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 <h1>Clang Language Extensions</h1>

 <ul>
 <li><a href="#intro">Introduction</a></li>
 <li><a href="#feature_check">Feature Checking Macros</a></li>
 <li><a href="#builtinmacros">Builtin Macros</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>
 <li><a href="#targetspecific">Target-Specific Extensions</a>
   <ul>
   <li><a href="#x86-specific">X86/X86-64 Language Extensions</a></li>
   </ul>
 </li>
 <li><a href="#analyzerspecific">Static Analysis-Specific Extensions</a>
   <ul>
     <li><a href="#analyzerattributes">Analyzer Attributes</a></li>
   </ul>
 </li>
 </ul>

 <!-- ======================================================================= -->
 <h2 id="intro">Introduction</h2>
 <!-- ======================================================================= -->

 <p>This document describes the language extensions provided by Clang.  In
 addition to the language 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="feature_check">Feature Checking Macros</h2>
 <!-- ======================================================================= -->

 <p>Language extensions can be very useful, but only if you know you can depend
 on them.  In order to allow fine-grain features checks, we support two builtin
 function-like macros.  This allows you to directly test for a feature in your
 code without having to resort to something like autoconf or fragile "compiler
 version checks".</p>

 <!-- ======================================================================= -->
 <h3 id="__has_builtin">__has_builtin</h3>
 <!-- ======================================================================= -->

 <p>This function-like macro takes a single identifier argument that is the name
 of a builtin function.  It evaluates to 1 if the builtin is supported or 0 if
 not.  It can be used like this:</p>

 <blockquote>
 <pre>
 #ifndef __has_builtin         // Optional of course.
   #define __has_builtin(x) 0  // Compatibility with non-clang compilers.
 #endif

 ...
 #if __has_builtin(__builtin_trap)
   __builtin_trap();
 #else
   abort();
 #endif
 ...
 </pre>
 </blockquote>


 <!-- ======================================================================= -->
 <h3 id="__has_feature">__has_feature</h3>
 <!-- ======================================================================= -->

 <p>This function-like macro takes a single identifier argument that is the name
 of a feature.  It evaluates to 1 if the feature is supported or 0 if not.  It
 can be used like this:</p>

 <blockquote>
 <pre>
 #ifndef __has_feature         // Optional of course.
   #define __has_feature(x) 0  // Compatibility with non-clang compilers.
 #endif

 ...
 #if __has_feature(attribute_overloadable) || \
     __has_feature(blocks)
 ...
 #endif
 ...
 </pre>
 </blockquote>

 <p>The feature tag is described along with the language feature below.</p>


 <!-- ======================================================================= -->
 <h2 id="builtinmacros">Builtin Macros</h2>
 <!-- ======================================================================= -->

 <p>__BASE_FILE__, __INCLUDE_LEVEL__, __TIMESTAMP__, __COUNTER__</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>

 <p>Query for this feature with __has_feature(attribute_ext_vector_type).</p>

 <!-- ======================================================================= -->
 <h2 id="blocks">Blocks</h2>
 <!-- ======================================================================= -->

 <p>The syntax and high level language feature description is in <a
 href="BlockLanguageSpec.txt">BlockLanguageSpec.txt</a>.  Implementation and ABI
 details for the clang implementation are in <a
 href="BlockImplementation.txt">BlockImplementation.txt</a>.</p>


 <p>Query for this feature with __has_feature(blocks).</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 &lt;math.h&gt;
 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>

 <p>Query for this feature with __has_feature(attribute_overloadable).</p>


 <!-- ======================================================================= -->
 <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>&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_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>&lt;xmmintrin.h&gt;</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>

 <!-- ======================================================================= -->
 <h2 id="targetspecific">Target-Specific Extensions</h2>
 <!-- ======================================================================= -->

 <p>Clang supports some language features conditionally on some targets.</p>

 <!-- ======================================================================= -->
 <h3 id="x86-specific">X86/X86-64 Language Extensions</h3>
 <!-- ======================================================================= -->

 <p>The X86 backend has these language extensions:</p>

 <!-- ======================================================================= -->
 <h4 id="x86-gs-segment">Memory references off the GS segment</h4>
 <!-- ======================================================================= -->

 <p>Annotating a pointer with address space #256 causes it to  be code generated
 relative to the X86 GS segment register, and address space #257 causes it to be
 relative to the X86 FS segment.  Note that this is a very very low-level
 feature that should only be used if you know what you're doing (for example in
 an OS kernel).</p>

 <p>Here is an example:</p>

 <pre>
 #define GS_RELATIVE __attribute__((address_space(256)))
 int foo(int GS_RELATIVE *P) {
   return *P;
 }
 </pre>

