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  18 <h1>Clang Language Extensions</h1>
  19
  20 <ul>
  21 <li><a href="#intro">Introduction</a></li>
  22 <li><a href="#feature_check">Feature Checking Macros</a></li>
  23 <li><a href="#has_include">Include File Checking Macros</a></li>
  24 <li><a href="#builtinmacros">Builtin Macros</a></li>
  25 <li><a href="#vectors">Vectors and Extended Vectors</a></li>
  26 <li><a href="#deprecated">Messages on <tt>deprecated</tt> and <tt>unavailable</tt> attributes</a></li>
  27 <li><a href="#attributes-on-enumerators">Attributes on enumerators</a></li>
  28 <li><a href="#checking_language_features">Checks for Standard Language Features</a></li>
  29   <ul>
  30   <li><a href="#cxx_exceptions">C++ exceptions</a></li>
  31   <li><a href="#cxx_rtti">C++ RTTI</a></li>
  32   </ul>
  33 <li><a href="#checking_upcoming_features">Checks for Upcoming Standard Language Features</a></li>
  34   <ul>
  35   <li><a href="#cxx_attributes">C++0x attributes</a></li>
  36   <li><a href="#cxx_decltype">C++0x <tt>decltype()</tt></a></li>
  37   <li><a href="#cxx_default_function_template_args">C++0x default template arguments in function templates</a></li>
  38   <li><a href="#cxx_deleted_functions">C++0x deleted functions</a></li>
  39   <li><a href="#cxx_lambdas">C++0x lambdas</a></li>
  40   <li><a href="#cxx_nullptr">C++0x nullptr</a></li>
  41   <li><a href="#cxx_rvalue_references">C++0x rvalue references</a></li>
  42   <li><a href="#cxx_reference_qualified_functions">C++0x reference-qualified functions</a></li>
  43   <li><a href="#cxx_static_assert">C++0x <tt>static_assert()</tt></a></li>
  44   <li><a href="#cxx_auto_type">C++0x type inference</a></li>
  45   <li><a href="#cxx_variadic_templates">C++0x variadic templates</a></li>
  46   <li><a href="#cxx_inline_namespaces">C++0x inline namespaces</a></li>
  47   <li><a href="#cxx_strong_enums">C++0x strongly-typed enumerations</a></li>
  48   <li><a href="#cxx_trailing_return">C++0x trailing return type</a></li>
  49   </ul>
  50 <li><a href="#checking_type_traits">Checks for Type Traits</a></li>
  51 <li><a href="#blocks">Blocks</a></li>
  52 <li><a href="#overloading-in-c">Function Overloading in C</a></li>
  53 <li><a href="#builtins">Builtin Functions</a>
  54   <ul>
  55   <li><a href="#__builtin_shufflevector">__builtin_shufflevector</a></li>
  56   <li><a href="#__builtin_unreachable">__builtin_unreachable</a></li>
  57  </ul>
  58 </li>
  59 <li><a href="#targetspecific">Target-Specific Extensions</a>
  60   <ul>
  61   <li><a href="#x86-specific">X86/X86-64 Language Extensions</a></li>
  62   </ul>
  63 </li>
  64 <li><a href="#analyzerspecific">Static Analysis-Specific Extensions</a>
  65   <ul>
  66     <li><a href="#analyzerattributes">Analyzer Attributes</a></li>
  67   </ul>
  68 </li>
  69 </ul>
  70
  71 <!-- ======================================================================= -->
  72 <h2 id="intro">Introduction</h2>
  73 <!-- ======================================================================= -->
  74
  75 <p>This document describes the language extensions provided by Clang.  In
  76 addition to the language extensions listed here, Clang aims to support a broad
  77 range of GCC extensions.  Please see the <a
  78 href="http://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html">GCC manual</a> for
  79 more information on these extensions.</p>
  80
  81 <!-- ======================================================================= -->
  82 <h2 id="feature_check">Feature Checking Macros</h2>
  83 <!-- ======================================================================= -->
  84
  85 <p>Language extensions can be very useful, but only if you know you can depend
  86 on them.  In order to allow fine-grain features checks, we support two builtin
  87 function-like macros.  This allows you to directly test for a feature in your
  88 code without having to resort to something like autoconf or fragile "compiler
  89 version checks".</p>
  90
  91 <!-- ======================================================================= -->
  92 <h3 id="__has_builtin">__has_builtin</h3>
  93 <!-- ======================================================================= -->
  94
  95 <p>This function-like macro takes a single identifier argument that is the name
  96 of a builtin function.  It evaluates to 1 if the builtin is supported or 0 if
  97 not.  It can be used like this:</p>
  98
  99 <blockquote>
 100 <pre>
 101 #ifndef __has_builtin         // Optional of course.
 102   #define __has_builtin(x) 0  // Compatibility with non-clang compilers.
