1 ============================
2 Clang Compiler User's Manual
3 ============================
11 The Clang Compiler is an open-source compiler for the C family of
12 programming languages, aiming to be the best in class implementation of
13 these languages. Clang builds on the LLVM optimizer and code generator,
14 allowing it to provide high-quality optimization and code generation
15 support for many targets. For more general information, please see the
16 `Clang Web Site <http://clang.llvm.org>`_ or the `LLVM Web
17 Site <http://llvm.org>`_.
19 This document describes important notes about using Clang as a compiler
20 for an end-user, documenting the supported features, command line
21 options, etc. If you are interested in using Clang to build a tool that
22 processes code, please see :doc:`InternalsManual`. If you are interested in the
23 `Clang Static Analyzer <http://clang-analyzer.llvm.org>`_, please see its web
26 Clang is designed to support the C family of programming languages,
27 which includes :ref:`C <c>`, :ref:`Objective-C <objc>`, :ref:`C++ <cxx>`, and
28 :ref:`Objective-C++ <objcxx>` as well as many dialects of those. For
29 language-specific information, please see the corresponding language
32 - :ref:`C Language <c>`: K&R C, ANSI C89, ISO C90, ISO C94 (C89+AMD1), ISO
34 - :ref:`Objective-C Language <objc>`: ObjC 1, ObjC 2, ObjC 2.1, plus
35 variants depending on base language.
36 - :ref:`C++ Language <cxx>`
37 - :ref:`Objective C++ Language <objcxx>`
39 In addition to these base languages and their dialects, Clang supports a
40 broad variety of language extensions, which are documented in the
41 corresponding language section. These extensions are provided to be
42 compatible with the GCC, Microsoft, and other popular compilers as well
43 as to improve functionality through Clang-specific features. The Clang
44 driver and language features are intentionally designed to be as
45 compatible with the GNU GCC compiler as reasonably possible, easing
46 migration from GCC to Clang. In most cases, code "just works".
47 Clang also provides an alternative driver, :ref:`clang-cl`, that is designed
48 to be compatible with the Visual C++ compiler, cl.exe.
50 In addition to language specific features, Clang has a variety of
51 features that depend on what CPU architecture or operating system is
52 being compiled for. Please see the :ref:`Target-Specific Features and
53 Limitations <target_features>` section for more details.
55 The rest of the introduction introduces some basic :ref:`compiler
56 terminology <terminology>` that is used throughout this manual and
57 contains a basic :ref:`introduction to using Clang <basicusage>` as a
58 command line compiler.
65 Front end, parser, backend, preprocessor, undefined behavior,
73 Intro to how to use a C compiler for newbies.
75 compile + link compile then link debug info enabling optimizations
76 picking a language to use, defaults to C11 by default. Autosenses based
77 on extension. using a makefile
82 This section is generally an index into other sections. It does not go
83 into depth on the ones that are covered by other sections. However, the
84 first part introduces the language selection and other high level
85 options like :option:`-c`, :option:`-g`, etc.
87 Options to Control Error and Warning Messages
88 ---------------------------------------------
92 Turn warnings into errors.
94 .. This is in plain monospaced font because it generates the same label as
95 .. -Werror, and Sphinx complains.
99 Turn warning "foo" into an error.
101 .. option:: -Wno-error=foo
103 Turn warning "foo" into an warning even if :option:`-Werror` is specified.
107 Enable warning "foo".
111 Disable warning "foo".
115 Disable all diagnostics.
117 .. option:: -Weverything
119 :ref:`Enable all diagnostics. <diagnostics_enable_everything>`
121 .. option:: -pedantic
123 Warn on language extensions.
125 .. option:: -pedantic-errors
127 Error on language extensions.
129 .. option:: -Wsystem-headers
131 Enable warnings from system headers.
133 .. option:: -ferror-limit=123
135 Stop emitting diagnostics after 123 errors have been produced. The default is
136 20, and the error limit can be disabled with :option:`-ferror-limit=0`.
138 .. option:: -ftemplate-backtrace-limit=123
140 Only emit up to 123 template instantiation notes within the template
141 instantiation backtrace for a single warning or error. The default is 10, and
142 the limit can be disabled with :option:`-ftemplate-backtrace-limit=0`.
144 .. _cl_diag_formatting:
146 Formatting of Diagnostics
147 ^^^^^^^^^^^^^^^^^^^^^^^^^
149 Clang aims to produce beautiful diagnostics by default, particularly for
150 new users that first come to Clang. However, different people have
151 different preferences, and sometimes Clang is driven not by a human,
152 but by a program that wants consistent and easily parsable output. For
153 these cases, Clang provides a wide range of options to control the exact
154 output format of the diagnostics that it generates.
156 .. _opt_fshow-column:
158 **-f[no-]show-column**
159 Print column number in diagnostic.
161 This option, which defaults to on, controls whether or not Clang
162 prints the column number of a diagnostic. For example, when this is
163 enabled, Clang will print something like:
167 test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
172 When this is disabled, Clang will print "test.c:28: warning..." with
175 The printed column numbers count bytes from the beginning of the
176 line; take care if your source contains multibyte characters.
178 .. _opt_fshow-source-location:
180 **-f[no-]show-source-location**
181 Print source file/line/column information in diagnostic.
183 This option, which defaults to on, controls whether or not Clang
184 prints the filename, line number and column number of a diagnostic.
185 For example, when this is enabled, Clang will print something like:
189 test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
194 When this is disabled, Clang will not print the "test.c:28:8: "
197 .. _opt_fcaret-diagnostics:
199 **-f[no-]caret-diagnostics**
200 Print source line and ranges from source code in diagnostic.
201 This option, which defaults to on, controls whether or not Clang
202 prints the source line, source ranges, and caret when emitting a
203 diagnostic. For example, when this is enabled, Clang will print
208 test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
213 **-f[no-]color-diagnostics**
214 This option, which defaults to on when a color-capable terminal is
215 detected, controls whether or not Clang prints diagnostics in color.
217 When this option is enabled, Clang will use colors to highlight
218 specific parts of the diagnostic, e.g.,
220 .. nasty hack to not lose our dignity
225 <b><span style="color:black">test.c:28:8: <span style="color:magenta">warning</span>: extra tokens at end of #endif directive [-Wextra-tokens]</span></b>
227 <span style="color:green">^</span>
228 <span style="color:green">//</span>
231 When this is disabled, Clang will just print:
235 test.c:2:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
240 **-fansi-escape-codes**
241 Controls whether ANSI escape codes are used instead of the Windows Console
242 API to output colored diagnostics. This option is only used on Windows and
245 .. option:: -fdiagnostics-format=clang/msvc/vi
247 Changes diagnostic output format to better match IDEs and command line tools.
249 This option controls the output format of the filename, line number,
250 and column printed in diagnostic messages. The options, and their
251 affect on formatting a simple conversion diagnostic, follow:
256 t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int'
261 t.c(3,11) : warning: conversion specifies type 'char *' but the argument has type 'int'
266 t.c +3:11: warning: conversion specifies type 'char *' but the argument has type 'int'
268 .. _opt_fdiagnostics-show-option:
270 **-f[no-]diagnostics-show-option**
271 Enable ``[-Woption]`` information in diagnostic line.
273 This option, which defaults to on, controls whether or not Clang
274 prints the associated :ref:`warning group <cl_diag_warning_groups>`
275 option name when outputting a warning diagnostic. For example, in
280 test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
285 Passing **-fno-diagnostics-show-option** will prevent Clang from
286 printing the [:ref:`-Wextra-tokens <opt_Wextra-tokens>`] information in
287 the diagnostic. This information tells you the flag needed to enable
288 or disable the diagnostic, either from the command line or through
289 :ref:`#pragma GCC diagnostic <pragma_GCC_diagnostic>`.
291 .. _opt_fdiagnostics-show-category:
293 .. option:: -fdiagnostics-show-category=none/id/name
295 Enable printing category information in diagnostic line.
297 This option, which defaults to "none", controls whether or not Clang
298 prints the category associated with a diagnostic when emitting it.
299 Each diagnostic may or many not have an associated category, if it
300 has one, it is listed in the diagnostic categorization field of the
301 diagnostic line (in the []'s).
