Changeset 119bb6a for doc/papers
- Timestamp:
- Feb 9, 2018, 4:30:14 PM (7 years ago)
- Branches:
- ADT, aaron-thesis, arm-eh, ast-experimental, 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
- Children:
- 92f8e18
- Parents:
- 04ba967
- File:
-
- 1 edited
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doc/papers/general/Paper.tex
r04ba967 r119bb6a 303 303 \end{lstlisting} 304 304 Here, the single name @MAX@ replaces all the C type-specific names: @SHRT_MAX@, @INT_MAX@, @DBL_MAX@. 305 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.306 In addition, several operations are defined in terms values @0@ and @1@, \eg:307 \begin{lstlisting}308 int x;309 if (x) x++ $\C{// if (x != 0) x += 1;}$310 \end{lstlisting}311 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.312 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.313 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.314 315 305 316 306 \subsection{Traits} … … 1569 1559 \section{Literals} 1570 1560 1561 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. 1562 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. 1571 1563 1572 1564 \subsection{0/1} 1573 1565 1574 \TODO{Some text already at the end of Section~\ref{sec:poly-fns}} 1566 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. 1567 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. 1568 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. 1569 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. 1570 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)@. 1571 1572 \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. 1573 In addition, several operations are defined in terms values @0@ and @1@, \eg: 1574 \begin{lstlisting} 1575 int x; 1576 if (x) x++ $\C{// if (x != 0) x += 1;}$ 1577 \end{lstlisting} 1578 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. 1579 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. 1580 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. 1575 1581 1576 1582 … … 1602 1608 \end{cfa} 1603 1609 }% 1604 1605 1610 1606 1611 \section{Evaluation} … … 1778 1783 Finally, we demonstrate that \CFA performance for some idiomatic cases is better than C and close to \CC, showing the design is practically applicable. 1779 1784 1780 There is ongoing work on a wide range of \CFA feature extensions, including reference types, arrays with size, exceptions, concurrent primitives and modules.1785 There is ongoing work on a wide range of \CFA feature extensions, including arrays with size, exceptions, concurrent primitives, modules, and user-defined conversions. 1781 1786 (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.) 1782 1787 In addition, there are interesting future directions for the polymorphism design.
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