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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>
John McCall48209082010-11-08 19:48:17 +0000[diff] [blame^]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>
Ted Kremenek87774fd2009-12-03 02:04:01 +0000[diff] [blame]28<li><a href="#checking_language_features">Checks for Standard Language Features</a></li>
Ted Kremenek22c34102009-12-03 02:05:57 +0000[diff] [blame]29 <ul>
30 <li><a href="#cxx_exceptions">C++ exceptions</a></li>
31 <li><a href="#cxx_rtti">C++ RTTI</a></li>
32 </ul>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]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>
Sebastian Redlf6c09772010-08-31 23:28:47 +0000[diff] [blame]45 <li><a href="#cxx_inline_namespaces">C++0x inline namespaces</a></li>
Douglas Gregor1274ccd2010-10-08 23:50:27 +0000[diff] [blame]46 <li><a href="#cxx_strong_enums">C++0x strongly-typed enumerations</a></li>
Douglas Gregordab60ad2010-10-01 18:44:50 +0000[diff] [blame]47 <li><a href="#cxx_trailing_return">C++0x trailing return type</a></li>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]48 </ul>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]49<li><a href="#blocks">Blocks</a></li>
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]50<li><a href="#overloading-in-c">Function Overloading in C</a></li>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]51<li><a href="#builtins">Builtin Functions</a>
52 <ul>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]53 <li><a href="#__builtin_shufflevector">__builtin_shufflevector</a></li>
Chris Lattner21190d52009-09-21 03:09:59 +0000[diff] [blame]54 <li><a href="#__builtin_unreachable">__builtin_unreachable</a></li>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]55 </ul>
56</li>
Chris Lattner1177f912009-04-09 19:58:15 +0000[diff] [blame]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>
Ted Kremeneked869312009-04-10 05:03:33 +0000[diff] [blame]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>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]67</ul>
68
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]69<!-- ======================================================================= -->
70<h2 id="intro">Introduction</h2>
71<!-- ======================================================================= -->
72
73<p>This document describes the language extensions provided by Clang. In
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]74addition to the language extensions listed here, Clang aims to support a broad
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]75range of GCC extensions. Please see the <a
76href="http://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html">GCC manual</a> for
77more information on these extensions.</p>
78
79<!-- ======================================================================= -->
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]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
84on them. In order to allow fine-grain features checks, we support two builtin
85function-like macros. This allows you to directly test for a feature in your
86code without having to resort to something like autoconf or fragile "compiler
87version 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
94of a builtin function. It evaluates to 1 if the builtin is supported or 0 if
95not. 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
119of a feature. It evaluates to 1 if the feature is supported or 0 if not. It
120can 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
John Thompson92bd8c72009-11-02 22:28:12 +0000[diff] [blame]139<!-- ======================================================================= -->
Anders Carlssoncae50952010-10-20 02:31:43 +0000[diff] [blame]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
144of an attribute. It evaluates to 1 if the attribute is supported or 0 if not. It
145can 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<!-- ======================================================================= -->
John Thompson92bd8c72009-11-02 22:28:12 +0000[diff] [blame]164<h2 id="has_include">Include File Checking Macros</h2>
165<!-- ======================================================================= -->
166
167<p>Not all developments systems have the same include files.