303 For example, a format string warning will produce these three
304 renditions based on the setting of this option:
308 t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int' [-Wformat]
309 t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int' [-Wformat,1]
310 t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int' [-Wformat,Format String]
312 This category can be used by clients that want to group diagnostics
313 by category, so it should be a high level category. We want dozens
314 of these, not hundreds or thousands of them.
316 .. _opt_fdiagnostics-fixit-info:
318 **-f[no-]diagnostics-fixit-info**
319 Enable "FixIt" information in the diagnostics output.
321 This option, which defaults to on, controls whether or not Clang
322 prints the information on how to fix a specific diagnostic
323 underneath it when it knows. For example, in this output:
327 test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
332 Passing **-fno-diagnostics-fixit-info** will prevent Clang from
333 printing the "//" line at the end of the message. This information
334 is useful for users who may not understand what is wrong, but can be
335 confusing for machine parsing.
337 .. _opt_fdiagnostics-print-source-range-info:
339 **-fdiagnostics-print-source-range-info**
340 Print machine parsable information about source ranges.
341 This option makes Clang print information about source ranges in a machine
342 parsable format after the file/line/column number information. The
343 information is a simple sequence of brace enclosed ranges, where each range
344 lists the start and end line/column locations. For example, in this output:
348 exprs.c:47:15:{47:8-47:14}{47:17-47:24}: error: invalid operands to binary expression ('int *' and '_Complex float')
349 P = (P-42) + Gamma*4;
352 The {}'s are generated by -fdiagnostics-print-source-range-info.
354 The printed column numbers count bytes from the beginning of the
355 line; take care if your source contains multibyte characters.
357 .. option:: -fdiagnostics-parseable-fixits
359 Print Fix-Its in a machine parseable form.
361 This option makes Clang print available Fix-Its in a machine
362 parseable format at the end of diagnostics. The following example
363 illustrates the format:
367 fix-it:"t.cpp":{7:25-7:29}:"Gamma"
369 The range printed is a half-open range, so in this example the
370 characters at column 25 up to but not including column 29 on line 7
371 in t.cpp should be replaced with the string "Gamma". Either the
372 range or the replacement string may be empty (representing strict
373 insertions and strict erasures, respectively). Both the file name
374 and the insertion string escape backslash (as "\\\\"), tabs (as
375 "\\t"), newlines (as "\\n"), double quotes(as "\\"") and
376 non-printable characters (as octal "\\xxx").
378 The printed column numbers count bytes from the beginning of the
379 line; take care if your source contains multibyte characters.
381 .. option:: -fno-elide-type
383 Turns off elision in template type printing.
385 The default for template type printing is to elide as many template
386 arguments as possible, removing those which are the same in both
387 template types, leaving only the differences. Adding this flag will
388 print all the template arguments. If supported by the terminal,
389 highlighting will still appear on differing arguments.
395 t.cc:4:5: note: candidate function not viable: no known conversion from 'vector<map<[...], map<float, [...]>>>' to 'vector<map<[...], map<double, [...]>>>' for 1st argument;
401 t.cc:4:5: note: candidate function not viable: no known conversion from 'vector<map<int, map<float, int>>>' to 'vector<map<int, map<double, int>>>' for 1st argument;
403 .. option:: -fdiagnostics-show-template-tree
405 Template type diffing prints a text tree.
407 For diffing large templated types, this option will cause Clang to
408 display the templates as an indented text tree, one argument per
409 line, with differences marked inline. This is compatible with
416 t.cc:4:5: note: candidate function not viable: no known conversion from 'vector<map<[...], map<float, [...]>>>' to 'vector<map<[...], map<double, [...]>>>' for 1st argument;
418 With :option:`-fdiagnostics-show-template-tree`:
422 t.cc:4:5: note: candidate function not viable: no known conversion for 1st argument;
430 .. _cl_diag_warning_groups:
432 Individual Warning Groups
433 ^^^^^^^^^^^^^^^^^^^^^^^^^
435 TODO: Generate this from tblgen. Define one anchor per warning group.
437 .. _opt_wextra-tokens:
439 .. option:: -Wextra-tokens
441 Warn about excess tokens at the end of a preprocessor directive.
443 This option, which defaults to on, enables warnings about extra
444 tokens at the end of preprocessor directives. For example:
448 test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
452 These extra tokens are not strictly conforming, and are usually best
453 handled by commenting them out.
455 .. option:: -Wambiguous-member-template
457 Warn about unqualified uses of a member template whose name resolves to
458 another template at the location of the use.
460 This option, which defaults to on, enables a warning in the
465 template<typename T> struct set{};
466 template<typename T> struct trait { typedef const T& type; };
468 template<typename T> void set(typename trait<T>::type value) {}
475 C++ [basic.lookup.classref] requires this to be an error, but,
476 because it's hard to work around, Clang downgrades it to a warning
479 .. option:: -Wbind-to-temporary-copy
481 Warn about an unusable copy constructor when binding a reference to a
484 This option enables warnings about binding a
485 reference to a temporary when the temporary doesn't have a usable
486 copy constructor. For example:
493 NonCopyable(const NonCopyable&);
495 void foo(const NonCopyable&);
497 foo(NonCopyable()); // Disallowed in C++98; allowed in C++11.
502 struct NonCopyable2 {
504 NonCopyable2(NonCopyable2&);
506 void foo(const NonCopyable2&);
508 foo(NonCopyable2()); // Disallowed in C++98; allowed in C++11.
511 Note that if ``NonCopyable2::NonCopyable2()`` has a default argument
512 whose instantiation produces a compile error, that error will still
513 be a hard error in C++98 mode even if this warning is turned off.
515 Options to Control Clang Crash Diagnostics
516 ------------------------------------------
518 As unbelievable as it may sound, Clang does crash from time to time.
519 Generally, this only occurs to those living on the `bleeding
520 edge <http://llvm.org/releases/download.html#svn>`_. Clang goes to great
521 lengths to assist you in filing a bug report. Specifically, Clang
522 generates preprocessed source file(s) and associated run script(s) upon
523 a crash. These files should be attached to a bug report to ease
524 reproducibility of the failure. Below are the command line options to
525 control the crash diagnostics.
527 .. option:: -fno-crash-diagnostics
529 Disable auto-generation of preprocessed source files during a clang crash.
531 The -fno-crash-diagnostics flag can be helpful for speeding the process
532 of generating a delta reduced test case.
534 Options to Emit Optimization Reports
535 ------------------------------------
537 Optimization reports trace, at a high-level, all the major decisions
538 done by compiler transformations. For instance, when the inliner
539 decides to inline function ``foo()`` into ``bar()``, or the loop unroller
540 decides to unroll a loop N times, or the vectorizer decides to
541 vectorize a loop body.
543 Clang offers a family of flags which the optimizers can use to emit
544 a diagnostic in three cases:
546 1. When the pass makes a transformation (:option:`-Rpass`).
548 2. When the pass fails to make a transformation (:option:`-Rpass-missed`).
550 3. When the pass determines whether or not to make a transformation
551 (:option:`-Rpass-analysis`).
553 NOTE: Although the discussion below focuses on :option:`-Rpass`, the exact
554 same options apply to :option:`-Rpass-missed` and :option:`-Rpass-analysis`.
556 Since there are dozens of passes inside the compiler, each of these flags
557 take a regular expression that identifies the name of the pass which should
558 emit the associated diagnostic. For example, to get a report from the inliner,
559 compile the code with:
561 .. code-block:: console
563 $ clang -O2 -Rpass=inline code.cc -o code
564 code.cc:4:25: remark: foo inlined into bar [-Rpass=inline]
565 int bar(int j) { return foo(j, j - 2); }
568 Note that remarks from the inliner are identified with `[-Rpass=inline]`.
569 To request a report from every optimization pass, you should use
570 :option:`-Rpass=.*` (in fact, you can use any valid POSIX regular
571 expression). However, do not expect a report from every transformation
572 made by the compiler. Optimization remarks do not really make sense
573 outside of the major transformations (e.g., inlining, vectorization,
574 loop optimizations) and not every optimization pass supports this
580 1. Optimization remarks that refer to function names will display the
581 mangled name of the function. Since these remarks are emitted by the
582 back end of the compiler, it does not know anything about the input
583 language, nor its mangling rules.
