Changeset f9c7d27 for doc/theses/aaron_moss_PhD/phd
- Timestamp:
- Sep 17, 2018, 11:43:41 AM (6 years ago)
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- 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
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- 3271166
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- doc/theses/aaron_moss_PhD/phd
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doc/theses/aaron_moss_PhD/phd/background.tex
r3271166 rf9c7d27 21 21 It is important to note that \CFA{} is not an object-oriented language. 22 22 This is a deliberate choice intended to maintain the applicability of the mental model and language idioms already possessed by C programmers. 23 This choice is in marked contrast to \CC{}, which, though it has backward-compatibility with C on the source code level, is a much larger and more complex language, and requires extensive developer re-training before they canwrite idiomatic, efficient code in \CC{}'s object-oriented paradigm.23 This choice is in marked contrast to \CC{}, which, though it has backward-compatibility with C on the source code level, is a much larger and more complex language, and requires extensive developer re-training to write idiomatic, efficient code in \CC{}'s object-oriented paradigm. 24 24 25 25 \CFA{} does have a system of implicit type conversions derived from C's ``usual arithmetic conversions''; while these conversions may be thought of as something like an inheritance hierarchy, the underlying semantics are significantly different and such an analogy is loose at best. … … 62 62 struct counter { int x; }; 63 63 64 counter& `++?`(counter& c) { ++c.x; return c; } $\C {// pre-increment}$65 counter `?++`(counter& c) { $\C {// post-increment}$64 counter& `++?`(counter& c) { ++c.x; return c; } $\C[2in]{// pre-increment}$ 65 counter `?++`(counter& c) { $\C[2in]{// post-increment}$ 66 66 counter tmp = c; ++c; return tmp; 67 67 } 68 bool `?<?`(const counter& a, const counter& b) { $\C {// comparison}$68 bool `?<?`(const counter& a, const counter& b) { $\C[2in]{// comparison}$ 69 69 return a.x < b.x; 70 70 } … … 91 91 One benefit of this design is that it allows polymorphic functions to be separately compiled. 92 92 The forward declaration !forall(otype T) T identity(T);! uniquely defines a single callable function, which may be implemented in a different file. 93 The fact that there is only one implementation of each polymorphic function also reduces compile times relative to the template-expansion approach taken by \CC{}, as well as reducing binary sizes and runtime pressure on instruction cache atby re-using a single version of each function.93 The fact that there is only one implementation of each polymorphic function also reduces compile times relative to the template-expansion approach taken by \CC{}, as well as reducing binary sizes and runtime pressure on instruction cache by re-using a single version of each function. 94 94 95 95 \subsubsection{Type Assertions} … … 117 117 118 118 This version of !twice! works for any type !S! that has an addition operator defined for it, and it could be used to satisfy the type assertion on !four_times!. 119 \CFACC{} accomplishes this by creating a wrapper function calling !twice //(2)! with !S! bound to !double!, then providing this wrapper function to !four_times!\footnote{\lstinline{twice // (2)} could also have had a type parameter named \lstinline{T}; \CFA{} specifies renaming of the type parameters, which would avoid the name conflict with the type variable \lstinline{T} of \lstinline{four_times}}.120 121 Finding appropriate functions to satisfy type assertions is essentially a recursive case of expression resolution, as it takes a name (that of the type assertion) and attempts to match it to a suitable declaration \emph{in the current scope}.122 If a polymorphic function can be used to satisfy one of its own type assertions, this recursion may not terminate, as it is possible that that function is examined as a candidate for its own typeassertion unboundedly repeatedly.119 \CFACC{} accomplishes this by creating a wrapper function calling !twice//(2)! with !S! bound to !double!, then providing this wrapper function to !four_times!\footnote{\lstinline{twice // (2)} could also have had a type parameter named \lstinline{T}; \CFA{} specifies renaming of the type parameters, which would avoid the name conflict with the type variable \lstinline{T} of \lstinline{four_times}}. 