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17
18<h1>Clang Language Extensions</h1>
19
20<ul>
21<li><a href="#intro">Introduction</a></li>
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]22<li><a href="#feature_check">Feature Checking Macros</a></li>
John Thompson92bd8c72009-11-02 22:28:12 +0000[diff] [blame]23<li><a href="#has_include">Include File Checking Macros</a></li>
Chris Lattner81edc9f2009-04-13 02:45:46 +0000[diff] [blame]24<li><a href="#builtinmacros">Builtin Macros</a></li>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]25<li><a href="#vectors">Vectors and Extended Vectors</a></li>
Ted Kremenek87774fd2009-12-03 02:04:01 +0000[diff] [blame]26<li><a href="#checking_language_features">Checks for Standard Language Features</a></li>
Ted Kremenek22c34102009-12-03 02:05:57 +0000[diff] [blame]27 <ul>
28 <li><a href="#cxx_exceptions">C++ exceptions</a></li>
29 <li><a href="#cxx_rtti">C++ RTTI</a></li>
30 </ul>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame^]31<li><a href="#checking_upcoming_features">Checks for Upcoming Standard Language Features</a></li>
32 <ul>
33 <li><a href="#cxx_attributes">C++0x attributes</a></li>
34 <li><a href="#cxx_decltype">C++0x <tt>decltype()</tt></a></li>
35 <li><a href="#cxx_deleted_functions">C++0x deleted functions</a></li>
36 <li><a href="#cxx_concepts">C++ TR concepts</a></li>
37 <li><a href="#cxx_lambdas">C++0x lambdas</a></li>
38 <li><a href="#cxx_nullptr">C++0x nullptr</a></li>
39 <li><a href="#cxx_rvalue_references">C++0x rvalue references</a></li>
40 <li><a href="#cxx_static_assert">C++0x <tt>static_assert()</tt></a></li>
41 <li><a href="#cxx_auto_type">C++0x type inference</a></li>
42 <li><a href="#cxx_variadic_templates">C++0x variadic templates</a></li>
43 </ul>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]44<li><a href="#blocks">Blocks</a></li>
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]45<li><a href="#overloading-in-c">Function Overloading in C</a></li>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]46<li><a href="#builtins">Builtin Functions</a>
47 <ul>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]48 <li><a href="#__builtin_shufflevector">__builtin_shufflevector</a></li>
Chris Lattner21190d52009-09-21 03:09:59 +0000[diff] [blame]49 <li><a href="#__builtin_unreachable">__builtin_unreachable</a></li>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]50 </ul>
51</li>
Chris Lattner1177f912009-04-09 19:58:15 +0000[diff] [blame]52<li><a href="#targetspecific">Target-Specific Extensions</a>
53 <ul>
54 <li><a href="#x86-specific">X86/X86-64 Language Extensions</a></li>
55 </ul>
56</li>
Ted Kremeneked869312009-04-10 05:03:33 +0000[diff] [blame]57<li><a href="#analyzerspecific">Static Analysis-Specific Extensions</a>
58 <ul>
59 <li><a href="#analyzerattributes">Analyzer Attributes</a></li>
60 </ul>
61</li>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]62</ul>
63
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]64<!-- ======================================================================= -->
65<h2 id="intro">Introduction</h2>
66<!-- ======================================================================= -->
67
68<p>This document describes the language extensions provided by Clang. In
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]69addition to the language extensions listed here, Clang aims to support a broad
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]70range of GCC extensions. Please see the <a
71href="http://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html">GCC manual</a> for
72more information on these extensions.</p>
73
74<!-- ======================================================================= -->
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]75<h2 id="feature_check">Feature Checking Macros</h2>
76<!-- ======================================================================= -->
77
78<p>Language extensions can be very useful, but only if you know you can depend
79on them. In order to allow fine-grain features checks, we support two builtin
80function-like macros. This allows you to directly test for a feature in your
81code without having to resort to something like autoconf or fragile "compiler
82version checks".</p>
83
84<!-- ======================================================================= -->
85<h3 id="__has_builtin">__has_builtin</h3>
86<!-- ======================================================================= -->
87
88<p>This function-like macro takes a single identifier argument that is the name
89of a builtin function. It evaluates to 1 if the builtin is supported or 0 if
90not. It can be used like this:</p>
91
92<blockquote>
93<pre>
94#ifndef __has_builtin // Optional of course.