585 2. Some source locations are not displayed correctly. The front end has
586 a more detailed source location tracking than the locations included
587 in the debug info (e.g., the front end can locate code inside macro
588 expansions). However, the locations used by :option:`-Rpass` are
589 translated from debug annotations. That translation can be lossy,
590 which results in some remarks having no location information.
594 Clang options that that don't fit neatly into other categories.
598 When emitting a dependency file, use formatting conventions appropriate
599 for NMake or Jom. Ignored unless another option causes Clang to emit a
602 When Clang emits a dependency file (e.g., you supplied the -M option)
603 most filenames can be written to the file without any special formatting.
604 Different Make tools will treat different sets of characters as "special"
605 and use different conventions for telling the Make tool that the character
606 is actually part of the filename. Normally Clang uses backslash to "escape"
607 a special character, which is the convention used by GNU Make. The -MV
608 option tells Clang to put double-quotes around the entire filename, which
609 is the convention used by NMake and Jom.
612 Language and Target-Independent Features
613 ========================================
615 Controlling Errors and Warnings
616 -------------------------------
618 Clang provides a number of ways to control which code constructs cause
619 it to emit errors and warning messages, and how they are displayed to
622 Controlling How Clang Displays Diagnostics
623 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
625 When Clang emits a diagnostic, it includes rich information in the
626 output, and gives you fine-grain control over which information is
627 printed. Clang has the ability to print this information, and these are
628 the options that control it:
630 #. A file/line/column indicator that shows exactly where the diagnostic
631 occurs in your code [:ref:`-fshow-column <opt_fshow-column>`,
632 :ref:`-fshow-source-location <opt_fshow-source-location>`].
633 #. A categorization of the diagnostic as a note, warning, error, or
635 #. A text string that describes what the problem is.
636 #. An option that indicates how to control the diagnostic (for
637 diagnostics that support it)
638 [:ref:`-fdiagnostics-show-option <opt_fdiagnostics-show-option>`].
639 #. A :ref:`high-level category <diagnostics_categories>` for the diagnostic
640 for clients that want to group diagnostics by class (for diagnostics
642 [:ref:`-fdiagnostics-show-category <opt_fdiagnostics-show-category>`].
643 #. The line of source code that the issue occurs on, along with a caret
644 and ranges that indicate the important locations
645 [:ref:`-fcaret-diagnostics <opt_fcaret-diagnostics>`].
646 #. "FixIt" information, which is a concise explanation of how to fix the
647 problem (when Clang is certain it knows)
648 [:ref:`-fdiagnostics-fixit-info <opt_fdiagnostics-fixit-info>`].
649 #. A machine-parsable representation of the ranges involved (off by
651 [:ref:`-fdiagnostics-print-source-range-info <opt_fdiagnostics-print-source-range-info>`].
653 For more information please see :ref:`Formatting of
654 Diagnostics <cl_diag_formatting>`.
659 All diagnostics are mapped into one of these 6 classes:
668 .. _diagnostics_categories:
670 Diagnostic Categories
671 ^^^^^^^^^^^^^^^^^^^^^
673 Though not shown by default, diagnostics may each be associated with a
674 high-level category. This category is intended to make it possible to
675 triage builds that produce a large number of errors or warnings in a
678 Categories are not shown by default, but they can be turned on with the
679 :ref:`-fdiagnostics-show-category <opt_fdiagnostics-show-category>` option.
680 When set to "``name``", the category is printed textually in the
681 diagnostic output. When it is set to "``id``", a category number is
682 printed. The mapping of category names to category id's can be obtained
683 by running '``clang --print-diagnostic-categories``'.
685 Controlling Diagnostics via Command Line Flags
686 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
688 TODO: -W flags, -pedantic, etc
690 .. _pragma_gcc_diagnostic:
692 Controlling Diagnostics via Pragmas
693 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
695 Clang can also control what diagnostics are enabled through the use of
696 pragmas in the source code. This is useful for turning off specific
697 warnings in a section of source code. Clang supports GCC's pragma for
698 compatibility with existing source code, as well as several extensions.
700 The pragma may control any warning that can be used from the command
701 line. Warnings may be set to ignored, warning, error, or fatal. The
702 following example code will tell Clang or GCC to ignore the -Wall
707 #pragma GCC diagnostic ignored "-Wall"
709 In addition to all of the functionality provided by GCC's pragma, Clang
710 also allows you to push and pop the current warning state. This is
711 particularly useful when writing a header file that will be compiled by
712 other people, because you don't know what warning flags they build with.
714 In the below example :option:`-Wmultichar` is ignored for only a single line of
715 code, after which the diagnostics return to whatever state had previously
720 #pragma clang diagnostic push
721 #pragma clang diagnostic ignored "-Wmultichar"
723 char b = 'df'; // no warning.
725 #pragma clang diagnostic pop
727 The push and pop pragmas will save and restore the full diagnostic state
728 of the compiler, regardless of how it was set. That means that it is
729 possible to use push and pop around GCC compatible diagnostics and Clang
730 will push and pop them appropriately, while GCC will ignore the pushes
731 and pops as unknown pragmas. It should be noted that while Clang
732 supports the GCC pragma, Clang and GCC do not support the exact same set
733 of warnings, so even when using GCC compatible #pragmas there is no
734 guarantee that they will have identical behaviour on both compilers.
736 In addition to controlling warnings and errors generated by the compiler, it is
737 possible to generate custom warning and error messages through the following
742 // The following will produce warning messages
743 #pragma message "some diagnostic message"
744 #pragma GCC warning "TODO: replace deprecated feature"
746 // The following will produce an error message
747 #pragma GCC error "Not supported"
749 These pragmas operate similarly to the ``#warning`` and ``#error`` preprocessor
750 directives, except that they may also be embedded into preprocessor macros via
751 the C99 ``_Pragma`` operator, for example:
756 #define DEFER(M,...) M(__VA_ARGS__)
757 #define CUSTOM_ERROR(X) _Pragma(STR(GCC error(X " at line " DEFER(STR,__LINE__))))
759 CUSTOM_ERROR("Feature not available");
761 Controlling Diagnostics in System Headers
762 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
764 Warnings are suppressed when they occur in system headers. By default,
765 an included file is treated as a system header if it is found in an
766 include path specified by ``-isystem``, but this can be overridden in
769 The ``system_header`` pragma can be used to mark the current file as
770 being a system header. No warnings will be produced from the location of
771 the pragma onwards within the same file.
775 char a = 'xy'; // warning
777 #pragma clang system_header
779 char b = 'ab'; // no warning
781 The :option:`--system-header-prefix=` and :option:`--no-system-header-prefix=`
782 command-line arguments can be used to override whether subsets of an include
783 path are treated as system headers. When the name in a ``#include`` directive
784 is found within a header search path and starts with a system prefix, the
785 header is treated as a system header. The last prefix on the
786 command-line which matches the specified header name takes precedence.
789 .. code-block:: console
791 $ clang -Ifoo -isystem bar --system-header-prefix=x/ \
792 --no-system-header-prefix=x/y/
794 Here, ``#include "x/a.h"`` is treated as including a system header, even
795 if the header is found in ``foo``, and ``#include "x/y/b.h"`` is treated
796 as not including a system header, even if the header is found in
799 A ``#include`` directive which finds a file relative to the current
800 directory is treated as including a system header if the including file
801 is treated as a system header.
803 .. _diagnostics_enable_everything:
805 Enabling All Diagnostics
806 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
808 In addition to the traditional ``-W`` flags, one can enable **all**
809 diagnostics by passing :option:`-Weverything`. This works as expected
811 :option:`-Werror`, and also includes the warnings from :option:`-pedantic`.