120 121 Finding appropriate functions to satisfy type assertions is essentially a recursive case of expression resolution, as it takes a name (that of the type assertion) and attempts to match it to a suitable declaration in the current scope. 122 If a polymorphic function can be used to satisfy one of its own type assertions, this recursion may not terminate, as it is possible that that function is examined as a candidate for its own assertion unboundedly repeatedly. 123 123 To avoid such infinite loops, \CFACC{} imposes a fixed limit on the possible depth of recursion, similar to that employed by most \CC{} compilers for template expansion; this restriction means that there are some semantically well-typed expressions that cannot be resolved by \CFACC{}. 124 \TODO{Update this with final state} One contribution made in the course of this thesis was modifying \CFACC{} to use the more flexible expression resolution algorithm for assertion matching, rather than the previous simplerapproach of unification on the types of the functions.124 \TODO{Update this with final state} One contribution made in the course of this thesis was modifying \CFACC{} to use the more flexible expression resolution algorithm for assertion matching, rather than the simpler but limited previous approach of unification on the types of the functions. 125 125 126 126 \subsubsection{Deleted Declarations} … … 175 175 \begin{cfa} 176 176 trait pointer_like(`otype Ptr, otype El`) { 177 El& *?(Ptr); $\C{ Ptr can be dereferenced to El}$177 El& *?(Ptr); $\C{// Ptr can be dereferenced to El}$ 178 178 }; 179 179 180 180 struct list { 181 181 int value; 182 list* next; $\C{ may omit struct on type names}$182 list* next; $\C{// may omit struct on type names}$ 183 183 }; 184 184 … … 200 200 201 201 In addition to the multiple interpretations of an expression produced by name overloading and polymorphic functions, for backward compatibility \CFA{} must support all of the implicit conversions present in C, producing further candidate interpretations for expressions. 202 As mentioned above, C does not have an inheritance hierarchy of types, but the C standard's rules for the ``usual arithmetic conversions' '\cit{} define which of the built-in tyhpes are implicitly convertable to which other types, and the relative cost of any pair of such conversions from a single source type.203 \CFA{} adds to the usual arithmetic conversions rules defining the cost of binding a polymorphic type variable in a function call; such bindings are cheaper than any \emph{unsafe} (narrowing) conversion, \eg{} !int! to !char!, but more expensive than any \emph{safe} (widening) conversion, \eg{} !int! to !double!.202 As mentioned above, C does not have an inheritance hierarchy of types, but the C standard's rules for the ``usual arithmetic conversions'\cit{} define which of the built-in types are implicitly convertible to which other types, and the relative cost of any pair of such conversions from a single source type. 203 \CFA{} adds rules to the usual arithmetic conversions defining the cost of binding a polymorphic type variable in a function call; such bindings are cheaper than any \emph{unsafe} (narrowing) conversion, \eg{} !int! to !char!, but more expensive than any \emph{safe} (widening) conversion, \eg{} !int! to !double!. 204 204 One contribution of this thesis, discussed in Section \TODO{add to resolution chapter}, is a number of refinements to this cost model to more efficiently resolve polymorphic function calls. 205 205 … … 208 208 Note that which subexpression interpretation is minimal-cost may require contextual information to disambiguate. 209 209 For instance, in the example in Section~\ref{overloading-sec}, !max(max, -max)! cannot be unambiguously resolved, but !int m = max(max, -max)! has a single minimal-cost resolution. 210 While the interpretation !int m = (int)max((double)max, -(double)max)! is also a valid interpretation, it is not minimal-cost due to the unsafe cast from the !double! result of !max! to the!int!\footnote{The two \lstinline{double} casts function as type ascriptions selecting \lstinline{double max} rather than casts from \lstinline{int max} to \lstinline{double}, and as such are zero-cost.}.211 These contextual effects make the expression resolution problem for \CFA{} both theoretically and practically difficult, but the observation driving the work in Chapter~\ref{resolution-chap} is that of the many top-level expressions in a given program, most will likely be straightforward and idiomatic so that programmers writing and maintaining the code can easily understand them; it follows that effective heuristics for common cases can bring down compiler runtime enough that a small proportion of harder-to-resolve expressions should not increase compiler runtime or memory usage inordinately.210 While the interpretation !int m = (int)max((double)max, -(double)max)! is also a valid interpretation, it is not minimal-cost due to the unsafe cast from the !double! result of !max! to !int!