168The <a href="#__has_include">__has_include</a> and
169<a href="#__has_include_next">__has_include_next</a> macros allow you to
170check for the existence of an include file before doing
171a 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
178is the name of an include file. It evaluates to 1 if the file can
179be 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
202is the name of an include file. It is like __has_include except that it
203looks for the second instance of the given file found in the include
204paths. It evaluates to 1 if the second instance of the file can
205be 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,
223and will issue a warning if used in the top-level compilation
224file. A warning will also be issued if an absolute path
225is used in the file argument.</p>
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]226
227<!-- ======================================================================= -->
Chris Lattner81edc9f2009-04-13 02:45:46 +0000[diff] [blame]228<h2 id="builtinmacros">Builtin Macros</h2>
229<!-- ======================================================================= -->
230
Douglas Gregor4290fbd2010-04-30 02:51:06 +0000[diff] [blame]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>
Chris Lattner81edc9f2009-04-13 02:45:46 +0000[diff] [blame]269
270<!-- ======================================================================= -->
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]271<h2 id="vectors">Vectors and Extended Vectors</h2>
272<!-- ======================================================================= -->
273
Owen Andersond2bf0cd2010-01-27 01:22:36 +0000[diff] [blame]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
277syntax and other tidbits as seen in OpenCL. An example is:</p>
278
279<blockquote>
280<pre>
281typedef float float4 <b>__attribute__((ext_vector_type(4)))</b>;
282typedef float float2 <b>__attribute__((ext_vector_type(2)))</b>;
283
284float4 foo(float2 a, float2 b) {
285 float4 c;
286 c.xz = a;
287 c.yw = b;
288 return c;
289}
John McCall48209082010-11-08 19:48:17 +0000[diff] [blame^]290</pre>
Owen Andersond2bf0cd2010-01-27 01:22:36 +0000[diff] [blame]291</blockquote>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]292
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]293<p>Query for this feature with __has_feature(attribute_ext_vector_type).</p>
294
Owen Andersond2bf0cd2010-01-27 01:22:36 +0000[diff] [blame]295<p>See also <a href="#__builtin_shufflevector">__builtin_shufflevector</a>.</p>
296
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]297<!-- ======================================================================= -->
John McCall48209082010-11-08 19:48:17 +0000[diff] [blame^]298<h2 id="deprecated">Messages on <tt>deprecated</tt> and <tt>unavailable</tt> Attributes</h2>
Fariborz Jahanianc784dc12010-10-06 23:12:32 +0000[diff] [blame]299<!-- ======================================================================= -->
300
John McCall48209082010-11-08 19:48:17 +0000[diff] [blame^]301<p>An optional string message can be added to the <tt>deprecated</tt>
302and <tt>unavailable</tt> attributes. For example:</p>
Fariborz Jahanianc784dc12010-10-06 23:12:32 +0000[diff] [blame]303
John McCall48209082010-11-08 19:48:17 +0000[diff] [blame^]304<blockquote>
305<pre>void explode(void) __attribute__((deprecated("extremely unsafe!!!")));</pre>
306</blockquote>
307
308<p>If the deprecated or unavailable declaration is used, the message
309will be incorporated into the appropriate diagnostic:</p>
310
311<blockquote>
312<pre>harmless.c:4:3: warning: 'explode' is deprecated: extremely unsafe!!! [-Wdeprecated-declarations]
313 explode();
314 ^</pre>
315</blockquote>
316
317<p>Query for this feature
318with <tt>__has_feature(attribute_deprecated_with_message)</tt>
319and <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.
326This allows enumerators to be deprecated, made unavailable, etc. The
327attribute must appear after the enumerator name and before any
328initializer, 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
340individual enumerators.</p>
341
342<p>Query for this feature with <tt>__has_feature(enumerator_attributes)</tt>.</p>
Fariborz Jahanianc784dc12010-10-06 23:12:32 +0000[diff] [blame]343
344<!-- ======================================================================= -->
Ted Kremenek87774fd2009-12-03 02:04:01 +0000[diff] [blame]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
349enabled. Those features are listed here.</p>
350
Ted Kremenek22c34102009-12-03 02:05:57 +0000[diff] [blame]351<h3 id="cxx_exceptions">C++ exceptions</h3>
Ted Kremenek87774fd2009-12-03 02:04:01 +0000[diff] [blame]352
Ted Kremenek22c34102009-12-03 02:05:57 +0000[diff] [blame]353<p>Use <tt>__has_feature(cxx_exceptions)</tt> to determine if C++ exceptions have been enabled. For
354example, compiling code with <tt>-fexceptions</tt> enables C++ exceptions.</p>
Ted Kremenek87774fd2009-12-03 02:04:01 +0000[diff] [blame]355
Ted Kremenek22c34102009-12-03 02:05:57 +0000[diff] [blame]356<h3 id="cxx_rtti">C++ RTTI</h3>
Ted Kremenek87774fd2009-12-03 02:04:01 +0000[diff] [blame]357
Ted Kremenek0eb95602009-12-03 02:06:43 +0000[diff] [blame]358<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]359compiling code with <tt>-fno-rtti</tt> disables the use of RTTI.</p>
Ted Kremenek87774fd2009-12-03 02:04:01 +0000[diff] [blame]360
361<!-- ======================================================================= -->