813 Note that when combined with :option:`-w` (which disables all warnings), that
816 Controlling Static Analyzer Diagnostics
817 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
819 While not strictly part of the compiler, the diagnostics from Clang's
820 `static analyzer <http://clang-analyzer.llvm.org>`_ can also be
821 influenced by the user via changes to the source code. See the available
822 `annotations <http://clang-analyzer.llvm.org/annotations.html>`_ and the
824 page <http://clang-analyzer.llvm.org/faq.html#exclude_code>`_ for more
827 .. _usersmanual-precompiled-headers:
832 `Precompiled headers <http://en.wikipedia.org/wiki/Precompiled_header>`__
833 are a general approach employed by many compilers to reduce compilation
834 time. The underlying motivation of the approach is that it is common for
835 the same (and often large) header files to be included by multiple
836 source files. Consequently, compile times can often be greatly improved
837 by caching some of the (redundant) work done by a compiler to process
838 headers. Precompiled header files, which represent one of many ways to
839 implement this optimization, are literally files that represent an
840 on-disk cache that contains the vital information necessary to reduce
841 some of the work needed to process a corresponding header file. While
842 details of precompiled headers vary between compilers, precompiled
843 headers have been shown to be highly effective at speeding up program
844 compilation on systems with very large system headers (e.g., Mac OS X).
846 Generating a PCH File
847 ^^^^^^^^^^^^^^^^^^^^^
849 To generate a PCH file using Clang, one invokes Clang with the
850 :option:`-x <language>-header` option. This mirrors the interface in GCC
851 for generating PCH files:
853 .. code-block:: console
855 $ gcc -x c-header test.h -o test.h.gch
856 $ clang -x c-header test.h -o test.h.pch
861 A PCH file can then be used as a prefix header when a :option:`-include`
862 option is passed to ``clang``:
864 .. code-block:: console
866 $ clang -include test.h test.c -o test
868 The ``clang`` driver will first check if a PCH file for ``test.h`` is
869 available; if so, the contents of ``test.h`` (and the files it includes)
870 will be processed from the PCH file. Otherwise, Clang falls back to
871 directly processing the content of ``test.h``. This mirrors the behavior
876 Clang does *not* automatically use PCH files for headers that are directly
877 included within a source file. For example:
879 .. code-block:: console
881 $ clang -x c-header test.h -o test.h.pch
884 $ clang test.c -o test
886 In this example, ``clang`` will not automatically use the PCH file for
887 ``test.h`` since ``test.h`` was included directly in the source file and not
888 specified on the command line using :option:`-include`.
890 Relocatable PCH Files
891 ^^^^^^^^^^^^^^^^^^^^^
893 It is sometimes necessary to build a precompiled header from headers
894 that are not yet in their final, installed locations. For example, one
895 might build a precompiled header within the build tree that is then
896 meant to be installed alongside the headers. Clang permits the creation
897 of "relocatable" precompiled headers, which are built with a given path
898 (into the build directory) and can later be used from an installed
901 To build a relocatable precompiled header, place your headers into a
902 subdirectory whose structure mimics the installed location. For example,
903 if you want to build a precompiled header for the header ``mylib.h``
904 that will be installed into ``/usr/include``, create a subdirectory
905 ``build/usr/include`` and place the header ``mylib.h`` into that
906 subdirectory. If ``mylib.h`` depends on other headers, then they can be
907 stored within ``build/usr/include`` in a way that mimics the installed
910 Building a relocatable precompiled header requires two additional
911 arguments. First, pass the ``--relocatable-pch`` flag to indicate that
912 the resulting PCH file should be relocatable. Second, pass
913 :option:`-isysroot /path/to/build`, which makes all includes for your library
914 relative to the build directory. For example:
916 .. code-block:: console
918 # clang -x c-header --relocatable-pch -isysroot /path/to/build /path/to/build/mylib.h mylib.h.pch
920 When loading the relocatable PCH file, the various headers used in the
921 PCH file are found from the system header root. For example, ``mylib.h``
922 can be found in ``/usr/include/mylib.h``. If the headers are installed
923 in some other system root, the :option:`-isysroot` option can be used provide
924 a different system root from which the headers will be based. For
925 example, :option:`-isysroot /Developer/SDKs/MacOSX10.4u.sdk` will look for
926 ``mylib.h`` in ``/Developer/SDKs/MacOSX10.4u.sdk/usr/include/mylib.h``.
928 Relocatable precompiled headers are intended to be used in a limited
929 number of cases where the compilation environment is tightly controlled
930 and the precompiled header cannot be generated after headers have been
933 .. _controlling-code-generation:
935 Controlling Code Generation
936 ---------------------------
938 Clang provides a number of ways to control code generation. The options
941 **-f[no-]sanitize=check1,check2,...**
942 Turn on runtime checks for various forms of undefined or suspicious
945 This option controls whether Clang adds runtime checks for various
946 forms of undefined or suspicious behavior, and is disabled by
947 default. If a check fails, a diagnostic message is produced at
948 runtime explaining the problem. The main checks are:
950 - .. _opt_fsanitize_address:
952 ``-fsanitize=address``:
953 :doc:`AddressSanitizer`, a memory error
955 - .. _opt_fsanitize_thread:
957 ``-fsanitize=thread``: :doc:`ThreadSanitizer`, a data race detector.
958 - .. _opt_fsanitize_memory:
960 ``-fsanitize=memory``: :doc:`MemorySanitizer`,
961 an *experimental* detector of uninitialized reads. Not ready for
963 - .. _opt_fsanitize_undefined:
965 ``-fsanitize=undefined``: :doc:`UndefinedBehaviorSanitizer`,
966 a fast and compatible undefined behavior checker.
968 - ``-fsanitize=dataflow``: :doc:`DataFlowSanitizer`, a general data
970 - ``-fsanitize=cfi``: :doc:`control flow integrity <ControlFlowIntegrity>`
971 checks. Requires ``-flto``.
972 - ``-fsanitize=safe-stack``: :doc:`safe stack <SafeStack>`
973 protection against stack-based memory corruption errors.
975 There are more fine-grained checks available: see
976 the :ref:`list <ubsan-checks>` of specific kinds of
977 undefined behavior that can be detected. Checks for :doc:`ControlFlowIntegrity`
980 - ``-fsanitize=cfi-cast-strict``: Enables :ref:`strict cast checks
982 - ``-fsanitize=cfi-derived-cast``: Base-to-derived cast to the wrong
983 dynamic type. Requires ``-flto``.
984 - ``-fsanitize=cfi-unrelated-cast``: Cast from ``void*`` or another
985 unrelated type to the wrong dynamic type. Requires ``-flto``.
986 - ``-fsanitize=cfi-nvcall``: Non-virtual call via an object whose vptr is of
987 the wrong dynamic type. Requires ``-flto``.
988 - ``-fsanitize=cfi-vcall``: Virtual call via an object whose vptr is of the
989 wrong dynamic type. Requires ``-flto``.
991 You can turn off or modify checks for certain source files, functions
992 or even variables by providing a special file:
994 - ``-fsanitize-blacklist=/path/to/blacklist/file``: disable or modify
995 sanitizer checks for objects listed in the file. See
996 :doc:`SanitizerSpecialCaseList` for file format description.
997 - ``-fno-sanitize-blacklist``: don't use blacklist file, if it was
998 specified earlier in the command line.
1000 Extra features of MemorySanitizer (require explicit
1001 ``-fsanitize=memory``):
1003 - ``-fsanitize-memory-track-origins[=level]``: Enables origin tracking in
1004 MemorySanitizer. Adds a second section to MemorySanitizer
1005 reports pointing to the heap or stack allocation the
1006 uninitialized bits came from. Slows down execution by additional
1009 Possible values for level are 0 (off), 1, 2 (default). Level 2
1010 adds more sections to MemorySanitizer reports describing the
1011 order of memory stores the uninitialized value went
1012 through. This mode may use extra memory in programs that copy
1013 uninitialized memory a lot.
1014 - ``-fsanitize-memory-use-after-dtor``: Enables use-after-destruction
1015 detection in MemorySanitizer. After invocation of the destructor,
1016 the object is considered no longer readable. Facilitates the
1017 detection of use-after-destroy bugs.
