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<title>Clang Language Extensions</title>
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<div id="content">
<h1>Clang Language Extensions</h1>
<ul>
<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="#builtins">Builtin Functions</a>
<ul>
<li><a href="#__builtin_overload">__builtin_overload</a></li>
<li><a href="#__builtin_shufflevector">__builtin_shufflevector</a></li>
</ul>
</li>
</ul>
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<h2 id="intro">Introduction</h2>
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<p>This document describes the language extensions provided by Clang. In
addition to the langauge extensions listed here, Clang aims to support a broad
range of GCC extensions. Please see the <a
href="http://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html">GCC manual</a> for
more information on these extensions.</p>
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<h2 id="vectors">Vectors and Extended Vectors</h2>
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<p>Supports the GCC vector extensions, plus some stuff like V[1]. ext_vector
with V.xyzw syntax and other tidbits. See also <a
href="#__builtin_shufflevector">__builtin_shufflevector</a>.</p>
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<h2 id="blocks">Blocks</h2>
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<p>The idea, syntax, and semantics.</p>
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<h2 id="builtins">Builtin Functions</h2>
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<p>Clang supports a number of builtin library functions with the same syntax as
GCC, including things like <tt>__builtin_nan</tt>,
<tt>__builtin_constant_p</tt>, <tt>__builtin_choose_expr</tt>,
<tt>__builtin_types_compatible_p</tt>, <tt>__sync_fetch_and_add</tt>, etc. In
addition to the GCC builtins, Clang supports a number of builtins that GCC does
not, which are listed here.</p>
<p>Please note that Clang does not and will not support all of the GCC builtins
for vector operations. Instead of using builtins, you should use the functions
defined in target-specific header files like <tt>&lt;xmmintrin.h&gt;</tt>, which
define portable wrappers for these. Many of the Clang versions of these
functions are implemented directly in terms of <a href="#vectors">extended
vector support</a> instead of builtins, in order to reduce the number of
builtins that we need to implement.</p>
<!-- ======================================================================= -->
<h3 id="__builtin_overload">__builtin_overload</h3>
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<p><tt>__builtin_overload</tt> is used to implement type-generic "overloaded"
functions in C. This builtin is used to implement the <tt>&lt;tgmath.h&gt;</tt>
header file, but is intended to be usable for a broad variety of other similar
situations, like the <tt>&lt;altivec.h&gt;</tt> header.
</p>
<p><b>Syntax:</b></p>
<pre>
__builtin_overload(FnNameStr, PromotionRuleStr, NumArgs, arg1, arg2, ...
overloadcandidate1, overloadcandidate2, ...)
</pre>
<p><b>Examples:</b></p>
<pre>
#define sin(x) \
(__builtin_overload("sin", "tgmath", 1, x, sinf, sin, sinl,
csinf, csin, csinl)(x))
#define fma(x,y,z) \
(__builtin_overload("fma", "tgmath", 3, x, y, z, fmaf, fma, fmal)(x,y,z))
#define ldexp(x, y) \
(__builtin_overload("ldexp", "tgmath1", 2, x, 0, ldexpf, ldexp, ldexpl)(x,y))
</pre>
<p><b>Description:</b></p>
<p>The first argument to __builtin_overload is a string that is the name of the
"function" being implemented. This is used to produce diagnostics that make
sense to the user. For example, if you accidentally pass a pointer argument to
"sin" in GCC, it emits 6 errors about incompatible types. This name allows
Clang to diagnose the error in a way the user can understand.
</p>
<p>The second argument is a string that indicates a set of promotion rules to
apply to the arguments before prototype matching occurs. The currently
supported rules are:</p>
<dl>
<dt>tgmath</dt>
<dd>Follow the rules of C99 7.22 to determine a single common type, and use it
for every argument.</dd>
<dt>tgmath1</dt>
<dd>Follow the rules of C99 7.22 to determine a single common type of just the
first argument (e.g. treat ints as doubles).</dd>
</dl>
<p>The third argument is an integer that specifies the arity of the function
being overloaded. After this are N expression arguments which are promoted
according to the rules specified by the promotion rule string.</p>
<p>The final arguments are functions or function pointers with different
signatures. __builtin_overload will match and evaluate to the first function
pointer whose signature is compatible and does not cause value truncation of
any arguments to the function.</p>
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<h3 id="__builtin_shufflevector">__builtin_shufflevector</h3>
<!-- ======================================================================= -->
<p>todo describe me.</p>
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