 103 #endif
 104
 105 ...
 106 #if __has_builtin(__builtin_trap)
 107   __builtin_trap();
 108 #else
 109   abort();
 110 #endif
 111 ...
 112 </pre>
 113 </blockquote>
 114
 115
 116 <!-- ======================================================================= -->
 117 <h3 id="__has_feature">__has_feature</h3>
 118 <!-- ======================================================================= -->
 119
 120 <p>This function-like macro takes a single identifier argument that is the name
 121 of a feature.  It evaluates to 1 if the feature is supported or 0 if not.  It
 122 can be used like this:</p>
 123
 124 <blockquote>
 125 <pre>
 126 #ifndef __has_feature         // Optional of course.
 127   #define __has_feature(x) 0  // Compatibility with non-clang compilers.
 128 #endif
 129
 130 ...
 131 #if __has_feature(attribute_overloadable) || \
 132     __has_feature(blocks)
 133 ...
 134 #endif
 135 ...
 136 </pre>
 137 </blockquote>
 138
 139 <p>The feature tag is described along with the language feature below.</p>
 140
 141 <!-- ======================================================================= -->
 142 <h3 id="__has_attribute">__has_attribute</h3>
 143 <!-- ======================================================================= -->
 144
 145 <p>This function-like macro takes a single identifier argument that is the name
 146 of an attribute.  It evaluates to 1 if the attribute is supported or 0 if not.  It
 147 can be used like this:</p>
 148
 149 <blockquote>
 150 <pre>
 151 #ifndef __has_attribute         // Optional of course.
 152   #define __has_attribute(x) 0  // Compatibility with non-clang compilers.
 153 #endif
 154
 155 ...
 156 #if __has_attribute(always_inline)
 157 #define ALWAYS_INLINE __attribute__((always_inline))
 158 #else
 159 #define ALWAYS_INLINE
 160 #endif
 161 ...
 162 </pre>
 163 </blockquote>
 164
 165 <!-- ======================================================================= -->
 166 <h2 id="has_include">Include File Checking Macros</h2>
 167 <!-- ======================================================================= -->
 168
 169 <p>Not all developments systems have the same include files.
 170 The <a href="#__has_include">__has_include</a> and
 171 <a href="#__has_include_next">__has_include_next</a> macros allow you to
 172 check for the existence of an include file before doing
 173 a possibly failing #include directive.</p>
 174
 175 <!-- ======================================================================= -->
 176 <h3 id="__has_include">__has_include</h3>
 177 <!-- ======================================================================= -->
 178
 179 <p>This function-like macro takes a single file name string argument that
 180 is the name of an include file.  It evaluates to 1 if the file can
 181 be found using the include paths, or 0 otherwise:</p>
 182
 183 <blockquote>
 184 <pre>
 185 // Note the two possible file name string formats.
 186 #if __has_include("myinclude.h") && __has_include(&lt;stdint.h&gt;)
 187 # include "myinclude.h"
 188 #endif
 189
 190 // To avoid problem with non-clang compilers not having this macro.
 191 #if defined(__has_include) && __has_include("myinclude.h")
 192 # include "myinclude.h"
 193 #endif
 194 </pre>
 195 </blockquote>
 196
 197 <p>To test for this feature, use #if defined(__has_include).</p>
 198
 199 <!-- ======================================================================= -->
 200 <h3 id="__has_include_next">__has_include_next</h3>
 201 <!-- ======================================================================= -->
 202
 203 <p>This function-like macro takes a single file name string argument that
 204 is the name of an include file.  It is like __has_include except that it
 205 looks for the second instance of the given file found in the include
 206 paths.  It evaluates to 1 if the second instance of the file can
 207 be found using the include paths, or 0 otherwise:</p>
 208
 209 <blockquote>
 210 <pre>
 211 // Note the two possible file name string formats.
 212 #if __has_include_next("myinclude.h") && __has_include_next(&lt;stdint.h&gt;)
 213 # include_next "myinclude.h"
 214 #endif
 215
 216 // To avoid problem with non-clang compilers not having this macro.