1019 Setting the MSAN_OPTIONS=poison_in_dtor=1 enables the poisoning of
1020 memory at runtime. Any subsequent access to the destroyed object
1021 fails at runtime. This feature is still experimental, but this
1022 environment variable must be set to 1 in order for the above flag
1025 The ``-fsanitize=`` argument must also be provided when linking, in
1026 order to link to the appropriate runtime library. When using
1027 ``-fsanitize=vptr`` (or a group that includes it, such as
1028 ``-fsanitize=undefined``) with a C++ program, the link must be
1029 performed by ``clang++``, not ``clang``, in order to link against the
1030 C++-specific parts of the runtime library.
1032 It is not possible to combine more than one of the ``-fsanitize=address``,
1033 ``-fsanitize=thread``, and ``-fsanitize=memory`` checkers in the same
1036 **-f[no-]sanitize-recover=check1,check2,...**
1038 Controls which checks enabled by ``-fsanitize=`` flag are non-fatal.
1039 If the check is fatal, program will halt after the first error
1040 of this kind is detected and error report is printed.
1042 By default, non-fatal checks are those enabled by
1043 :doc:`UndefinedBehaviorSanitizer`,
1044 except for ``-fsanitize=return`` and ``-fsanitize=unreachable``. Some
1045 sanitizers may not support recovery (or not support it by default
1046 e.g. :doc:`AddressSanitizer`), and always crash the program after the issue
1049 Note that the ``-fsanitize-trap`` flag has precedence over this flag.
1050 This means that if a check has been configured to trap elsewhere on the
1051 command line, or if the check traps by default, this flag will not have
1052 any effect unless that sanitizer's trapping behavior is disabled with
1053 ``-fno-sanitize-trap``.
1055 For example, if a command line contains the flags ``-fsanitize=undefined
1056 -fsanitize-trap=undefined``, the flag ``-fsanitize-recover=alignment``
1057 will have no effect on its own; it will need to be accompanied by
1058 ``-fno-sanitize-trap=alignment``.
1060 **-f[no-]sanitize-trap=check1,check2,...**
1062 Controls which checks enabled by the ``-fsanitize=`` flag trap. This
1063 option is intended for use in cases where the sanitizer runtime cannot
1064 be used (for instance, when building libc or a kernel module), or where
1065 the binary size increase caused by the sanitizer runtime is a concern.
1067 This flag is only compatible with ``local-bounds``,
1068 ``unsigned-integer-overflow``, sanitizers in the ``cfi`` group and
1069 sanitizers in the ``undefined`` group other than ``vptr``. If this flag
1070 is supplied together with ``-fsanitize=undefined``, the ``vptr`` sanitizer
1071 will be implicitly disabled.
1073 This flag is enabled by default for sanitizers in the ``cfi`` group.
1075 **-f[no-]sanitize-coverage=[type,features,...]**
1077 Enable simple code coverage in addition to certain sanitizers.
1078 See :doc:`SanitizerCoverage` for more details.
1080 .. option:: -fsanitize-undefined-trap-on-error
1082 Deprecated alias for ``-fsanitize-trap=undefined``.
1084 .. option:: -fno-assume-sane-operator-new
1086 Don't assume that the C++'s new operator is sane.
1088 This option tells the compiler to do not assume that C++'s global
1089 new operator will always return a pointer that does not alias any
1090 other pointer when the function returns.
1092 .. option:: -ftrap-function=[name]
1094 Instruct code generator to emit a function call to the specified
1095 function name for ``__builtin_trap()``.
1097 LLVM code generator translates ``__builtin_trap()`` to a trap
1098 instruction if it is supported by the target ISA. Otherwise, the
1099 builtin is translated into a call to ``abort``. If this option is
1100 set, then the code generator will always lower the builtin to a call
1101 to the specified function regardless of whether the target ISA has a
1102 trap instruction. This option is useful for environments (e.g.
1103 deeply embedded) where a trap cannot be properly handled, or when
1104 some custom behavior is desired.
1106 .. option:: -ftls-model=[model]
1108 Select which TLS model to use.
1110 Valid values are: ``global-dynamic``, ``local-dynamic``,
1111 ``initial-exec`` and ``local-exec``. The default value is
1112 ``global-dynamic``. The compiler may use a different model if the
1113 selected model is not supported by the target, or if a more
1114 efficient model can be used. The TLS model can be overridden per
1115 variable using the ``tls_model`` attribute.
1117 .. option:: -femulated-tls
1119 Select emulated TLS model, which overrides all -ftls-model choices.
1121 In emulated TLS mode, all access to TLS variables are converted to
1122 calls to __emutls_get_address in the runtime library.
1124 .. option:: -mhwdiv=[values]
1126 Select the ARM modes (arm or thumb) that support hardware division
1129 Valid values are: ``arm``, ``thumb`` and ``arm,thumb``.
1130 This option is used to indicate which mode (arm or thumb) supports
1131 hardware division instructions. This only applies to the ARM
1134 .. option:: -m[no-]crc
1136 Enable or disable CRC instructions.
1138 This option is used to indicate whether CRC instructions are to
1139 be generated. This only applies to the ARM architecture.
1141 CRC instructions are enabled by default on ARMv8.
1143 .. option:: -mgeneral-regs-only
1145 Generate code which only uses the general purpose registers.
1147 This option restricts the generated code to use general registers
1148 only. This only applies to the AArch64 architecture.
1150 **-f[no-]max-unknown-pointer-align=[number]**
1151 Instruct the code generator to not enforce a higher alignment than the given
1152 number (of bytes) when accessing memory via an opaque pointer or reference.
1153 This cap is ignored when directly accessing a variable or when the pointee
1154 type has an explicit “aligned” attribute.
1156 The value should usually be determined by the properties of the system allocator.
1157 Some builtin types, especially vector types, have very high natural alignments;
1158 when working with values of those types, Clang usually wants to use instructions
1159 that take advantage of that alignment. However, many system allocators do
1160 not promise to return memory that is more than 8-byte or 16-byte-aligned. Use
1161 this option to limit the alignment that the compiler can assume for an arbitrary
1162 pointer, which may point onto the heap.
1164 This option does not affect the ABI alignment of types; the layout of structs and
1165 unions and the value returned by the alignof operator remain the same.
1167 This option can be overridden on a case-by-case basis by putting an explicit
1168 “aligned” alignment on a struct, union, or typedef. For example:
1170 .. code-block:: console
1172 #include <immintrin.h>
1173 // Make an aligned typedef of the AVX-512 16-int vector type.
1174 typedef __v16si __aligned_v16si __attribute__((aligned(64)));
1176 void initialize_vector(__aligned_v16si *v) {
1177 // The compiler may assume that ‘v’ is 64-byte aligned, regardless of the
1178 // value of -fmax-unknown-pointer-align.
1182 Profile Guided Optimization
1183 ---------------------------
1185 Profile information enables better optimization. For example, knowing that a
1186 branch is taken very frequently helps the compiler make better decisions when
1187 ordering basic blocks. Knowing that a function ``foo`` is called more
1188 frequently than another function ``bar`` helps the inliner.
1190 Clang supports profile guided optimization with two different kinds of
1191 profiling. A sampling profiler can generate a profile with very low runtime
1192 overhead, or you can build an instrumented version of the code that collects
1193 more detailed profile information. Both kinds of profiles can provide execution
1194 counts for instructions in the code and information on branches taken and
1195 function invocation.
1197 Regardless of which kind of profiling you use, be careful to collect profiles
1198 by running your code with inputs that are representative of the typical
1199 behavior. Code that is not exercised in the profile will be optimized as if it
1200 is unimportant, and the compiler may make poor optimization choices for code
1201 that is disproportionately used while profiling.
1203 Differences Between Sampling and Instrumentation
1204 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1206 Although both techniques are used for similar purposes, there are important
1207 differences between the two:
1209 1. Profile data generated with one cannot be used by the other, and there is no
1210 conversion tool that can convert one to the other. So, a profile generated
1211 via ``-fprofile-instr-generate`` must be used with ``-fprofile-instr-use``.
1212 Similarly, sampling profiles generated by external profilers must be
1213 converted and used with ``-fprofile-sample-use``.
