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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="#has_include">Include File 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="#deprecated">Messages on <tt>deprecated</tt> and <tt>unavailable</tt> attributes</a></li>
 <li><a href="#attributes-on-enumerators">Attributes on enumerators</a></li>
 <li><a href="#checking_language_features">Checks for Standard Language Features</a></li>
   <ul>
   <li><a href="#cxx_exceptions">C++ exceptions</a></li>
   <li><a href="#cxx_rtti">C++ RTTI</a></li>
   </ul>
 <li><a href="#checking_upcoming_features">Checks for Upcoming Standard Language Features</a></li>
   <ul>
   <li><a href="#cxx_attributes">C++0x attributes</a></li>
   <li><a href="#cxx_decltype">C++0x <tt>decltype()</tt></a></li>
   <li><a href="#cxx_default_function_template_args">C++0x default template arguments in function templates</a></li>
   <li><a href="#cxx_deleted_functions">C++0x deleted functions</a></li>
   <li><a href="#cxx_lambdas">C++0x lambdas</a></li>
   <li><a href="#cxx_nullptr">C++0x nullptr</a></li>
   <li><a href="#cxx_rvalue_references">C++0x rvalue references</a></li>
   <li><a href="#cxx_reference_qualified_functions">C++0x reference-qualified functions</a></li>
   <li><a href="#cxx_static_assert">C++0x <tt>static_assert()</tt></a></li>
   <li><a href="#cxx_auto_type">C++0x type inference</a></li>
   <li><a href="#cxx_variadic_templates">C++0x variadic templates</a></li>
   <li><a href="#cxx_inline_namespaces">C++0x inline namespaces</a></li>
   <li><a href="#cxx_strong_enums">C++0x strongly-typed enumerations</a></li>
   <li><a href="#cxx_trailing_return">C++0x trailing return type</a></li>
   </ul>
 <li><a href="#checking_type_traits">Checks for Type Traits</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>
   <li><a href="#__builtin_unreachable">__builtin_unreachable</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>

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

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

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

 ...
 #if __has_attribute(always_inline)
 #define ALWAYS_INLINE __attribute__((always_inline))
 #else
 #define ALWAYS_INLINE
 #endif
 ...
 </pre>
 </blockquote>

 <!-- ======================================================================= -->
 <h2 id="has_include">Include File Checking Macros</h2>
 <!-- ======================================================================= -->

 <p>Not all developments systems have the same include files.
 The <a href="#__has_include">__has_include</a> and
 <a href="#__has_include_next">__has_include_next</a> macros allow you to
 check for the existence of an include file before doing
 a possibly failing #include directive.</p>

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

 <p>This function-like macro takes a single file name string argument that
 is the name of an include file.  It evaluates to 1 if the file can
 be found using the include paths, or 0 otherwise:</p>

 <blockquote>
 <pre>
 // Note the two possible file name string formats.
 #if __has_include("myinclude.h") && __has_include(&lt;stdint.h&gt;)
 # include "myinclude.h"
 #endif

 // To avoid problem with non-clang compilers not having this macro.
 #if defined(__has_include) && __has_include("myinclude.h")
 # include "myinclude.h"
 #endif
 </pre>
 </blockquote>

 <p>To test for this feature, use #if defined(__has_include).</p>

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

 <p>This function-like macro takes a single file name string argument that
 is the name of an include file.  It is like __has_include except that it
 looks for the second instance of the given file found in the include
 paths.  It evaluates to 1 if the second instance of the file can
 be found using the include paths, or 0 otherwise:</p>

 <blockquote>
 <pre>
 // Note the two possible file name string formats.
 #if __has_include_next("myinclude.h") && __has_include_next(&lt;stdint.h&gt;)
 # include_next "myinclude.h"
 #endif

 // To avoid problem with non-clang compilers not having this macro.
 #if defined(__has_include_next) && __has_include_next("myinclude.h")
 # include_next "myinclude.h"
 #endif
 </pre>
 </blockquote>

 <p>Note that __has_include_next, like the GNU extension
 #include_next directive, is intended for use in headers only,
 and will issue a warning if used in the top-level compilation
 file.  A warning will also be issued if an absolute path
 is used in the file argument.</p>

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

 <dl>
   <dt><code>__BASE_FILE__</code></dt>
   <dd>Defined to a string that contains the name of the main input
   file passed to Clang.</dd>

   <dt><code>__COUNTER__</code></dt>
   <dd>Defined to an integer value that starts at zero and is
   incremented each time the <code>__COUNTER__</code> macro is
   expanded.</dd>

   <dt><code>__INCLUDE_LEVEL__</code></dt>
   <dd>Defined to an integral value that is the include depth of the
   file currently being translated. For the main file, this value is
   zero.</dd>