 <p>Which compiles to (on X86-32):</p>

 <pre>
 _foo:
 	movl	4(%esp), %eax
 	movl	%gs:(%eax), %eax
 	ret
 </pre>

 <!-- ======================================================================= -->
 <h2 id="analyzerspecific">Static Analysis-Specific Extensions</h2>
 <!-- ======================================================================= -->

 <p>Clang supports additional attributes that are useful for documenting program
 invariants and rules for static analysis tools. The extensions documented here
 are used by the <a
 href="http://clang.llvm.org/StaticAnalysis.html">path-sensitive static analyzer
 engine</a> that is part of Clang's Analysis library.</p>

 <!-- ======================================================================= -->
 <h3 id="analyzerattributes">Analyzer Attributes</h3>
 <!-- ======================================================================= -->

 <h4 id="attr_analyzer_noreturn"><tt>analyzer_noreturn</tt></h4>

 <p>Clang's static analysis engine understands the standard <tt>noreturn</tt>
 attribute. This attribute, which is typically affixed to a function prototype,
 indicates that a call to a given function never returns. Function prototypes for
 common functions like <tt>exit</tt> are typically annotated with this attribute,
 as well as a variety of common assertion handlers. Users can educate the static
 analyzer about their own custom assertion handles (thus cutting down on false
 positives due to false paths) by marking their own &quot;panic&quot; functions
 with this attribute.</p>

 <p>While useful, <tt>noreturn</tt> is not applicable in all cases. Sometimes
 there are special functions that for all intents and purposes should be
 considered panic functions (i.e., they are only called when an internal program
 error occurs) but may actually return so that the program can fail gracefully.
 The <tt>analyzer_noreturn</tt> attribute allows one to annotate such functions
 as being interpreted as &quot;no return&quot; functions by the analyzer (thus
 pruning bogus paths) but will not affect compilation (as in the case of
 <tt>noreturn</tt>).</p>

 <p><b>Usage</b>: The <tt>analyzer_noreturn</tt> attribute can be placed in the
 same places where the <tt>noreturn</tt> attribute can be placed. It is commonly
 placed at the end of function prototypes:</p>

 <pre>
   void foo() <b>__attribute__((analyzer_noreturn))</b>;
 </pre>

 <p>Query for this feature with __has_feature(attribute_analyzer_noreturn).</p>


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	<div id="content">

	<h1>Clang Language Extensions</h1>

	<ul>
	<li><a href="#intro">Introduction</a></li>
	<li><a href="#feature_check">Feature Checking Macros</a></li>
	<li><a href="#builtinmacros">Builtin Macros</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>
	<li><a href="#targetspecific">Target-Specific Extensions</a>
	<ul>
	<li><a href="#x86-specific">X86/X86-64 Language Extensions</a></li>
	</ul>
	</li>
	<li><a href="#analyzerspecific">Static Analysis-Specific Extensions</a>
	<ul>
	<li><a href="#analyzerattributes">Analyzer Attributes</a></li>
	</ul>
	</li>
	</ul>

	<!-- ======================================================================= -->
	<h2 id="intro">Introduction</h2>
	<!-- ======================================================================= -->

	<p>This document describes the language extensions provided by Clang. In
	addition to the language 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="feature_check">Feature Checking Macros</h2>
	<!-- ======================================================================= -->

	<p>Language extensions can be very useful, but only if you know you can depend
	on them. In order to allow fine-grain features checks, we support two builtin
	function-like macros. This allows you to directly test for a feature in your
	code without having to resort to something like autoconf or fragile "compiler
	version checks".</p>

	<!-- ======================================================================= -->
	<h3 id="__has_builtin">__has_builtin</h3>
	<!-- ======================================================================= -->

	<p>This function-like macro takes a single identifier argument that is the name
	of a builtin function. It evaluates to 1 if the builtin is supported or 0 if
	not. It can be used like this:</p>

	<blockquote>
	<pre>
	#ifndef __has_builtin // Optional of course.
	#define __has_builtin(x) 0 // Compatibility with non-clang compilers.
	#endif

	...
	#if __has_builtin(__builtin_trap)
	__builtin_trap();
	#else
	abort();
	#endif
	...
	</pre>
	</blockquote>


	<!-- ======================================================================= -->
	<h3 id="__has_feature">__has_feature</h3>
	<!-- ======================================================================= -->

	<p>This function-like macro takes a single identifier argument that is the name
	of a feature. It evaluates to 1 if the feature is supported or 0 if not. It
	can be used like this:</p>