95 #define __has_builtin(x) 0 // Compatibility with non-clang compilers.
96#endif
97
98...
99#if __has_builtin(__builtin_trap)
100 __builtin_trap();
101#else
102 abort();
103#endif
104...
105</pre>
106</blockquote>
107
108
109<!-- ======================================================================= -->
110<h3 id="__has_feature">__has_feature</h3>
111<!-- ======================================================================= -->
112
113<p>This function-like macro takes a single identifier argument that is the name
114of a feature. It evaluates to 1 if the feature is supported or 0 if not. It
115can be used like this:</p>
116
117<blockquote>
118<pre>
119#ifndef __has_feature // Optional of course.
120 #define __has_feature(x) 0 // Compatibility with non-clang compilers.
121#endif
122
123...
124#if __has_feature(attribute_overloadable) || \
125 __has_feature(blocks)
126...
127#endif
128...
129</pre>
130</blockquote>
131
132<p>The feature tag is described along with the language feature below.</p>
133
John Thompson92bd8c72009-11-02 22:28:12 +0000[diff] [blame]134<!-- ======================================================================= -->
135<h2 id="has_include">Include File Checking Macros</h2>
136<!-- ======================================================================= -->
137
138<p>Not all developments systems have the same include files.
139The <a href="#__has_include">__has_include</a> and
140<a href="#__has_include_next">__has_include_next</a> macros allow you to
141check for the existence of an include file before doing
142a possibly failing #include directive.</p>
143
144<!-- ======================================================================= -->
145<h3 id="__has_include">__has_include</h3>
146<!-- ======================================================================= -->
147
148<p>This function-like macro takes a single file name string argument that
149is the name of an include file. It evaluates to 1 if the file can
150be found using the include paths, or 0 otherwise:</p>
151
152<blockquote>
153<pre>
154// Note the two possible file name string formats.
155#if __has_include("myinclude.h") && __has_include(&lt;stdint.h&gt;)
156# include "myinclude.h"
157#endif
158
159// To avoid problem with non-clang compilers not having this macro.
160#if defined(__has_include) && __has_include("myinclude.h")
161# include "myinclude.h"
162#endif
163</pre>
164</blockquote>
165
166<p>To test for this feature, use #if defined(__has_include).</p>
167
168<!-- ======================================================================= -->
169<h3 id="__has_include_next">__has_include_next</h3>
170<!-- ======================================================================= -->
171
172<p>This function-like macro takes a single file name string argument that
173is the name of an include file. It is like __has_include except that it
174looks for the second instance of the given file found in the include
175paths. It evaluates to 1 if the second instance of the file can
176be found using the include paths, or 0 otherwise:</p>
177
178<blockquote>
179<pre>
180// Note the two possible file name string formats.
181#if __has_include_next("myinclude.h") && __has_include_next(&lt;stdint.h&gt;)
182# include_next "myinclude.h"
183#endif
184
185// To avoid problem with non-clang compilers not having this macro.