1215 2. Instrumentation profile data can be used for code coverage analysis and
1218 3. Sampling profiles can only be used for optimization. They cannot be used for
1219 code coverage analysis. Although it would be technically possible to use
1220 sampling profiles for code coverage, sample-based profiles are too
1221 coarse-grained for code coverage purposes; it would yield poor results.
1223 4. Sampling profiles must be generated by an external tool. The profile
1224 generated by that tool must then be converted into a format that can be read
1225 by LLVM. The section on sampling profilers describes one of the supported
1226 sampling profile formats.
1229 Using Sampling Profilers
1230 ^^^^^^^^^^^^^^^^^^^^^^^^
1232 Sampling profilers are used to collect runtime information, such as
1233 hardware counters, while your application executes. They are typically
1234 very efficient and do not incur a large runtime overhead. The
1235 sample data collected by the profiler can be used during compilation
1236 to determine what the most executed areas of the code are.
1238 Using the data from a sample profiler requires some changes in the way
1239 a program is built. Before the compiler can use profiling information,
1240 the code needs to execute under the profiler. The following is the
1241 usual build cycle when using sample profilers for optimization:
1243 1. Build the code with source line table information. You can use all the
1244 usual build flags that you always build your application with. The only
1245 requirement is that you add ``-gline-tables-only`` or ``-g`` to the
1246 command line. This is important for the profiler to be able to map
1247 instructions back to source line locations.
1249 .. code-block:: console
1251 $ clang++ -O2 -gline-tables-only code.cc -o code
1253 2. Run the executable under a sampling profiler. The specific profiler
1254 you use does not really matter, as long as its output can be converted
1255 into the format that the LLVM optimizer understands. Currently, there
1256 exists a conversion tool for the Linux Perf profiler
1257 (https://perf.wiki.kernel.org/), so these examples assume that you
1258 are using Linux Perf to profile your code.
1260 .. code-block:: console
1262 $ perf record -b ./code
1264 Note the use of the ``-b`` flag. This tells Perf to use the Last Branch
1265 Record (LBR) to record call chains. While this is not strictly required,
1266 it provides better call information, which improves the accuracy of
1269 3. Convert the collected profile data to LLVM's sample profile format.
1270 This is currently supported via the AutoFDO converter ``create_llvm_prof``.
1271 It is available at http://github.com/google/autofdo. Once built and
1272 installed, you can convert the ``perf.data`` file to LLVM using
1275 .. code-block:: console
1277 $ create_llvm_prof --binary=./code --out=code.prof
1279 This will read ``perf.data`` and the binary file ``./code`` and emit
1280 the profile data in ``code.prof``. Note that if you ran ``perf``
1281 without the ``-b`` flag, you need to use ``--use_lbr=false`` when
1282 calling ``create_llvm_prof``.
1284 4. Build the code again using the collected profile. This step feeds
1285 the profile back to the optimizers. This should result in a binary
1286 that executes faster than the original one. Note that you are not
1287 required to build the code with the exact same arguments that you
1288 used in the first step. The only requirement is that you build the code
1289 with ``-gline-tables-only`` and ``-fprofile-sample-use``.
1291 .. code-block:: console
1293 $ clang++ -O2 -gline-tables-only -fprofile-sample-use=code.prof code.cc -o code
1296 Sample Profile Formats
1297 """"""""""""""""""""""
1299 Since external profilers generate profile data in a variety of custom formats,
1300 the data generated by the profiler must be converted into a format that can be
1301 read by the backend. LLVM supports three different sample profile formats:
1303 1. ASCII text. This is the easiest one to generate. The file is divided into
1304 sections, which correspond to each of the functions with profile
1305 information. The format is described below. It can also be generated from
1306 the binary or gcov formats using the ``llvm-profdata`` tool.
1308 2. Binary encoding. This uses a more efficient encoding that yields smaller
1309 profile files. This is the format generated by the ``create_llvm_prof`` tool
1310 in http://github.com/google/autofdo.
1312 3. GCC encoding. This is based on the gcov format, which is accepted by GCC. It
1313 is only interesting in environments where GCC and Clang co-exist. This
1314 encoding is only generated by the ``create_gcov`` tool in
1315 http://github.com/google/autofdo. It can be read by LLVM and
1316 ``llvm-profdata``, but it cannot be generated by either.
1318 If you are using Linux Perf to generate sampling profiles, you can use the
1319 conversion tool ``create_llvm_prof`` described in the previous section.
1320 Otherwise, you will need to write a conversion tool that converts your
1321 profiler's native format into one of these three.
1324 Sample Profile Text Format
1325 """"""""""""""""""""""""""
1327 This section describes the ASCII text format for sampling profiles. It is,
1328 arguably, the easiest one to generate. If you are interested in generating any
1329 of the other two, consult the ``ProfileData`` library in in LLVM's source tree
1330 (specifically, ``include/llvm/ProfileData/SampleProfReader.h``).
1332 .. code-block:: console
1334 function1:total_samples:total_head_samples
1335 offset1[.discriminator]: number_of_samples [fn1:num fn2:num ... ]
1336 offset2[.discriminator]: number_of_samples [fn3:num fn4:num ... ]
1338 offsetN[.discriminator]: number_of_samples [fn5:num fn6:num ... ]
1339 offsetA[.discriminator]: fnA:num_of_total_samples
1340 offsetA1[.discriminator]: number_of_samples [fn7:num fn8:num ... ]
1341 offsetA1[.discriminator]: number_of_samples [fn9:num fn10:num ... ]
1342 offsetB[.discriminator]: fnB:num_of_total_samples
1343 offsetB1[.discriminator]: number_of_samples [fn11:num fn12:num ... ]
1345 This is a nested tree in which the identation represents the nesting level
1346 of the inline stack. There are no blank lines in the file. And the spacing
1347 within a single line is fixed. Additional spaces will result in an error
1348 while reading the file.
1350 Any line starting with the '#' character is completely ignored.
1352 Inlined calls are represented with indentation. The Inline stack is a
1353 stack of source locations in which the top of the stack represents the
1354 leaf function, and the bottom of the stack represents the actual
1355 symbol to which the instruction belongs.
1357 Function names must be mangled in order for the profile loader to
1358 match them in the current translation unit. The two numbers in the
1359 function header specify how many total samples were accumulated in the
1360 function (first number), and the total number of samples accumulated
1361 in the prologue of the function (second number). This head sample
1362 count provides an indicator of how frequently the function is invoked.
1364 There are two types of lines in the function body.
1366 - Sampled line represents the profile information of a source location.
1367 ``offsetN[.discriminator]: number_of_samples [fn5:num fn6:num ... ]``
1369 - Callsite line represents the profile information of an inlined callsite.
1370 ``offsetA[.discriminator]: fnA:num_of_total_samples``
1372 Each sampled line may contain several items. Some are optional (marked
1375 a. Source line offset. This number represents the line number
1376 in the function where the sample was collected. The line number is
1377 always relative to the line where symbol of the function is
1378 defined. So, if the function has its header at line 280, the offset
1379 13 is at line 293 in the file.
1381 Note that this offset should never be a negative number. This could
1382 happen in cases like macros. The debug machinery will register the
1383 line number at the point of macro expansion. So, if the macro was
1384 expanded in a line before the start of the function, the profile
1385 converter should emit a 0 as the offset (this means that the optimizers
1386 will not be able to associate a meaningful weight to the instructions
1389 b. [OPTIONAL] Discriminator. This is used if the sampled program
1390 was compiled with DWARF discriminator support
1391 (http://wiki.dwarfstd.org/index.php?title=Path_Discriminators).
1392 DWARF discriminators are unsigned integer values that allow the
1393 compiler to distinguish between multiple execution paths on the
1394 same source line location.
1396 For example, consider the line of code ``if (cond) foo(); else bar();``.
1397 If the predicate ``cond`` is true 80% of the time, then the edge
1398 into function ``foo`` should be considered to be taken most of the
1399 time. But both calls to ``foo`` and ``bar`` are at the same source
1400 line, so a sample count at that line is not sufficient. The
1401 compiler needs to know which part of that line is taken more
1404 This is what discriminators provide. In this case, the calls to
1405 ``foo`` and ``bar`` will be at the same line, but will have
1406 different discriminator values. This allows the compiler to correctly
1407 set edge weights into ``foo`` and ``bar``.
