Changeset 119bb6a

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Timestamp:
Feb 9, 2018, 4:30:14 PM (5 years ago)
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aaron-thesis, arm-eh, cleanup-dtors, deferred_resn, demangler, enum, forall-pointer-decay, jacob/cs343-translation, jenkins-sandbox, master, new-ast, new-ast-unique-expr, new-env, no_list, persistent-indexer, pthread-emulation, qualifiedEnum, resolv-new, with_gc
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92f8e18
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04ba967
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Started 0/1 section of paper

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 r04ba967 \end{lstlisting} Here, the single name @MAX@ replaces all the C type-specific names: @SHRT_MAX@, @INT_MAX@, @DBL_MAX@. As well, restricted constant overloading is allowed for the values @0@ and @1@, which have special status in C, \eg the value @0@ is both an integer and a pointer literal, so its meaning depends on context. In addition, several operations are defined in terms values @0@ and @1@, \eg: \begin{lstlisting} int x; if (x) x++                                                                      $\C{// if (x != 0) x += 1;}$ \end{lstlisting} Every @if@ and iteration statement in C compares the condition with @0@, and every increment and decrement operator is semantically equivalent to adding or subtracting the value @1@ and storing the result. Due to these rewrite rules, the values @0@ and @1@ have the types @zero_t@ and @one_t@ in \CFA, which allows overloading various operations for new types that seamlessly connect to all special @0@ and @1@ contexts. The types @zero_t@ and @one_t@ have special built in implicit conversions to the various integral types, and a conversion to pointer types for @0@, which allows standard C code involving @0@ and @1@ to work as normal. \subsection{Traits} \section{Literals} C already includes limited polymorphism for literals -- @0@ can be either an integer or a pointer literal, depending on context, while the syntactic forms of literals of the various integer and floating-point types are very similar, differing from each other only in suffix. In keeping with the general \CFA approach of adding features while respecting the C way'' of doing things, we have extended both C's polymorphic zero and typed literal syntax to interoperate with user-defined types, while maintaining a backwards-compatible semantics. \subsection{0/1} \TODO{Some text already at the end of Section~\ref{sec:poly-fns}} In C, @0@ has the special property that it is the only false'' value; by the standard, any value which compares equal to @0@ is false, while any value that compares unequal to @0@ is true. As such, an expression @x@ in any boolean context (such as the condition of an @if@ or @while@ statement, or the arguments to an @&&@, @||@, or ternary operator) can be rewritten as @x != 0@ without changing its semantics. The operator overloading feature of \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. To provide this precision, \CFA introduces a new type @zero_t@ as type type of literal @0@ (somewhat analagous to @nullptr_t@ and @nullptr@ in \CCeleven); @zero_t@ can only take the value @0@, but has implicit conversions to the integer and pointer types so that standard C code involving @0@ continues to work properly. With this addition, the \CFA compiler rewrites @if (x)@ and similar expressions to @if (x != 0)@ or the appropriate analogue, and any type @T@ can be made truthy'' by defining a single function @int ?!=?(T, zero_t)@. \TODO{Clean up and integrate this paragraph} As well, restricted constant overloading is allowed for the values @0@ and @1@, which have special status in C, \eg the value @0@ is both an integer and a pointer literal, so its meaning depends on context. In addition, several operations are defined in terms values @0@ and @1@, \eg: \begin{lstlisting} int x; if (x) x++                                                                      $\C{// if (x != 0) x += 1;}$ \end{lstlisting} Every @if@ and iteration statement in C compares the condition with @0@, and every increment and decrement operator is semantically equivalent to adding or subtracting the value @1@ and storing the result. Due to these rewrite rules, the values @0@ and @1@ have the types @zero_t@ and @one_t@ in \CFA, which allows overloading various operations for new types that seamlessly connect to all special @0@ and @1@ contexts. The types @zero_t@ and @one_t@ have special built in implicit conversions to the various integral types, and a conversion to pointer types for @0@, which allows standard C code involving @0@ and @1@ to work as normal. \end{cfa} }% \section{Evaluation} Finally, we demonstrate that \CFA performance for some idiomatic cases is better than C and close to \CC, showing the design is practically applicable. There is ongoing work on a wide range of \CFA feature extensions, including reference types, arrays with size, exceptions, concurrent primitives and modules. There is ongoing work on a wide range of \CFA feature extensions, including arrays with size, exceptions, concurrent primitives, modules, and user-defined conversions. (While all examples in the paper compile and run, a public beta-release of \CFA will take another 8--12 months to finalize these additional extensions.) In addition, there are interesting future directions for the polymorphism design.