@ to @int@ on the final assignment, whereas the second has an early conversion to @int@. 402 \CFA minimizes the number of conversions and their potential to lose information; 403 hence, it selects the first approach, which corresponds with C programmer intuition. 395 404 396 405 Crucial to the design of a new programming language are the libraries to access thousands of external software features. 397 Like \CC, \CFA inherits a massive compatible library -base, where other programming languages must rewrite or provide fragile inter-language communication with C.398 A simple example is leveraging the existing type-unsafe (@void *@) C @bsearch@ to binary search a sorted float array :406 Like \CC, \CFA inherits a massive compatible library base, where other programming languages must rewrite or provide fragile interlanguage communication with C. 407 A simple example is leveraging the existing type-unsafe (@void *@) C @bsearch@ to binary search a sorted float array. 399 408 \begin{cfa} 400 409 void * bsearch( const void * key, const void * base, size_t nmemb, size_t size, … … 406 415 double * val = (double *)bsearch( &key, vals, 10, sizeof(vals[0]), comp ); $\C{// search sorted array}$ 407 416 \end{cfa} 408 which can be augmented simply with generalized, type-safe, \CFA-overloaded wrappers: 417 This can be augmented simply with generalized, type-safe, \CFA-overloaded wrappers. 409 418 \begin{cfa} 410 419 forall( otype T | { int ?<?( T, T ); } ) T * bsearch( T key, const T * arr, size_t size ) { … … 420 429 \end{cfa} 421 430 The nested function @comp@ provides the hidden interface from typed \CFA to untyped (@void *@) C, plus the cast of the result. 422 Providing a hidden @comp@ function in \CC is awkward as lambdas do not use C calling-conventions and template declarations cannot appear at block scope. 423 As well, an alternate kind of return is made available: position versus pointer to found element. 424 \CC's type-system cannot disambiguate between the two versions of @bsearch@ because it does not use the return type in overload resolution, nor can \CC separately compile a template @bsearch@. 431 % FIX 432 Providing a hidden @comp@ function in \CC is awkward as lambdas do not use C calling conventions and template declarations cannot appear in block scope. 433 In addition, an alternate kind of return is made available: position versus pointer to found element. 434 \CC's type system cannot disambiguate between the two versions of @bsearch@ because it does not use the return type in overload resolution, nor can \CC separately compile a template @bsearch@. 425 435 426 436 \CFA has replacement libraries condensing hundreds of existing C functions into tens of \CFA overloaded functions, all without rewriting the actual computations (see Section~\ref{sec:libraries}). … … 432 442 \end{cfa} 433 443 434 Call -site inferencing and nested functions provide a localized form of inheritance.444 Call site inferencing and nested functions provide a localized form of inheritance. 435 445 For example, the \CFA @qsort@ only sorts in ascending order using @<@. 436 However, it is trivial to locally change this behavio ur:446 However, it is trivial to locally change this behavior. 437 447 \begin{cfa} 438 448 forall( otype T | { int ?<?( T, T ); } ) void qsort( const T * arr, size_t size ) { /* use C qsort */ } 439 449 int main() { 440 int ?<?( double x, double y ) { return x `>` y; } $\C{// locally override behavio ur}$450 int ?<?( double x, double y ) { return x `>` y; } $\C{// locally override behavior}$ 441 451 qsort( vals, 10 ); $\C{// descending sort}$ 442 452 } 443 453 \end{cfa} 444 454 The local version of @?<?@ performs @?>?@ overriding the built-in @?<?@ so it is passed to @qsort@. 445 Hence, programmers can easily form local environments, adding and modifying appropriate functions, to maximize reuse of other existing functions and types.446 447 To reduce duplication, it is possible to distribute a group of @forall@ (and storage-class qualifiers) over functions/types, s o each block declaration is prefixed by the group (see example in Appendix~\ref{s:CforallStack}).455 Therefore, programmers can easily form local environments, adding and modifying appropriate functions, to maximize the reuse of other existing functions and types. 456 457 To reduce duplication, it is possible to distribute a group of @forall@ (and storage-class qualifiers) over functions/types, such that each block declaration is prefixed by the group (see the example in Appendix~\ref{s:CforallStack}). 448 458 \begin{cfa} 449 459 forall( otype `T` ) { $\C{// distribution block, add forall qualifier to declarations}$ … … 456 466 457 467 458 \vspace*{-2pt}459 468 \subsection{Traits} 460 469 461 \CFA provides \newterm{traits} to name a group of type assertions, where the trait name allows specifying the same set of assertions in multiple locations, preventing repetition mistakes at each function declaration: 462 470 \CFA provides \newterm{traits} to name a group of type assertions, where the trait name allows specifying the same set of assertions in multiple locations, preventing repetition mistakes at each function declaration. 463 471 \begin{cquote} 464 472 \lstDeleteShortInline@% … … 487 495 \end{cquote} 488 496 489 Note ,the @sumable@ trait does not include a copy constructor needed for the right side of @?+=?@ and return;490 it is provided by @otype@, which is syntactic sugar for the following trait :497 Note that the @sumable@ trait does not include a copy constructor needed for the right side of @?+=?@ and return; 498 it is provided by @otype@, which is syntactic sugar for the following trait. 491 499 \begin{cfa} 492 500 trait otype( dtype T | sized(T) ) { // sized is a pseudo-trait for types with known size and alignment … … 497 505 }; 498 506 \end{cfa} 499 Given the information provided for an @otype@, variables of polymorphic type can be treated as if they were a complete type: stack -allocatable, default or copy-initialized, assigned, and deleted.500 501 In summation, the \CFA type -system uses \newterm{nominal typing} for concrete types, matching with the C type-system, and \newterm{structural typing} for polymorphic types.507 Given the information provided for an @otype@, variables of polymorphic type can be treated as if they were a complete type: stack allocatable, default or copy initialized, assigned, and deleted. 508 509 In summation, the \CFA type system uses \newterm{nominal typing} for concrete types, matching with the C type system, and \newterm{structural typing} for polymorphic types. 502 510 Hence, trait names play no part in type equivalence; 503 511 the names are simply macros for a list of polymorphic assertions, which are expanded at usage sites. 504 Nevertheless, trait names form a logical subtype -hierarchy with @dtype@ at the top, where traits often contain overlapping assertions, \eg operator @+@.505 Traits are used like interfaces in Java or abstract base -classes in \CC, but without the nominal inheritance-relationships.506 Instead, each polymorphic function (or generic type) defines the structural type needed for its execution (polymorphic type -key), and this key is fulfilled at each call site from the lexical environment, which is similar toGo~\cite{Go} interfaces.507 Hence, new lexical scopes and nested functions are used extensively to create local subtypes, as in the @qsort@ example, without having to manage a nominal -inheritance hierarchy.512 Nevertheless, trait names form a logical subtype hierarchy with @dtype@ at the top, where traits often contain overlapping assertions, \eg operator @+@. 513 Traits are used like interfaces in Java or abstract base classes in \CC, but without the nominal inheritance relationships. 514 Instead, each polymorphic function (or generic type) defines the structural type needed for its execution (polymorphic type key), and this key is fulfilled at each call site from the lexical environment, which is similar to the Go~\cite{Go} interfaces. 515 Hence, new lexical scopes and nested functions are used extensively to create local subtypes, as in the @qsort@ example, without having to manage a nominal inheritance hierarchy. 508 516 % (Nominal inheritance can be approximated with traits using marker variables or functions, as is done in Go.) 509 517 … … 536 544 537 545 A significant shortcoming of standard C is the lack of reusable type-safe abstractions for generic data structures and algorithms. 538 Broadly speaking, there are three approaches to implement abstract data -structures in C.539 One approach is to write bespoke data -structures for each context in which they are needed.540 While this approach is flexible and supports integration with the C type -checker and tooling, it is also tedious and error-prone, especially for more complex data structures.541 A second approach is to use @void *@-based polymorphism, \eg the C standard -library functions @bsearch@ and @qsort@, which allowreuse of code with common functionality.542 However, basing all polymorphism on @void *@ eliminates the type -checker's ability to ensure that argument types are properly matched, often requiring a number of extra function parameters, pointer indirection, and dynamic allocation that is not otherwiseneeded.543 A third approach to generic code is to use preprocessor macros, which does allow the generated code to be both generic and type -checked, but errors may be difficult to interpret.546 Broadly speaking, there are three approaches to implement abstract data structures in C. 547 One approach is to write bespoke data structures for each context in which they are needed. 548 While this approach is flexible and supports integration with the C type checker and tooling, it is also tedious and error prone, especially for more complex data structures. 549 A second approach is to use @void *@-based polymorphism, \eg the C standard library functions @bsearch@ and @qsort@, which allow for the reuse of code with common functionality. 550 However, basing all polymorphism on @void *@ eliminates the type checker's ability to ensure that argument types are properly matched, often requiring a number of extra function parameters, pointer indirection, and dynamic allocation that is otherwise not needed. 551 A third approach to generic code is to use preprocessor macros, which does allow the generated code to be both generic and type checked, but errors may be difficult to interpret. 544 552 Furthermore, writing and using preprocessor macros is unnatural and inflexible. 545 553 546 \CC, Java, and other languages use \newterm{generic types} to produce type-safe abstract data -types.547 \CFA generic types integrate efficiently and naturally with the existing polymorphic functions, while retaining backward scompatibility with C and providing separate compilation.554 \CC, Java, and other languages use \newterm{generic types} to produce type-safe abstract data types. 555 \CFA generic types integrate efficiently and naturally with the existing polymorphic functions, while retaining backward compatibility with C and providing separate compilation. 548 556 However, for known concrete parameters, the generic-type definition can be inlined, like \CC templates. 549 557 550 A generic type can be declared by placing a @forall@ specifier on a @struct@ or @union@ declaration , and instantiated using a parenthesized list of types after the type name:558 A generic type can be declared by placing a @forall@ specifier on a @struct@ or @union@ declaration and instantiated using a parenthesized list of types after the type name. 551 559 \begin{cquote} 552 560 \lstDeleteShortInline@% … … 576 584 577 585 \CFA classifies generic types as either \newterm{concrete} or \newterm{dynamic}. 578 Concrete types have a fixed memory layout regardless of type parameters, wh iledynamic types vary in memory layout depending on their type parameters.586 Concrete types have a fixed memory layout regardless of type parameters, whereas dynamic types vary in memory layout depending on their type parameters. 579 587 A \newterm{dtype-static} type has polymorphic parameters but is still concrete. 580 588 Polymorphic pointers are an example of dtype-static types; 581 given some type variable @T@, @T@ is a polymorphic type, as is @T *@, but @T *@ has a fixed size and can thereforebe represented by @void *@ in code generation.582 583 \CFA generic types also allow checked argument -constraints.584 For example, the following declaration of a sorted set -type ensures the set key supports equality and relational comparison:589 given some type variable @T@, @T@ is a polymorphic type, as is @T *@, but @T *@ has a fixed size and can, therefore, be represented by @void *@ in code generation. 590 591 \CFA generic types also allow checked argument constraints. 592 For example, the following declaration of a sorted set type ensures the set key supports equality and relational comparison. 585 593 \begin{cfa} 586 594 forall( otype Key | { _Bool ?==?(Key, Key); _Bool ?<?(Key, Key); } ) struct sorted_set; … … 588 596 589 597 590 \subsection{Concrete Generic-Types}591 592 The \CFA translator template -expands concrete generic-types into new structure types, affording maximal inlining.593 To enable inter -operation among equivalent instantiations of a generic type, the translator saves the set of instantiations currently in scope and reuses the generated structure declarations where appropriate.594 A function declaration that accepts or returns a concrete generic -type produces a declaration for the instantiated structure in the same scope, which all callers may reuse.595 For example, the concrete instantiation for @pair( const char *, int )@ is :598 \subsection{Concrete generic types} 599 600 The \CFA translator template expands concrete generic types into new structure types, affording maximal inlining. 601 To enable interoperation among equivalent instantiations of a generic type, the translator saves the set of instantiations currently in scope and reuses the generated structure declarations where appropriate. 602 A function declaration that accepts or returns a concrete generic type produces a declaration for the instantiated structure in the same scope, which all callers may reuse. 603 For example, the concrete instantiation for @pair( const char *, int )@ is 596 604 \begin{cfa} 597 605 struct _pair_conc0 { … … 600 608 \end{cfa} 601 609 602 A concrete generic -type with dtype-static parameters is also expanded to a structure type, but this type is used for all matching instantiations.603 In the above example, the @pair( F *, T * )@ parameter to @value@ is such a type; its expansion is below and it is used as the type of the variables @q@ and @r@ as well, with casts for member access where appropriate:610 A concrete generic type with dtype-static parameters is also expanded to a structure type, but this type is used for all matching instantiations. 611 In the above example, the @pair( F *, T * )@ parameter to @value@ is such a type; its expansion is below, and it is used as the type of the variables @q@ and @r@ as well, with casts for member access where appropriate. 604 612 \begin{cfa} 605 613 struct _pair_conc1 { … … 609 617 610 618 611 \subsection{Dynamic Generic-Types} 612 613 Though \CFA implements concrete generic-types efficiently, it also has a fully general system for dynamic generic types. 614 As mentioned in Section~\ref{sec:poly-fns}, @otype@ function parameters (in fact all @sized@ polymorphic parameters) come with implicit size and alignment parameters provided by the caller. 615 Dynamic generic-types also have an \newterm{offset array} containing structure-member offsets. 616 A dynamic generic-@union@ needs no such offset array, as all members are at offset 0, but size and alignment are still necessary. 617 Access to members of a dynamic structure is provided at runtime via base-displacement addressing with the structure pointer and the member offset (similar to the @offsetof@ macro), moving a compile-time offset calculation to runtime. 619 \subsection{Dynamic generic types} 620 621 Though \CFA implements concrete generic types efficiently, it also has a fully general system for dynamic generic types. 622 As mentioned in Section~\ref{sec:poly-fns}, @otype@ function parameters (in fact, all @sized@ polymorphic parameters) come with implicit size and alignment parameters provided by the caller. 623 Dynamic generic types also have an \newterm{offset array} containing structure-member offsets. 624 A dynamic generic @union@ needs no such offset array, as all members are at offset 0, but size and alignment are still necessary. 625 Access to members of a dynamic structure is provided at runtime via base displacement addressing 626 % FIX 627 using the structure pointer and the member offset (similar to the @offsetof@ macro), moving a compile-time offset calculation to runtime. 618 628 619 629 The offset arrays are statically generated where possible. 620 If a dynamic generic -type is declared to be passed or returned by value from a polymorphic function, the translator can safely assume the generic type is complete (\ie has a known layout) at any call-site, and the offset array is passed from the caller;630 If a dynamic generic type is declared to be passed or returned by value from a polymorphic function, the translator can safely assume that the generic type is complete (\ie has a known layout) at any call site, and the offset array is passed from the caller; 621 631 if the generic type is concrete at the call site, the elements of this offset array can even be statically generated using the C @offsetof@ macro. 622 As an example, the body of the second @value@ function is implemented as :632 As an example, the body of the second @value@ function is implemented as 623 633 \begin{cfa} 624 634 _assign_T( _retval, p + _offsetof_pair[1] ); $\C{// return *p.second}$ 625 635 \end{cfa} 626 @_assign_T@ is passed in as an implicit parameter from @otype T@, and takes two @T *@ (@void *@ in the generated code), a destination and a source; @_retval@ is the pointer to a caller-allocated buffer for the return value, the usual \CFA method to handle dynamically-sized return types. 627 @_offsetof_pair@ is the offset array passed into @value@; this array is generated at the call site as: 636 \newpage 637 \noindent 638 Here, @_assign_T@ is passed in as an implicit parameter from @otype T@, and takes two @T *@ (@void *@ in the generated code), a destination and a source, and @_retval@ is the pointer to a caller-allocated buffer for the return value, the usual \CFA method to handle dynamically sized return types. 