 217 #if defined(__has_include_next) && __has_include_next("myinclude.h")
 218 # include_next "myinclude.h"
 219 #endif
 220 </pre>
 221 </blockquote>
 222
 223 <p>Note that __has_include_next, like the GNU extension
 224 #include_next directive, is intended for use in headers only,
 225 and will issue a warning if used in the top-level compilation
 226 file.  A warning will also be issued if an absolute path
 227 is used in the file argument.</p>
 228
 229 <!-- ======================================================================= -->
 230 <h2 id="builtinmacros">Builtin Macros</h2>
 231 <!-- ======================================================================= -->
 232
 233 <dl>
 234   <dt><code>__BASE_FILE__</code></dt>
 235   <dd>Defined to a string that contains the name of the main input
 236   file passed to Clang.</dd>
 237
 238   <dt><code>__COUNTER__</code></dt>
 239   <dd>Defined to an integer value that starts at zero and is
 240   incremented each time the <code>__COUNTER__</code> macro is
 241   expanded.</dd>
 242
 243   <dt><code>__INCLUDE_LEVEL__</code></dt>
 244   <dd>Defined to an integral value that is the include depth of the
 245   file currently being translated. For the main file, this value is
 246   zero.</dd>
 247
 248   <dt><code>__TIMESTAMP__</code></dt>
 249   <dd>Defined to the date and time of the last modification of the
 250   current source file.</dd>
 251
 252   <dt><code>__clang__</code></dt>
 253   <dd>Defined when compiling with Clang</dd>
 254
 255   <dt><code>__clang_major__</code></dt>
 256   <dd>Defined to the major version number of Clang (e.g., the 2 in
 257   2.0.1).</dd>
 258
 259   <dt><code>__clang_minor__</code></dt>
 260   <dd>Defined to the minor version number of Clang (e.g., the 0 in
 261   2.0.1).</dd>
 262
 263   <dt><code>__clang_patchlevel__</code></dt>
 264   <dd>Defined to the patch level of Clang (e.g., the 1 in 2.0.1).</dd>
 265
 266   <dt><code>__clang_version__</code></dt>
 267   <dd>Defined to a string that captures the Clang version, including
 268   the Subversion tag or revision number, e.g., "1.5 (trunk
 269   102332)".</dd>
 270 </dl>
 271
 272 <!-- ======================================================================= -->
 273 <h2 id="vectors">Vectors and Extended Vectors</h2>
 274 <!-- ======================================================================= -->
 275
 276 <p>Supports the GCC vector extensions, plus some stuff like V[1].</p>
 277
 278 <p>Also supports <tt>ext_vector</tt>, which additionally support for V.xyzw
 279 syntax and other tidbits as seen in OpenCL. An example is:</p>
 280
 281 <blockquote>
 282 <pre>
 283 typedef float float4 <b>__attribute__((ext_vector_type(4)))</b>;
 284 typedef float float2 <b>__attribute__((ext_vector_type(2)))</b>;
 285
 286 float4 foo(float2 a, float2 b) {
 287   float4 c;
 288   c.xz = a;
 289   c.yw = b;
 290   return c;
 291 }
 292 </pre>
 293 </blockquote>
 294
 295 <p>Query for this feature with __has_feature(attribute_ext_vector_type).</p>
 296
 297 <p>See also <a href="#__builtin_shufflevector">__builtin_shufflevector</a>.</p>
 298
 299 <!-- ======================================================================= -->
 300 <h2 id="deprecated">Messages on <tt>deprecated</tt> and <tt>unavailable</tt> Attributes</h2>
 301 <!-- ======================================================================= -->
 302
 303 <p>An optional string message can be added to the <tt>deprecated</tt>
 304 and <tt>unavailable</tt> attributes.  For example:</p>
 305
 306 <blockquote>
 307 <pre>void explode(void) __attribute__((deprecated("extremely unsafe, use 'combust' instead!!!")));</pre>
 308 </blockquote>
 309
 310 <p>If the deprecated or unavailable declaration is used, the message
 311 will be incorporated into the appropriate diagnostic:</p>
 312
 313 <blockquote>
 314 <pre>harmless.c:4:3: warning: 'explode' is deprecated: extremely unsafe, use 'combust' instead!!! [-Wdeprecated-declarations]
 315   explode();
 316   ^</pre>
 317 </blockquote>
 318
 319 <p>Query for this feature
 320 with <tt>__has_feature(attribute_deprecated_with_message)</tt>
 321 and <tt>__has_feature(attribute_unavailable_with_message)</tt>.</p>
 322
 323 <!-- ======================================================================= -->
 324 <h2 id="attributes-on-enumerators">Attributes on Enumerators</h2>
 325 <!-- ======================================================================= -->
 326
 327 <p>Clang allows attributes to be written on individual enumerators.
