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<title>Clang Language Extensions</title>
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<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="#overloading-in-c">Function Overloading in C</a></li>
<li><a href="#builtins">Builtin Functions</a>
<ul>
<li><a href="#__builtin_shufflevector">__builtin_shufflevector</a></li>
</ul>
</li>
</ul>
<!-- ======================================================================= -->
<h2 id="intro">Introduction</h2>
<!-- ======================================================================= -->
<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>
<!-- ======================================================================= -->
<h2 id="vectors">Vectors and Extended Vectors</h2>
<!-- ======================================================================= -->
<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>
<!-- ======================================================================= -->
<h2 id="blocks">Blocks</h2>
<!-- ======================================================================= -->
<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 marked <tt>overloadable</tt> must have
prototypes. Therefore, the following code is ill-formed:</p>
<blockquote>
<pre>
int h() __attribute__((overloadable)); <i>// error: h does not have a prototype</i>
</pre>
</blockquote>
<p>However, <tt>overloadable</tt> functions are allowed to use a
ellipsis even if there are no named parameters (as is permitted in C++). This feature is particularly useful when combined with the <tt>unavailable</tt> attribute:</p>
<blockquote>
<pre>
void honeypot(...) __attribute__((overloadable, unavailable)); <i>// calling me is an error</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>
<!-- ======================================================================= -->
<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><xmmintrin.h></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_shufflevector">__builtin_shufflevector</h3>
<!-- ======================================================================= -->
<p><tt>__builtin_shufflevector</tt> is used to expression generic vector
permutation/shuffle/swizzle operations. This builtin is also very important for
the implementation of various target-specific header files like
<tt><xmmintrin.h></tt>.
</p>
<p><b>Syntax:</b></p>
<pre>
__builtin_shufflevector(vec1, vec2, index1, index2, ...)
</pre>
<p><b>Examples:</b></p>
<pre>
// Identity operation - return 4-element vector V1.
__builtin_shufflevector(V1, V1, 0, 1, 2, 3)
// "Splat" element 0 of V1 into a 4-element result.
__builtin_shufflevector(V1, V1, 0, 0, 0, 0)
// Reverse 4-element vector V1.
__builtin_shufflevector(V1, V1, 3, 2, 1, 0)
// Concatenate every other element of 4-element vectors V1 and V2.
__builtin_shufflevector(V1, V2, 0, 2, 4, 6)
// Concatenate every other element of 8-element vectors V1 and V2.
__builtin_shufflevector(V1, V2, 0, 2, 4, 6, 8, 10, 12, 14)
</pre>
<p><b>Description:</b></p>
<p>The first two arguments to __builtin_shufflevector are vectors that have the
same element type. The remaining arguments are a list of integers that specify
the elements indices of the first two vectors that should be extracted and
returned in a new vector. These element indices are numbered sequentially
starting with the first vector, continuing into the second vector. Thus, if
vec1 is a 4-element vector, index 5 would refer to the second element of vec2.
</p>
<p>The result of __builtin_shufflevector is a vector
with the same element type as vec1/vec2 but that has an element count equal to
the number of indices specified.
</p>
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