639 @_offsetof_pair@ is the offset array passed into @value@; 640 this array is generated at the call site as 628 641 \begin{cfa} 629 642 size_t _offsetof_pair[] = { offsetof( _pair_conc0, first ), offsetof( _pair_conc0, second ) } 630 643 \end{cfa} 631 644 632 In some cases the offset arrays cannot be statically generated.633 For instance, modularity is generally provided in C by including an opaque forward -declaration of a structure and associated accessor and mutator functions in a header file, with the actual implementations in a separately-compiled @.c@ file.634 \CFA supports this pattern for generic types, but the caller does not know the actual layout or size of the dynamic generic -type,and only holds it by a pointer.645 In some cases, the offset arrays cannot be statically generated. 646 For instance, modularity is generally provided in C by including an opaque forward declaration of a structure and associated accessor and mutator functions in a header file, with the actual implementations in a separately compiled @.c@ file. 647 \CFA supports this pattern for generic types, but the caller does not know the actual layout or size of the dynamic generic type and only holds it by a pointer. 635 648 The \CFA translator automatically generates \newterm{layout functions} for cases where the size, alignment, and offset array of a generic struct cannot be passed into a function from that function's caller. 636 649 These layout functions take as arguments pointers to size and alignment variables and a caller-allocated array of member offsets, as well as the size and alignment of all @sized@ parameters to the generic structure (un@sized@ parameters are forbidden from being used in a context that affects layout). … … 642 655 Whether a type is concrete, dtype-static, or dynamic is decided solely on the @forall@'s type parameters. 643 656 This design allows opaque forward declarations of generic types, \eg @forall(otype T)@ @struct Box@ -- like in C, all uses of @Box(T)@ can be separately compiled, and callers from other translation units know the proper calling conventions to use. 644 If the definition of a structure type is included in deciding whether a generic type is dynamic or concrete, some further types may be recognized as dtype-static (\eg @forall(otype T)@ @struct unique_ptr { T * p }@ does not depend on @T@ for its layout, but the existence of an @otype@ parameter means that it \emph{could}.), but preserving separate compilation (and the associated C compatibility) in the existing design is judged to be an appropriate trade-off. 657 If the definition of a structure type is included in deciding whether a generic type is dynamic or concrete, some further types may be recognized as dtype-static (\eg @forall(otype T)@ @struct unique_ptr { T * p }@ does not depend on @T@ for its layout, but the existence of an @otype@ parameter means that it \emph{could}.); 658 however, preserving separate compilation (and the associated C compatibility) in the existing design is judged to be an appropriate trade-off. 645 659 646 660 … … 655 669 } 656 670 \end{cfa} 657 Since @pair( T *, T * )@ is a concrete type, there are no implicit parameters passed to @lexcmp@, so the generated code is identical to a function written in standard C using @void *@, yet the \CFA version is type-checked to ensure the members of both pairs and the arguments to the comparison function match in type. 658 659 Another useful pattern enabled by reused dtype-static type instantiations is zero-cost \newterm{tag-structures}. 660 Sometimes information is only used for type-checking and can be omitted at runtime, \eg: 671 Since @pair( T *, T * )@ is a concrete type, there are no implicit parameters passed to @lexcmp@; 672 hence, the generated code is identical to a function written in standard C using @void *@, yet the \CFA version is type checked to ensure members of both pairs and arguments to the comparison function match in type. 673 674 Another useful pattern enabled by reused dtype-static type instantiations is zero-cost \newterm{tag structures}. 675 Sometimes, information is only used for type checking and can be omitted at runtime. 661 676 \begin{cquote} 662 677 \lstDeleteShortInline@% … … 677 692 half_marathon; 678 693 scalar(litres) two_pools = pool + pool; 679 `marathon + pool;` // ERROR, mismatched types694 `marathon + pool;` // ERROR, mismatched types 680 695 \end{cfa} 681 696 \end{tabular} 682 697 \lstMakeShortInline@% 683 698 \end{cquote} 684 @scalar@ is a dtype-static type, so all uses have a single structure definition, containing @unsigned long@, and can share the same implementations of common functions like @?+?@. 699 Here, @scalar@ is a dtype-static type; 700 hence, all uses have a single structure definition, containing @unsigned long@, and can share the same implementations of common functions like @?+?@. 685 701 These implementations may even be separately compiled, unlike \CC template functions. 686 However, the \CFA type -checker ensures matching types are used by all calls to @?+?@, preventing nonsensical computations like adding a length to a volume.702 However, the \CFA type checker ensures matching types are used by all calls to @?+?@, preventing nonsensical computations like adding a length to a volume. 687 703 688 704 … … 690 706 \label{sec:tuples} 691 707 692 In many languages, functions can return at mostone value;708 In many languages, functions can return, at most, one value; 693 709 however, many operations have multiple outcomes, some exceptional. 694 710 Consider C's @div@ and @remquo@ functions, which return the quotient and remainder for a division of integer and float values, respectively. … … 701 717 double r = remquo( 13.5, 5.2, &q ); $\C{// return remainder, alias quotient}$ 702 718 \end{cfa} 703 @div@ aggregates the quotient/remainder in a structure, while@remquo@ aliases a parameter to an argument.719 Here, @div@ aggregates the quotient/remainder in a structure, whereas @remquo@ aliases a parameter to an argument. 704 720 Both approaches are awkward. 705 Alternatively, a programming language can directly support returning multiple values, \eg in \CFA: 721 % FIX 722 Alternatively, a programming language can directly support returning multiple values, \eg \CFA provides the following. 706 723 \begin{cfa} 707 724 [ int, int ] div( int num, int den ); $\C{// return two integers}$ … … 714 731 This approach is straightforward to understand and use; 715 732 therefore, why do few programming languages support this obvious feature or provide it awkwardly? 716 To answer, there are complex consequences that cascade through multiple aspects of the language, especially the type -system.717 This section show these consequences and how \CFA handles them.733 To answer, there are complex consequences that cascade through multiple aspects of the language, especially the type system. 734 This section shows these consequences and how \CFA handles them. 718 735 719 736 720 737 \subsection{Tuple Expressions} 721 738 722 The addition of multiple-return-value functions (MRVF ) are \emph{useless} without a syntax for accepting multiple values at the call-site.739 The addition of multiple-return-value functions (MRVFs) is \emph{useless} without a syntax for accepting multiple values at the call site. 723 740 The simplest mechanism for capturing the return values is variable assignment, allowing the values to be retrieved directly. 724 741 As such, \CFA allows assigning multiple values from a function into multiple variables, using a square-bracketed list of lvalue expressions (as above), called a \newterm{tuple}. 725 742 726 However, functions also use \newterm{composition} (nested calls), with the direct consequence that MRVFs must also support composition to be orthogonal with single-returning-value functions (SRVF ), \eg:743 However, functions also use \newterm{composition} (nested calls), with the direct consequence that MRVFs must also support composition to be orthogonal with single-returning-value functions (SRVFs), \eg, \CFA provides the following. 727 744 \begin{cfa} 728 745 printf( "%d %d\n", div( 13, 5 ) ); $\C{// return values seperated into arguments}$ 729 746 \end{cfa} 730 747 Here, the values returned by @div@ are composed with the call to @printf@ by flattening the tuple into separate arguments. 731 However, the \CFA type-system must support significantly more complex composition :748 However, the \CFA type-system must support significantly more complex composition. 732 749 \begin{cfa} 733 750 [ int, int ] foo$\(_1\)$( int ); $\C{// overloaded foo functions}$ … … 736 753 `bar`( foo( 3 ), foo( 3 ) ); 737 754 \end{cfa} 738 The type-resolver only has the tuple return-types to resolve the call to @bar@ as the @foo@ parameters are identical, which involves unifying the possible @foo@ functions with @bar@'s parameter list. 739 No combination of @foo@s are an exact match with @bar@'s parameters, so the resolver applies C conversions. 755 The type resolver only has the tuple return types to resolve the call to @bar@ as the @foo@ parameters are identical, which involves unifying the possible @foo@ functions with @bar@'s parameter list. 756 No combination of @foo@s is an exact match with @bar@'s parameters; 757 thus, the resolver applies C conversions. 758 % FIX 740 759 The minimal cost is @bar( foo@$_1$@( 3 ), foo@$_2$@( 3 ) )@, giving (@int@, {\color{ForestGreen}@int@}, @double@) to (@int@, {\color{ForestGreen}@double@}, @double@) with one {\color{ForestGreen}safe} (widening) conversion from @int@ to @double@ versus ({\color{red}@double@}, {\color{ForestGreen}@int@}, {\color{ForestGreen}@int@}) to ({\color{red}@int@}, {\color{ForestGreen}@double@}, {\color{ForestGreen}@double@}) with one {\color{red}unsafe} (narrowing) conversion from @double@ to @int@ and two safe conversions. 741 760 742 761 743 \subsection{Tuple Variables}762 \subsection{Tuple variables} 744 763 745 764 An important observation from function composition is that new variable names are not required to initialize parameters from an MRVF. 746 \CFA also allows declaration of tuple variables that can be initialized from an MRVF, since it can be awkward to declare multiple variables of different types, \eg: 765 \CFA also allows declaration of tuple variables that can be initialized from an MRVF, since it can be awkward to declare multiple variables of different types. 766 \newpage 747 767 \begin{cfa} 748 768 [ int, int ] qr = div( 13, 5 ); $\C{// tuple-variable declaration and initialization}$ 749 769 [ double, double ] qr = div( 13.5, 5.2 ); 750 770 \end{cfa} 751 where the tuple variable-name serves the same purpose as the parameter name(s).771 Here, the tuple variable name serves the same purpose as the parameter name(s). 752 772 Tuple variables can be composed of any types, except for array types, since array sizes are generally unknown in C. 753 773 754 One way to access the tuple -variable components is with assignment or composition:774 One way to access the tuple variable components is with assignment or composition. 755 775 \begin{cfa} 756 776 [ q, r ] = qr; $\C{// access tuple-variable components}$ 757 777 printf( "%d %d\n", qr ); 758 778 \end{cfa} 759 \CFA also supports \newterm{tuple indexing} to access single components of a tuple expression :779 \CFA also supports \newterm{tuple indexing} to access single components of a tuple expression. 760 780 \begin{cfa} 761 781 [int, int] * p = &qr; $\C{// tuple pointer}$ … … 768 788 769 789 770 \subsection{Flattening and Restructuring}790 \subsection{Flattening and restructuring} 771 791 772 792 In function call contexts, tuples support implicit flattening and restructuring conversions. 773 793 Tuple flattening recursively expands a tuple into the list of its basic components. 774 Tuple structuring packages a list of expressions into a value of tuple type , \eg:794 Tuple structuring packages a list of expressions into a value of tuple type. 775 795 \begin{cfa} 776 796 int f( int, int ); … … 783 803 h( x, y ); $\C{// flatten and structure}$ 784 804 \end{cfa} 785 In the call to @f@, @x@ is implicitly flattened so the components of @x@ are passed as t he two arguments.805 In the call to @f@, @x@ is implicitly flattened so the components of @x@ are passed as two arguments. 786 806 In the call to @g@, the values @y@ and @10@ are structured into a single argument of type @[int, int]@ to match the parameter type of @g@. 787 807 Finally, in the call to @h@, @x@ is flattened to yield an argument list of length 3, of which the first component of @x@ is passed as the first parameter of @h@, and the second component of @x@ and @y@ are structured into the second argument of type @[int, int]@. 788 The flexible structure of tuples permits a simple and expressive function call syntax to work seamlessly with both SRVF and MRVF, and with any number of arguments of arbitrarily complex structure. 789 790 791 \subsection{Tuple Assignment} 792 808 The flexible structure of tuples permits a simple and expressive function call syntax to work seamlessly with both SRVFs and MRVFs with any number of arguments of arbitrarily complex structure. 809 810 811 \subsection{Tuple assignment} 812 813 \enlargethispage{-10pt} 793 814 An assignment where the left side is a tuple type is called \newterm{tuple assignment}. 794 There are two kinds of tuple assignment depending on whether the right side of the assignment operator has a tuple type or a non -tuple type, called \newterm{multiple} and \newterm{mass assignment}, respectively.815 There are two kinds of tuple assignment depending on whether the right side of the assignment operator has a tuple type or a nontuple type, called \newterm{multiple} and \newterm{mass assignment}, respectively. 795 816 \begin{cfa} 796 817 int x = 10; … … 802 823 [y, x] = 3.14; $\C{// mass assignment}$ 803 824 \end{cfa} 804 Both kinds of tuple assignment have parallel semantics, so that each value on the left and right side is evaluated before any assignments occur.825 Both kinds of tuple assignment have parallel semantics, so that each value on the left and right sides is evaluated before any assignments occur. 805 826 As a result, it is possible to swap the values in two variables without explicitly creating any temporary variables or calling a function, \eg, @[x, y] = [y, x]@. 806 827 This semantics means mass assignment differs from C cascading assignment (\eg @a = b = c@) in that conversions are applied in each individual assignment, which prevents data loss from the chain of conversions that can happen during a cascading assignment. 807 For example, @[y, x] = 3.14@ performs the assignments @y = 3.14@ and @x = 3.14@, yielding @y == 3.14@ and @x == 3@; 808 whereas, C cascading assignment @y = x = 3.14@ performs the assignments @x = 3.14@ and @y = x@, yielding @3@ in @y@ and @x@. 828 For example, @[y, x] = 3.14@ performs the assignments @y = 3.14@ and @x = 3.14@, yielding @y == 3.14@ and @x == 3@, whereas C cascading assignment @y = x = 3.14@ performs the assignments @x = 3.14@ and @y = x@, yielding @3@ in @y@ and @x@. 809 829 Finally, tuple assignment is an expression where the result type is the type of the left-hand side of the assignment, just like all other assignment expressions in C. 810 This example shows mass, multiple, and cascading assignment used in one expression :830 This example shows mass, multiple, and cascading assignment used in one expression. 811 831 \begin{cfa} 812 832 [void] f( [int, int] ); … … 815 835 816 836 817 \subsection{Member Access}818 819 It is also possible to access multiple members from a single expression using a \newterm{member -access}.820 The result is a single tuple-valued expression whose type is the tuple of the types of the members , \eg:837 \subsection{Member access} 838 839 It is also possible to access multiple members from a single expression using a \newterm{member access}. 840 The result is a single tuple-valued expression whose type is the tuple of the types of the members. 821 841 \begin{cfa} 822 842 struct S { int x; double y; char * z; } s; … … 832 852 [int, int, int] y = x.[2, 0, 2]; $\C{// duplicate: [y.0, y.1, y.2] = [x.2, x.0.x.2]}$ 833 853 \end{cfa} 834 It is also possible for a member access to contain other member accesses , \eg:854 It is also possible for a member access to contain other member accesses. 835 855 \begin{cfa} 836 856 struct A { double i; int j; }; … … 899 919 900 920 Tuples also integrate with \CFA polymorphism as a kind of generic type. 901 Due to the implicit flattening and structuring conversions involved in argument passing, @otype@ and @dtype@ parameters are restricted to matching only with non -tuple types, \eg:921 Due to the implicit flattening and structuring conversions involved in argument passing, @otype@ and @dtype@ parameters are restricted to matching only with nontuple types. 902 922 \begin{cfa} 903 923 forall( otype T, dtype U ) void f( T x, U * y ); 904 924 f( [5, "hello"] ); 905 925 \end{cfa} 906 where@[5, "hello"]@ is flattened, giving argument list @5, "hello"@, and @T@ binds to @int@ and @U@ binds to @const char@.926 Here, @[5, "hello"]@ is flattened, giving argument list @5, "hello"@, and @T@ binds to @int@ and @U@ binds to @const char@. 907 927 Tuples, however, may contain polymorphic components. 908 928 For example, a plus operator can be written to sum two triples. … … 922 942 g( 5, 10.21 ); 923 943 \end{cfa} 944 \newpage 924 945 Hence, function parameter and return lists are flattened for the purposes of type unification allowing the example to pass expression resolution. 925 946 This relaxation is possible by extending the thunk scheme described by Bilson~\cite{Bilson03}. … … 932 953 933 954 934 \subsection{Variadic Tuples}955 \subsection{Variadic tuples} 935 956 \label{sec:variadic-tuples} 936 957 937 To define variadic functions, \CFA adds a new kind of type parameter, @ttype@ (tuple type).938 Matching against a @ttype@ parameter consumes all remaining argument components and packages them into a tuple, binding to the resulting tuple of types.939 In a given parameter list, there must be at mostone @ttype@ parameter that occurs last, which matches normal variadic semantics, with a strong feeling of similarity to \CCeleven variadic templates.958 To define variadic functions, \CFA adds a new kind of type parameter, \ie @ttype@ (tuple type). 959 Matching against a @ttype@ parameter consumes all the remaining argument components and packages them into a tuple, binding to the resulting tuple of types. 960 In a given parameter list, there must be, at most, one @ttype@ parameter that occurs last, which matches normal variadic semantics, with a strong feeling of similarity to \CCeleven variadic templates. 940 961 As such, @ttype@ variables are also called \newterm{argument packs}. 941 962 … … 943 964 Since nothing is known about a parameter pack by default, assertion parameters are key to doing anything meaningful. 944 965 Unlike variadic templates, @ttype@ polymorphic functions can be separately compiled. 945 For example, a generalized @sum@ function:966 For example, the following is a generalized @sum@ function. 946 967 \begin{cfa} 947 968 int sum$\(_0\)$() { return 0; } … … 952 973 \end{cfa} 953 974 Since @sum@\(_0\) does not accept any arguments, it is not a valid candidate function for the call @sum(10, 20, 30)@. 