 328 This allows enumerators to be deprecated, made unavailable, etc.  The
 329 attribute must appear after the enumerator name and before any
 330 initializer, like so:</p>
 331
 332 <blockquote>
 333 <pre>enum OperationMode {
 334   OM_Invalid,
 335   OM_Normal,
 336   OM_Terrified __attribute__((deprecated)),
 337   OM_AbortOnError __attribute__((deprecated)) = 4
 338 };</pre>
 339 </blockquote>
 340
 341 <p>Attributes on the <tt>enum</tt> declaration do not apply to
 342 individual enumerators.</p>
 343
 344 <p>Query for this feature with <tt>__has_feature(enumerator_attributes)</tt>.</p>
 345
 346 <!-- ======================================================================= -->
 347 <h2 id="checking_language_features">Checks for Standard Language Features</h2>
 348 <!-- ======================================================================= -->
 349
 350 <p>The <tt>__has_feature</tt> macro can be used to query if certain standard language features are
 351 enabled.  Those features are listed here.</p>
 352
 353 <h3 id="cxx_exceptions">C++ exceptions</h3>
 354
 355 <p>Use <tt>__has_feature(cxx_exceptions)</tt> to determine if C++ exceptions have been enabled. For
 356 example, compiling code with <tt>-fexceptions</tt> enables C++ exceptions.</p>
 357
 358 <h3 id="cxx_rtti">C++ RTTI</h3>
 359
 360 <p>Use <tt>__has_feature(cxx_rtti)</tt> to determine if C++ RTTI has been enabled. For example,
 361 compiling code with <tt>-fno-rtti</tt> disables the use of RTTI.</p>
 362
 363 <!-- ======================================================================= -->
 364 <h2 id="checking_upcoming_features">Checks for Upcoming Standard Language Features</h2>
 365 <!-- ======================================================================= -->
 366
 367 <p>The <tt>__has_feature</tt> macro can be used to query if certain upcoming
 368 standard language features are enabled.  Those features are listed here.</p>
 369
 370 <p>Currently, all features listed here are slated for inclusion in the upcoming
 371 C++0x standard. As a result, all the features that clang supports are enabled
 372 with the <tt>-std=c++0x</tt> option when compiling C++ code. Features that are
 373 not yet implemented will be noted.</p>
 374
 375 <h3 id="cxx_decltype">C++0x <tt>decltype()</tt></h3>
 376
 377 <p>Use <tt>__has_feature(cxx_decltype)</tt> to determine if support for the
 378 <tt>decltype()</tt> specifier is enabled.</p>
 379
 380 <h3 id="cxx_attributes">C++0x attributes</h3>
 381
 382 <p>Use <tt>__has_feature(cxx_attributes)</tt> to determine if support for
 383 attribute parsing with C++0x's square bracket notation is enabled.</p>
 384
 385 <h3 id="cxx_default_function_template_args">C++0x default template arguments in function templates</h3>
 386
 387 <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>
 388
 389 <h3 id="cxx_deleted_functions">C++0x deleted functions</tt></h3>
 390
 391 <p>Use <tt>__has_feature(cxx_deleted_functions)</tt> to determine if support for
 392 deleted function definitions (with <tt>= delete</tt>) is enabled.</p>
 393
 394 <h3 id="cxx_lambdas">C++0x lambdas</h3>
 395
 396 <p>Use <tt>__has_feature(cxx_lambdas)</tt> to determine if support for
 397 lambdas is enabled. clang does not currently implement this feature.</p>
 398
 399 <h3 id="cxx_nullptr">C++0x <tt>nullptr</tt></h3>
 400
 401 <p>Use <tt>__has_feature(cxx_nullptr)</tt> to determine if support for
 402 <tt>nullptr</tt> is enabled. clang does not yet fully implement this
 403 feature.</p>
 404
 405 <h3 id="cxx_reference_qualified_functions">C++0x reference-qualified functions</h3>
 406 <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>
 407
 408 <h3 id="cxx_rvalue_references">C++0x rvalue references</tt></h3>
 409
 410 <p>Use <tt>__has_feature(cxx_rvalue_references)</tt> to determine if support for
 411 rvalue references is enabled. </p>
 412
 413 <h3 id="cxx_static_assert">C++0x <tt>static_assert()</tt></h3>
 414
 415 <p>Use <tt>__has_feature(cxx_static_assert)</tt> to determine if support for
 416 compile-time assertions using <tt>static_assert</tt> is enabled.</p>
 417
 418 <h3 id="cxx_auto_type">C++0x type inference</h3>
 419
 420 <p>Use <tt>__has_feature(cxx_auto_type)</tt> to determine C++0x type inference
 421 is supported using the <tt>auto</tt> specifier. If this is disabled,
 422 <tt>auto</tt> will instead be a storage class specifier, as in C or C++98.