\footnote{The two \lstinline{double} casts function as type ascriptions selecting \lstinline{double max} rather than casts from \lstinline{int max} to \lstinline{double}, and as such are zero-cost.}. 211 These contextual effects make the expression resolution problem for \CFA{} both theoretically and practically difficult, but the observation driving the work in Chapter~\ref{resolution-chap} is that of the many top-level expressions in a given program, most are straightforward and idiomatic so that programmers writing and maintaining the code can easily understand them; it follows that effective heuristics for common cases can bring down compiler runtime enough that a small proportion of harder-to-resolve expressions does not inordinately increase overall compiler runtime or memory usage. 212 212 213 213 \subsection{Type Features} \label{type-features-sec} 214 214 215 The name overloading and polymorphism features of \CFA{} have the greatest effect on language design and compiler runtime, but there are a number of other features in the type system which have a smaller effect but are useful for code examples. 216 These features are described here. 217 215 218 \subsubsection{Reference Types} 216 219 217 % TODO mention contribution on reference rebind 218 219 \subsubsection{Lifetime Management} 220 One of the key ergonomic improvements in \CFA{} is reference types, designed and implemented by Robert Schluntz\cite{Schluntz17}. 221 Given some type !T!, a !T&! (``reference to !T!'') is essentially an automatically dereferenced pointer. 222 These types allow seamless pass-by-reference for function parameters, without the extraneous dereferencing syntax present in C; they also allow easy easy aliasing of nested values with a similarly convenient syntax. 223 A particular improvement is removing syntactic special cases for operators which take or return mutable values; for example, the use !a += b! of a compound assignment operator now matches its signature, !int& ?+=?(int&, int)!, as opposed to the previous syntactic special cases to automatically take the address of the first argument to !+=! and to mark its return value as mutable. 224 225 The C standard makes heavy use of the concept of \emph{lvalue}, an expression with a memory address; its complement, \emph{rvalue} (a non-addressable expression) is not explicitly named. 226 In \CFA{}, the distinction between lvalue and rvalue can be reframed in terms of reference and non-reference types, with the benefit of being able to express the difference in user code. 227 \CFA{} references preserve the existing qualifier-dropping implicit lvalue-to-rvalue conversion from C (\eg{} a !const volatile int&! can be implicitly copied to a bare !int!) 228 To make reference types more easily usable in legacy pass-by-value code, \CFA{} also adds an implicit rvalue-to-lvalue conversion, implemented by storing the value in a fresh compiler-generated temporary variable and passing a reference to that temporary. 229 To mitigate the ``!const! hell'' problem present in \CC{}, there is also a qualifier-dropping lvalue-to-lvalue conversion, also implemented by copying into a temporary: 230 231 \begin{cfa} 232 const int magic = 42; 233 234 void inc_print( int& x ) { printf("%d\n", ++x); } 235 236 print_inc( magic ); $\C{// legal; implicitly generated code in red below:}$ 237 238 `int tmp = magic;` $\C{// to safely strip const-qualifier}$ 239 `print_inc( tmp );` $\C{// tmp is incremented, magic is unchanged}$ 240 \end{cfa} 241 242 Despite the similar syntax, \CFA{} references are significantly more flexible than \CC{} references. 243 The primary issue with \CC{} references is that it is impossible to extract the address of the reference variable rather than the address of the referred-to variable. 244 This breaks a number of the usual compositional properties of the \CC{} type system, \eg{} a reference cannot be re-bound to another variable, nor is it possible to take a pointer to, array of, or reference to a reference. 245 \CFA{} supports all of these use cases \TODO{test array} without further added syntax. 246 The key to this syntax-free feature support is an observation made by the author that the address of a reference is a lvalue. 