1409 c. Number of samples. This is an integer quantity representing the
1410 number of samples collected by the profiler at this source
1413 d. [OPTIONAL] Potential call targets and samples. If present, this
1414 line contains a call instruction. This models both direct and
1415 number of samples. For example,
1417 .. code-block:: console
1419 130: 7 foo:3 bar:2 baz:7
1421 The above means that at relative line offset 130 there is a call
1422 instruction that calls one of ``foo()``, ``bar()`` and ``baz()``,
1423 with ``baz()`` being the relatively more frequently called target.
1425 As an example, consider a program with the call chain ``main -> foo -> bar``.
1426 When built with optimizations enabled, the compiler may inline the
1427 calls to ``bar`` and ``foo`` inside ``main``. The generated profile
1428 could then be something like this:
1430 .. code-block:: console
1438 This profile indicates that there were a total of 35,504 samples
1439 collected in main. All of those were at line 1 (the call to ``foo``).
1440 Of those, 31,977 were spent inside the body of ``bar``. The last line
1441 of the profile (``2: 0``) corresponds to line 2 inside ``main``. No
1442 samples were collected there.
1444 Profiling with Instrumentation
1445 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1447 Clang also supports profiling via instrumentation. This requires building a
1448 special instrumented version of the code and has some runtime
1449 overhead during the profiling, but it provides more detailed results than a
1450 sampling profiler. It also provides reproducible results, at least to the
1451 extent that the code behaves consistently across runs.
1453 Here are the steps for using profile guided optimization with
1456 1. Build an instrumented version of the code by compiling and linking with the
1457 ``-fprofile-instr-generate`` option.
1459 .. code-block:: console
1461 $ clang++ -O2 -fprofile-instr-generate code.cc -o code
1463 2. Run the instrumented executable with inputs that reflect the typical usage.
1464 By default, the profile data will be written to a ``default.profraw`` file
1465 in the current directory. You can override that default by setting the
1466 ``LLVM_PROFILE_FILE`` environment variable to specify an alternate file.
1467 Any instance of ``%p`` in that file name will be replaced by the process
1468 ID, so that you can easily distinguish the profile output from multiple
1471 .. code-block:: console
1473 $ LLVM_PROFILE_FILE="code-%p.profraw" ./code
1475 3. Combine profiles from multiple runs and convert the "raw" profile format to
1476 the input expected by clang. Use the ``merge`` command of the
1477 ``llvm-profdata`` tool to do this.
1479 .. code-block:: console
1481 $ llvm-profdata merge -output=code.profdata code-*.profraw
1483 Note that this step is necessary even when there is only one "raw" profile,
1484 since the merge operation also changes the file format.
1486 4. Build the code again using the ``-fprofile-instr-use`` option to specify the
1487 collected profile data.
1489 .. code-block:: console
1491 $ clang++ -O2 -fprofile-instr-use=code.profdata code.cc -o code
1493 You can repeat step 4 as often as you like without regenerating the
1494 profile. As you make changes to your code, clang may no longer be able to
1495 use the profile data. It will warn you when this happens.
1497 Profile generation and use can also be controlled by the GCC-compatible flags
1498 ``-fprofile-generate`` and ``-fprofile-use``. Although these flags are
1499 semantically equivalent to their GCC counterparts, they *do not* handle
1500 GCC-compatible profiles. They are only meant to implement GCC's semantics
1501 with respect to profile creation and use.
1503 .. option:: -fprofile-generate[=<dirname>]
1505 Without any other arguments, ``-fprofile-generate`` behaves identically to
1506 ``-fprofile-instr-generate``. When given a directory name, it generates the
1507 profile file ``default.profraw`` in the directory named ``dirname``. If
1508 ``dirname`` does not exist, it will be created at runtime. The environment
1509 variable ``LLVM_PROFILE_FILE`` can be used to override the directory and
1510 filename for the profile file at runtime. For example,
1512 .. code-block:: console
1514 $ clang++ -O2 -fprofile-generate=yyy/zzz code.cc -o code
1516 When ``code`` is executed, the profile will be written to the file
1517 ``yyy/zzz/default.profraw``. This can be altered at runtime via the
1518 ``LLVM_PROFILE_FILE`` environment variable:
1520 .. code-block:: console
1522 $ LLVM_PROFILE_FILE=/tmp/myprofile/code.profraw ./code
1524 The above invocation will produce the profile file
1525 ``/tmp/myprofile/code.profraw`` instead of ``yyy/zzz/default.profraw``.
1526 Notice that ``LLVM_PROFILE_FILE`` overrides the directory *and* the file
1527 name for the profile file.
1529 .. option:: -fprofile-use[=<pathname>]
1531 Without any other arguments, ``-fprofile-use`` behaves identically to
1532 ``-fprofile-instr-use``. Otherwise, if ``pathname`` is the full path to a
1533 profile file, it reads from that file. If ``pathname`` is a directory name,
1534 it reads from ``pathname/default.profdata``.
1536 Disabling Instrumentation
1537 ^^^^^^^^^^^^^^^^^^^^^^^^^
1539 In certain situations, it may be useful to disable profile generation or use
1540 for specific files in a build, without affecting the main compilation flags
1541 used for the other files in the project.
1543 In these cases, you can use the flag ``-fno-profile-instr-generate`` (or
1544 ``-fno-profile-generate``) to disable profile generation, and
1545 ``-fno-profile-instr-use`` (or ``-fno-profile-use``) to disable profile use.
1547 Note that these flags should appear after the corresponding profile
1548 flags to have an effect.
1550 Controlling Size of Debug Information
1551 -------------------------------------
1553 Debug info kind generated by Clang can be set by one of the flags listed
1554 below. If multiple flags are present, the last one is used.
1558 Don't generate any debug info (default).
1560 .. option:: -gline-tables-only
1562 Generate line number tables only.
1564 This kind of debug info allows to obtain stack traces with function names,
1565 file names and line numbers (by such tools as ``gdb`` or ``addr2line``). It
1566 doesn't contain any other data (e.g. description of local variables or
1567 function parameters).
1569 .. option:: -fstandalone-debug
1571 Clang supports a number of optimizations to reduce the size of debug
1572 information in the binary. They work based on the assumption that
1573 the debug type information can be spread out over multiple
1574 compilation units. For instance, Clang will not emit type
1575 definitions for types that are not needed by a module and could be
1576 replaced with a forward declaration. Further, Clang will only emit
1577 type info for a dynamic C++ class in the module that contains the
1578 vtable for the class.
1580 The **-fstandalone-debug** option turns off these optimizations.
1581 This is useful when working with 3rd-party libraries that don't come
1582 with debug information. Note that Clang will never emit type
1583 information for types that are not referenced at all by the program.
1585 .. option:: -fno-standalone-debug
1587 On Darwin **-fstandalone-debug** is enabled by default. The
1588 **-fno-standalone-debug** option can be used to get to turn on the
1589 vtable-based optimization described above.
1593 Generate complete debug info.
1595 Comment Parsing Options
1596 -----------------------
1598 Clang parses Doxygen and non-Doxygen style documentation comments and attaches
1599 them to the appropriate declaration nodes. By default, it only parses
1600 Doxygen-style comments and ignores ordinary comments starting with ``//`` and
1603 .. option:: -Wdocumentation
1605 Emit warnings about use of documentation comments. This warning group is off
1608 This includes checking that ``\param`` commands name parameters that actually
1609 present in the function signature, checking that ``\returns`` is used only on
1610 functions that actually return a value etc.
1612 .. option:: -Wno-documentation-unknown-command
1614 Don't warn when encountering an unknown Doxygen command.
1616 .. option:: -fparse-all-comments
1618 Parse all comments as documentation comments (including ordinary comments
1619 starting with ``//`` and ``/*``).
1621 .. option:: -fcomment-block-commands=[commands]
1623 Define custom documentation commands as block commands. This allows Clang to
1624 construct the correct AST for these custom commands, and silences warnings
1625 about unknown commands. Several commands must be separated by a comma
1626 *without trailing space*; e.g. ``-fcomment-block-commands=foo,bar`` defines
1627 custom commands ``\foo`` and ``\bar``.