 423 Clang does not currently implement this feature.</p>
 424
 425 <h3 id="cxx_variadic_templates">C++0x variadic templates</h3>
 426
 427 <p>Use <tt>__has_feature(cxx_variadic_templates)</tt> to determine if support
 428 for variadic templates is enabled.</p>
 429
 430 <h3 id="cxx_inline_namespaces">C++0x inline namespaces</h3>
 431
 432 <p>Use <tt>__has_feature(cxx_inline_namespaces)</tt> to determine if support for
 433 inline namespaces is enabled.</p>
 434
 435 <h3 id="cxx_trailing_return">C++0x trailing return type</h3>
 436
 437 <p>Use <tt>__has_feature(cxx_trailing_return)</tt> to determine if support for
 438 the alternate function declaration syntax with trailing return type is enabled.</p>
 439
 440 <h3 id="cxx_strong_enums">C++0x strongly typed enumerations</h3>
 441
 442 <p>Use <tt>__has_feature(cxx_strong_enums)</tt> to determine if support for
 443 strongly typed, scoped enumerations is enabled.</p>
 444
 445 <!-- ======================================================================= -->
 446 <h2 id="checking_type_traits">Checks for Type Traits</h2>
 447 <!-- ======================================================================= -->
 448
 449 <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:
 450 <blockquote>
 451 <pre>
 452 #if __has_feature(is_convertible_to)
 453 template&lt;typename From, typename To&gt;
 454 struct is_convertible_to {
 455   static const bool value = __is_convertible_to(From, To);
 456 };
 457 #else
 458 // Emulate type trait
 459 #endif
 460 </pre>
 461 </blockquote>
 462
 463 <p>The following type traits are supported by Clang:</p>
 464 <ul>
 465   <li><code>__has_nothrow_assign</code> (GNU, Microsoft)</li>
 466   <li><code>__has_nothrow_copy</code> (GNU, Microsoft)</li>
 467   <li><code>__has_nothrow_constructor</code> (GNU, Microsoft)</li>
 468   <li><code>__has_trivial_assign</code> (GNU, Microsoft)</li>
 469   <li><code>__has_trivial_copy</code> (GNU, Microsoft)</li>
 470   <li><code>__has_trivial_constructor</code> (GNU, Microsoft)</li>
 471   <li><code>__has_trivial_destructor</code> (GNU, Microsoft)</li>
 472   <li><code>__has_virtual_destructor</code> (GNU, Microsoft)</li>
 473   <li><code>__is_abstract</code> (GNU, Microsoft)</li>
 474   <li><code>__is_base_of</code> (GNU, Microsoft)</li>
 475   <li><code>__is_class</code> (GNU, Microsoft)</li>
 476   <li><code>__is_convertible_to</code> (Microsoft)</li>
 477   <li><code>__is_empty</code> (GNU, Microsoft)</li>
 478   <li><code>__is_enum</code> (GNU, Microsoft)</li>
 479   <li><code>__is_pod</code> (GNU, Microsoft)</li>
 480   <li><code>__is_polymorphic</code> (GNU, Microsoft)</li>
 481   <li><code>__is_union</code> (GNU, Microsoft)</li>
 482   <li><code>__is_literal(type)</code>: Determines whether the given type is a literal type</li>
 483 </ul>
 484
 485 <!-- ======================================================================= -->
 486 <h2 id="blocks">Blocks</h2>
 487 <!-- ======================================================================= -->
 488
 489 <p>The syntax and high level language feature description is in <a
 490 href="BlockLanguageSpec.txt">BlockLanguageSpec.txt</a>.  Implementation and ABI
 491 details for the clang implementation are in <a
 492 href="Block-ABI-Apple.txt">Block-ABI-Apple.txt</a>.</p>
 493
 494
 495 <p>Query for this feature with __has_feature(blocks).</p>
 496
 497 <!-- ======================================================================= -->
 498 <h2 id="overloading-in-c">Function Overloading in C</h2>
 499 <!-- ======================================================================= -->
 500
 501 <p>Clang provides support for C++ function overloading in C. Function
 502 overloading in C is introduced using the <tt>overloadable</tt> attribute. For
 503 example, one might provide several overloaded versions of a <tt>tgsin</tt>
 504 function that invokes the appropriate standard function computing the sine of a
 505 value with <tt>float</tt>, <tt>double</tt>, or <tt>long double</tt>
 506 precision:</p>
 507
 508 <blockquote>
 509 <pre>
 510 #include &lt;math.h&gt;
 511 float <b>__attribute__((overloadable))</b> tgsin(float x) { return sinf(x); }
 512 double <b>__attribute__((overloadable))</b> tgsin(double x) { return sin(x); }
 513 long double <b>__attribute__((overloadable))</b> tgsin(long double x) { return sinl(x); }
 514 </pre>
 515 </blockquote>
 516
 517 <p>Given these declarations, one can call <tt>tgsin</tt> with a
 518 <tt>float</tt> value to receive a <tt>float</tt> result, with a
 519 <tt>double</tt> to receive a <tt>double</tt> result, etc. Function
 520 overloading in C follows the rules of C++ function overloading to pick
 521 the best overload given the call arguments, with a few C-specific
 522 semantics:</p>
 523 <ul>
 524   <li>Conversion from <tt>float</tt> or <tt>double</tt> to <tt>long
 525   double</tt> is ranked as a floating-point promotion (per C99) rather