186#if defined(__has_include_next) && __has_include_next("myinclude.h")
187# include_next "myinclude.h"
188#endif
189</pre>
190</blockquote>
191
192<p>Note that __has_include_next, like the GNU extension
193#include_next directive, is intended for use in headers only,
194and will issue a warning if used in the top-level compilation
195file. A warning will also be issued if an absolute path
196is used in the file argument.</p>
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]197
198<!-- ======================================================================= -->
Chris Lattner81edc9f2009-04-13 02:45:46 +0000[diff] [blame]199<h2 id="builtinmacros">Builtin Macros</h2>
200<!-- ======================================================================= -->
201
202<p>__BASE_FILE__, __INCLUDE_LEVEL__, __TIMESTAMP__, __COUNTER__</p>
203
204<!-- ======================================================================= -->
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]205<h2 id="vectors">Vectors and Extended Vectors</h2>
206<!-- ======================================================================= -->
207
208<p>Supports the GCC vector extensions, plus some stuff like V[1]. ext_vector
209with V.xyzw syntax and other tidbits. See also <a
210href="#__builtin_shufflevector">__builtin_shufflevector</a>.</p>
211
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]212<p>Query for this feature with __has_feature(attribute_ext_vector_type).</p>
213
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]214<!-- ======================================================================= -->
Ted Kremenek87774fd2009-12-03 02:04:01 +0000[diff] [blame]215<h2 id="checking_language_features">Checks for Standard Language Features</h2>
216<!-- ======================================================================= -->
217
218<p>The <tt>__has_feature</tt> macro can be used to query if certain standard language features are
219enabled. Those features are listed here.</p>
220
Ted Kremenek22c34102009-12-03 02:05:57 +0000[diff] [blame]221<h3 id="cxx_exceptions">C++ exceptions</h3>
Ted Kremenek87774fd2009-12-03 02:04:01 +0000[diff] [blame]222
Ted Kremenek22c34102009-12-03 02:05:57 +0000[diff] [blame]223<p>Use <tt>__has_feature(cxx_exceptions)</tt> to determine if C++ exceptions have been enabled. For
224example, compiling code with <tt>-fexceptions</tt> enables C++ exceptions.</p>
Ted Kremenek87774fd2009-12-03 02:04:01 +0000[diff] [blame]225
Ted Kremenek22c34102009-12-03 02:05:57 +0000[diff] [blame]226<h3 id="cxx_rtti">C++ RTTI</h3>
Ted Kremenek87774fd2009-12-03 02:04:01 +0000[diff] [blame]227
Ted Kremenek0eb95602009-12-03 02:06:43 +0000[diff] [blame]228<p>Use <tt>__has_feature(cxx_rtti)</tt> to determine if C++ RTTI has been enabled. For example,
Ted Kremenek22c34102009-12-03 02:05:57 +0000[diff] [blame]229compiling code with <tt>-fno-rtti</tt> disables the use of RTTI.</p>
Ted Kremenek87774fd2009-12-03 02:04:01 +0000[diff] [blame]230
231<!-- ======================================================================= -->
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame^]232<h2 id="checking_upcoming_features">Checks for Upcoming Standard Language Features</h2>
233<!-- ======================================================================= -->
234
235<p>The <tt>__has_feature</tt> macro can be used to query if certain upcoming
236standard language features are enabled. Those features are listed here.</p>
237
238<p>Currently, all features listed here are slated for inclusion in the upcoming
239C++0x standard. As a result, all the features that clang supports are enabled
240with the <tt>-std=c++0x</tt> option when compiling C++ code. Features that are
241not yet implemented will be noted.</p>
242
243<h3 id="cxx_decltype">C++0x <tt>decltype()</tt></h3>
244
245<p>Use <tt>__has_feature(cxx_decltype)</tt> to determine if support for the
246<tt>decltype()</tt> specifier is enabled.</p>
247
248<h3 id="cxx_attributes">C++0x attributes</h3>
249
250<p>Use <tt>__has_feature(cxx_attributes)</tt> to determine if support for
251attribute parsing with C++0x's square bracket notation is enabled.
252
253<h3 id="cxx_deleted_functions">C++0x deleted functions</tt></h3>
254
255<p>Use <tt>__has_feature(cxx_deleted_functions)</tt> to determine if support for
256deleted function definitions (with <tt>= delete</tt>) is enabled.
257
258<h3 id="cxx_concepts">C++ TR <tt>concepts</tt></h3>
259
260<p>Use <tt>__has_feature(cxx_lambdas)</tt> to determine if support for
261concepts is enabled. clang does not currently implement this feature.
262
263<h3 id="cxx_lambdas">C++0x <tt>nullptr</tt></h3>
264
265<p>Use <tt>__has_feature(cxx_lambdas)</tt> to determine if support for
266lambdas is enabled. clang does not currently implement this feature.
267
268<h3 id="cxx_nullptr">C++0x <tt>nullptr</tt></h3>
269
270<p>Use <tt>__has_feature(cxx_nullptr)</tt> to determine if support for
271<tt>nullptr</tt> is enabled. clang does not yet fully implement this feature.