954 In order to call @sum@\(_1\), @10@ is matched with @x@, and the argument resolution moves on to the argument pack @rest@, which consumes the remainder of the argument list and @Params@ is bound to @[20, 30]@.975 In order to call @sum@\(_1\), @10@ is matched with @x@, and the argument resolution moves on to the argument pack @rest@, which consumes the remainder of the argument list, and @Params@ is bound to @[20, 30]@. 955 976 The process continues until @Params@ is bound to @[]@, requiring an assertion @int sum()@, which matches @sum@\(_0\) and terminates the recursion. 956 977 Effectively, this algorithm traces as @sum(10, 20, 30)@ $\rightarrow$ @10 + sum(20, 30)@ $\rightarrow$ @10 + (20 + sum(30))@ $\rightarrow$ @10 + (20 + (30 + sum()))@ $\rightarrow$ @10 + (20 + (30 + 0))@. 957 978 958 It is reasonable to take the @sum@ function a step further to enforce a minimum number of arguments :979 It is reasonable to take the @sum@ function a step further to enforce a minimum number of arguments. 959 980 \begin{cfa} 960 981 int sum( int x, int y ) { return x + y; } … … 963 984 } 964 985 \end{cfa} 965 One more step permits the summation of any sumable type with all arguments of the same type :986 One more step permits the summation of any sumable type with all arguments of the same type. 966 987 \begin{cfa} 967 988 trait sumable( otype T ) { … … 992 1013 This example showcases a variadic-template-like decomposition of the provided argument list. 993 1014 The individual @print@ functions allow printing a single element of a type. 994 The polymorphic @print@ allows printing any list of types, where aseach individual type has a @print@ function.1015 The polymorphic @print@ allows printing any list of types, where each individual type has a @print@ function. 995 1016 The individual print functions can be used to build up more complicated @print@ functions, such as @S@, which cannot be done with @printf@ in C. 996 1017 This mechanism is used to seamlessly print tuples in the \CFA I/O library (see Section~\ref{s:IOLibrary}). 997 1018 998 1019 Finally, it is possible to use @ttype@ polymorphism to provide arbitrary argument forwarding functions. 999 For example, it is possible to write @new@ as a library function :1020 For example, it is possible to write @new@ as a library function. 1000 1021 \begin{cfa} 1001 1022 forall( otype R, otype S ) void ?{}( pair(R, S) *, R, S ); … … 1006 1027 \end{cfa} 1007 1028 The @new@ function provides the combination of type-safe @malloc@ with a \CFA constructor call, making it impossible to forget constructing dynamically allocated objects. 1008 This function provides the type -safety of @new@ in \CC, without the need to specify the allocated type again, thanksto return-type inference.1029 This function provides the type safety of @new@ in \CC, without the need to specify the allocated type again, due to return-type inference. 1009 1030 1010 1031 … … 1012 1033 1013 1034 Tuples are implemented in the \CFA translator via a transformation into \newterm{generic types}. 1014 For each $N$, the first time an $N$-tuple is seen in a scope a generic type with $N$ type parameters is generated, \eg: 1035 For each $N$, the first time an $N$-tuple is seen in a scope, a generic type with $N$ type parameters is generated. 1036 For example, the following 1015 1037 \begin{cfa} 1016 1038 [int, int] f() { … … 1019 1041 } 1020 1042 \end{cfa} 1021 is transformed into :1043 is transformed into 1022 1044 \begin{cfa} 1023 1045 forall( dtype T0, dtype T1 | sized(T0) | sized(T1) ) struct _tuple2 { … … 1085 1107 1086 1108 The various kinds of tuple assignment, constructors, and destructors generate GNU C statement expressions. 1087 A variable is generated to store the value produced by a statement expression, since its members may need to be constructed with a non -trivial constructor and it may need to be referred to multiple time, \eg in a unique expression.1109 A variable is generated to store the value produced by a statement expression, since its members may need to be constructed with a nontrivial constructor and it may need to be referred to multiple time, \eg in a unique expression. 1088 1110 The use of statement expressions allows the translator to arbitrarily generate additional temporary variables as needed, but binds the implementation to a non-standard extension of the C language. 1089 1111 However, there are other places where the \CFA translator makes use of GNU C extensions, such as its use of nested functions, so this restriction is not new. … … 1093 1115 \section{Control Structures} 1094 1116 1095 \CFA identifies inconsistent, problematic, and missing control structures in C, a ndextends, modifies, and adds control structures to increase functionality and safety.1096 1097 1098 \subsection{\texorpdfstring{\protect\lstinline@if@ Statement}{if Statement}}1099 1100 The @if@ expression allows declarations, similar to @for@ declaration expression:1117 \CFA identifies inconsistent, problematic, and missing control structures in C, as well as extends, modifies, and adds control structures to increase functionality and safety. 1118 1119 1120 \subsection{\texorpdfstring{\protect\lstinline@if@ statement}{if statement}} 1121 1122 The @if@ expression allows declarations, similar to the @for@ declaration expression. 1101 1123 \begin{cfa} 1102 1124 if ( int x = f() ) ... $\C{// x != 0}$ … … 1105 1127 \end{cfa} 1106 1128 Unless a relational expression is specified, each variable is compared not equal to 0, which is the standard semantics for the @if@ expression, and the results are combined using the logical @&&@ operator.\footnote{\CC only provides a single declaration always compared not equal to 0.} 1107 The scope of the declaration(s) is local to the @if@ statement but exist within both the ``then'' and ``else'' clauses.1108 1109 1110 \subsection{\texorpdfstring{\protect\lstinline@switch@ Statement}{switch Statement}}1129 The scope of the declaration(s) is local to the @if@ statement but exists within both the ``then'' and ``else'' clauses. 1130 1131 1132 \subsection{\texorpdfstring{\protect\lstinline@switch@ statement}{switch statement}} 1111 1133 1112 1134 There are a number of deficiencies with the C @switch@ statements: enumerating @case@ lists, placement of @case@ clauses, scope of the switch body, and fall through between case clauses. 1113 1135 1114 C has no shorthand for specifying a list of case values, whether the list is non -contiguous or contiguous\footnote{C provides this mechanism via fall through.}.1115 \CFA provides a shorthand for a non -contiguous list:1136 C has no shorthand for specifying a list of case values, whether the list is noncontiguous or contiguous\footnote{C provides this mechanism via fall through.}. 1137 \CFA provides a shorthand for a noncontiguous list: 1116 1138 \begin{cquote} 1117 1139 \lstDeleteShortInline@% … … 1128 1150 \lstMakeShortInline@% 1129 1151 \end{cquote} 1130 for a contiguous list:\footnote{gcc has the same mechanism but awkward syntax, \lstinline@2 ...42@, as a space is required after a number, otherwise the first period is a decimal point.} 1152 for a contiguous list:\footnote{gcc has the same mechanism but awkward syntax, \lstinline@2 ...42@, as a space is required after a number; 1153 otherwise, the first period is a decimal point.} 1131 1154 \begin{cquote} 1132 1155 \lstDeleteShortInline@% … … 1159 1182 } 1160 1183 \end{cfa} 1161 \CFA precludes this form of transfer \emph{into} a control structure because it causes undefined behaviour, especially with respect to missed initialization, and provides very limited functionality.1162 1163 C allows placement of declaration within the @switch@ body and unreachable code at the start, resulting in undefined behaviour:1184 \CFA precludes this form of transfer \emph{into} a control structure because it causes an undefined behavior, especially with respect to missed initialization, and provides very limited functionality. 1185 1186 C allows placement of declaration within the @switch@ body and unreachable code at the start, resulting in an undefined behavior. 1164 1187 \begin{cfa} 1165 1188 switch ( x ) { … … 1178 1201 1179 1202 C @switch@ provides multiple entry points into the statement body, but once an entry point is selected, control continues across \emph{all} @case@ clauses until the end of the @switch@ body, called \newterm{fall through}; 1180 @case@ clauses are made disjoint by the @break@ statement. 1203 @case@ clauses are made disjoint by the @break@ 1204 \newpage 1205 \noindent 1206 statement. 1181 1207 While fall through \emph{is} a useful form of control flow, it does not match well with programmer intuition, resulting in errors from missing @break@ statements. 1182 For backward s compatibility, \CFA provides a \emph{new} control structure,@choose@, which mimics @switch@, but reverses the meaning of fall through (see Figure~\ref{f:ChooseSwitchStatements}), similar to Go.1208 For backward compatibility, \CFA provides a \emph{new} control structure, \ie @choose@, which mimics @switch@, but reverses the meaning of fall through (see Figure~\ref{f:ChooseSwitchStatements}), similar to Go. 1183 1209 1184 1210 \begin{figure} 1185 1211 \centering 1212 \fontsize{9bp}{11bp}\selectfont 1186 1213 \lstDeleteShortInline@% 1187 1214 \begin{tabular}{@{}l|@{\hspace{\parindentlnth}}l@{}} … … 1220 1247 \end{tabular} 1221 1248 \lstMakeShortInline@% 1222 \caption{\lstinline|choose| versus \lstinline|switch| Statements}1249 \caption{\lstinline|choose| versus \lstinline|switch| statements} 1223 1250 \label{f:ChooseSwitchStatements} 1251 \vspace*{-11pt} 1224 1252 \end{figure} 1225 1253 1226 Finally, Figure~\ref{f:FallthroughStatement} shows @fallthrough@ may appear in contexts other than terminating a @case@ clause , and have an explicit transfer label allowing separate cases but common final-code for a set of cases.1254 Finally, Figure~\ref{f:FallthroughStatement} shows @fallthrough@ may appear in contexts other than terminating a @case@ clause and have an explicit transfer label allowing separate cases but common final code for a set of cases. 1227 1255 The target label must be below the @fallthrough@ and may not be nested in a control structure, \ie @fallthrough@ cannot form a loop, and the target label must be at the same or higher level as the containing @case@ clause and located at the same level as a @case@ clause; 1228 1256 the target label may be case @default@, but only associated with the current @switch@/@choose@ statement. … … 1230 1258 \begin{figure} 1231 1259 \centering 1260 \fontsize{9bp}{11bp}\selectfont 1232 1261 \lstDeleteShortInline@% 1233 1262 \begin{tabular}{@{}l|@{\hspace{\parindentlnth}}l@{}} … … 1258 1287 \end{tabular} 1259 1288 \lstMakeShortInline@% 1260 \caption{\lstinline|fallthrough| Statement}1289 \caption{\lstinline|fallthrough| statement} 1261 1290 \label{f:FallthroughStatement} 1291 \vspace*{-11pt} 1262 1292 \end{figure} 1263 1293 1264 1294 1265 \subsection{\texorpdfstring{Labelled \protect\lstinline@continue@ / \protect\lstinline@break@}{Labelled continue / break}} 1295 \vspace*{-8pt} 1296 \subsection{\texorpdfstring{Labeled \protect\lstinline@continue@ / \protect\lstinline@break@}{Labeled continue / break}} 1266 1297 1267 1298 While C provides @continue@ and @break@ statements for altering control flow, both are restricted to one level of nesting for a particular control structure. 1268 Unfortunately, this restriction forces programmers to use @goto@ to achieve the equivalent control -flow for more than one level of nesting.1269 To prevent having to switch to the @goto@, \CFA extends the @continue@ and @break@ with a target label to support static multi-level exit~\cite{Buhr85}, as in Java.1299 Unfortunately, this restriction forces programmers to use @goto@ to achieve the equivalent control flow for more than one level of nesting. 1300 To prevent having to switch to the @goto@, \CFA extends @continue@ and @break@ with a target label to support static multilevel exit~\cite{Buhr85}, as in Java. 1270 1301 For both @continue@ and @break@, the target label must be directly associated with a @for@, @while@ or @do@ statement; 1271 1302 for @break@, the target label can also be associated with a @switch@, @if@ or compound (@{}@) statement. 1272 Figure~\ref{f:MultiLevelExit} shows @continue@ and @break@ indicating the specific control structure ,and the corresponding C program using only @goto@ and labels.1273 The innermost loop has 7 exit points, which cause continuation or termination of one or more of the 7 nested control-structures.1303 Figure~\ref{f:MultiLevelExit} shows @continue@ and @break@ indicating the specific control structure and the corresponding C program using only @goto@ and labels. 1304 The innermost loop has seven exit points, which cause a continuation or termination of one or more of the seven nested control structures. 1274 1305 1275 1306 \begin{figure} 1307 \fontsize{9bp}{11bp}\selectfont 1276 1308 \lstDeleteShortInline@% 1277 1309 \begin{tabular}{@{\hspace{\parindentlnth}}l|@{\hspace{\parindentlnth}}l@{\hspace{\parindentlnth}}l@{}} … … 1338 1370 \end{tabular} 1339 1371 \lstMakeShortInline@% 1340 \caption{Multi -level Exit}1372 \caption{Multilevel exit} 1341 1373 \label{f:MultiLevelExit} 1374 \vspace*{-5pt} 1342 1375 \end{figure} 1343 1376 1344 With respect to safety, both label led @continue@ and @break@ are a @goto@ restricted in the following ways:1345 \begin{ itemize}1377 With respect to safety, both labeled @continue@ and @break@ are @goto@ restricted in the following ways. 1378 \begin{list}{$\bullet$}{\topsep=4pt\itemsep=0pt\parsep=0pt} 1346 1379 \item 1347 1380 They cannot create a loop, which means only the looping constructs cause looping. … … 1349 1382 \item 1350 1383 They cannot branch into a control structure. 1351 This restriction prevents missing declarations and/or initializations at the start of a control structure resulting in undefined behaviour. 1352 \end{itemize} 1353 The advantage of the labelled @continue@/@break@ is allowing static multi-level exits without having to use the @goto@ statement, and tying control flow to the target control structure rather than an arbitrary point in a program. 1354 Furthermore, the location of the label at the \emph{beginning} of the target control structure informs the reader (eye candy) that complex control-flow is occurring in the body of the control structure. 1384 This restriction prevents missing declarations and/or initializations at the start of a control structure resulting in an undefined behavior. 1385 \end{list} 1386 The advantage of the labeled @continue@/@break@ is allowing static multilevel exits without having to use the @goto@ statement and tying control flow to the target control structure rather than an arbitrary point in a program. 1387 Furthermore, the location of the label at the \emph{beginning} of the target control structure informs the reader (eye candy) that complex control flow is 1388 occurring in the body of the control structure. 1355 1389 With @goto@, the label is at the end of the control structure, which fails to convey this important clue early enough to the reader. 1356 Finally, using an explicit target for the transfer instead of an implicit target allows new constructs to be added or removed without affecting existing constructs.1390 Finally, using an explicit target for the transfer instead of an implicit target allows new constructs to be added or removed without affecting the existing constructs. 1357 1391 Otherwise, the implicit targets of the current @continue@ and @break@, \ie the closest enclosing loop or @switch@, change as certain constructs are added or removed. 1358 1392 1359 1393 1360 \subsection{Exception Handling} 1361 1362 The following framework for \CFA exception-handling is in place, excluding some runtime type-information and virtual functions. 1394 \vspace*{-5pt} 1395 \subsection{Exception handling} 1396 1397 The following framework for \CFA exception handling is in place, excluding some runtime type information and virtual functions. 1363 1398 \CFA provides two forms of exception handling: \newterm{fix-up} and \newterm{recovery} (see Figure~\ref{f:CFAExceptionHandling})~\cite{Buhr92b,Buhr00a}. 1364 Both mechanisms provide dynamic call to a handler using dynamic name -lookup, where fix-up has dynamic return and recovery has static return from the handler.1399 Both mechanisms provide dynamic call to a handler using dynamic name lookup, where fix-up has dynamic return and recovery has static return from the handler. 1365 1400 \CFA restricts exception types to those defined by aggregate type @exception@. 1366 1401 The form of the raise dictates the set of handlers examined during propagation: \newterm{resumption propagation} (@resume@) only examines resumption handlers (@catchResume@); \newterm{terminating propagation} (@throw@) only examines termination handlers (@catch@). 1367 If @resume@ or @throw@ ha ve no exception type, it is a reresume/rethrow, meaning the currentlyexception continues propagation.1402 If @resume@ or @throw@ has no exception type, it is a reresume/rethrow, which means that the current exception continues propagation. 1368 1403 If there is no current exception, the reresume/rethrow results in a runtime error. 1369 1404 1370 1405 \begin{figure} 1406 \fontsize{9bp}{11bp}\selectfont 1407 \lstDeleteShortInline@% 1371 1408 \begin{cquote} 1372 \lstDeleteShortInline@%1373 1409 \begin{tabular}{@{}l|@{\hspace{\parindentlnth}}l@{}} 1374 1410 \multicolumn{1}{@{}c|@{\hspace{\parindentlnth}}}{\textbf{Resumption}} & \multicolumn{1}{c@{}}{\textbf{Termination}} \\ … … 1401 1437 \end{cfa} 1402 1438 \end{tabular} 1403 \lstMakeShortInline@%1404 1439 \end{cquote} 1405 \caption{\CFA Exception Handling} 1440 \lstMakeShortInline@% 1441 \caption{\CFA exception handling} 1406 1442 \label{f:CFAExceptionHandling} 1443 \vspace*{-5pt} 1407 1444 \end{figure} 1408 1445 1409 The set of exception types in a list of catch clause may include both a resumption and termination handler:1446 The set of exception types in a list of catch clauses may include both a resumption and a termination handler. 1410 1447 \begin{cfa} 1411 1448 try { … … 1421 1458 The termination handler is available because the resumption propagation did not unwind the stack. 1422 1459 1423 An additional feature is conditional matching in a catch clause :1460 An additional feature is conditional matching in a catch clause. 1424 1461 \begin{cfa} 1425 1462 try { … … 1430 1467 catch ( IOError err ) { ... } $\C{// handler error from other files}$ 1431 1468 \end{cfa} 1432 where the throw inserts the failing file-handle into the I/O exception.1433 Conditional catch cannot be trivially mimicked by other mechanisms because once an exception is caught, handler clauses in that @try@ statement are no longer eligible. .1434 1435 The resumption raise can specify an alternate stack on which to raise an exception, called a \newterm{nonlocal raise} :1469 Here, the throw inserts the failing file handle into the I/O exception. 1470 Conditional catch cannot be trivially mimicked by other mechanisms because once an exception is caught, handler clauses in that @try@ statement are no longer eligible. 