 526   than as a floating-point conversion (as in C++).</li>
 527
 528   <li>A conversion from a pointer of type <tt>T*</tt> to a pointer of type
 529   <tt>U*</tt> is considered a pointer conversion (with conversion
 530   rank) if <tt>T</tt> and <tt>U</tt> are compatible types.</li>
 531
 532   <li>A conversion from type <tt>T</tt> to a value of type <tt>U</tt>
 533   is permitted if <tt>T</tt> and <tt>U</tt> are compatible types. This
 534   conversion is given "conversion" rank.</li>
 535 </ul>
 536
 537 <p>The declaration of <tt>overloadable</tt> functions is restricted to
 538 function declarations and definitions. Most importantly, if any
 539 function with a given name is given the <tt>overloadable</tt>
 540 attribute, then all function declarations and definitions with that
 541 name (and in that scope) must have the <tt>overloadable</tt>
 542 attribute. This rule even applies to redeclarations of functions whose original
 543 declaration had the <tt>overloadable</tt> attribute, e.g.,</p>
 544
 545 <blockquote>
 546 <pre>
 547 int f(int) __attribute__((overloadable));
 548 float f(float); <i>// error: declaration of "f" must have the "overloadable" attribute</i>
 549
 550 int g(int) __attribute__((overloadable));
 551 int g(int) { } <i>// error: redeclaration of "g" must also have the "overloadable" attribute</i>
 552 </pre>
 553 </blockquote>
 554
 555 <p>Functions marked <tt>overloadable</tt> must have
 556 prototypes. Therefore, the following code is ill-formed:</p>
 557
 558 <blockquote>
 559 <pre>
 560 int h() __attribute__((overloadable)); <i>// error: h does not have a prototype</i>
 561 </pre>
 562 </blockquote>
 563
 564 <p>However, <tt>overloadable</tt> functions are allowed to use a
 565 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>
 566
 567 <blockquote>
 568 <pre>
 569 void honeypot(...) __attribute__((overloadable, unavailable)); <i>// calling me is an error</i>
 570 </pre>
 571 </blockquote>
 572
 573 <p>Functions declared with the <tt>overloadable</tt> attribute have
 574 their names mangled according to the same rules as C++ function
 575 names. For example, the three <tt>tgsin</tt> functions in our
 576 motivating example get the mangled names <tt>_Z5tgsinf</tt>,
 577 <tt>_Z5tgsind</tt>, and <tt>_Z5tgsine</tt>, respectively. There are two
 578 caveats to this use of name mangling:</p>
 579
 580 <ul>
 581
 582   <li>Future versions of Clang may change the name mangling of
 583   functions overloaded in C, so you should not depend on an specific
 584   mangling. To be completely safe, we strongly urge the use of
 585   <tt>static inline</tt> with <tt>overloadable</tt> functions.</li>
 586
 587   <li>The <tt>overloadable</tt> attribute has almost no meaning when
 588   used in C++, because names will already be mangled and functions are
 589   already overloadable. However, when an <tt>overloadable</tt>
 590   function occurs within an <tt>extern "C"</tt> linkage specification,
 591   it's name <i>will</i> be mangled in the same way as it would in
 592   C.</li>
 593 </ul>
 594
 595 <p>Query for this feature with __has_feature(attribute_overloadable).</p>
 596
 597
 598 <!-- ======================================================================= -->
 599 <h2 id="builtins">Builtin Functions</h2>
 600 <!-- ======================================================================= -->
 601
 602 <p>Clang supports a number of builtin library functions with the same syntax as
 603 GCC, including things like <tt>__builtin_nan</tt>,
 604 <tt>__builtin_constant_p</tt>, <tt>__builtin_choose_expr</tt>,
 605 <tt>__builtin_types_compatible_p</tt>, <tt>__sync_fetch_and_add</tt>, etc.  In
 606 addition to the GCC builtins, Clang supports a number of builtins that GCC does
 607 not, which are listed here.</p>
 608
 609 <p>Please note that Clang does not and will not support all of the GCC builtins
 610 for vector operations.  Instead of using builtins, you should use the functions
 611 defined in target-specific header files like <tt>&lt;xmmintrin.h&gt;</tt>, which
 612 define portable wrappers for these.  Many of the Clang versions of these
 613 functions are implemented directly in terms of <a href="#vectors">extended
 614 vector support</a> instead of builtins, in order to reduce the number of
 615 builtins that we need to implement.</p>
 616
 617 <!-- ======================================================================= -->
 618 <h3 id="__builtin_shufflevector">__builtin_shufflevector</h3>
 619 <!-- ======================================================================= -->
 620
 621 <p><tt>__builtin_shufflevector</tt> is used to express generic vector
 622 permutation/shuffle/swizzle operations. This builtin is also very important for
 623 the implementation of various target-specific header files like
 624 <tt>&lt;xmmintrin.h&gt;</tt>.