272
273<h3 id="cxx_rvalue_references">C++0x rvalue references</tt></h3>
274
275<p>Use <tt>__has_feature(cxx_rvalue_references)</tt> to determine if support for
276rvalue references is enabled. clang does not yet fully implement this feature.
277
278<h3 id="cxx_static_assert">C++0x <tt>static_assert()</tt></h3>
279
280<p>Use <tt>__has_feature(cxx_static_assert)</tt> to determine if support for
281compile-time assertions using <tt>static_assert</tt> is enabled.</p>
282
283<h3 id="cxx_auto_type">C++0x type inference</h3>
284
285<p>Use <tt>__has_feature(cxx_auto_type)</tt> to determine C++0x type inference
286is supported using the <tt>auto</tt> specifier. If this is disabled,
287<tt>auto</tt> will instead be a storage class specifier, as in C or C++98.</p>
288
289<h3 id="cxx_variadic_templates">C++0x variadic templates</tt></h3>
290
291<p>Use <tt>__has_feature(cxx_variadic_templates)</tt> to determine if support
292for templates taking any number of arguments with the ellipsis notation is
293enabled. clang does not yet fully implement this feature.</p>
294
295<!-- ======================================================================= -->
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]296<h2 id="blocks">Blocks</h2>
297<!-- ======================================================================= -->
298
Chris Lattnera7dbdf52009-03-09 07:03:22 +0000[diff] [blame]299<p>The syntax and high level language feature description is in <a
300href="BlockLanguageSpec.txt">BlockLanguageSpec.txt</a>. Implementation and ABI
301details for the clang implementation are in <a
302href="BlockImplementation.txt">BlockImplementation.txt</a>.</p>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]303
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]304
305<p>Query for this feature with __has_feature(blocks).</p>
306
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]307<!-- ======================================================================= -->
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]308<h2 id="overloading-in-c">Function Overloading in C</h2>
309<!-- ======================================================================= -->
310
Chris Lattnerf161d412009-02-13 21:51:45 +0000[diff] [blame]311<p>Clang provides support for C++ function overloading in C. Function
312overloading in C is introduced using the <tt>overloadable</tt> attribute. For
313example, one might provide several overloaded versions of a <tt>tgsin</tt>
314function that invokes the appropriate standard function computing the sine of a
315value with <tt>float</tt>, <tt>double</tt>, or <tt>long double</tt>
316precision:</p>
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]317
318<blockquote>
319<pre>
320#include &lt;math.h&gt;
321float <b>__attribute__((overloadable))</b> tgsin(float x) { return sinf(x); }
322double <b>__attribute__((overloadable))</b> tgsin(double x) { return sin(x); }
323long double <b>__attribute__((overloadable))</b> tgsin(long double x) { return sinl(x); }
324</pre>
325</blockquote>
326
327<p>Given these declarations, one can call <tt>tgsin</tt> with a
328<tt>float</tt> value to receive a <tt>float</tt> result, with a
329<tt>double</tt> to receive a <tt>double</tt> result, etc. Function
330overloading in C follows the rules of C++ function overloading to pick
331the best overload given the call arguments, with a few C-specific
332semantics:</p>
333<ul>
334 <li>Conversion from <tt>float</tt> or <tt>double</tt> to <tt>long
335 double</tt> is ranked as a floating-point promotion (per C99) rather
336 than as a floating-point conversion (as in C++).</li>
337
338 <li>A conversion from a pointer of type <tt>T*</tt> to a pointer of type
339 <tt>U*</tt> is considered a pointer conversion (with conversion
340 rank) if <tt>T</tt> and <tt>U</tt> are compatible types.</li>
341
342 <li>A conversion from type <tt>T</tt> to a value of type <tt>U</tt>
343 is permitted if <tt>T</tt> and <tt>U</tt> are compatible types. This
344 conversion is given "conversion" rank.</li>
345</ul>
346
347<p>The declaration of <tt>overloadable</tt> functions is restricted to
348function declarations and definitions. Most importantly, if any
349function with a given name is given the <tt>overloadable</tt>
350attribute, then all function declarations and definitions with that
351name (and in that scope) must have the <tt>overloadable</tt>
Chris Lattnerf161d412009-02-13 21:51:45 +0000[diff] [blame]352attribute. This rule even applies to redeclarations of functions whose original