1471 1472 The resumption raise can specify an alternate stack on which to raise an exception, called a \newterm{nonlocal raise}. 1436 1473 \begin{cfa} 1437 1474 resume( $\emph{exception-type}$, $\emph{alternate-stack}$ ) … … 1441 1478 Nonlocal raise is restricted to resumption to provide the exception handler the greatest flexibility because processing the exception does not unwind its stack, allowing it to continue after the handler returns. 1442 1479 1443 To facilitate nonlocal raise, \CFA provides dynamic enabling and disabling of nonlocal exception -propagation.1444 The constructs for controlling propagation of nonlocal exceptions are the @enable@ and the @disable@ blocks:1480 To facilitate nonlocal raise, \CFA provides dynamic enabling and disabling of nonlocal exception propagation. 1481 The constructs for controlling propagation of nonlocal exceptions are the @enable@ and @disable@ blocks. 1445 1482 \begin{cquote} 1446 1483 \lstDeleteShortInline@% … … 1448 1485 \begin{cfa} 1449 1486 enable $\emph{exception-type-list}$ { 1450 // allow non -local raise1487 // allow nonlocal raise 1451 1488 } 1452 1489 \end{cfa} … … 1454 1491 \begin{cfa} 1455 1492 disable $\emph{exception-type-list}$ { 1456 // disallow non -local raise1493 // disallow nonlocal raise 1457 1494 } 1458 1495 \end{cfa} … … 1462 1499 The arguments for @enable@/@disable@ specify the exception types allowed to be propagated or postponed, respectively. 1463 1500 Specifying no exception type is shorthand for specifying all exception types. 1464 Both @enable@ and @disable@ blocks can be nested, turning propagation on/off on entry, and on exit, the specified exception types are restored to their prior state. 1465 Coroutines and tasks start with non-local exceptions disabled, allowing handlers to be put in place, before non-local exceptions are explicitly enabled. 1501 Both @enable@ and @disable@ blocks can be nested; 1502 turning propagation on/off on entry and on exit, the specified exception types are restored to their prior state. 1503 Coroutines and tasks start with nonlocal exceptions disabled, allowing handlers to be put in place, before nonlocal exceptions are explicitly enabled. 1466 1504 \begin{cfa} 1467 1505 void main( mytask & t ) { $\C{// thread starts here}$ 1468 // non -local exceptions disabled1469 try { $\C{// establish handles for non -local exceptions}$1470 enable { $\C{// allow non -local exception delivery}$1506 // nonlocal exceptions disabled 1507 try { $\C{// establish handles for nonlocal exceptions}$ 1508 enable { $\C{// allow nonlocal exception delivery}$ 1471 1509 // task body 1472 1510 } … … 1476 1514 \end{cfa} 1477 1515 1478 Finally, \CFA provides a Java like @finally@ clause after the catch clauses:1516 Finally, \CFA provides a Java-like @finally@ clause after the catch clauses. 1479 1517 \begin{cfa} 1480 1518 try { … … 1485 1523 } 1486 1524 \end{cfa} 1487 The finally clause is always executed, i.e., if the try block ends normally or if an exception is raised.1525 The finally clause is always executed, \ie, if the try block ends normally or if an exception is raised. 1488 1526 If an exception is raised and caught, the handler is run before the finally clause. 1489 1527 Like a destructor (see Section~\ref{s:ConstructorsDestructors}), a finally clause can raise an exception but not if there is an exception being propagated. 1490 Mimicking the @finally@ clause with mechanisms like R AII is non-trivial when there are multiple types and local accesses.1491 1492 1493 \subsection{\texorpdfstring{\protect\lstinline{with} Statement}{with Statement}}1528 Mimicking the @finally@ clause with mechanisms like Resource Aquisition Is Initialization (RAII) is nontrivial when there are multiple types and local accesses. 1529 1530 1531 \subsection{\texorpdfstring{\protect\lstinline{with} statement}{with statement}} 1494 1532 \label{s:WithStatement} 1495 1533 1496 Heterogeneous data isoften aggregated into a structure/union.1497 To reduce syntactic noise, \CFA provides a @with@ statement (see Pascal~\cite[\S~4.F]{Pascal}) to elide aggregate member-qualification by opening a scope containing the member identifiers.1534 Heterogeneous data are often aggregated into a structure/union. 1535 To reduce syntactic noise, \CFA provides a @with@ statement (see section~4.F in the Pascal User Manual and Report~\cite{Pascal}) to elide aggregate member qualification by opening a scope containing the member identifiers. 1498 1536 \begin{cquote} 1499 1537 \vspace*{-\baselineskip}%??? … … 1523 1561 Object-oriented programming languages only provide implicit qualification for the receiver. 1524 1562 1525 In detail, the @with@ statement has the form :1563 In detail, the @with@ statement has the form 1526 1564 \begin{cfa} 1527 1565 $\emph{with-statement}$: … … 1529 1567 \end{cfa} 1530 1568 and may appear as the body of a function or nested within a function body. 1531 Each expression in the expression -list provides a type and object.1569 Each expression in the expression list provides a type and object. 1532 1570 The type must be an aggregate type. 1533 1571 (Enumerations are already opened.) 1534 The object is the implicit qualifier for the open structure -members.1572 The object is the implicit qualifier for the open structure members. 1535 1573 1536 1574 All expressions in the expression list are open in parallel within the compound statement, which is different from Pascal, which nests the openings from left to right. 1537 The difference between parallel and nesting occurs for members with the same name and type :1575 The difference between parallel and nesting occurs for members with the same name and type. 1538 1576 \begin{cfa} 1539 1577 struct S { int `i`; int j; double m; } s, w; $\C{// member i has same type in structure types S and T}$ … … 1549 1587 } 1550 1588 \end{cfa} 1551 For parallel semantics, both @s.i@ and @t.i@ are visible , so@i@ is ambiguous without qualification;1552 for nested semantics, @t.i@ hides @s.i@ , so@i@ implies @t.i@.1589 For parallel semantics, both @s.i@ and @t.i@ are visible and, therefore, @i@ is ambiguous without qualification; 1590 for nested semantics, @t.i@ hides @s.i@ and, therefore, @i@ implies @t.i@. 1553 1591 \CFA's ability to overload variables means members with the same name but different types are automatically disambiguated, eliminating most qualification when opening multiple aggregates. 1554 1592 Qualification or a cast is used to disambiguate. 1555 1593 1556 There is an interesting problem between parameters and the function -body @with@, \eg:1594 There is an interesting problem between parameters and the function body @with@. 1557 1595 \begin{cfa} 1558 1596 void ?{}( S & s, int i ) with ( s ) { $\C{// constructor}$ … … 1560 1598 } 1561 1599 \end{cfa} 1562 Here, the assignment @s.i = i@ means @s.i = s.i@, which is meaningless, and there is no mechanism to qualify the parameter @i@, making the assignment impossible using the function -body @with@.1563 To solve this problem, parameters are treated like an initialized aggregate :1600 Here, the assignment @s.i = i@ means @s.i = s.i@, which is meaningless, and there is no mechanism to qualify the parameter @i@, making the assignment impossible using the function body @with@. 1601 To solve this problem, parameters are treated like an initialized aggregate 1564 1602 \begin{cfa} 1565 1603 struct Params { … … 1568 1606 } params; 1569 1607 \end{cfa} 1570 and implicitly opened \emph{after} a function-body open, to give them higher priority: 1608 \newpage 1609 and implicitly opened \emph{after} a function body open, to give them higher priority 1571 1610 \begin{cfa} 1572 1611 void ?{}( S & s, int `i` ) with ( s ) `{` `with( $\emph{\color{red}params}$ )` { … … 1574 1613 } `}` 1575 1614 \end{cfa} 1576 Finally, a cast may be used to disambiguate among overload variables in a @with@ expression :1615 Finally, a cast may be used to disambiguate among overload variables in a @with@ expression 1577 1616 \begin{cfa} 1578 1617 with ( w ) { ... } $\C{// ambiguous, same name and no context}$ 1579 1618 with ( (S)w ) { ... } $\C{// unambiguous, cast}$ 1580 1619 \end{cfa} 1581 and @with@ expressions may be complex expressions with type reference (see Section~\ref{s:References}) to aggregate :1620 and @with@ expressions may be complex expressions with type reference (see Section~\ref{s:References}) to aggregate 1582 1621 \begin{cfa} 1583 1622 struct S { int i, j; } sv; … … 1603 1642 \CFA attempts to correct and add to C declarations, while ensuring \CFA subjectively ``feels like'' C. 1604 1643 An important part of this subjective feel is maintaining C's syntax and procedural paradigm, as opposed to functional and object-oriented approaches in other systems languages such as \CC and Rust. 1605 Maintaining the C approach means that C coding -patterns remain not only useable but idiomatic in \CFA, reducing the mental burden of retraining C programmers and switching between C and \CFA development.1644 Maintaining the C approach means that C coding patterns remain not only useable but idiomatic in \CFA, reducing the mental burden of retraining C programmers and switching between C and \CFA development. 1606 1645 Nevertheless, some features from other approaches are undeniably convenient; 1607 1646 \CFA attempts to adapt these features to the C paradigm. 1608 1647 1609 1648 1610 \subsection{Alternative Declaration Syntax}1649 \subsection{Alternative declaration syntax} 1611 1650 1612 1651 C declaration syntax is notoriously confusing and error prone. 1613 For example, many C programmers are confused by a declaration as simple as :1652 For example, many C programmers are confused by a declaration as simple as the following. 1614 1653 \begin{cquote} 1615 1654 \lstDeleteShortInline@% … … 1623 1662 \lstMakeShortInline@% 1624 1663 \end{cquote} 1625 Is this an array of 5 pointers to integers or a pointer to an array of 5integers?1664 Is this an array of five pointers to integers or a pointer to an array of five integers? 1626 1665 If there is any doubt, it implies productivity and safety issues even for basic programs. 1627 1666 Another example of confusion results from the fact that a function name and its parameters are embedded within the return type, mimicking the way the return value is used at the function's call site. 1628 For example, a function returning a pointer to an array of integers is defined and used in the following way :1667 For example, a function returning a pointer to an array of integers is defined and used in the following way. 1629 1668 \begin{cfa} 1630 1669 int `(*`f`())[`5`]` {...}; $\C{// definition}$ … … 1634 1673 While attempting to make the two contexts consistent is a laudable goal, it has not worked out in practice. 1635 1674 1636 \CFA provides its own type, variable and function declarations, using a different syntax~\cite[pp.~856--859]{Buhr94a}. 1637 The new declarations place qualifiers to the left of the base type, while C declarations place qualifiers to the right. 1675 \newpage 1676 \CFA provides its own type, variable, and function declarations, using a different syntax~\cite[pp.~856--859]{Buhr94a}. 1677 The new declarations place qualifiers to the left of the base type, whereas C declarations place qualifiers to the right. 1638 1678 The qualifiers have the same meaning but are ordered left to right to specify a variable's type. 1639 1679 \begin{cquote} … … 1661 1701 \lstMakeShortInline@% 1662 1702 \end{cquote} 1663 The only exception is bit-field specification, which always appear to the right of the base type.1703 The only exception is bit-field specification, which always appears to the right of the base type. 1664 1704 % Specifically, the character @*@ is used to indicate a pointer, square brackets @[@\,@]@ are used to represent an array or function return value, and parentheses @()@ are used to indicate a function parameter. 1665 1705 However, unlike C, \CFA type declaration tokens are distributed across all variables in the declaration list. 1666 For instance, variables @x@ and @y@ of type pointer to integer are defined in \CFA as follows:1706 For instance, variables @x@ and @y@ of type pointer to integer are defined in \CFA as 1667 1707 \begin{cquote} 1668 1708 \lstDeleteShortInline@% … … 1727 1767 \end{comment} 1728 1768 1729 All specifiers (@extern@, @static@, \etc) and qualifiers (@const@, @volatile@, \etc) are used in the normal way with the new declarations and also appear left to right , \eg:1769 All specifiers (@extern@, @static@, \etc) and qualifiers (@const@, @volatile@, \etc) are used in the normal way with the new declarations and also appear left to right. 1730 1770 \begin{cquote} 1731 1771 \lstDeleteShortInline@% 1732 1772 \begin{tabular}{@{}l@{\hspace{2\parindentlnth}}l@{\hspace{2\parindentlnth}}l@{}} 1733 1773 \multicolumn{1}{@{}c@{\hspace{2\parindentlnth}}}{\textbf{\CFA}} & \multicolumn{1}{c@{\hspace{2\parindentlnth}}}{\textbf{C}} \\ 1734 \begin{cfa} 1774 \begin{cfa}[basicstyle=\linespread{0.9}\fontsize{9bp}{12bp}\selectfont\sf] 1735 1775 extern const * const int x; 1736 1776 static const * [5] const int y; 1737 1777 \end{cfa} 1738 1778 & 1739 \begin{cfa} 1779 \begin{cfa}[basicstyle=\linespread{0.9}\fontsize{9bp}{12bp}\selectfont\sf] 1740 1780 int extern const * const x; 1741 1781 static const int (* const y)[5] 1742 1782 \end{cfa} 1743 1783 & 1744 \begin{cfa} 1784 \begin{cfa}[basicstyle=\linespread{0.9}\fontsize{9bp}{12bp}\selectfont\sf] 1745 1785 // external const pointer to const int 1746 1786 // internal const pointer to array of 5 const int … … 1750 1790 \end{cquote} 1751 1791 Specifiers must appear at the start of a \CFA function declaration\footnote{\label{StorageClassSpecifier} 1752 The placement of a storage-class specifier other than at the beginning of the declaration specifiers in a declaration is an obsolescent feature .~\cite[\S~6.11.5(1)]{C11}}.1792 The placement of a storage-class specifier other than at the beginning of the declaration specifiers in a declaration is an obsolescent feature (see section~6.11.5(1) in ISO/IEC 9899~\cite{C11}).}. 1753 1793 1754 1794 The new declaration syntax can be used in other contexts where types are required, \eg casts and the pseudo-function @sizeof@: … … 1771 1811 1772 1812 The syntax of the new function-prototype declaration follows directly from the new function-definition syntax; 1773 a s well, parameter names are optional, \eg:1813 also, parameter names are optional. 1774 1814 \begin{cfa} 1775 1815 [ int x ] f ( /* void */ ); $\C[2.5in]{// returning int with no parameters}$ … … 1779 1819 [ * int, int ] j ( int ); $\C{// returning pointer to int and int with int parameter}$ 1780 1820 \end{cfa} 1781 This syntax allows a prototype declaration to be created by cutting and pasting source text from the function-definition header (or vice versa).1782 Like C, it is possible to declare multiple function -prototypes in a single declaration, where the return type is distributed across \emph{all} function names in the declaration list, \eg:1821 This syntax allows a prototype declaration to be created by cutting and pasting the source text from the function-definition header (or vice versa). 1822 Like C, it is possible to declare multiple function prototypes in a single declaration, where the return type is distributed across \emph{all} function names in the declaration list. 1783 1823 \begin{cquote} 1784 1824 \lstDeleteShortInline@% … … 1795 1835 \lstMakeShortInline@% 1796 1836 \end{cquote} 1797 where\CFA allows the last function in the list to define its body.1798 1799 The syntax for pointers to \CFA functions specifies the pointer name on the right , \eg:1837 Here, \CFA allows the last function in the list to define its body. 1838 1839 The syntax for pointers to \CFA functions specifies the pointer name on the right. 1800 1840 \begin{cfa} 1801 1841 * [ int x ] () fp; $\C{// pointer to function returning int with no parameters}$ … … 1804 1844 * [ * int, int ] ( int ) jp; $\C{// pointer to function returning pointer to int and int with int parameter}\CRT$ 1805 1845 \end{cfa} 1806 Note, the name of the function pointer is specified last, as for other variable declarations. 1807 1808 Finally, new \CFA declarations may appear together with C declarations in the same program block, but cannot be mixed within a specific declaration. 1809 Therefore, a programmer has the option of either continuing to use traditional C declarations or take advantage of the new style. 1810 Clearly, both styles need to be supported for some time due to existing C-style header-files, particularly for UNIX-like systems. 1846 \newpage 1847 \noindent 1848 Note that the name of the function pointer is specified last, as for other variable declarations. 1849 1850 Finally, new \CFA declarations may appear together with C declarations in the same program block but cannot be mixed within a specific declaration. 1851 Therefore, a programmer has the option of either continuing to use traditional C declarations or taking advantage of the new style. 1852 Clearly, both styles need to be supported for some time due to existing C-style header files, particularly for UNIX-like systems. 1811 1853 1812 1854 … … 1816 1858 All variables in C have an \newterm{address}, a \newterm{value}, and a \newterm{type}; 1817 1859 at the position in the program's memory denoted by the address, there exists a sequence of bits (the value), with the length and semantic meaning of this bit sequence defined by the type. 1818 The C type -system does not always track the relationship between a value and its address;1819 a value that does not have a corresponding address is called a \newterm{rvalue} (for ``right-hand value''), while a value that does have an address is called a\newterm{lvalue} (for ``left-hand value'').1820 For example, in @int x; x = 42;@ the variable expression @x@ on the left-hand -side of the assignment is a lvalue, while the constant expression @42@ on the right-hand-side of the assignment is arvalue.1821 Despite the nomenclature of ``left-hand'' and ``right-hand'', an expression's classification as lvalue or rvalue is entirely dependent on whether it has an address or not; in imperative programming, the address of a value is used for both reading and writing (mutating) a value, and as such, lvalues can be convertedto rvalues and read from, but rvalues cannot be mutated because they lack a location to store the updated value.1860 The C type system does not always track the relationship between a value and its address; 1861 a value that does not have a corresponding address is called an \newterm{rvalue} (for ``right-hand value''), whereas a value that does have an address is called an \newterm{lvalue} (for ``left-hand value''). 1862 For example, in @int x; x = 42;@ the variable expression @x@ on the left-hand side of the assignment is an lvalue, whereas the constant expression @42@ on the right-hand side of the assignment is an rvalue. 