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]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
366standard 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
369C++0x standard. As a result, all the features that clang supports are enabled
370with the <tt>-std=c++0x</tt> option when compiling C++ code. Features that are
371not 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
Sebastian Redlf6c09772010-08-31 23:28:47 +0000[diff] [blame]381attribute parsing with C++0x's square bracket notation is enabled.</p>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]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
Sebastian Redlf6c09772010-08-31 23:28:47 +0000[diff] [blame]386deleted function definitions (with <tt>= delete</tt>) is enabled.</p>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]387
Nick Lewycky1444aef2010-04-23 06:09:40 +0000[diff] [blame]388<h3 id="cxx_concepts">C++ TR concepts</h3>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]389
Nick Lewycky1444aef2010-04-23 06:09:40 +0000[diff] [blame]390<p>Use <tt>__has_feature(cxx_concepts)</tt> to determine if support for
Sebastian Redlf6c09772010-08-31 23:28:47 +0000[diff] [blame]391concepts is enabled. clang does not currently implement this feature.</p>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]392
Douglas Gregor9cc90a32010-01-13 16:27:49 +0000[diff] [blame]393<h3 id="cxx_lambdas">C++0x lambdas</h3>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]394
395<p>Use <tt>__has_feature(cxx_lambdas)</tt> to determine if support for
Sebastian Redlf6c09772010-08-31 23:28:47 +0000[diff] [blame]396lambdas is enabled. clang does not currently implement this feature.</p>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]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
Sebastian Redlf6c09772010-08-31 23:28:47 +0000[diff] [blame]401<tt>nullptr</tt> is enabled. clang does not yet fully implement this
402feature.</p>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]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
Sebastian Redlf6c09772010-08-31 23:28:47 +0000[diff] [blame]407rvalue references is enabled. clang does not yet fully implement this
408feature.</p>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]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
413compile-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
418is 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
Sebastian Redlf6c09772010-08-31 23:28:47 +0000[diff] [blame]421<h3 id="cxx_variadic_templates">C++0x variadic templates</h3>
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]422
423<p>Use <tt>__has_feature(cxx_variadic_templates)</tt> to determine if support
424for templates taking any number of arguments with the ellipsis notation is
425enabled. clang does not yet fully implement this feature.</p>
426
Sebastian Redlf6c09772010-08-31 23:28:47 +0000[diff] [blame]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
430inline namespaces is enabled.</p>
431
Douglas Gregordab60ad2010-10-01 18:44:50 +0000[diff] [blame]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
435the alternate function declaration syntax with trailing return type is enabled.</p>
436
Douglas Gregor1274ccd2010-10-08 23:50:27 +0000[diff] [blame]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
440strongly typed, scoped enumerations is enabled.</p>
441
Sean Hunt4ef4c6b2010-01-13 08:31:49 +0000[diff] [blame]442<!-- ======================================================================= -->
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]443<h2 id="blocks">Blocks</h2>
444<!-- ======================================================================= -->
445
Chris Lattnera7dbdf52009-03-09 07:03:22 +0000[diff] [blame]446<p>The syntax and high level language feature description is in <a
447href="BlockLanguageSpec.txt">BlockLanguageSpec.txt</a>. Implementation and ABI
448details for the clang implementation are in <a
Chris Lattner5d7650b2010-03-16 21:43:03 +0000[diff] [blame]449href="Block-ABI-Apple.txt">Block-ABI-Apple.txt</a>.</p>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]450
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]451
452<p>Query for this feature with __has_feature(blocks).</p>
453
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]454<!-- ======================================================================= -->
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]455<h2 id="overloading-in-c">Function Overloading in C</h2>
456<!-- ======================================================================= -->
457
Chris Lattnerf161d412009-02-13 21:51:45 +0000[diff] [blame]458<p>Clang provides support for C++ function overloading in C. Function
459overloading in C is introduced using the <tt>overloadable</tt> attribute. For
460example, one might provide several overloaded versions of a <tt>tgsin</tt>
461function that invokes the appropriate standard function computing the sine of a
462value with <tt>float</tt>, <tt>double</tt>, or <tt>long double</tt>
463precision:</p>
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]464
465<blockquote>
466<pre>
467#include &lt;math.h&gt;
468float <b>__attribute__((overloadable))</b> tgsin(float x) { return sinf(x); }
469double <b>__attribute__((overloadable))</b> tgsin(double x) { return sin(x); }
470long 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