353declaration had the <tt>overloadable</tt> attribute, e.g.,</p>
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]354
355<blockquote>
356<pre>
357int f(int) __attribute__((overloadable));
358float f(float); <i>// error: declaration of "f" must have the "overloadable" attribute</i>
359
360int g(int) __attribute__((overloadable));
361int g(int) { } <i>// error: redeclaration of "g" must also have the "overloadable" attribute</i>
362</pre>
363</blockquote>
364
Douglas Gregor965acbb2009-02-18 07:07:28 +0000[diff] [blame]365<p>Functions marked <tt>overloadable</tt> must have
366prototypes. Therefore, the following code is ill-formed:</p>
367
368<blockquote>
369<pre>
370int h() __attribute__((overloadable)); <i>// error: h does not have a prototype</i>
371</pre>
372</blockquote>
373
374<p>However, <tt>overloadable</tt> functions are allowed to use a
375ellipsis 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>
376
377<blockquote>
378<pre>
Chris Lattner02246802009-02-18 22:27:46 +0000[diff] [blame]379void honeypot(...) __attribute__((overloadable, unavailable)); <i>// calling me is an error</i>
Douglas Gregor965acbb2009-02-18 07:07:28 +0000[diff] [blame]380</pre>
381</blockquote>
382
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]383<p>Functions declared with the <tt>overloadable</tt> attribute have
384their names mangled according to the same rules as C++ function
385names. For example, the three <tt>tgsin</tt> functions in our
386motivating example get the mangled names <tt>_Z5tgsinf</tt>,
387<tt>_Z5tgsind</tt>, and <tt>Z5tgsine</tt>, respectively. There are two
388caveats to this use of name mangling:</p>
389
390<ul>
391
392 <li>Future versions of Clang may change the name mangling of
393 functions overloaded in C, so you should not depend on an specific
394 mangling. To be completely safe, we strongly urge the use of
395 <tt>static inline</tt> with <tt>overloadable</tt> functions.</li>
396
397 <li>The <tt>overloadable</tt> attribute has almost no meaning when
398 used in C++, because names will already be mangled and functions are
399 already overloadable. However, when an <tt>overloadable</tt>
400 function occurs within an <tt>extern "C"</tt> linkage specification,
401 it's name <i>will</i> be mangled in the same way as it would in
402 C.</li>
403</ul>
404
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]405<p>Query for this feature with __has_feature(attribute_overloadable).</p>
406
407
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]408<!-- ======================================================================= -->
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]409<h2 id="builtins">Builtin Functions</h2>
410<!-- ======================================================================= -->
411
412<p>Clang supports a number of builtin library functions with the same syntax as
413GCC, including things like <tt>__builtin_nan</tt>,
414<tt>__builtin_constant_p</tt>, <tt>__builtin_choose_expr</tt>,
415<tt>__builtin_types_compatible_p</tt>, <tt>__sync_fetch_and_add</tt>, etc. In
416addition to the GCC builtins, Clang supports a number of builtins that GCC does
417not, which are listed here.</p>
418
419<p>Please note that Clang does not and will not support all of the GCC builtins
420for vector operations. Instead of using builtins, you should use the functions
421defined in target-specific header files like <tt>&lt;xmmintrin.h&gt;</tt>, which
422define portable wrappers for these. Many of the Clang versions of these
423functions are implemented directly in terms of <a href="#vectors">extended
424vector support</a> instead of builtins, in order to reduce the number of
425builtins that we need to implement.</p>
426
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]427<!-- ======================================================================= -->
Chris Lattner6f72da52009-02-13 20:00:20 +0000[diff] [blame]428<h3 id="__builtin_shufflevector">__builtin_shufflevector</h3>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]429<!-- ======================================================================= -->
430
Chris Lattneraad826b2009-09-16 18:56:12 +0000[diff] [blame]431<p><tt>__builtin_shufflevector</tt> is used to express generic vector
Chris Lattner6f72da52009-02-13 20:00:20 +0000[diff] [blame]432permutation/shuffle/swizzle operations. This builtin is also very important for
433the implementation of various target-specific header files like
434<tt>&lt;xmmintrin.h&gt;</tt>.