1863 Despite the nomenclature of ``left-hand'' and ``right-hand'', an expression's classification as an lvalue or an rvalue is entirely dependent on whether it has an address or not; in imperative programming, the address of a value is used for both reading and writing (mutating) a value, and as such, lvalues can be converted into rvalues and read from, but rvalues cannot be mutated because they lack a location to store the updated value. 1822 1864 1823 1865 Within a lexical scope, lvalue expressions have an \newterm{address interpretation} for writing a value or a \newterm{value interpretation} to read a value. 1824 For example, in @x = y@, @x@ has an address interpretation, wh ile@y@ has a value interpretation.1866 For example, in @x = y@, @x@ has an address interpretation, whereas @y@ has a value interpretation. 1825 1867 While this duality of interpretation is useful, C lacks a direct mechanism to pass lvalues between contexts, instead relying on \newterm{pointer types} to serve a similar purpose. 1826 1868 In C, for any type @T@ there is a pointer type @T *@, the value of which is the address of a value of type @T@. 1827 A pointer rvalue can be explicitly \newterm{dereferenced} to the pointed-to lvalue with the dereference operator @*?@, while the rvalue representing the address of a lvalue can be obtained with the address-of operator @&?@. 1828 1869 A pointer rvalue can be explicitly \newterm{dereferenced} to the pointed-to lvalue with the dereference operator @*?@, whereas the rvalue representing the address of an lvalue can be obtained with the address-of operator @&?@. 1829 1870 \begin{cfa} 1830 1871 int x = 1, y = 2, * p1, * p2, ** p3; … … 1834 1875 *p2 = ((*p1 + *p2) * (**p3 - *p1)) / (**p3 - 15); 1835 1876 \end{cfa} 1836 1837 1877 Unfortunately, the dereference and address-of operators introduce a great deal of syntactic noise when dealing with pointed-to values rather than pointers, as well as the potential for subtle bugs because of pointer arithmetic. 1838 1878 For both brevity and clarity, it is desirable for the compiler to figure out how to elide the dereference operators in a complex expression such as the assignment to @*p2@ above. 1839 However, since C defines a number of forms of \newterm{pointer arithmetic}, two similar expressions involving pointers to arithmetic types (\eg @*p1 + x@ and @p1 + x@) may each have well-defined but distinct semantics, introducing the possibility that a programmer may write one when they mean the other ,and precluding any simple algorithm for elision of dereference operators.1879 However, since C defines a number of forms of \newterm{pointer arithmetic}, two similar expressions involving pointers to arithmetic types (\eg @*p1 + x@ and @p1 + x@) may each have well-defined but distinct semantics, introducing the possibility that a programmer may write one when they mean the other and precluding any simple algorithm for elision of dereference operators. 1840 1880 To solve these problems, \CFA introduces reference types @T &@; 1841 a @T &@ has exactly the same value as a @T *@, but where the @T *@ takes the address interpretation by default, a @T &@ takes the value interpretation by default, as below: 1842 1881 a @T &@ has exactly the same value as a @T *@, but where the @T *@ takes the address interpretation by default, a @T &@ takes the value interpretation by default, as below. 1843 1882 \begin{cfa} 1844 1883 int x = 1, y = 2, & r1, & r2, && r3; … … 1848 1887 r2 = ((r1 + r2) * (r3 - r1)) / (r3 - 15); $\C{// implicit dereferencing}$ 1849 1888 \end{cfa} 1850 1851 1889 Except for auto-dereferencing by the compiler, this reference example is exactly the same as the previous pointer example. 1852 Hence, a reference behaves like a variable name -- an lvalue expression which is interpreted as a value --but also has the type system track the address of that value.1853 One way to conceptualize a reference is via a rewrite rule, where the compiler inserts a dereference operator before the reference variable for each reference qualifier in the reference variable declaration , so the previous example implicitly acts like:1854 1890 Hence, a reference behaves like a variable name---an lvalue expression that is interpreted as a value---but also has the type system track the address of that value. 1891 One way to conceptualize a reference is via a rewrite rule, where the compiler inserts a dereference operator before the reference variable for each reference qualifier in the reference variable declaration; 1892 thus, the previous example implicitly acts like the following. 1855 1893 \begin{cfa} 1856 1894 `*`r2 = ((`*`r1 + `*`r2) * (`**`r3 - `*`r1)) / (`**`r3 - 15); 1857 1895 \end{cfa} 1858 1859 1896 References in \CFA are similar to those in \CC, with important improvements, which can be seen in the example above. 1860 1897 Firstly, \CFA does not forbid references to references. 1861 This provides a much more orthogonal design for library implementors, obviating the need for workarounds such as @std::reference_wrapper@.1898 This provides a much more orthogonal design for library \mbox{implementors}, obviating the need for workarounds such as @std::reference_wrapper@. 1862 1899 Secondly, \CFA references are rebindable, whereas \CC references have a fixed address. 1863 Rebinding allows \CFA references to be default -initialized (\eg to a null pointer\footnote{1864 While effort has been made into non-null reference checking in \CC and Java, the exercise seems moot for any non -managed languages (C/\CC), given that it only handles one of many different error situations, \eg using a pointer after its storage is deleted.}) and point to different addresses throughout their lifetime, like pointers.1900 Rebinding allows \CFA references to be default initialized (\eg to a null pointer\footnote{ 1901 While effort has been made into non-null reference checking in \CC and Java, the exercise seems moot for any nonmanaged languages (C/\CC), given that it only handles one of many different error situations, \eg using a pointer after its storage is deleted.}) and point to different addresses throughout their lifetime, like pointers. 1865 1902 Rebinding is accomplished by extending the existing syntax and semantics of the address-of operator in C. 1866 1903 1867 In C, the address of a lvalue is always a rvalue, as in general that address is not stored anywhere in memory,and does not itself have an address.1868 In \CFA, the address of a @T &@ is a lvalue @T *@, as the address of the underlying @T@ is stored in the reference,and can thus be mutated there.1904 In C, the address of an lvalue is always an rvalue, as, in general, that address is not stored anywhere in memory and does not itself have an address. 1905 In \CFA, the address of a @T &@ is an lvalue @T *@, as the address of the underlying @T@ is stored in the reference and can thus be mutated there. 1869 1906 The result of this rule is that any reference can be rebound using the existing pointer assignment semantics by assigning a compatible pointer into the address of the reference, \eg @&r1 = &x;@ above. 1870 1907 This rebinding occurs to an arbitrary depth of reference nesting; 1871 1908 loosely speaking, nested address-of operators produce a nested lvalue pointer up to the depth of the reference. 1872 1909 These explicit address-of operators can be thought of as ``cancelling out'' the implicit dereference operators, \eg @(&`*`)r1 = &x@ or @(&(&`*`)`*`)r3 = &(&`*`)r1@ or even @(&`*`)r2 = (&`*`)`*`r3@ for @&r2 = &r3@. 1873 More precisely: 1910 The precise rules are 1874 1911 \begin{itemize} 1875 1912 \item 1876 if @R@ is an rvalue of type @T &@$_1\cdots$ @&@$_r$, where $r \ge 1$ references (@&@ symbols), than @&R@ has type @T `*`&@$_{\color{red}2}\cdots$ @&@$_{\color{red}r}$, \ie @T@ pointer with $r-1$ references (@&@ symbols).1913 If @R@ is an rvalue of type @T &@$_1\cdots$ @&@$_r$, where $r \ge 1$ references (@&@ symbols), than @&R@ has type @T `*`&@$_{\color{red}2}\cdots$ @&@$_{\color{red}r}$, \ie @T@ pointer with $r-1$ references (@&@ symbols). 1877 1914 \item 1878 if @L@ is an lvalue of type @T &@$_1\cdots$ @&@$_l$, where $l \ge 0$ references (@&@ symbols), than @&L@ has type @T `*`&@$_{\color{red}1}\cdots$ @&@$_{\color{red}l}$, \ie @T@ pointer with $l$ references (@&@ symbols).1915 If @L@ is an lvalue of type @T &@$_1\cdots$ @&@$_l$, where $l \ge 0$ references (@&@ symbols), than @&L@ has type @T `*`&@$_{\color{red}1}\cdots$ @&@$_{\color{red}l}$, \ie @T@ pointer with $l$ references (@&@ symbols). 1879 1916 \end{itemize} 1880 Since pointers and references share the same internal representation, code using either is equally performant; in fact the \CFA compiler converts references to pointers internally, and the choice between them is made solely on convenience, \eg many pointer or value accesses. 1917 Since pointers and references share the same internal representation, code using either is equally performant; 1918 in fact, the \CFA compiler converts references into pointers internally, and the choice between them is made solely on convenience, \eg many pointer or value accesses. 1881 1919 1882 1920 By analogy to pointers, \CFA references also allow cv-qualifiers such as @const@: … … 1893 1931 There are three initialization contexts in \CFA: declaration initialization, argument/parameter binding, and return/temporary binding. 1894 1932 In each of these contexts, the address-of operator on the target lvalue is elided. 1895 The syntactic motivation is clearest when considering overloaded operator -assignment, \eg @int ?+=?(int &, int)@; given @int x, y@, the expected call syntax is @x += y@, not @&x += y@.1896 1897 More generally, this initialization of references from lvalues rather than pointers is an instance of a ``lvalue-to-reference'' conversion rather than an elision of the address-of operator;1933 The syntactic motivation is clearest when considering overloaded operator assignment, \eg @int ?+=?(int &, int)@; given @int x, y@, the expected call syntax is @x += y@, not @&x += y@. 1934 1935 More generally, this initialization of references from lvalues rather than pointers is an instance of an ``lvalue-to-reference'' conversion rather than an elision of the address-of operator; 1898 1936 this conversion is used in any context in \CFA where an implicit conversion is allowed. 1899 Similarly, use of athe value pointed to by a reference in an rvalue context can be thought of as a ``reference-to-rvalue'' conversion, and \CFA also includes a qualifier-adding ``reference-to-reference'' conversion, analogous to the @T *@ to @const T *@ conversion in standard C.1900 The final reference conversion included in \CFA is ``rvalue-to-reference'' conversion, implemented by means of an implicit temporary.1937 Similarly, use of the value pointed to by a reference in an rvalue context can be thought of as a ``reference-to-rvalue'' conversion, and \CFA also includes a qualifier-adding ``reference-to-reference'' conversion, analogous to the @T *@ to @const T *@ conversion in standard C. 1938 The final reference conversion included in \CFA is an ``rvalue-to-reference'' conversion, implemented by means of an implicit temporary. 1901 1939 When an rvalue is used to initialize a reference, it is instead used to initialize a hidden temporary value with the same lexical scope as the reference, and the reference is initialized to the address of this temporary. 1902 1940 \begin{cfa} … … 1906 1944 f( 3, x + y, (S){ 1.0, 7.0 }, (int [3]){ 1, 2, 3 } ); $\C{// pass rvalue to lvalue \(\Rightarrow\) implicit temporary}$ 1907 1945 \end{cfa} 1908 This allows complex values to be succinctly and efficiently passed to functions, without the syntactic overhead of explicit definition of a temporary variable or the runtime cost of pass-by-value. 1909 \CC allows a similar binding, but only for @const@ references; the more general semantics of \CFA are an attempt to avoid the \newterm{const poisoning} problem~\cite{Taylor10}, in which addition of a @const@ qualifier to one reference requires a cascading chain of added qualifiers. 1910 1911 1912 \subsection{Type Nesting} 1913 1914 Nested types provide a mechanism to organize associated types and refactor a subset of members into a named aggregate (\eg sub-aggregates @name@, @address@, @department@, within aggregate @employe@). 1915 Java nested types are dynamic (apply to objects), \CC are static (apply to the \lstinline[language=C++]@class@), and C hoists (refactors) nested types into the enclosing scope, meaning there is no need for type qualification. 1916 Since \CFA in not object-oriented, adopting dynamic scoping does not make sense; 1917 instead \CFA adopts \CC static nesting, using the member-selection operator ``@.@'' for type qualification, as does Java, rather than the \CC type-selection operator ``@::@'' (see Figure~\ref{f:TypeNestingQualification}). 1946 This allows complex values to be succinctly and efficiently passed to functions, without the syntactic overhead of the explicit definition of a temporary variable or the runtime cost of pass-by-value. 1947 \CC allows a similar binding, but only for @const@ references; the more general semantics of \CFA are an attempt to avoid the \newterm{const poisoning} problem~\cite{Taylor10}, in which the addition of a @const@ qualifier to one reference requires a cascading chain of added qualifiers. 1948 1949 1950 \subsection{Type nesting} 1951 1952 Nested types provide a mechanism to organize associated types and refactor a subset of members into a named aggregate (\eg subaggregates @name@, @address@, @department@, within aggregate @employe@). 1953 Java nested types are dynamic (apply to objects), \CC are static (apply to the \lstinline[language=C++]@class@), and C hoists (refactors) nested types into the enclosing scope, which means there is no need for type qualification. 1954 Since \CFA in not object oriented, adopting dynamic scoping does not make sense; 1955 instead, \CFA adopts \CC static nesting, using the member-selection operator ``@.@'' for type qualification, as does Java, rather than the \CC type-selection operator ``@::@'' (see Figure~\ref{f:TypeNestingQualification}). 1956 In the C left example, types @C@, @U@ and @T@ are implicitly hoisted outside of type @S@ into the containing block scope. 1957 In the \CFA right example, the types are not hoisted and accessible. 1958 1918 1959 \begin{figure} 1919 1960 \centering 1961 \fontsize{9bp}{11bp}\selectfont\sf 1920 1962 \lstDeleteShortInline@% 1921 1963 \begin{tabular}{@{}l@{\hspace{3em}}l|l@{}} … … 1979 2021 \end{tabular} 1980 2022 \lstMakeShortInline@% 1981 \caption{Type Nesting / Qualification}2023 \caption{Type nesting / qualification} 1982 2024 \label{f:TypeNestingQualification} 2025 \vspace*{-8pt} 1983 2026 \end{figure} 1984 In the C left example, types @C@, @U@ and @T@ are implicitly hoisted outside of type @S@ into the containing block scope. 1985 In the \CFA right example, the types are not hoisted and accessible. 1986 1987 1988 \subsection{Constructors and Destructors} 2027 2028 2029 \vspace*{-8pt} 2030 \subsection{Constructors and destructors} 1989 2031 \label{s:ConstructorsDestructors} 1990 2032 1991 One of the strengths (and weaknesses) of C is memory-management control, allowing resource release to be precisely specified versus unknown release with garbage-collected memory -management.2033 One of the strengths (and weaknesses) of C is memory-management control, allowing resource release to be precisely specified versus unknown release with garbage-collected memory management. 1992 2034 However, this manual approach is verbose, and it is useful to manage resources other than memory (\eg file handles) using the same mechanism as memory. 1993 \CC addresses these issues using R esource Aquisition Is Initialization (RAII), implemented by means of \newterm{constructor} and \newterm{destructor} functions;2035 \CC addresses these issues using RAII, implemented by means of \newterm{constructor} and \newterm{destructor} functions; 1994 2036 \CFA adopts constructors and destructors (and @finally@) to facilitate RAII. 1995 While constructors and destructors are a common feature of object-oriented programming -languages, they are an independent capability allowing \CFA to adopt them while retaining a procedural paradigm.1996 Specifically, \CFA constructors and destructors are denoted by name and first parameter -type versus name and nesting in an aggregate type.2037 While constructors and destructors are a common feature of object-oriented programming languages, they are an independent capability allowing \CFA to adopt them while retaining a procedural paradigm. 2038 Specifically, \CFA constructors and destructors are denoted by name and first parameter type versus name and nesting in an aggregate type. 1997 2039 Constructor calls seamlessly integrate with existing C initialization syntax, providing a simple and familiar syntax to C programmers and allowing constructor calls to be inserted into legacy C code with minimal code changes. 1998 2040 … … 2003 2045 The constructor and destructor have return type @void@, and the first parameter is a reference to the object type to be constructed or destructed. 2004 2046 While the first parameter is informally called the @this@ parameter, as in object-oriented languages, any variable name may be used. 2005 Both constructors and destructors allow additional parameters after the @this@ parameter for specifying values for initialization/de -initialization\footnote{2006 Destruction parameters are useful for specifying storage-management actions, such as de -initialize but not deallocate.}.2007 \begin{cfa} 2047 Both constructors and destructors allow additional parameters after the @this@ parameter for specifying values for initialization/deinitialization\footnote{ 2048 Destruction parameters are useful for specifying storage-management actions, such as deinitialize but not deallocate.}. 2049 \begin{cfa}[basicstyle=\linespread{0.9}\fontsize{9bp}{11bp}\selectfont\sf] 2008 2050 struct VLA { int size, * data; }; $\C{// variable length array of integers}$ 2009 2051 void ?{}( VLA & vla ) with ( vla ) { size = 10; data = alloc( size ); } $\C{// default constructor}$ … … 2014 2056 \end{cfa} 2015 2057 @VLA@ is a \newterm{managed type}\footnote{ 2016 A managed type affects the runtime environment versus a self-contained type.}: a type requiring a non -trivial constructor or destructor, or with a member of a managed type.2058 A managed type affects the runtime environment versus a self-contained type.}: a type requiring a nontrivial constructor or destructor, or with a member of a managed type. 2017 2059 A managed type is implicitly constructed at allocation and destructed at deallocation to ensure proper interaction with runtime resources, in this case, the @data@ array in the heap. 2018 For details of the code-generation placement of implicit constructor and destructor calls among complex executable statements see~\cite[\S~2.2]{Schluntz17}.2019 2020 \CFA also provides syntax for \newterm{initialization} and \newterm{copy} :2060 For details of the code-generation placement of implicit constructor and destructor calls among complex executable statements, see section~2.2 in the work of Schlintz~\cite{Schluntz17}. 