 625 </p>
 626
 627 <p><b>Syntax:</b></p>
 628
 629 <pre>
 630 __builtin_shufflevector(vec1, vec2, index1, index2, ...)
 631 </pre>
 632
 633 <p><b>Examples:</b></p>
 634
 635 <pre>
 636   // Identity operation - return 4-element vector V1.
 637   __builtin_shufflevector(V1, V1, 0, 1, 2, 3)
 638
 639   // "Splat" element 0 of V1 into a 4-element result.
 640   __builtin_shufflevector(V1, V1, 0, 0, 0, 0)
 641
 642   // Reverse 4-element vector V1.
 643   __builtin_shufflevector(V1, V1, 3, 2, 1, 0)
 644
 645   // Concatenate every other element of 4-element vectors V1 and V2.
 646   __builtin_shufflevector(V1, V2, 0, 2, 4, 6)
 647
 648   // Concatenate every other element of 8-element vectors V1 and V2.
 649   __builtin_shufflevector(V1, V2, 0, 2, 4, 6, 8, 10, 12, 14)
 650 </pre>
 651
 652 <p><b>Description:</b></p>
 653
 654 <p>The first two arguments to __builtin_shufflevector are vectors that have the
 655 same element type.  The remaining arguments are a list of integers that specify
 656 the elements indices of the first two vectors that should be extracted and
 657 returned in a new vector.  These element indices are numbered sequentially
 658 starting with the first vector, continuing into the second vector.  Thus, if
 659 vec1 is a 4-element vector, index 5 would refer to the second element of vec2.
 660 </p>
 661
 662 <p>The result of __builtin_shufflevector is a vector
 663 with the same element type as vec1/vec2 but that has an element count equal to
 664 the number of indices specified.
 665 </p>
 666
 667 <p>Query for this feature with __has_builtin(__builtin_shufflevector).</p>
 668
 669 <!-- ======================================================================= -->
 670 <h3 id="__builtin_unreachable">__builtin_unreachable</h3>
 671 <!-- ======================================================================= -->
 672
 673 <p><tt>__builtin_unreachable</tt> is used to indicate that a specific point in
 674 the program cannot be reached, even if the compiler might otherwise think it
 675 can.  This is useful to improve optimization and eliminates certain warnings.
 676 For example, without the <tt>__builtin_unreachable</tt> in the example below,
 677 the compiler assumes that the inline asm can fall through and prints a "function
 678 declared 'noreturn' should not return" warning.
 679 </p>
 680
 681 <p><b>Syntax:</b></p>
 682
 683 <pre>
 684 __builtin_unreachable()
 685 </pre>
 686
 687 <p><b>Example of Use:</b></p>
 688
 689 <pre>
 690 void myabort(void) __attribute__((noreturn));
 691 void myabort(void) {
 692     asm("int3");
 693     __builtin_unreachable();
 694 }
 695 </pre>
 696
 697 <p><b>Description:</b></p>
 698
 699 <p>The __builtin_unreachable() builtin has completely undefined behavior.  Since
 700 it has undefined behavior, it is a statement that it is never reached and the
 701 optimizer can take advantage of this to produce better code.  This builtin takes
 702 no arguments and produces a void result.
 703 </p>
 704
 705 <p>Query for this feature with __has_builtin(__builtin_unreachable).</p>
 706
 707
 708 <!-- ======================================================================= -->
 709 <h2 id="targetspecific">Target-Specific Extensions</h2>
 710 <!-- ======================================================================= -->
 711
 712 <p>Clang supports some language features conditionally on some targets.</p>
 713
 714 <!-- ======================================================================= -->
 715 <h3 id="x86-specific">X86/X86-64 Language Extensions</h3>
 716 <!-- ======================================================================= -->
 717
 718 <p>The X86 backend has these language extensions:</p>
 719
 720 <!-- ======================================================================= -->
 721 <h4 id="x86-gs-segment">Memory references off the GS segment</h4>
 722 <!-- ======================================================================= -->
 723
 724 <p>Annotating a pointer with address space #256 causes it to  be code generated
 725 relative to the X86 GS segment register, and address space #257 causes it to be
 726 relative to the X86 FS segment.  Note that this is a very very low-level
 727 feature that should only be used if you know what you're doing (for example in
 728 an OS kernel).</p>
 729
 730 <p>Here is an example:</p>
 731
 732 <pre>
 733 #define GS_RELATIVE __attribute__((address_space(256)))
 734 int foo(int GS_RELATIVE *P) {
 735   return *P;
 736 }
 737 </pre>
 738
 739 <p>Which compiles to (on X86-32):</p>
 740
 741 <pre>
 742 _foo:
 743         movl    4(%esp), %eax
 744         movl    %gs:(%eax), %eax
 745         ret
 746 </pre>
 747
 748 <!-- ======================================================================= -->