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]435</p>
436
437<p><b>Syntax:</b></p>
438
439<pre>
Chris Lattner6f72da52009-02-13 20:00:20 +0000[diff] [blame]440__builtin_shufflevector(vec1, vec2, index1, index2, ...)
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]441</pre>
442
443<p><b>Examples:</b></p>
444
445<pre>
Chris Lattner6f72da52009-02-13 20:00:20 +0000[diff] [blame]446 // Identity operation - return 4-element vector V1.
447 __builtin_shufflevector(V1, V1, 0, 1, 2, 3)
448
449 // "Splat" element 0 of V1 into a 4-element result.
450 __builtin_shufflevector(V1, V1, 0, 0, 0, 0)
451
452 // Reverse 4-element vector V1.
453 __builtin_shufflevector(V1, V1, 3, 2, 1, 0)
454
455 // Concatenate every other element of 4-element vectors V1 and V2.
456 __builtin_shufflevector(V1, V2, 0, 2, 4, 6)
457
458 // Concatenate every other element of 8-element vectors V1 and V2.
459 __builtin_shufflevector(V1, V2, 0, 2, 4, 6, 8, 10, 12, 14)
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]460</pre>
461
462<p><b>Description:</b></p>
463
Chris Lattner6f72da52009-02-13 20:00:20 +0000[diff] [blame]464<p>The first two arguments to __builtin_shufflevector are vectors that have the
465same element type. The remaining arguments are a list of integers that specify
466the elements indices of the first two vectors that should be extracted and
467returned in a new vector. These element indices are numbered sequentially
468starting with the first vector, continuing into the second vector. Thus, if
469vec1 is a 4-element vector, index 5 would refer to the second element of vec2.
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]470</p>
471
Chris Lattner6f72da52009-02-13 20:00:20 +0000[diff] [blame]472<p>The result of __builtin_shufflevector is a vector
473with the same element type as vec1/vec2 but that has an element count equal to
474the number of indices specified.
475</p>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]476
Chris Lattner21190d52009-09-21 03:09:59 +0000[diff] [blame]477<p>Query for this feature with __has_builtin(__builtin_shufflevector).</p>
478
479<!-- ======================================================================= -->
480<h3 id="__builtin_unreachable">__builtin_unreachable</h3>
481<!-- ======================================================================= -->
482
483<p><tt>__builtin_unreachable</tt> is used to indicate that a specific point in
484the program cannot be reached, even if the compiler might otherwise think it
485can. This is useful to improve optimization and eliminates certain warnings.
486For example, without the <tt>__builtin_unreachable</tt> in the example below,
487the compiler assumes that the inline asm can fall through and prints a "function
488declared 'noreturn' should not return" warning.
489</p>
490
491<p><b>Syntax:</b></p>
492
493<pre>
494__builtin_unreachable()
495</pre>
496
497<p><b>Example of Use:</b></p>
498
499<pre>
500void myabort(void) __attribute__((noreturn));
501void myabort(void) {
502 asm("int3");
503 __builtin_unreachable();
504}
505</pre>
506
507<p><b>Description:</b></p>
508
509<p>The __builtin_unreachable() builtin has completely undefined behavior. Since
510it has undefined behavior, it is a statement that it is never reached and the
511optimizer can take advantage of this to produce better code. This builtin takes
512no arguments and produces a void result.