247 In C, the address-of operator !&x! can only be applied to lvalue expressions, and always produces an immutable rvalue; \CFA{} supports reference re-binding by assignment to the address of a reference, and pointers to references by repeating the address-of operator: 248 249 \begin{cfa} 250 int x = 2, y = 3; 251 int& r = x; $\C{// r aliases x}$ 252 &r = &y; $\C{// r now aliases y}$ 253 int** p = &&r; $\C{// p points to r}$ 254 \end{cfa} 255 256 For better compatibility with C, the \CFA{} team has chosen not to differentiate function overloads based on top-level reference types, and as such their contribution to the difficulty of \CFA{} expression resolution is largely restricted to the implementation details of normalization conversions and adapters. 257 258 \subsubsection{Resource Management} 259 260 \CFA{} also supports the RAII (``Resource Acquisition is Initialization'') idiom originated by \CC{}, thanks to the object lifetime work of Robert Schluntz\cite{Schluntz17}. 261 This idiom allows a safer and more principled approach to resource management by tying acquisition of a resource to object initialization, with the corresponding resource release executed automatically at object finalization. 262 A wide variety of conceptual resources may be conveniently managed by this scheme, including heap memory, file handles, and software locks. 263 264 \CFA{}'s implementation of RAII is based on special constructor and destructor operators, available via the !x{ ... }! constructor syntax and !^x{ ... }! destructor syntax. 265 Each type has an overridable compiler-generated zero-argument constructor, copy constructor, assignment operator, and destructor, as well as a field-wise constructor for each appropriate prefix of the member fields of !struct! types. 266 For !struct! types the default versions of these operators call their equivalents on each field of the !struct!. 267 The main implication of these object lifetime functions for expression resolution is that they are all included as implicit type assertions for !otype! type variables, with a secondary effect being an increase in code size due to the compiler-generated operators. 268 Due to these implicit type assertions, assertion resolution is pervasive in \CFA{} polymorphic functions, even those without explicit type assertions. 269 Implicitly-generated code is shown in red in the following example: 270 271 \begin{cfa} 272 struct kv { 273 int key; 274 char* value; 275 }; 276 277 `void ?{} (kv& this) {` $\C[3in]{// default constructor}$ 278 ` this.key{};` $\C[3in]{// call recursively on members}$ 279 ` this.value{}; 280 } 281 282 void ?{} (kv& this, int key) {` $\C[3in]{// partial field constructor}$ 283 ` this.key{ key }; 284 this.value{};` $\C[3in]{// default-construct missing fields}$ 285 `} 286 287 void ?{} (kv& this, int key, char* value) {` $\C[3in]{// complete field constructor}$ 288 ` this.key{ key }; 289 this.value{ value }; 290 } 291 292 void ?{} (kv& this, kv that) {` $\C[3in]{// copy constructor}$ 293 ` this.key{ that.key }; 294 this.value{ that.value }; 295 } 296 297 kv ?=? (kv& this, kv that) {` $\C[3in]{// assignment operator}$ 298 ` this.key = that.key; 299 this.value = that.value; 300 } 301 302 void ^?{} (kv& this) {` $\C[3in]{// destructor}$ 303 ` ^this.key{}; 304 ^this.value{}; 305 }` 306 307 forall(otype T `| { void ?{}(T&); void ?{}(T&, T); T ?=?(T&, T); void ^?{}(T&); }`) 308 void foo(T); 309 \end{cfa} 220 310 221 311 \subsubsection{0 and 1 Literals} 312 313 % TODO mention own motivating contribution 314 315 % TODO mention future work in user-defined implicit conversions 316 317 \subsubsection{Tuple Types} 318 319 % TODO "precludes some matching strategies" -
doc/theses/aaron_moss_PhD/phd/cfa-macros.tex
r3271166 rf9c7d27 20 20 \newcommand{\LstCommentStyle}[1]{{\lst@basicstyle{\lst@commentstyle{#1}}}} 21 21 22 \newcommand{\C}[2][ 2in]{\hfill\makebox[#1][l]{\LstCommentStyle{#2}}}22 \newcommand{\C}[2][3.5in]{\hfill\makebox[#1][l]{\LstCommentStyle{#2}}} 23 23 24 24 % CFA programming language, based on ANSI C (with some gcc additions) -
doc/theses/aaron_moss_PhD/phd/generic-types.tex
r3271166 rf9c7d27 5 5 6 6 % TODO discuss layout function algorithm, application to separate compilation 7 % TODO put a static const field in for _n_fields for each generic, describe utility for separate compilation 7 8 8 9 % TODO mention impetus for zero_t design 9 10 10 11 % TODO mention use in tuple-type implementation 12 13 % TODO pull benchmarks from Moss et al.
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