	<blockquote>
	<pre>
	#ifndef __has_feature // Optional of course.
	#define __has_feature(x) 0 // Compatibility with non-clang compilers.
	#endif

	...
	#if __has_feature(attribute_overloadable) \|\| \
	__has_feature(blocks)
	...
	#endif
	...
	</pre>
	</blockquote>

	<p>The feature tag is described along with the language feature below.</p>


	<!-- ======================================================================= -->
	<h2 id="builtinmacros">Builtin Macros</h2>
	<!-- ======================================================================= -->

	<p>__BASE_FILE__, __INCLUDE_LEVEL__, __TIMESTAMP__, __COUNTER__</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>

	<p>Query for this feature with __has_feature(attribute_ext_vector_type).</p>

	<!-- ======================================================================= -->
	<h2 id="blocks">Blocks</h2>
	<!-- ======================================================================= -->

	<p>The syntax and high level language feature description is in <a
	href="BlockLanguageSpec.txt">BlockLanguageSpec.txt</a>. Implementation and ABI
	details for the clang implementation are in <a
	href="BlockImplementation.txt">BlockImplementation.txt</a>.</p>


	<p>Query for this feature with __has_feature(blocks).</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>

	<p>Query for this feature with __has_feature(attribute_overloadable).</p>


	<!-- ======================================================================= -->
	<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>

	<!-- ======================================================================= -->
	<h2 id="targetspecific">Target-Specific Extensions</h2>
	<!-- ======================================================================= -->

	<p>Clang supports some language features conditionally on some targets.</p>

	<!-- ======================================================================= -->
	<h3 id="x86-specific">X86/X86-64 Language Extensions</h3>
	<!-- ======================================================================= -->

	<p>The X86 backend has these language extensions:</p>

	<!-- ======================================================================= -->
	<h4 id="x86-gs-segment">Memory references off the GS segment</h4>
	<!-- ======================================================================= -->

	<p>Annotating a pointer with address space #256 causes it to be code generated
	relative to the X86 GS segment register, and address space #257 causes it to be
	relative to the X86 FS segment. Note that this is a very very low-level
	feature that should only be used if you know what you're doing (for example in
	an OS kernel).</p>

	<p>Here is an example:</p>

	<pre>
	#define GS_RELATIVE __attribute__((address_space(256)))
	int foo(int GS_RELATIVE *P) {
	return *P;
	}
	</pre>

	<p>Which compiles to (on X86-32):</p>

	<pre>
	_foo:
	movl 4(%esp), %eax
	movl %gs:(%eax), %eax
	ret
	</pre>

	<!-- ======================================================================= -->
	<h2 id="analyzerspecific">Static Analysis-Specific Extensions</h2>
	<!-- ======================================================================= -->

	<p>Clang supports additional attributes that are useful for documenting program
	invariants and rules for static analysis tools. The extensions documented here
	are used by the <a
	href="http://clang.llvm.org/StaticAnalysis.html">path-sensitive static analyzer
	engine</a> that is part of Clang's Analysis library.</p>

	<!-- ======================================================================= -->
	<h3 id="analyzerattributes">Analyzer Attributes</h3>
	<!-- ======================================================================= -->

	<h4 id="attr_analyzer_noreturn"><tt>analyzer_noreturn</tt></h4>

	<p>Clang's static analysis engine understands the standard <tt>noreturn</tt>
	attribute. This attribute, which is typically affixed to a function prototype,
	indicates that a call to a given function never returns. Function prototypes for
	common functions like <tt>exit</tt> are typically annotated with this attribute,
	as well as a variety of common assertion handlers. Users can educate the static
	analyzer about their own custom assertion handles (thus cutting down on false
	positives due to false paths) by marking their own "panic" functions
	with this attribute.</p>

	<p>While useful, <tt>noreturn</tt> is not applicable in all cases. Sometimes
	there are special functions that for all intents and purposes should be
	considered panic functions (i.e., they are only called when an internal program
	error occurs) but may actually return so that the program can fail gracefully.
	The <tt>analyzer_noreturn</tt> attribute allows one to annotate such functions
	as being interpreted as "no return" functions by the analyzer (thus
	pruning bogus paths) but will not affect compilation (as in the case of
	<tt>noreturn</tt>).</p>

	<p><b>Usage</b>: The <tt>analyzer_noreturn</tt> attribute can be placed in the
	same places where the <tt>noreturn</tt> attribute can be placed. It is commonly
	placed at the end of function prototypes:</p>

	<pre>
	void foo() <b>__attribute__((analyzer_noreturn))</b>;
	</pre>

	<p>Query for this feature with __has_feature(attribute_analyzer_noreturn).</p>


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