477overloading in C follows the rules of C++ function overloading to pick
478the best overload given the call arguments, with a few C-specific
479semantics:</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
495function declarations and definitions. Most importantly, if any
496function with a given name is given the <tt>overloadable</tt>
497attribute, then all function declarations and definitions with that
498name (and in that scope) must have the <tt>overloadable</tt>
Chris Lattnerf161d412009-02-13 21:51:45 +0000[diff] [blame]499attribute. This rule even applies to redeclarations of functions whose original
500declaration had the <tt>overloadable</tt> attribute, e.g.,</p>
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]501
502<blockquote>
503<pre>
504int f(int) __attribute__((overloadable));
505float f(float); <i>// error: declaration of "f" must have the "overloadable" attribute</i>
506
507int g(int) __attribute__((overloadable));
508int g(int) { } <i>// error: redeclaration of "g" must also have the "overloadable" attribute</i>
509</pre>
510</blockquote>
511
Douglas Gregor965acbb2009-02-18 07:07:28 +0000[diff] [blame]512<p>Functions marked <tt>overloadable</tt> must have
513prototypes. Therefore, the following code is ill-formed:</p>
514
515<blockquote>
516<pre>
517int 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
522ellipsis 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>
Chris Lattner02246802009-02-18 22:27:46 +0000[diff] [blame]526void honeypot(...) __attribute__((overloadable, unavailable)); <i>// calling me is an error</i>
Douglas Gregor965acbb2009-02-18 07:07:28 +0000[diff] [blame]527</pre>
528</blockquote>
529
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]530<p>Functions declared with the <tt>overloadable</tt> attribute have
531their names mangled according to the same rules as C++ function
532names. For example, the three <tt>tgsin</tt> functions in our
533motivating example get the mangled names <tt>_Z5tgsinf</tt>,
534<tt>_Z5tgsind</tt>, and <tt>Z5tgsine</tt>, respectively. There are two
535caveats 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
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]552<p>Query for this feature with __has_feature(attribute_overloadable).</p>
553
554
Douglas Gregorcb54d432009-02-13 00:57:04 +0000[diff] [blame]555<!-- ======================================================================= -->
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]556<h2 id="builtins">Builtin Functions</h2>
557<!-- ======================================================================= -->
558
559<p>Clang supports a number of builtin library functions with the same syntax as
560GCC, 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
563addition to the GCC builtins, Clang supports a number of builtins that GCC does
564not, which are listed here.</p>
565
566<p>Please note that Clang does not and will not support all of the GCC builtins
567for vector operations. Instead of using builtins, you should use the functions
568defined in target-specific header files like <tt>&lt;xmmintrin.h&gt;</tt>, which
569define portable wrappers for these. Many of the Clang versions of these
570functions are implemented directly in terms of <a href="#vectors">extended
571vector support</a> instead of builtins, in order to reduce the number of
572builtins that we need to implement.</p>
573
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]574<!-- ======================================================================= -->
Chris Lattner6f72da52009-02-13 20:00:20 +0000[diff] [blame]575<h3 id="__builtin_shufflevector">__builtin_shufflevector</h3>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]576<!-- ======================================================================= -->
577
Chris Lattneraad826b2009-09-16 18:56:12 +0000[diff] [blame]578<p><tt>__builtin_shufflevector</tt> is used to express generic vector
Chris Lattner6f72da52009-02-13 20:00:20 +0000[diff] [blame]579permutation/shuffle/swizzle operations. This builtin is also very important for
580the implementation of various target-specific header files like
581<tt>&lt;xmmintrin.h&gt;</tt>.
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]582</p>
583
584<p><b>Syntax:</b></p>
585
586<pre>
Chris Lattner6f72da52009-02-13 20:00:20 +0000[diff] [blame]587__builtin_shufflevector(vec1, vec2, index1, index2, ...)
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]588</pre>
589
590<p><b>Examples:</b></p>
591
592<pre>
Chris Lattner6f72da52009-02-13 20:00:20 +0000[diff] [blame]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)
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]607</pre>
608
609<p><b>Description:</b></p>
610
Chris Lattner6f72da52009-02-13 20:00:20 +0000[diff] [blame]611<p>The first two arguments to __builtin_shufflevector are vectors that have the
612same element type. The remaining arguments are a list of integers that specify
613the elements indices of the first two vectors that should be extracted and
614returned in a new vector. These element indices are numbered sequentially
615starting with the first vector, continuing into the second vector. Thus, if
616vec1 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]617</p>
618
Chris Lattner6f72da52009-02-13 20:00:20 +0000[diff] [blame]619<p>The result of __builtin_shufflevector is a vector
620with the same element type as vec1/vec2 but that has an element count equal to
621the number of indices specified.