2061 2062 \CFA also provides syntax for \newterm{initialization} and \newterm{copy}. 2021 2063 \begin{cfa} 2022 2064 void ?{}( VLA & vla, int size, char fill = '\0' ) { $\C{// initialization}$ … … 2027 2069 } 2028 2070 \end{cfa} 2029 (Note ,the example is purposely simplified using shallow-copy semantics.)2030 An initialization constructor -call has the same syntax as a C initializer, except the initialization values are passed as arguments to a matching constructor (number and type of paremeters).2071 (Note that the example is purposely simplified using shallow-copy semantics.) 2072 An initialization constructor call has the same syntax as a C initializer, except that the initialization values are passed as arguments to a matching constructor (number and type of parameters). 2031 2073 \begin{cfa} 2032 2074 VLA va = `{` 20, 0 `}`, * arr = alloc()`{` 5, 0 `}`; 2033 2075 \end{cfa} 2034 Note , the use of a \newterm{constructor expression} to initialize the storage from the dynamic storage-allocation.2076 Note the use of a \newterm{constructor expression} to initialize the storage from the dynamic storage allocation. 2035 2077 Like \CC, the copy constructor has two parameters, the second of which is a value parameter with the same type as the first parameter; 2036 2078 appropriate care is taken to not recursively call the copy constructor when initializing the second parameter. … … 2038 2080 \CFA constructors may be explicitly called, like Java, and destructors may be explicitly called, like \CC. 2039 2081 Explicit calls to constructors double as a \CC-style \emph{placement syntax}, useful for construction of members in user-defined constructors and reuse of existing storage allocations. 2040 Like the other operators in \CFA, there is a concise syntax for constructor/destructor function calls :2082 Like the other operators in \CFA, there is a concise syntax for constructor/destructor function calls. 2041 2083 \begin{cfa} 2042 2084 { … … 2054 2096 To provide a uniform type interface for @otype@ polymorphism, the \CFA compiler automatically generates a default constructor, copy constructor, assignment operator, and destructor for all types. 2055 2097 These default functions can be overridden by user-generated versions. 2056 For compatibility with the standard behavio ur of C, the default constructor and destructor for all basic, pointer, and reference types do nothing, whilethe copy constructor and assignment operator are bitwise copies;2057 if default zero -initialization is desired, the default constructors can be overridden.2098 For compatibility with the standard behavior of C, the default constructor and destructor for all basic, pointer, and reference types do nothing, whereas the copy constructor and assignment operator are bitwise copies; 2099 if default zero initialization is desired, the default constructors can be overridden. 2058 2100 For user-generated types, the four functions are also automatically generated. 2059 2101 @enum@ types are handled the same as their underlying integral type, and unions are also bitwise copied and no-op initialized and destructed. 2060 2102 For compatibility with C, a copy constructor from the first union member type is also defined. 2061 For @struct@ types, each of the four functions areimplicitly defined to call their corresponding functions on each member of the struct.2062 To better simulate the behavio ur of C initializers, a set of \newterm{member constructors} is also generated for structures.2063 A constructor is generated for each non -empty prefix of a structure's member-list to copy-construct the members passed as parameters and default-construct the remaining members.2103 For @struct@ types, each of the four functions is implicitly defined to call their corresponding functions on each member of the struct. 2104 To better simulate the behavior of C initializers, a set of \newterm{member constructors} is also generated for structures. 2105 A constructor is generated for each nonempty prefix of a structure's member list to copy-construct the members passed as parameters and default-construct the remaining members. 2064 2106 To allow users to limit the set of constructors available for a type, when a user declares any constructor or destructor, the corresponding generated function and all member constructors for that type are hidden from expression resolution; 2065 similarly, the generated default constructor is hidden upon declaration of any constructor.2107 similarly, the generated default constructor is hidden upon the declaration of any constructor. 2066 2108 These semantics closely mirror the rule for implicit declaration of constructors in \CC\cite[p.~186]{ANSI98:C++}. 2067 2109 2068 In some circumstance programmers may not wish to have implicit constructor and destructor generation and calls. 2069 In these cases, \CFA provides the initialization syntax \lstinline|S x `@=` {}|, and the object becomes unmanaged, so implicit constructor and destructor calls are not generated. 2110 In some circumstance, programmers may not wish to have implicit constructor and destructor generation and calls. 2111 In these cases, \CFA provides the initialization syntax \lstinline|S x `@=` {}|, and the object becomes unmanaged; 2112 hence, implicit \mbox{constructor} and destructor calls are not generated. 2070 2113 Any C initializer can be the right-hand side of an \lstinline|@=| initializer, \eg \lstinline|VLA a @= { 0, 0x0 }|, with the usual C initialization semantics. 2071 2114 The same syntax can be used in a compound literal, \eg \lstinline|a = (VLA)`@`{ 0, 0x0 }|, to create a C-style literal. 2072 The point of \lstinline|@=| is to provide a migration path from legacy C code to \CFA, by providing a mechanism to incrementally convert to implicit initialization.2115 The point of \lstinline|@=| is to provide a migration path from legacy C code to \CFA, by providing a mechanism to incrementally convert into implicit initialization. 2073 2116 2074 2117 … … 2078 2121 \section{Literals} 2079 2122 2080 C already includes limited polymorphism for literals -- @0@ can be either an integer or a pointer literal, depending on context, whilethe syntactic forms of literals of the various integer and float types are very similar, differing from each other only in suffix.2081 In keeping with the general \CFA approach of adding features while respecting the ``C -style'' of doing things, C's polymorphic constants and typed literal syntax are extended to interoperate with user-defined types, while maintaining a backwards-compatible semantics.2123 C already includes limited polymorphism for literals---@0@ can be either an integer or a pointer literal, depending on context, whereas the syntactic forms of literals of the various integer and float types are very similar, differing from each other only in suffix. 2124 In keeping with the general \CFA approach of adding features while respecting the ``C style'' of doing things, C's polymorphic constants and typed literal syntax are extended to interoperate with user-defined types, while maintaining a backward-compatible semantics. 2082 2125 2083 2126 A simple example is allowing the underscore, as in Ada, to separate prefixes, digits, and suffixes in all \CFA constants, \eg @0x`_`1.ffff`_`ffff`_`p`_`128`_`l@, where the underscore is also the standard separator in C identifiers. 2084 \CC uses a single quote as a separator but it is restricted among digits, precluding its use in the literal prefix or suffix, \eg @0x1.ffff@@`'@@ffffp128l@, and causes problems with most IDEs, which must be extended to deal with this alternate use of the single quote.2127 \CC uses a single quote as a separator, but it is restricted among digits, precluding its use in the literal prefix or suffix, \eg @0x1.ffff@@`'@@ffffp128l@, and causes problems with most integrated development environments (IDEs), which must be extended to deal with this alternate use of the single quote. 2085 2128 2086 2129 … … 2125 2168 2126 2169 In C, @0@ has the special property that it is the only ``false'' value; 2127 by the standard, any value that compares equal to @0@ is false, wh ileany value that compares unequal to @0@ is true.2128 As such, an expression @x@ in any boolean context (such as the condition of an @if@ or @while@ statement, or the arguments to @&&@, @||@, or @?:@\,) can be rewritten as @x != 0@ without changing its semantics.2170 by the standard, any value that compares equal to @0@ is false, whereas any value that compares unequal to @0@ is true. 2171 As such, an expression @x@ in any Boolean context (such as the condition of an @if@ or @while@ statement, or the arguments to @&&@, @||@, or @?:@\,) can be rewritten as @x != 0@ without changing its semantics. 2129 2172 Operator overloading in \CFA provides a natural means to implement this truth-value comparison for arbitrary types, but the C type system is not precise enough to distinguish an equality comparison with @0@ from an equality comparison with an arbitrary integer or pointer. 2130 2173 To provide this precision, \CFA introduces a new type @zero_t@ as the type of literal @0@ (somewhat analagous to @nullptr_t@ and @nullptr@ in \CCeleven); … … 2132 2175 With this addition, \CFA rewrites @if (x)@ and similar expressions to @if ( (x) != 0 )@ or the appropriate analogue, and any type @T@ is ``truthy'' by defining an operator overload @int ?!=?( T, zero_t )@. 2133 2176 \CC makes types truthy by adding a conversion to @bool@; 2134 prior to the addition of explicit cast operators in \CCeleven, this approach had the pitfall of making truthy types transitively convert ableto any numeric type;2177 prior to the addition of explicit cast operators in \CCeleven, this approach had the pitfall of making truthy types transitively convertible into any numeric type; 2135 2178 \CFA avoids this issue. 2136 2179 … … 2143 2186 2144 2187 2145 \subsection{User Literals}2188 \subsection{User literals} 2146 2189 2147 2190 For readability, it is useful to associate units to scale literals, \eg weight (stone, pound, kilogram) or time (seconds, minutes, hours). 2148 The left of Figure~\ref{f:UserLiteral} shows the \CFA alternative call -syntax (postfix: literal argument before function name), using the backquote, to convert basic literals into user literals.2191 The left of Figure~\ref{f:UserLiteral} shows the \CFA alternative call syntax (postfix: literal argument before function name), using the backquote, to convert basic literals into user literals. 2149 2192 The backquote is a small character, making the unit (function name) predominate. 2150 For examples, the multi -precision integer-type in Section~\ref{s:MultiPrecisionIntegers} has user literals:2193 For examples, the multiprecision integer type in Section~\ref{s:MultiPrecisionIntegers} has the following user literals. 2151 2194 {\lstset{language=CFA,moredelim=**[is][\color{red}]{|}{|},deletedelim=**[is][]{`}{`}} 2152 2195 \begin{cfa} … … 2154 2197 y = "12345678901234567890123456789"|`mp| + "12345678901234567890123456789"|`mp|; 2155 2198 \end{cfa} 2156 Because \CFA uses a standard function, all types and literals are applicable, as well as overloading and conversions, where @?`@ denotes a postfix-function name and @`@ denotes a postfix-function call.2199 Because \CFA uses a standard function, all types and literals are applicable, as well as overloading and conversions, where @?`@ denotes a postfix-function name and @`@ denotes a postfix-function call. 2157 2200 }% 2158 2201 \begin{cquote} … … 2196 2239 \end{cquote} 2197 2240 2198 The right of Figure~\ref{f:UserLiteral} shows the equivalent \CC version using the underscore for the call -syntax.2241 The right of Figure~\ref{f:UserLiteral} shows the equivalent \CC version using the underscore for the call syntax. 2199 2242 However, \CC restricts the types, \eg @unsigned long long int@ and @long double@ to represent integral and floating literals. 2200 2243 After which, user literals must match (no conversions); … … 2203 2246 \begin{figure} 2204 2247 \centering 2248 \fontsize{9bp}{11bp}\selectfont 2205 2249 \lstset{language=CFA,moredelim=**[is][\color{red}]{|}{|},deletedelim=**[is][]{`}{`}} 2206 2250 \lstDeleteShortInline@% … … 2258 2302 \end{tabular} 2259 2303 \lstMakeShortInline@% 2260 \caption{User Literal}2304 \caption{User literal} 2261 2305 \label{f:UserLiteral} 2262 2306 \end{figure} … … 2266 2310 \label{sec:libraries} 2267 2311 2268 As stated in Section~\ref{sec:poly-fns}, \CFA inherits a large corpus of library code, where other programming languages must rewrite or provide fragile inter -language communication with C.2312 As stated in Section~\ref{sec:poly-fns}, \CFA inherits a large corpus of library code, where other programming languages must rewrite or provide fragile interlanguage communication with C. 2269 2313 \CFA has replacement libraries condensing hundreds of existing C names into tens of \CFA overloaded names, all without rewriting the actual computations. 2270 In many cases, the interface is an inline wrapper providing overloading during compilation but zero cost at runtime.2314 In many cases, the interface is an inline wrapper providing overloading during compilation but of zero cost at runtime. 2271 2315 The following sections give a glimpse of the interface reduction to many C libraries. 2272 2316 In many cases, @signed@/@unsigned@ @char@, @short@, and @_Complex@ functions are available (but not shown) to ensure expression computations remain in a single type, as conversions can distort results. … … 2276 2320 2277 2321 C library @limits.h@ provides lower and upper bound constants for the basic types. 2278 \CFA name overloading is used to condense these typed constants , \eg:2322 \CFA name overloading is used to condense these typed constants. 2279 2323 \begin{cquote} 2280 2324 \lstDeleteShortInline@% … … 2295 2339 \lstMakeShortInline@% 2296 2340 \end{cquote} 2297 The result is a significant reduction in names to access typed constants , \eg:2341 The result is a significant reduction in names to access typed constants. 2298 2342 \begin{cquote} 2299 2343 \lstDeleteShortInline@% … … 2321 2365 2322 2366 C library @math.h@ provides many mathematical functions. 2323 \CFA function overloading is used to condense these mathematical functions , \eg:2367 \CFA function overloading is used to condense these mathematical functions. 2324 2368 \begin{cquote} 2325 2369 \lstDeleteShortInline@% … … 2340 2384 \lstMakeShortInline@% 2341 2385 \end{cquote} 2342 The result is a significant reduction in names to access math functions , \eg:2386 The result is a significant reduction in names to access math functions. 2343 2387 \begin{cquote} 2344 2388 \lstDeleteShortInline@% … … 2359 2403 \lstMakeShortInline@% 2360 2404 \end{cquote} 2361 While \Celeven has type-generic math ~\cite[\S~7.25]{C11}in @tgmath.h@ to provide a similar mechanism, these macros are limited, matching a function name with a single set of floating type(s).2405 While \Celeven has type-generic math (see section~7.25 of the ISO/IEC 9899\cite{C11}) in @tgmath.h@ to provide a similar mechanism, these macros are limited, matching a function name with a single set of floating type(s). 2362 2406 For example, it is impossible to overload @atan@ for both one and two arguments; 2363 instead the names @atan@ and @atan2@ are required (see Section~\ref{s:NameOverloading}).2364 The key observation is that only a restricted set of type-generic macros areprovided for a limited set of function names, which do not generalize across the type system, as in \CFA.2407 instead, the names @atan@ and @atan2@ are required (see Section~\ref{s:NameOverloading}). 2408 The key observation is that only a restricted set of type-generic macros is provided for a limited set of function names, which do not generalize across the type system, as in \CFA. 2365 2409 2366 2410 … … 2368 2412 2369 2413 C library @stdlib.h@ provides many general functions. 2370 \CFA function overloading is used to condense these utility functions , \eg:2414 \CFA function overloading is used to condense these utility functions. 2371 2415 \begin{cquote} 2372 2416 \lstDeleteShortInline@% … … 2387 2431 \lstMakeShortInline@% 2388 2432 \end{cquote} 2389 The result is a significant reduction in names to access utility functions, \eg:2433 The result is a significant reduction in names to access the utility functions. 2390 2434 \begin{cquote} 2391 2435 \lstDeleteShortInline@% … … 2406 2450 \lstMakeShortInline@% 2407 2451 \end{cquote} 2408 In addit on, there are polymorphic functions, like @min@ and @max@, that work on any type with operators@?<?@ or @?>?@.2452 In addition, there are polymorphic functions, like @min@ and @max@, that work on any type with operator @?<?@ or @?>?@. 2409 2453 2410 2454 The following shows one example where \CFA \emph{extends} an existing standard C interface to reduce complexity and provide safety. 2411 C/\Celeven provide a number of complex and overlapping storage-management operation to support the following capabilities:2412 \begin{ description}%[topsep=3pt,itemsep=2pt,parsep=0pt]2455 C/\Celeven provide a number of complex and overlapping storage-management operations to support the following capabilities. 2456 \begin{list}{}{\itemsep=0pt\parsep=0pt\labelwidth=0pt\leftmargin\parindent\itemindent-\leftmargin\let\makelabel\descriptionlabel} 2413 2457 \item[fill] 2414 2458 an allocation with a specified character. 2415 2459 \item[resize] 2416 2460 an existing allocation to decrease or increase its size. 2417 In either case, new storage may or may not be allocated and, if there is a new allocation, as much data from the existing allocation iscopied.2461 In either case, new storage may or may not be allocated, and if there is a new allocation, as much data from the existing allocation are copied. 2418 2462 For an increase in storage size, new storage after the copied data may be filled. 2463 \newpage 2419 2464 \item[align] 2420 2465 an allocation on a specified memory boundary, \eg, an address multiple of 64 or 128 for cache-line purposes. … … 2422 2467 allocation with a specified number of elements. 2423 2468 An array may be filled, resized, or aligned. 2424 \end{ description}2425 Table~\ref{t:StorageManagementOperations} shows the capabilities provided by C/\Celeven allocation -functions and how all the capabilities can be combined into two \CFA functions.2426 \CFA storage-management functions extend the C equivalents by overloading, providing shallow type -safety, and removing the need to specify the base allocation-size.2427 Figure~\ref{f:StorageAllocation} contrasts \CFA and C storage -allocation performing the same operations with the same type safety.2469 \end{list} 2470 Table~\ref{t:StorageManagementOperations} shows the capabilities provided by C/\Celeven allocation functions and how all the capabilities can be combined into two \CFA functions. 