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