622</p>
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]623
Chris Lattner21190d52009-09-21 03:09:59 +0000[diff] [blame]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
631the program cannot be reached, even if the compiler might otherwise think it
632can. This is useful to improve optimization and eliminates certain warnings.
633For example, without the <tt>__builtin_unreachable</tt> in the example below,
634the compiler assumes that the inline asm can fall through and prints a "function
635declared '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>
647void myabort(void) __attribute__((noreturn));
648void 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
657it has undefined behavior, it is a statement that it is never reached and the
658optimizer can take advantage of this to produce better code. This builtin takes
659no arguments and produces a void result.
660</p>
661
662<p>Query for this feature with __has_builtin(__builtin_unreachable).</p>
663
664
Chris Lattner1177f912009-04-09 19:58:15 +0000[diff] [blame]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
Chris Lattnera021e7c2009-05-05 18:54:47 +0000[diff] [blame]682relative to the X86 GS segment register, and address space #257 causes it to be
683relative to the X86 FS segment. Note that this is a very very low-level
684feature that should only be used if you know what you're doing (for example in
685an OS kernel).</p>
Chris Lattner1177f912009-04-09 19:58:15 +0000[diff] [blame]686
687<p>Here is an example:</p>
688
689<pre>
690#define GS_RELATIVE __attribute__((address_space(256)))
691int 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
Ted Kremeneked869312009-04-10 05:03:33 +0000[diff] [blame]705<!-- ======================================================================= -->
706<h2 id="analyzerspecific">Static Analysis-Specific Extensions</h2>
707<!-- ======================================================================= -->
708
709<p>Clang supports additional attributes that are useful for documenting program
710invariants and rules for static analysis tools. The extensions documented here
711are used by the <a
712href="http://clang.llvm.org/StaticAnalysis.html">path-sensitive static analyzer
713engine</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>
Ted Kremenek4df21142009-04-10 05:04:22 +0000[diff] [blame]722attribute. This attribute, which is typically affixed to a function prototype,
723indicates that a call to a given function never returns. Function prototypes for
724common functions like <tt>exit</tt> are typically annotated with this attribute,
725as well as a variety of common assertion handlers. Users can educate the static
726analyzer about their own custom assertion handles (thus cutting down on false
727positives due to false paths) by marking their own &quot;panic&quot; functions
728with this attribute.</p>
Ted Kremeneked869312009-04-10 05:03:33 +0000[diff] [blame]729
730<p>While useful, <tt>noreturn</tt> is not applicable in all cases. Sometimes
Nick Lewycky625b5862009-06-14 04:08:08 +0000[diff] [blame]731there are special functions that for all intents and purposes should be
732considered panic functions (i.e., they are only called when an internal program
733error occurs) but may actually return so that the program can fail gracefully.
734The <tt>analyzer_noreturn</tt> attribute allows one to annotate such functions
735as being interpreted as &quot;no return&quot; functions by the analyzer (thus
Chris Lattner28935892009-04-10 05:54:56 +0000[diff] [blame]736pruning bogus paths) but will not affect compilation (as in the case of
Ted Kremeneked869312009-04-10 05:03:33 +0000[diff] [blame]737<tt>noreturn</tt>).</p>
738
739<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]740same places where the <tt>noreturn</tt> attribute can be placed. It is commonly
Ted Kremeneked869312009-04-10 05:03:33 +0000[diff] [blame]741placed at the end of function prototypes:</p>
742
743<pre>
744 void foo() <b>__attribute__((analyzer_noreturn))</b>;
Chris Lattner148772a2009-06-13 07:13:28 +0000[diff] [blame]745</pre>
746
747<p>Query for this feature with __has_feature(attribute_analyzer_noreturn).</p>
748
Ted Kremeneked869312009-04-10 05:03:33 +0000[diff] [blame]749
Chris Lattner5ce933f2009-02-09 08:46:11 +0000[diff] [blame]750</div>
751</body>
752</html>