513</p>
514
515<p>Query for this feature with __has_builtin(__builtin_unreachable).</p>
516
517
Chris Lattner1177f912009-04-09 19:58:15 +0000[diff] [blame]518<!-- ======================================================================= -->
519<h2 id="targetspecific">Target-Specific Extensions</h2>
520<!-- ======================================================================= -->
521
522<p>Clang supports some language features conditionally on some targets.</p>
523
524<!-- ======================================================================= -->
525<h3 id="x86-specific">X86/X86-64 Language Extensions</h3>
526<!-- ======================================================================= -->
527
528<p>The X86 backend has these language extensions:</p>
529
530<!-- ======================================================================= -->
531<h4 id="x86-gs-segment">Memory references off the GS segment</h4>
532<!-- ======================================================================= -->
533
534<p>Annotating a pointer with address space #256 causes it to be code generated
Chris Lattnera021e7c2009-05-05 18:54:47 +0000[diff] [blame]535relative to the X86 GS segment register, and address space #257 causes it to be
536relative to the X86 FS segment. Note that this is a very very low-level
537feature that should only be used if you know what you're doing (for example in
538an OS kernel).</p>
Chris Lattner1177f912009-04-09 19:58:15 +0000[diff] [blame]539
540<p>Here is an example:</p>
541
542<pre>
543#define GS_RELATIVE __attribute__((address_space(256)))
544int foo(int GS_RELATIVE *P) {
545 return *P;
546}
547</pre>
548
549<p>Which compiles to (on X86-32):</p>
550
551<pre>
552_foo:
553 movl 4(%esp), %eax
554 movl %gs:(%eax), %eax
555 ret
556</pre>
557
Ted Kremeneked869312009-04-10 05:03:33 +0000[diff] [blame]558<!-- ======================================================================= -->
559<h2 id="analyzerspecific">Static Analysis-Specific Extensions</h2>
560<!-- ======================================================================= -->
561
562<p>Clang supports additional attributes that are useful for documenting program
563invariants and rules for static analysis tools. The extensions documented here
564are used by the <a
565href="http://clang.llvm.org/StaticAnalysis.html">path-sensitive static analyzer
566engine</a> that is part of Clang's Analysis library.</p>
567
568<!-- ======================================================================= -->
569<h3 id="analyzerattributes">Analyzer Attributes</h3>
570<!-- ======================================================================= -->
571
572<h4 id="attr_analyzer_noreturn"><tt>analyzer_noreturn</tt></h4>
573
574<p>Clang's static analysis engine understands the standard <tt>noreturn</tt>
Ted Kremenek4df21142009-04-10 05:04:22 +0000[diff] [blame]575attribute. This attribute, which is typically affixed to a function prototype,
576indicates that a call to a given function never returns. Function prototypes for
577common functions like <tt>exit</tt> are typically annotated with this attribute,
578as well as a variety of common assertion handlers. Users can educate the static
579analyzer about their own custom assertion handles (thus cutting down on false
580positives due to false paths) by marking their own &quot;panic&quot; functions
581with this attribute.</p>
Ted Kremeneked869312009-04-10 05:03:33 +0000[diff] [blame]582
583<p>While useful, <tt>noreturn</tt> is not applicable in all cases. Sometimes
Nick Lewycky625b5862009-06-14 04:08:08 +0000[diff] [blame]584there are special functions that for all intents and purposes should be
585considered panic functions (i.e., they are only called when an internal program
586error occurs) but may actually return so that the program can fail gracefully.
587The <tt>analyzer_noreturn</tt> attribute allows one to annotate such functions
588as being interpreted as &quot;no return&quot; functions by the analyzer (thus
Chris Lattner28935892009-04-10 05:54:56 +0000[diff] [blame]589pruning bogus paths) but will not affect compilation (as in the case of
Ted Kremeneked869312009-04-10 05:03:33 +0000[diff] [blame]590<tt>noreturn</tt>).</p>
591
592<p><b>Usage</b>: The <tt>analyzer_noreturn</tt> attribute can be placed in the
Chris Lattner28935892009-04-10 05:54:56 +0000[diff] [blame]593same places where the <tt>noreturn</tt> attribute can be placed. It is commonly
Ted Kremeneked869312009-04-10 05:03:33 +0000[diff] [blame]594placed at the end of function prototypes:</p>
595
596<pre>
597 void foo() <b>__attribute__((analyzer_noreturn))</b>;
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]598</pre>
599
600<p>Query for this feature with __has_feature(attribute_analyzer_noreturn).</p>
601
Ted Kremeneked869312009-04-10 05:03:33 +0000[diff] [blame]602
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]603</div>
604</body>
605</html>