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diff --git a/docs/LanguageExtensions.html b/docs/LanguageExtensions.html index 1bccdbd2cc..e9f409ace9 100644 --- a/docs/LanguageExtensions.html +++ b/docs/LanguageExtensions.html @@ -21,6 +21,7 @@ td { <li><a href="#intro">Introduction</a></li> <li><a href="#vectors">Vectors and Extended Vectors</a></li> <li><a href="#blocks">Blocks</a></li> +<li><a href="#overloading-in-c">Function Overloading in C</a></li> <li><a href="#builtins">Builtin Functions</a> <ul> <li><a href="#__builtin_overload">__builtin_overload</a></li> @@ -55,6 +56,80 @@ href="#__builtin_shufflevector">__builtin_shufflevector</a>.</p> <p>The idea, syntax, and semantics.</p> <!-- ======================================================================= --> +<h2 id="overloading-in-c">Function Overloading in C</h2> +<!-- ======================================================================= --> + +<p>Clang provides support for C++ function overloading in C. Function overloading in C is introduced using the <tt>overloadable</tt> attribute. For example, one might provide several overloaded versions of a <tt>tgsin</tt> function that invokes the appropriate standard function computing the sine of a value with <tt>float</tt>, <tt>double</tt>, or <tt>long double</tt> precision:</p> + +<blockquote> +<pre> +#include <math.h> +float <b>__attribute__((overloadable))</b> tgsin(float x) { return sinf(x); } +double <b>__attribute__((overloadable))</b> tgsin(double x) { return sin(x); } +long double <b>__attribute__((overloadable))</b> tgsin(long double x) { return sinl(x); } +</pre> +</blockquote> + +<p>Given these declarations, one can call <tt>tgsin</tt> with a +<tt>float</tt> value to receive a <tt>float</tt> result, with a +<tt>double</tt> to receive a <tt>double</tt> result, etc. Function +overloading in C follows the rules of C++ function overloading to pick +the best overload given the call arguments, with a few C-specific +semantics:</p> +<ul> + <li>Conversion from <tt>float</tt> or <tt>double</tt> to <tt>long + double</tt> is ranked as a floating-point promotion (per C99) rather + than as a floating-point conversion (as in C++).</li> + + <li>A conversion from a pointer of type <tt>T*</tt> to a pointer of type + <tt>U*</tt> is considered a pointer conversion (with conversion + rank) if <tt>T</tt> and <tt>U</tt> are compatible types.</li> + + <li>A conversion from type <tt>T</tt> to a value of type <tt>U</tt> + is permitted if <tt>T</tt> and <tt>U</tt> are compatible types. This + conversion is given "conversion" rank.</li> +</ul> + +<p>The declaration of <tt>overloadable</tt> functions is restricted to +function declarations and definitions. Most importantly, if any +function with a given name is given the <tt>overloadable</tt> +attribute, then all function declarations and definitions with that +name (and in that scope) must have the <tt>overloadable</tt> +attribute. This rule even applies to redeclarations of functions whose original declaration had the <tt>overloadable</tt> attribute, e.g.,</p> + +<blockquote> +<pre> +int f(int) __attribute__((overloadable)); +float f(float); <i>// error: declaration of "f" must have the "overloadable" attribute</i> + +int g(int) __attribute__((overloadable)); +int g(int) { } <i>// error: redeclaration of "g" must also have the "overloadable" attribute</i> +</pre> +</blockquote> + +<p>Functions declared with the <tt>overloadable</tt> attribute have +their names mangled according to the same rules as C++ function +names. For example, the three <tt>tgsin</tt> functions in our +motivating example get the mangled names <tt>_Z5tgsinf</tt>, +<tt>_Z5tgsind</tt>, and <tt>Z5tgsine</tt>, respectively. There are two +caveats to this use of name mangling:</p> + +<ul> + + <li>Future versions of Clang may change the name mangling of + functions overloaded in C, so you should not depend on an specific + mangling. To be completely safe, we strongly urge the use of + <tt>static inline</tt> with <tt>overloadable</tt> functions.</li> + + <li>The <tt>overloadable</tt> attribute has almost no meaning when + used in C++, because names will already be mangled and functions are + already overloadable. However, when an <tt>overloadable</tt> + function occurs within an <tt>extern "C"</tt> linkage specification, + it's name <i>will</i> be mangled in the same way as it would in + C.</li> +</ul> + +<!-- ======================================================================= --> <h2 id="builtins">Builtin Functions</h2> <!-- ======================================================================= --> @@ -82,6 +157,7 @@ builtins that we need to implement.</p> functions in C. This builtin is used to implement the <tt><tgmath.h></tt> header file, but is intended to be usable for a broad variety of other similar situations, like the <tt><altivec.h></tt> header. +<b>In the future, we intend to eliminate <tt>__builtin_overload</tt> in favor of <a href="#overloading-in-c">function overloading in C</a>, which provides a better solution for type-generic "overloaded" functions.</b> </p> <p><b>Syntax:</b></p> |