   <dt><code>__TIMESTAMP__</code></dt>
   <dd>Defined to the date and time of the last modification of the
   current source file.</dd>

   <dt><code>__clang__</code></dt>
   <dd>Defined when compiling with Clang</dd>

   <dt><code>__clang_major__</code></dt>
   <dd>Defined to the major version number of Clang (e.g., the 2 in
   2.0.1).</dd>

   <dt><code>__clang_minor__</code></dt>
   <dd>Defined to the minor version number of Clang (e.g., the 0 in
   2.0.1).</dd>

   <dt><code>__clang_patchlevel__</code></dt>
   <dd>Defined to the patch level of Clang (e.g., the 1 in 2.0.1).</dd>

   <dt><code>__clang_version__</code></dt>
   <dd>Defined to a string that captures the Clang version, including
   the Subversion tag or revision number, e.g., "1.5 (trunk
   102332)".</dd>
 </dl>

 <!-- ======================================================================= -->
 <h2 id="vectors">Vectors and Extended Vectors</h2>
 <!-- ======================================================================= -->

 <p>Supports the GCC vector extensions, plus some stuff like V[1].</p>

 <p>Also supports <tt>ext_vector</tt>, which additionally support for V.xyzw
 syntax and other tidbits as seen in OpenCL. An example is:</p>

 <blockquote>
 <pre>
 typedef float float4 <b>__attribute__((ext_vector_type(4)))</b>;
 typedef float float2 <b>__attribute__((ext_vector_type(2)))</b>;

 float4 foo(float2 a, float2 b) {
   float4 c;
   c.xz = a;
   c.yw = b;
   return c;
 }
 </pre>
 </blockquote>

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

 <p>See also <a href="#__builtin_shufflevector">__builtin_shufflevector</a>.</p>

 <!-- ======================================================================= -->
 <h2 id="deprecated">Messages on <tt>deprecated</tt> and <tt>unavailable</tt> Attributes</h2>
 <!-- ======================================================================= -->

 <p>An optional string message can be added to the <tt>deprecated</tt>
 and <tt>unavailable</tt> attributes.  For example:</p>

 <blockquote>
 <pre>void explode(void) __attribute__((deprecated("extremely unsafe, use 'combust' instead!!!")));</pre>
 </blockquote>

 <p>If the deprecated or unavailable declaration is used, the message
 will be incorporated into the appropriate diagnostic:</p>

 <blockquote>
 <pre>harmless.c:4:3: warning: 'explode' is deprecated: extremely unsafe, use 'combust' instead!!! [-Wdeprecated-declarations]
   explode();
   ^</pre>
 </blockquote>

 <p>Query for this feature
 with <tt>__has_feature(attribute_deprecated_with_message)</tt>
 and <tt>__has_feature(attribute_unavailable_with_message)</tt>.</p>

 <!-- ======================================================================= -->
 <h2 id="attributes-on-enumerators">Attributes on Enumerators</h2>
 <!-- ======================================================================= -->

 <p>Clang allows attributes to be written on individual enumerators.
 This allows enumerators to be deprecated, made unavailable, etc.  The
 attribute must appear after the enumerator name and before any
 initializer, like so:</p>

 <blockquote>
 <pre>enum OperationMode {
   OM_Invalid,
   OM_Normal,
   OM_Terrified __attribute__((deprecated)),
   OM_AbortOnError __attribute__((deprecated)) = 4
 };</pre>
 </blockquote>

 <p>Attributes on the <tt>enum</tt> declaration do not apply to
 individual enumerators.</p>

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

 <!-- ======================================================================= -->
 <h2 id="checking_language_features">Checks for Standard Language Features</h2>
 <!-- ======================================================================= -->

 <p>The <tt>__has_feature</tt> macro can be used to query if certain standard language features are
 enabled.  Those features are listed here.</p>

 <h3 id="cxx_exceptions">C++ exceptions</h3>

 <p>Use <tt>__has_feature(cxx_exceptions)</tt> to determine if C++ exceptions have been enabled. For
 example, compiling code with <tt>-fexceptions</tt> enables C++ exceptions.</p>

 <h3 id="cxx_rtti">C++ RTTI</h3>

 <p>Use <tt>__has_feature(cxx_rtti)</tt> to determine if C++ RTTI has been enabled. For example,
 compiling code with <tt>-fno-rtti</tt> disables the use of RTTI.</p>

 <!-- ======================================================================= -->
 <h2 id="checking_upcoming_features">Checks for Upcoming Standard Language Features</h2>
 <!-- ======================================================================= -->

 <p>The <tt>__has_feature</tt> macro can be used to query if certain upcoming
 standard language features are enabled.  Those features are listed here.</p>

 <p>Currently, all features listed here are slated for inclusion in the upcoming
 C++0x standard. As a result, all the features that clang supports are enabled
 with the <tt>-std=c++0x</tt> option when compiling C++ code. Features that are
 not yet implemented will be noted.</p>