 749 <h2 id="analyzerspecific">Static Analysis-Specific Extensions</h2>
 750 <!-- ======================================================================= -->
 751
 752 <p>Clang supports additional attributes that are useful for documenting program
 753 invariants and rules for static analysis tools. The extensions documented here
 754 are used by the <a
 755 href="http://clang.llvm.org/StaticAnalysis.html">path-sensitive static analyzer
 756 engine</a> that is part of Clang's Analysis library.</p>
 757
 758 <!-- ======================================================================= -->
 759 <h3 id="analyzerattributes">Analyzer Attributes</h3>
 760 <!-- ======================================================================= -->
 761
 762 <h4 id="attr_analyzer_noreturn"><tt>analyzer_noreturn</tt></h4>
 763
 764 <p>Clang's static analysis engine understands the standard <tt>noreturn</tt>
 765 attribute. This attribute, which is typically affixed to a function prototype,
 766 indicates that a call to a given function never returns. Function prototypes for
 767 common functions like <tt>exit</tt> are typically annotated with this attribute,
 768 as well as a variety of common assertion handlers. Users can educate the static
 769 analyzer about their own custom assertion handles (thus cutting down on false
 770 positives due to false paths) by marking their own &quot;panic&quot; functions
 771 with this attribute.</p>
 772
 773 <p>While useful, <tt>noreturn</tt> is not applicable in all cases. Sometimes
 774 there are special functions that for all intents and purposes should be
 775 considered panic functions (i.e., they are only called when an internal program
 776 error occurs) but may actually return so that the program can fail gracefully.
 777 The <tt>analyzer_noreturn</tt> attribute allows one to annotate such functions
 778 as being interpreted as &quot;no return&quot; functions by the analyzer (thus
 779 pruning bogus paths) but will not affect compilation (as in the case of
 780 <tt>noreturn</tt>).</p>
 781
 782 <p><b>Usage</b>: The <tt>analyzer_noreturn</tt> attribute can be placed in the
 783 same places where the <tt>noreturn</tt> attribute can be placed. It is commonly
 784 placed at the end of function prototypes:</p>
 785
 786 <pre>
 787   void foo() <b>__attribute__((analyzer_noreturn))</b>;
 788 </pre>
 789
 790 <p>Query for this feature with __has_feature(attribute_analyzer_noreturn).</p>
 791
 792 <h4 id="attr_retain_release">Objective-C retaining behavior attributes</h4>
 793
 794 <p>In Objective-C, functions and methods are generally assumed to take
 795 and return objects with +0 retain counts, with some exceptions for
 796 special methods like <tt>+alloc</tt> and <tt>init</tt>.  However,
 797 there are exceptions, and so Clang provides attributes to allow these
 798 exceptions to be documented, which helps the analyzer find leaks (and
 799 ignore non-leaks).</p>
 800
 801 <p><b>Usage</b>: The <tt>ns_returns_retained</tt>, <tt>ns_returns_not_retained</tt>,
 802 <tt>ns_returns_autoreleased</tt>, <tt>cf_returns_retained</tt>,
 803 and <tt>cf_returns_not_retained</tt> attributes can be placed on
 804 methods and functions that return Objective-C or CoreFoundation
 805 objects.  They are commonly placed at the end of a function prototype
 806 or method declaration:</p>
 807
 808 <pre>
 809   id foo() <b>__attribute__((ns_returns_retained))</b>;
 810
 811   - (NSString*) bar: (int) x <b>__attribute__((ns_returns_retained))</b>;
 812 </pre>
 813
 814 <p>The <tt>*_returns_retained</tt> attributes specify that the
 815 returned object has a +1 retain count.
 816 The <tt>*_returns_not_retained</tt> attributes specify that the return
 817 object has a +0 retain count, even if the normal convention for its
 818 selector would be +1.  <tt>ns_returns_autoreleased</tt> specifies that the
 819 returned object is +0, but is guaranteed to live at least as long as the
 820 next flush of an autorelease pool.</p>
 821
 822 <p><b>Usage</b>: The <tt>ns_consumed</tt> and <tt>cf_consumed</tt>
 823 attributes can be placed on an parameter declaration; they specify
 824 that the argument is expected to have a +1 retain count, which will be
 825 balanced in some way by the function or method.
 826 The <tt>ns_consumes_self</tt> attribute can only be placed on an
 827 Objective-C method; it specifies that the method expects
 828 its <tt>self</tt> parameter to have a +1 retain count, which it will
 829 balance in some way.</p>
 830
 831 <pre>
 832   void <b>foo(__attribute__((ns_consumed))</b> NSString *string);
 833
 834   - (void) bar <b>__attribute__((ns_consumes_self))</b>;
 835   - (void) baz: (id) <b>__attribute__((ns_consumed))</b> x;
 836 </pre>
 837
 838 </div>
 839 </body>
 840 </html>