2471 \CFA storage-management functions extend the C equivalents by overloading, providing shallow type safety, and removing the need to specify the base allocation size. 2472 Figure~\ref{f:StorageAllocation} contrasts \CFA and C storage allocation performing the same operations with the same type safety. 2428 2473 2429 2474 \begin{table} 2430 \caption{Storage- Management Operations}2475 \caption{Storage-management operations} 2431 2476 \label{t:StorageManagementOperations} 2432 2477 \centering 2433 2478 \lstDeleteShortInline@% 2434 2479 \lstMakeShortInline~% 2435 \begin{tabular}{@{}r|r|l|l|l|l@{}} 2436 \multicolumn{1}{c}{}& & \multicolumn{1}{c|}{fill} & resize & align & array \\ 2437 \hline 2480 \begin{tabular}{@{}rrllll@{}} 2481 \multicolumn{1}{c}{}& & \multicolumn{1}{c}{fill} & resize & align & array \\ 2438 2482 C & ~malloc~ & no & no & no & no \\ 2439 2483 & ~calloc~ & yes (0 only) & no & no & yes \\ … … 2441 2485 & ~memalign~ & no & no & yes & no \\ 2442 2486 & ~posix_memalign~ & no & no & yes & no \\ 2443 \hline2444 2487 C11 & ~aligned_alloc~ & no & no & yes & no \\ 2445 \hline2446 2488 \CFA & ~alloc~ & yes/copy & no/yes & no & yes \\ 2447 2489 & ~align_alloc~ & yes & no & yes & yes \\ … … 2453 2495 \begin{figure} 2454 2496 \centering 2497 \fontsize{9bp}{11bp}\selectfont 2455 2498 \begin{cfa}[aboveskip=0pt,xleftmargin=0pt] 2456 2499 size_t dim = 10; $\C{// array dimension}$ … … 2490 2533 \end{tabular} 2491 2534 \lstMakeShortInline@% 2492 \caption{\CFA versus C Storage-Allocation}2535 \caption{\CFA versus C storage allocation} 2493 2536 \label{f:StorageAllocation} 2494 2537 \end{figure} 2495 2538 2496 2539 Variadic @new@ (see Section~\ref{sec:variadic-tuples}) cannot support the same overloading because extra parameters are for initialization. 2497 Hence, there are @new@ and @anew@ functions for single and array variables, and the fill value is the arguments to the constructor , \eg:2540 Hence, there are @new@ and @anew@ functions for single and array variables, and the fill value is the arguments to the constructor. 2498 2541 \begin{cfa} 2499 2542 struct S { int i, j; }; … … 2502 2545 S * as = anew( dim, 2, 3 ); $\C{// each array element initialized to 2, 3}$ 2503 2546 \end{cfa} 2504 Note ,\CC can only initialize array elements via the default constructor.2505 2506 Finally, the \CFA memory -allocator has \newterm{sticky properties} for dynamic storage: fill and alignment are remembered with an object's storage in the heap.2547 Note that \CC can only initialize array elements via the default constructor. 2548 2549 Finally, the \CFA memory allocator has \newterm{sticky properties} for dynamic storage: fill and alignment are remembered with an object's storage in the heap. 2507 2550 When a @realloc@ is performed, the sticky properties are respected, so that new storage is correctly aligned and initialized with the fill character. 2508 2551 … … 2511 2554 \label{s:IOLibrary} 2512 2555 2513 The goal of \CFA I/O is to simplify the common cases, while fully supporting polymorphism and user defined types in a consistent way.2556 The goal of \CFA I/O is to simplify the common cases, while fully supporting polymorphism and user-defined types in a consistent way. 2514 2557 The approach combines ideas from \CC and Python. 2515 2558 The \CFA header file for the I/O library is @fstream@. … … 2540 2583 \lstMakeShortInline@% 2541 2584 \end{cquote} 2542 The \CFA form has half the characters of the \CC form ,and is similar to Python I/O with respect to implicit separators.2585 The \CFA form has half the characters of the \CC form and is similar to Python I/O with respect to implicit separators. 2543 2586 Similar simplification occurs for tuple I/O, which prints all tuple values separated by ``\lstinline[showspaces=true]@, @''. 2544 2587 \begin{cfa} … … 2573 2616 \lstMakeShortInline@% 2574 2617 \end{cquote} 2575 There is a weak similarity between the \CFA logical-or operator and the Shell pipe -operator for moving data, where data flows in the correct direction for input butthe opposite direction for output.2618 There is a weak similarity between the \CFA logical-or operator and the Shell pipe operator for moving data, where data flow in the correct direction for input but in the opposite direction for output. 2576 2619 \begin{comment} 2577 2620 The implicit separator character (space/blank) is a separator not a terminator. … … 2594 2637 \end{itemize} 2595 2638 \end{comment} 2596 There are functions to set and get the separator string ,and manipulators to toggle separation on and off in the middle of output.2597 2598 2599 \subsection{Multi -precision Integers}2639 There are functions to set and get the separator string and manipulators to toggle separation on and off in the middle of output. 2640 2641 2642 \subsection{Multiprecision integers} 2600 2643 \label{s:MultiPrecisionIntegers} 2601 2644 2602 \CFA has an interface to the G MP multi-precision signed-integers~\cite{GMP}, similar to the \CC interface provided by GMP.2603 The \CFA interface wraps GMP functions into operator functions to make programming with multi -precision integers identical to using fixed-sized integers.2604 The \CFA type name for multi -precision signed-integers is @Int@ and the header file is @gmp@.2605 Figure~\ref{f:GMPInterface} shows a multi -precision factorial-program contrasting the GMP interface in \CFA and C.2606 2607 \begin{figure} 2645 \CFA has an interface to the GNU multiple precision (GMP) signed integers~\cite{GMP}, similar to the \CC interface provided by GMP. 2646 The \CFA interface wraps GMP functions into operator functions to make programming with multiprecision integers identical to using fixed-sized integers. 2647 The \CFA type name for multiprecision signed integers is @Int@ and the header file is @gmp@. 2648 Figure~\ref{f:GMPInterface} shows a multiprecision factorial program contrasting the GMP interface in \CFA and C. 2649 2650 \begin{figure}[b] 2608 2651 \centering 2652 \fontsize{9bp}{11bp}\selectfont 2609 2653 \lstDeleteShortInline@% 2610 2654 \begin{tabular}{@{}l@{\hspace{3\parindentlnth}}l@{}} … … 2637 2681 \end{tabular} 2638 2682 \lstMakeShortInline@% 2639 \caption{GMP Interface \CFA versus C}2683 \caption{GMP interface \CFA versus C} 2640 2684 \label{f:GMPInterface} 2641 2685 \end{figure} 2642 2686 2643 2687 2688 \vspace{-4pt} 2644 2689 \section{Polymorphism Evaluation} 2645 2690 \label{sec:eval} … … 2650 2695 % Though \CFA provides significant added functionality over C, these features have a low runtime penalty. 2651 2696 % In fact, it is shown that \CFA's generic programming can enable faster runtime execution than idiomatic @void *@-based C code. 2652 The experiment is a set of generic-stack micro -benchmarks~\cite{CFAStackEvaluation} in C, \CFA, and \CC (see implementations in Appendix~\ref{sec:BenchmarkStackImplementations}).2697 The experiment is a set of generic-stack microbenchmarks~\cite{CFAStackEvaluation} in C, \CFA, and \CC (see implementations in Appendix~\ref{sec:BenchmarkStackImplementations}). 2653 2698 Since all these languages share a subset essentially comprising standard C, maximal-performance benchmarks should show little runtime variance, differing only in length and clarity of source code. 2654 2699 A more illustrative comparison measures the costs of idiomatic usage of each language's features. 2655 Figure~\ref{fig:BenchmarkTest} shows the \CFA benchmark tests for a generic stack based on a singly linked -list.2700 Figure~\ref{fig:BenchmarkTest} shows the \CFA benchmark tests for a generic stack based on a singly linked list. 2656 2701 The benchmark test is similar for the other languages. 2657 2702 The experiment uses element types @int@ and @pair(short, char)@, and pushes $N=40M$ elements on a generic stack, copies the stack, clears one of the stacks, and finds the maximum value in the other stack. 2658 2703 2659 2704 \begin{figure} 2705 \fontsize{9bp}{11bp}\selectfont 2660 2706 \begin{cfa}[xleftmargin=3\parindentlnth,aboveskip=0pt,belowskip=0pt] 2661 2707 int main() { … … 2677 2723 } 2678 2724 \end{cfa} 2679 \caption{\protect\CFA Benchmark Test}2725 \caption{\protect\CFA benchmark test} 2680 2726 \label{fig:BenchmarkTest} 2727 \vspace*{-10pt} 2681 2728 \end{figure} 2682 2729 2683 The structure of each benchmark implemented is :C with @void *@-based polymorphism, \CFA with parametric polymorphism, \CC with templates, and \CC using only class inheritance for polymorphism, called \CCV.2730 The structure of each benchmark implemented is C with @void *@-based polymorphism, \CFA with parametric polymorphism, \CC with templates, and \CC using only class inheritance for polymorphism, called \CCV. 2684 2731 The \CCV variant illustrates an alternative object-oriented idiom where all objects inherit from a base @object@ class, mimicking a Java-like interface; 2685 hence runtime checks are necessary to safely down-cast objects.2686 The most notable difference among the implementations is in memory layout of generic types: \CFA and \CC inline the stack and pair elements into corresponding list and pair nodes, wh ile C and \CCV lack such a capability and instead must store generic objects via pointers to separately-allocated objects.2687 Note , the C benchmark uses unchecked casts as C has no runtime mechanism to perform such checks, while \CFA and \CC provide type-safety statically.2732 hence, runtime checks are necessary to safely downcast objects. 2733 The most notable difference among the implementations is in memory layout of generic types: \CFA and \CC inline the stack and pair elements into corresponding list and pair nodes, whereas C and \CCV lack such capability and, instead, must store generic objects via pointers to separately allocated objects. 2734 Note that the C benchmark uses unchecked casts as C has no runtime mechanism to perform such checks, whereas \CFA and \CC provide type safety statically. 2688 2735 2689 2736 Figure~\ref{fig:eval} and Table~\ref{tab:eval} show the results of running the benchmark in Figure~\ref{fig:BenchmarkTest} and its C, \CC, and \CCV equivalents. 2690 The graph plots the median of 5consecutive runs of each program, with an initial warm-up run omitted.2737 The graph plots the median of five consecutive runs of each program, with an initial warm-up run omitted. 2691 2738 All code is compiled at \texttt{-O2} by gcc or g++ 6.4.0, with all \CC code compiled as \CCfourteen. 2692 2739 The benchmarks are run on an Ubuntu 16.04 workstation with 16 GB of RAM and a 6-core AMD FX-6300 CPU with 3.5 GHz maximum clock frequency. … … 2694 2741 \begin{figure} 2695 2742 \centering 2696 \ input{timing}2697 \caption{Benchmark Timing Results (smaller is better)}2743 \resizebox{0.7\textwidth}{!}{\input{timing}} 2744 \caption{Benchmark timing results (smaller is better)} 2698 2745 \label{fig:eval} 2746 \vspace*{-10pt} 2699 2747 \end{figure} 2700 2748 2701 2749 \begin{table} 2750 \vspace*{-10pt} 2702 2751 \caption{Properties of benchmark code} 2703 2752 \label{tab:eval} 2704 2753 \centering 2754 \vspace*{-4pt} 2705 2755 \newcommand{\CT}[1]{\multicolumn{1}{c}{#1}} 2706 \begin{tabular}{ rrrrr}2707 & \CT{C} & \CT{\CFA} & \CT{\CC} & \CT{\CCV} \\ \hline2708 maximum memory usage (MB) & 10 ,001 & 2,502 & 2,503 & 11,253 \\2756 \begin{tabular}{lrrrr} 2757 & \CT{C} & \CT{\CFA} & \CT{\CC} & \CT{\CCV} \\ 2758 maximum memory usage (MB) & 10\,001 & 2\,502 & 2\,503 & 11\,253 \\ 2709 2759 source code size (lines) & 201 & 191 & 125 & 294 \\ 2710 2760 redundant type annotations (lines) & 27 & 0 & 2 & 16 \\ 2711 2761 binary size (KB) & 14 & 257 & 14 & 37 \\ 2712 2762 \end{tabular} 2763 \vspace*{-16pt} 2713 2764 \end{table} 2714 2765 2715 The C and \CCV variants are generally the slowest with the largest memory footprint, because of their less-efficient memory layout and the pointer-indirection necessary to implement generic types; 2766 \enlargethispage{-10pt} 2767 The C and \CCV variants are generally the slowest with the largest memory footprint, due to their less-efficient memory layout and the pointer indirection necessary to implement generic types; 2716 2768 this inefficiency is exacerbated by the second level of generic types in the pair benchmarks. 2717 By contrast, the \CFA and \CC variants run in roughly equivalent time for both the integer and pair because of equivalent storage layout, with the inlined libraries (\ie no separate compilation) and greater maturity of the \CC compiler contributing to its lead.2718 \CCV is slower than C largely due to the cost of runtime type -checking of down-casts (implemented with @dynamic_cast@);2769 By contrast, the \CFA and \CC variants run in roughly equivalent time for both the integer and pair because of the equivalent storage layout, with the inlined libraries (\ie no separate compilation) and greater maturity of the \CC compiler contributing to its lead. 2770 \CCV is slower than C largely due to the cost of runtime type checking of downcasts (implemented with @dynamic_cast@). 2719 2771 The outlier for \CFA, pop @pair@, results from the complexity of the generated-C polymorphic code. 2720 2772 The gcc compiler is unable to optimize some dead code and condense nested calls; … … 2722 2774 Finally, the binary size for \CFA is larger because of static linking with the \CFA libraries. 2723 2775 2724 \CFA is also competitive in terms of source code size, measured as a proxy for programmer effort. The line counts in Table~\ref{tab:eval} include implementations of @pair@ and @stack@ types for all four languages for purposes of direct comparison, though it should be noted that \CFA and \CC have pre-written data structures in their standard libraries that programmers would generally use instead. Use of these standard library types has minimal impact on the performance benchmarks, but shrinks the \CFA and \CC benchmarks to 39 and 42 lines, respectively.2776 \CFA is also competitive in terms of source code size, measured as a proxy for programmer effort. The line counts in Table~\ref{tab:eval} include implementations of @pair@ and @stack@ types for all four languages for purposes of direct comparison, although it should be noted that \CFA and \CC have prewritten data structures in their standard libraries that programmers would generally use instead. Use of these standard library types has minimal impact on the performance benchmarks, but shrinks the \CFA and \CC benchmarks to 39 and 42 lines, respectively. 2725 2777 The difference between the \CFA and \CC line counts is primarily declaration duplication to implement separate compilation; a header-only \CFA library would be similar in length to the \CC version. 2726 On the other hand, C does not have a generic collections -library in its standard distribution, resulting in frequent reimplementation of such collection types by C programmers.2727 \CCV does not use the \CC standard template library by construction , and in factincludes the definition of @object@ and wrapper classes for @char@, @short@, and @int@ in its line count, which inflates this count somewhat, as an actual object-oriented language would include these in the standard library;2778 On the other hand, C does not have a generic collections library in its standard distribution, resulting in frequent reimplementation of such collection types by C programmers. 2779 \CCV does not use the \CC standard template library by construction and, in fact, includes the definition of @object@ and wrapper classes for @char@, @short@, and @int@ in its line count, which inflates this count somewhat, as an actual object-oriented language would include these in the standard library; 2728 2780 with their omission, the \CCV line count is similar to C. 2729 2781 We justify the given line count by noting that many object-oriented languages do not allow implementing new interfaces on library types without subclassing or wrapper types, which may be similarly verbose. 2730 2782 2731 Line -count is a fairly rough measure of code complexity;2732 another important factor is how much type information the programmer must specify manually, especially where that information is not compiler -checked.2733 Such unchecked type information produces a heavier documentation burden and increased potential for runtime bugs ,and is much less common in \CFA than C, with its manually specified function pointer arguments and format codes, or \CCV, with its extensive use of un-type-checked downcasts, \eg @object@ to @integer@ when popping a stack.2783 Line count is a fairly rough measure of code complexity; 2784 another important factor is how much type information the programmer must specify manually, especially where that information is not compiler checked. 2785 Such unchecked type information produces a heavier documentation burden and increased potential for runtime bugs and is much less common in \CFA than C, with its manually specified function pointer arguments and format codes, or \CCV, with its extensive use of un-type-checked downcasts, \eg @object@ to @integer@ when popping a stack. 2734 2786 To quantify this manual typing, the ``redundant type annotations'' line in Table~\ref{tab:eval} counts the number of lines on which the type of a known variable is respecified, either as a format specifier, explicit downcast, type-specific function, or by name in a @sizeof@, struct literal, or @new@ expression. 2735 The \CC benchmark uses two redundant type annotations to create a new stack nodes, wh ilethe C and \CCV benchmarks have several such annotations spread throughout their code.2787 The \CC benchmark uses two redundant type annotations to create a new stack nodes, whereas the C and \CCV benchmarks have several such annotations spread throughout their code. 2736 2788 The \CFA benchmark is able to eliminate all redundant type annotations through use of the polymorphic @alloc@ function discussed in Section~\ref{sec:libraries}. 2737 2789 2738 We conjecture these results scale across most generic data-types as the underlying polymorphism implement is constant. 2739 2740 2790 We conjecture that these results scale across most generic data types as the underlying polymorphism implement is constant. 2791 2792 2793 \vspace*{-8pt} 2741 2794 \section{Related Work} 2742 2795 \label{s:RelatedWork} … … 2754 2807 \CC provides three disjoint polymorphic extensions to C: overloading, inheritance, and templates. 2755 2808 The overloading is restricted because resolution does not use the return type, inheritance requires learning object-oriented programming and coping with a restricted nominal-inheritance hierarchy, templates cannot be separately compiled resulting in compilation/code bloat and poor error messages, and determining how these mechanisms interact and which to use is confusing. 2756 In contrast, \CFA has a single facility for polymorphic code supporting type-safe separate -compilation of polymorphic functions and generic (opaque) types, which uniformly leverage the C procedural paradigm.2809 In contrast, \CFA has a single facility for polymorphic code supporting type-safe separate compilation of polymorphic functions and generic (opaque) types, which uniformly leverage the C procedural paradigm. 