 <h3 id="cxx_decltype">C++0x <tt>decltype()</tt></h3>

 <p>Use <tt>__has_feature(cxx_decltype)</tt> to determine if support for the
 <tt>decltype()</tt> specifier is enabled.</p>

 <h3 id="cxx_attributes">C++0x attributes</h3>

 <p>Use <tt>__has_feature(cxx_attributes)</tt> to determine if support for
 attribute parsing with C++0x's square bracket notation is enabled.</p>

 <h3 id="cxx_default_function_template_args">C++0x default template arguments in function templates</h3>

 <p>Use <tt>__has_feature(cxx_default_function_template_args)</tt> to determine if support for default template arguments in function templates is enabled.</p>

 <h3 id="cxx_deleted_functions">C++0x deleted functions</tt></h3>

 <p>Use <tt>__has_feature(cxx_deleted_functions)</tt> to determine if support for
 deleted function definitions (with <tt>= delete</tt>) is enabled.</p>

 <h3 id="cxx_lambdas">C++0x lambdas</h3>

 <p>Use <tt>__has_feature(cxx_lambdas)</tt> to determine if support for
 lambdas is enabled. clang does not currently implement this feature.</p>

 <h3 id="cxx_nullptr">C++0x <tt>nullptr</tt></h3>

 <p>Use <tt>__has_feature(cxx_nullptr)</tt> to determine if support for
 <tt>nullptr</tt> is enabled. clang does not yet fully implement this
 feature.</p>

 <h3 id="cxx_reference_qualified_functions">C++0x reference-qualified functions</h3>
 <p>Use <tt>__has_feature(cxx_reference_qualified_functions)</tt> to determine if support for reference-qualified functions (e.g., member functions with <code>&amp;</code> or <code>&amp;&amp;</code> applied to <code>*this</code>) is enabled.</p>

 <h3 id="cxx_rvalue_references">C++0x rvalue references</tt></h3>

 <p>Use <tt>__has_feature(cxx_rvalue_references)</tt> to determine if support for
 rvalue references is enabled. </p>

 <h3 id="cxx_static_assert">C++0x <tt>static_assert()</tt></h3>

 <p>Use <tt>__has_feature(cxx_static_assert)</tt> to determine if support for
 compile-time assertions using <tt>static_assert</tt> is enabled.</p>

 <h3 id="cxx_auto_type">C++0x type inference</h3>

 <p>Use <tt>__has_feature(cxx_auto_type)</tt> to determine C++0x type inference
 is supported using the <tt>auto</tt> specifier. If this is disabled,
 <tt>auto</tt> will instead be a storage class specifier, as in C or C++98.</p>

 <h3 id="cxx_variadic_templates">C++0x variadic templates</h3>

 <p>Use <tt>__has_feature(cxx_variadic_templates)</tt> to determine if support
 for variadic templates is enabled.</p>

 <h3 id="cxx_inline_namespaces">C++0x inline namespaces</h3>

 <p>Use <tt>__has_feature(cxx_inline_namespaces)</tt> to determine if support for
 inline namespaces is enabled.</p>

 <h3 id="cxx_trailing_return">C++0x trailing return type</h3>

 <p>Use <tt>__has_feature(cxx_trailing_return)</tt> to determine if support for
 the alternate function declaration syntax with trailing return type is enabled.</p>

 <h3 id="cxx_strong_enums">C++0x strongly typed enumerations</h3>

 <p>Use <tt>__has_feature(cxx_strong_enums)</tt> to determine if support for
 strongly typed, scoped enumerations is enabled.</p>

 <!-- ======================================================================= -->
 <h2 id="checking_type_traits">Checks for Type Traits</h2>
 <!-- ======================================================================= -->

 <p>Clang supports the <a hef="http://gcc.gnu.org/onlinedocs/gcc/Type-Traits.html">GNU C++ type traits</a> and a subset of the <a href="http://msdn.microsoft.com/en-us/library/ms177194(v=VS.100).aspx">Microsoft Visual C++ Type traits</a>. For each supported type trait <code>__X</code>, <code>__has_feature(X)</code> indicates the presence of the type trait. For example:
 <blockquote>
 <pre>
 #if __has_feature(is_convertible_to)
 template&lt;typename From, typename To&gt;
 struct is_convertible_to {
   static const bool value = __is_convertible_to(From, To);
 };
 #else
 // Emulate type trait
 #endif
 </pre>
 </blockquote>

 <p>The following type traits are supported by Clang:</p>
 <ul>
   <li><code>__has_nothrow_assign</code> (GNU, Microsoft)</li>
   <li><code>__has_nothrow_copy</code> (GNU, Microsoft)</li>
   <li><code>__has_nothrow_constructor</code> (GNU, Microsoft)</li>
   <li><code>__has_trivial_assign</code> (GNU, Microsoft)</li>
   <li><code>__has_trivial_copy</code> (GNU, Microsoft)</li>
   <li><code>__has_trivial_constructor</code> (GNU, Microsoft)</li>
   <li><code>__has_trivial_destructor</code> (GNU, Microsoft)</li>
   <li><code>__has_virtual_destructor</code> (GNU, Microsoft)</li>
   <li><code>__is_abstract</code> (GNU, Microsoft)</li>
   <li><code>__is_base_of</code> (GNU, Microsoft)</li>
   <li><code>__is_class</code> (GNU, Microsoft)</li>
   <li><code>__is_convertible_to</code> (Microsoft)</li>
   <li><code>__is_empty</code> (GNU, Microsoft)</li>
   <li><code>__is_enum</code> (GNU, Microsoft)</li>
   <li><code>__is_pod</code> (GNU, Microsoft)</li>
   <li><code>__is_polymorphic</code> (GNU, Microsoft)</li>
   <li><code>__is_union</code> (GNU, Microsoft)</li>
   <li><code>__is_literal(type)</code>: Determines whether the given type is a literal type</li>
 </ul>

 <!-- ======================================================================= -->
 <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="Block-ABI-Apple.txt">Block-ABI-Apple.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 express 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>

 <p>Query for this feature with __has_builtin(__builtin_shufflevector).</p>

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

 <p><tt>__builtin_unreachable</tt> is used to indicate that a specific point in
 the program cannot be reached, even if the compiler might otherwise think it
 can.  This is useful to improve optimization and eliminates certain warnings.
 For example, without the <tt>__builtin_unreachable</tt> in the example below,
 the compiler assumes that the inline asm can fall through and prints a "function
 declared 'noreturn' should not return" warning.
 </p>

 <p><b>Syntax:</b></p>

 <pre>
 __builtin_unreachable()
 </pre>

 <p><b>Example of Use:</b></p>

 <pre>
 void myabort(void) __attribute__((noreturn));
 void myabort(void) {
     asm("int3");
     __builtin_unreachable();
 }
 </pre>

 <p><b>Description:</b></p>

 <p>The __builtin_unreachable() builtin has completely undefined behavior.  Since
 it has undefined behavior, it is a statement that it is never reached and the
 optimizer can take advantage of this to produce better code.  This builtin takes
 no arguments and produces a void result.
 </p>

 <p>Query for this feature with __has_builtin(__builtin_unreachable).</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>

 <h4 id="attr_retain_release">Objective-C retaining behavior attributes</h4>

 <p>In Objective-C, functions and methods are generally assumed to take
 and return objects with +0 retain counts, with some exceptions for
 special methods like <tt>+alloc</tt> and <tt>init</tt>.  However,
 there are exceptions, and so Clang provides attributes to allow these
 exceptions to be documented, which helps the analyzer find leaks (and
 ignore non-leaks).</p>

 <p><b>Usage</b>: The <tt>ns_returns_retained</tt>, <tt>ns_returns_not_retained</tt>,
 <tt>ns_returns_autoreleased</tt>, <tt>cf_returns_retained</tt>,
 and <tt>cf_returns_not_retained</tt> attributes can be placed on
 methods and functions that return Objective-C or CoreFoundation
 objects.  They are commonly placed at the end of a function prototype
 or method declaration:</p>

 <pre>
   id foo() <b>__attribute__((ns_returns_retained))</b>;

   - (NSString*) bar: (int) x <b>__attribute__((ns_returns_retained))</b>;
 </pre>

 <p>The <tt>*_returns_retained</tt> attributes specify that the
 returned object has a +1 retain count.
 The <tt>*_returns_not_retained</tt> attributes specify that the return
 object has a +0 retain count, even if the normal convention for its
 selector would be +1.  <tt>ns_returns_autoreleased</tt> specifies that the
 returned object is +0, but is guaranteed to live at least as long as the
 next flush of an autorelease pool.</p>

 <p><b>Usage</b>: The <tt>ns_consumed</tt> and <tt>cf_consumed</tt>
 attributes can be placed on an parameter declaration; they specify
 that the argument is expected to have a +1 retain count, which will be
 balanced in some way by the function or method.
 The <tt>ns_consumes_self</tt> attribute can only be placed on an
 Objective-C method; it specifies that the method expects
 its <tt>self</tt> parameter to have a +1 retain count, which it will
 balance in some way.</p>

 <pre>
   void <b>foo(__attribute__((ns_consumed))</b> NSString *string);

   - (void) bar <b>__attribute__((ns_consumes_self))</b>;
   - (void) baz: (id) <b>__attribute__((ns_consumed))</b> x;
 </pre>

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