2757 2810 The key mechanism to support separate compilation is \CFA's \emph{explicit} use of assumed type properties. 2758 Until \CC concepts~\cite{C++Concepts} are standardized (anticipated for \CCtwenty), \CC provides no way to specifythe requirements of a generic function beyond compilation errors during template expansion;2811 Until \CC concepts~\cite{C++Concepts} are standardized (anticipated for \CCtwenty), \CC provides no way of specifying the requirements of a generic function beyond compilation errors during template expansion; 2759 2812 furthermore, \CC concepts are restricted to template polymorphism. 2760 2813 2761 2814 Cyclone~\cite{Grossman06} also provides capabilities for polymorphic functions and existential types, similar to \CFA's @forall@ functions and generic types. 2762 Cyclone existential types can include function pointers in a construct similar to a virtual function -table, but these pointers must be explicitly initialized at some point in the code,a tedious and potentially error-prone process.2815 Cyclone existential types can include function pointers in a construct similar to a virtual function table, but these pointers must be explicitly initialized at some point in the code, which is a tedious and potentially error-prone process. 2763 2816 Furthermore, Cyclone's polymorphic functions and types are restricted to abstraction over types with the same layout and calling convention as @void *@, \ie only pointer types and @int@. 2764 2817 In \CFA terms, all Cyclone polymorphism must be dtype-static. 2765 2818 While the Cyclone design provides the efficiency benefits discussed in Section~\ref{sec:generic-apps} for dtype-static polymorphism, it is more restrictive than \CFA's general model. 2766 Smith and Volpano~\cite{Smith98} present Polymorphic C, an ML dialect with polymorphic functions, C-like syntax, and pointer types; it lacks many of C's features, however, most notably structure types, and so is not a practical C replacement. 2819 Smith and Volpano~\cite{Smith98} present Polymorphic C, an ML dialect with polymorphic functions, C-like syntax, and pointer types; 2820 it lacks many of C's features, most notably structure types, and hence, is not a practical C replacement. 2767 2821 2768 2822 Objective-C~\cite{obj-c-book} is an industrially successful extension to C. 2769 However, Objective-C is a radical departure from C, using an object-oriented model with message -passing.2823 However, Objective-C is a radical departure from C, using an object-oriented model with message passing. 2770 2824 Objective-C did not support type-checked generics until recently \cite{xcode7}, historically using less-efficient runtime checking of object types. 2771 The GObject~\cite{GObject} framework also adds object-oriented programming with runtime type-checking and reference-counting garbage -collection to C;2772 these features are more intrusive additions than those provided by \CFA, in addition to the runtime overhead of reference -counting.2773 Vala~\cite{Vala} compiles to GObject-based C, adding the burden of learning a separate language syntax to the aforementioned demerits of GObject as a modernization path for existing C code -bases.2774 Java~\cite{Java8} included generic types in Java~5, which are type -checked at compilation and type-erased at runtime, similar to \CFA's.2775 However, in Java, each object carries its own table of method pointers, wh ile\CFA passes the method pointers separately to maintain a C-compatible layout.2825 The GObject~\cite{GObject} framework also adds object-oriented programming with runtime type-checking and reference-counting garbage collection to C; 2826 these features are more intrusive additions than those provided by \CFA, in addition to the runtime overhead of reference counting. 2827 Vala~\cite{Vala} compiles to GObject-based C, adding the burden of learning a separate language syntax to the aforementioned demerits of GObject as a modernization path for existing C code bases. 2828 Java~\cite{Java8} included generic types in Java~5, which are type checked at compilation and type erased at runtime, similar to \CFA's. 2829 However, in Java, each object carries its own table of method pointers, whereas \CFA passes the method pointers separately to maintain a C-compatible layout. 2776 2830 Java is also a garbage-collected, object-oriented language, with the associated resource usage and C-interoperability burdens. 2777 2831 2778 D~\cite{D}, Go, and Rust~\cite{Rust} are modern , compiled languages with abstraction features similar to \CFA traits, \emph{interfaces} in D and Goand \emph{traits} in Rust.2832 D~\cite{D}, Go, and Rust~\cite{Rust} are modern compiled languages with abstraction features similar to \CFA traits, \emph{interfaces} in D and Go, and \emph{traits} in Rust. 2779 2833 However, each language represents a significant departure from C in terms of language model, and none has the same level of compatibility with C as \CFA. 2780 2834 D and Go are garbage-collected languages, imposing the associated runtime overhead. 2781 2835 The necessity of accounting for data transfer between managed runtimes and the unmanaged C runtime complicates foreign-function interfaces to C. 2782 2836 Furthermore, while generic types and functions are available in Go, they are limited to a small fixed set provided by the compiler, with no language facility to define more. 2783 D restricts garbage collection to its own heap by default, wh ile Rust is not garbage-collected, and thushas a lighter-weight runtime more interoperable with C.2837 D restricts garbage collection to its own heap by default, whereas Rust is not garbage collected and, thus, has a lighter-weight runtime more interoperable with C. 2784 2838 Rust also possesses much more powerful abstraction capabilities for writing generic code than Go. 2785 On the other hand, Rust's borrow -checker provides strong safety guarantees but is complex and difficult to learn and imposes a distinctly idiomatic programming style.2839 On the other hand, Rust's borrow checker provides strong safety guarantees but is complex and difficult to learn and imposes a distinctly idiomatic programming style. 2786 2840 \CFA, with its more modest safety features, allows direct ports of C code while maintaining the idiomatic style of the original source. 2787 2841 2788 2842 2789 \subsection{Tuples/Variadics} 2790 2843 \vspace*{-18pt} 2844 \subsection{Tuples/variadics} 2845 2846 \vspace*{-5pt} 2791 2847 Many programming languages have some form of tuple construct and/or variadic functions, \eg SETL, C, KW-C, \CC, D, Go, Java, ML, and Scala. 2792 2848 SETL~\cite{SETL} is a high-level mathematical programming language, with tuples being one of the primary data types. 2793 2849 Tuples in SETL allow subscripting, dynamic expansion, and multiple assignment. 2794 C provides variadic functions through @va_list@ objects, but the programmer is responsible for managing the number of arguments and their types, so the mechanism is type unsafe. 2850 C provides variadic functions through @va_list@ objects, but the programmer is responsible for managing the number of arguments and their types; 2851 thus, the mechanism is type unsafe. 2795 2852 KW-C~\cite{Buhr94a}, a predecessor of \CFA, introduced tuples to C as an extension of the C syntax, taking much of its inspiration from SETL. 2796 2853 The main contributions of that work were adding MRVF, tuple mass and multiple assignment, and record-member access. 2797 \CCeleven introduced @std::tuple@ as a library variadic template structure.2854 \CCeleven introduced @std::tuple@ as a library variadic-template structure. 2798 2855 Tuples are a generalization of @std::pair@, in that they allow for arbitrary length, fixed-size aggregation of heterogeneous values. 2799 2856 Operations include @std::get<N>@ to extract values, @std::tie@ to create a tuple of references used for assignment, and lexicographic comparisons. 2800 \CCseventeen proposes \emph{structured bindings}~\cite{Sutter15} to eliminate pre -declaring variables anduse of @std::tie@ for binding the results.2801 This extension requires the use of @auto@ to infer the types of the new variables , so complicated expressions with a non-obvious type must be documented with some other mechanism.2857 \CCseventeen proposes \emph{structured bindings}~\cite{Sutter15} to eliminate predeclaring variables and the use of @std::tie@ for binding the results. 2858 This extension requires the use of @auto@ to infer the types of the new variables; hence, complicated expressions with a nonobvious type must be documented with some other mechanism. 2802 2859 Furthermore, structured bindings are not a full replacement for @std::tie@, as it always declares new variables. 2803 2860 Like \CC, D provides tuples through a library variadic-template structure. 2804 2861 Go does not have tuples but supports MRVF. 2805 Java's variadic functions appear similar to C's but are type -safe using homogeneous arrays, which are less useful than \CFA's heterogeneously-typed variadic functions.2862 Java's variadic functions appear similar to C's but are type safe using homogeneous arrays, which are less useful than \CFA's heterogeneously typed variadic functions. 2806 2863 Tuples are a fundamental abstraction in most functional programming languages, such as Standard ML~\cite{sml}, Haskell, and Scala~\cite{Scala}, which decompose tuples using pattern matching. 2807 2864 2808 2865 2866 \vspace*{-18pt} 2809 2867 \subsection{C Extensions} 2810 2868 2811 \CC is the best known C-based language, and is similar to \CFA in that both are extensions to C with source and runtime backwards compatibility. 2812 Specific difference between \CFA and \CC have been identified in prior sections, with a final observation that \CFA has equal or fewer tokens to express the same notion in many cases. 2869 \vspace*{-5pt} 2870 \CC is the best known C-based language and is similar to \CFA in that both are extensions to C with source and runtime backward compatibility. 2871 Specific differences between \CFA and \CC have been identified in prior sections, with a final observation that \CFA has equal or fewer tokens to express the same notion in many cases. 2813 2872 The key difference in design philosophies is that \CFA is easier for C programmers to understand by maintaining a procedural paradigm and avoiding complex interactions among extensions. 2814 2873 \CC, on the other hand, has multiple overlapping features (such as the three forms of polymorphism), many of which have complex interactions with its object-oriented design. 2815 As a result, \CC has a steep learning curve for even experienced C programmers, especially when attempting to maintain performance equivalent to C legacy -code.2816 2817 There are several other C extension -languages with less usage and even more dramatic changes than \CC.2818 Objective-Cand Cyclone are two other extensions to C with different design goals than \CFA, as discussed above.2874 As a result, \CC has a steep learning curve for even experienced C programmers, especially when attempting to maintain performance equivalent to C legacy code. 2875 2876 There are several other C extension languages with less usage and even more dramatic changes than \CC. 2877 \mbox{Objective-C} and Cyclone are two other extensions to C with different design goals than \CFA, as discussed above. 2819 2878 Other languages extend C with more focused features. 2820 2879 $\mu$\CC~\cite{uC++book}, CUDA~\cite{Nickolls08}, ispc~\cite{Pharr12}, and Sierra~\cite{Leissa14} add concurrent or data-parallel primitives to C or \CC; 2821 data-parallel features have not yet been added to \CFA, but are easily incorporated within its design, wh ileconcurrency primitives similar to those in $\mu$\CC have already been added~\cite{Delisle18}.2822 Finally, CCured~\cite{Necula02} and Ironclad \CC~\cite{DeLozier13} attempt to provide a more memory-safe C by annotating pointer types with garbage collection information; type-checked polymorphism in \CFA covers several of C's memory-safety issues, but more aggressive approaches such as annotating all pointer types with their nullability or requiring runtime garbage collection are contradictory to \CFA's backward scompatibility goals.2880 data-parallel features have not yet been added to \CFA, but are easily incorporated within its design, whereas concurrency primitives similar to those in $\mu$\CC have already been added~\cite{Delisle18}. 2881 Finally, CCured~\cite{Necula02} and Ironclad \CC~\cite{DeLozier13} attempt to provide a more memory-safe C by annotating pointer types with garbage collection information; type-checked polymorphism in \CFA covers several of C's memory-safety issues, but more aggressive approaches such as annotating all pointer types with their nullability or requiring runtime garbage collection are contradictory to \CFA's backward compatibility goals. 2823 2882 2824 2883 2825 2884 \section{Conclusion and Future Work} 2826 2885 2827 The goal of \CFA is to provide an evolutionary pathway for large C development -environments to be more productive and safer, while respecting the talent and skill of C programmers.2828 While other programming languages purport to be a better C, they are in factnew and interesting languages in their own right, but not C extensions.2829 The purpose of this paper is to introduce \CFA, and showcase language features that illustrate the \CFA type -system and approaches taken to achieve the goal of evolutionary C extension.2830 The contributions are a powerful type -system using parametric polymorphism and overloading, generic types, tuples, advanced control structures, and extended declarations, which all have complex interactions.2886 The goal of \CFA is to provide an evolutionary pathway for large C development environments to be more productive and safer, while respecting the talent and skill of C programmers. 2887 While other programming languages purport to be a better C, they are, in fact, new and interesting languages in their own right, but not C extensions. 2888 The purpose of this paper is to introduce \CFA, and showcase language features that illustrate the \CFA type system and approaches taken to achieve the goal of evolutionary C extension. 2889 The contributions are a powerful type system using parametric polymorphism and overloading, generic types, tuples, advanced control structures, and extended declarations, which all have complex interactions. 2831 2890 The work is a challenging design, engineering, and implementation exercise. 2832 2891 On the surface, the project may appear as a rehash of similar mechanisms in \CC. 2833 2892 However, every \CFA feature is different than its \CC counterpart, often with extended functionality, better integration with C and its programmers, and always supporting separate compilation. 2834 All of these new features are being used by the \CFA development -team to build the \CFA runtime-system.2893 All of these new features are being used by the \CFA development team to build the \CFA runtime system. 2835 2894 Finally, we demonstrate that \CFA performance for some idiomatic cases is better than C and close to \CC, showing the design is practically applicable. 2836 2895 2837 2896 While all examples in the paper compile and run, there are ongoing efforts to reduce compilation time, provide better debugging, and add more libraries; 2838 2897 when this work is complete in early 2019, a public beta release will be available at \url{https://github.com/cforall/cforall}. 2839 There is also new work on a number of \CFA features, including arrays with size, runtime type -information, virtual functions, user-defined conversions, and modules.2840 While \CFA polymorphic functions use dynamic virtual -dispatch with low runtime overhead (see Section~\ref{sec:eval}), it is not as low as \CC template-inlining.2841 Hence it may be beneficial to provide a mechanism for performance-sensitive code.2842 Two promising approaches are an @inline@ annotation at polymorphic function call sites to create a template -specialization of the function (provided the code is visible) or placing an @inline@ annotation on polymorphic function-definitions to instantiate a specialized version for some set of types (\CC template specialization).2843 These approaches are not mutually exclusive and allow performance optimizations to be applied only when necessary, without suffering global code -bloat.2844 In general, we believe separate compilation, producing smaller code, works well with loaded hardware -caches, which may offset the benefit of larger inlined-code.2898 There is also new work on a number of \CFA features, including arrays with size, runtime type information, virtual functions, user-defined conversions, and modules. 2899 While \CFA polymorphic functions use dynamic virtual dispatch with low runtime overhead (see Section~\ref{sec:eval}), it is not as low as \CC template inlining. 2900 Hence, it may be beneficial to provide a mechanism for performance-sensitive code. 2901 Two promising approaches are an @inline@ annotation at polymorphic function call sites to create a template specialization of the function (provided the code is visible) or placing an @inline@ annotation on polymorphic function definitions to instantiate a specialized version for some set of types (\CC template specialization). 2902 These approaches are not mutually exclusive and allow performance optimizations to be applied only when necessary, without suffering global code bloat. 2903 In general, we believe separate compilation, producing smaller code, works well with loaded hardware caches, which may offset the benefit of larger inlined code. 2845 2904 2846 2905 2847 2906 \section{Acknowledgments} 2848 2907 2849 The authors would like to recognize the design assistance of Glen Ditchfield, Richard Bilson, Thierry Delisle, Andrew Beach and Brice Dobry on the features described in this paper,and thank Magnus Madsen for feedback on the writing.2850 Funding for this project has been provided by Huawei Ltd.\ (\url{http://www.huawei.com}), and Aaron Moss and Peter Buhr are partially funded by the Natural Sciences and Engineering Research Council of Canada.2908 The authors would like to recognize the design assistance of Glen Ditchfield, Richard Bilson, Thierry Delisle, Andrew Beach, and Brice Dobry on the features described in this paper and thank Magnus Madsen for feedback on the writing. 2909 Funding for this project was provided by Huawei Ltd (\url{http://www.huawei.com}), and Aaron Moss and Peter Buhr were partially funded by the Natural Sciences and Engineering Research Council of Canada. 2851 2910 2852 2911 {% … … 2931 2990 2932 2991 2992 \enlargethispage{1000pt} 2933 2993 \subsection{\CFA} 2934 2994 \label{s:CforallStack} … … 2997 3057 2998 3058 3059 \newpage 2999 3060 \subsection{\CC} 3000 3061
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