1629 It is also possible to use ``-fcomment-block-commands`` several times; e.g.
1630 ``-fcomment-block-commands=foo -fcomment-block-commands=bar`` does the same
1638 The support for standard C in clang is feature-complete except for the
1639 C99 floating-point pragmas.
1641 Extensions supported by clang
1642 -----------------------------
1644 See :doc:`LanguageExtensions`.
1646 Differences between various standard modes
1647 ------------------------------------------
1649 clang supports the -std option, which changes what language mode clang
1650 uses. The supported modes for C are c89, gnu89, c94, c99, gnu99, c11,
1651 gnu11, and various aliases for those modes. If no -std option is
1652 specified, clang defaults to gnu11 mode. Many C99 and C11 features are
1653 supported in earlier modes as a conforming extension, with a warning. Use
1654 ``-pedantic-errors`` to request an error if a feature from a later standard
1655 revision is used in an earlier mode.
1657 Differences between all ``c*`` and ``gnu*`` modes:
1659 - ``c*`` modes define "``__STRICT_ANSI__``".
1660 - Target-specific defines not prefixed by underscores, like "linux",
1661 are defined in ``gnu*`` modes.
1662 - Trigraphs default to being off in ``gnu*`` modes; they can be enabled by
1663 the -trigraphs option.
1664 - The parser recognizes "asm" and "typeof" as keywords in ``gnu*`` modes;
1665 the variants "``__asm__``" and "``__typeof__``" are recognized in all
1667 - The Apple "blocks" extension is recognized by default in ``gnu*`` modes
1668 on some platforms; it can be enabled in any mode with the "-fblocks"
1670 - Arrays that are VLA's according to the standard, but which can be
1671 constant folded by the frontend are treated as fixed size arrays.
1672 This occurs for things like "int X[(1, 2)];", which is technically a
1673 VLA. ``c*`` modes are strictly compliant and treat these as VLAs.
1675 Differences between ``*89`` and ``*99`` modes:
1677 - The ``*99`` modes default to implementing "inline" as specified in C99,
1678 while the ``*89`` modes implement the GNU version. This can be
1679 overridden for individual functions with the ``__gnu_inline__``
1681 - Digraphs are not recognized in c89 mode.
1682 - The scope of names defined inside a "for", "if", "switch", "while",
1683 or "do" statement is different. (example: "``if ((struct x {int
1685 - ``__STDC_VERSION__`` is not defined in ``*89`` modes.
1686 - "inline" is not recognized as a keyword in c89 mode.
1687 - "restrict" is not recognized as a keyword in ``*89`` modes.
1688 - Commas are allowed in integer constant expressions in ``*99`` modes.
1689 - Arrays which are not lvalues are not implicitly promoted to pointers
1691 - Some warnings are different.
1693 Differences between ``*99`` and ``*11`` modes:
1695 - Warnings for use of C11 features are disabled.
1696 - ``__STDC_VERSION__`` is defined to ``201112L`` rather than ``199901L``.
1698 c94 mode is identical to c89 mode except that digraphs are enabled in
1699 c94 mode (FIXME: And ``__STDC_VERSION__`` should be defined!).
1701 GCC extensions not implemented yet
1702 ----------------------------------
1704 clang tries to be compatible with gcc as much as possible, but some gcc
1705 extensions are not implemented yet:
1707 - clang does not support #pragma weak (`bug
1708 3679 <http://llvm.org/bugs/show_bug.cgi?id=3679>`_). Due to the uses
1709 described in the bug, this is likely to be implemented at some point,
1711 - clang does not support decimal floating point types (``_Decimal32`` and
1712 friends) or fixed-point types (``_Fract`` and friends); nobody has
1713 expressed interest in these features yet, so it's hard to say when
1714 they will be implemented.
1715 - clang does not support nested functions; this is a complex feature
1716 which is infrequently used, so it is unlikely to be implemented
1717 anytime soon. In C++11 it can be emulated by assigning lambda
1718 functions to local variables, e.g:
1722 auto const local_function = [&](int parameter) {
1728 - clang does not support global register variables; this is unlikely to
1729 be implemented soon because it requires additional LLVM backend
1731 - clang does not support static initialization of flexible array
1732 members. This appears to be a rarely used extension, but could be
1733 implemented pending user demand.
1734 - clang does not support
1735 ``__builtin_va_arg_pack``/``__builtin_va_arg_pack_len``. This is
1736 used rarely, but in some potentially interesting places, like the
1737 glibc headers, so it may be implemented pending user demand. Note
1738 that because clang pretends to be like GCC 4.2, and this extension
1739 was introduced in 4.3, the glibc headers will not try to use this
1740 extension with clang at the moment.
1741 - clang does not support the gcc extension for forward-declaring
1742 function parameters; this has not shown up in any real-world code
1743 yet, though, so it might never be implemented.
1745 This is not a complete list; if you find an unsupported extension
1746 missing from this list, please send an e-mail to cfe-dev. This list
1747 currently excludes C++; see :ref:`C++ Language Features <cxx>`. Also, this
1748 list does not include bugs in mostly-implemented features; please see
1750 tracker <http://llvm.org/bugs/buglist.cgi?quicksearch=product%3Aclang+component%3A-New%2BBugs%2CAST%2CBasic%2CDriver%2CHeaders%2CLLVM%2BCodeGen%2Cparser%2Cpreprocessor%2CSemantic%2BAnalyzer>`_
1751 for known existing bugs (FIXME: Is there a section for bug-reporting
1752 guidelines somewhere?).
1754 Intentionally unsupported GCC extensions
1755 ----------------------------------------
1757 - clang does not support the gcc extension that allows variable-length
1758 arrays in structures. This is for a few reasons: one, it is tricky to
1759 implement, two, the extension is completely undocumented, and three,
1760 the extension appears to be rarely used. Note that clang *does*
1761 support flexible array members (arrays with a zero or unspecified
1762 size at the end of a structure).
1763 - clang does not have an equivalent to gcc's "fold"; this means that
1764 clang doesn't accept some constructs gcc might accept in contexts
1765 where a constant expression is required, like "x-x" where x is a
1767 - clang does not support ``__builtin_apply`` and friends; this extension
1768 is extremely obscure and difficult to implement reliably.
1772 Microsoft extensions
1773 --------------------
1775 clang has some experimental support for extensions from Microsoft Visual
1776 C++; to enable it, use the ``-fms-extensions`` command-line option. This is
1777 the default for Windows targets. Note that the support is incomplete.
1778 Some constructs such as ``dllexport`` on classes are ignored with a warning,
1779 and others such as `Microsoft IDL annotations
1780 <http://msdn.microsoft.com/en-us/library/8tesw2eh.aspx>`_ are silently
1783 clang has a ``-fms-compatibility`` flag that makes clang accept enough
1784 invalid C++ to be able to parse most Microsoft headers. For example, it
1785 allows `unqualified lookup of dependent base class members
1786 <http://clang.llvm.org/compatibility.html#dep_lookup_bases>`_, which is
1787 a common compatibility issue with clang. This flag is enabled by default
1788 for Windows targets.
1790 ``-fdelayed-template-parsing`` lets clang delay parsing of function template
1791 definitions until the end of a translation unit. This flag is enabled by
1792 default for Windows targets.
1794 - clang allows setting ``_MSC_VER`` with ``-fmsc-version=``. It defaults to
1795 1700 which is the same as Visual C/C++ 2012. Any number is supported
1796 and can greatly affect what Windows SDK and c++stdlib headers clang
1798 - clang does not support the Microsoft extension where anonymous record
1799 members can be declared using user defined typedefs.
1800 - clang supports the Microsoft ``#pragma pack`` feature for controlling
1801 record layout. GCC also contains support for this feature, however
1802 where MSVC and GCC are incompatible clang follows the MSVC
1804 - clang supports the Microsoft ``#pragma comment(lib, "foo.lib")`` feature for
1805 automatically linking against the specified library. Currently this feature
1806 only works with the Visual C++ linker.
1807 - clang supports the Microsoft ``#pragma comment(linker, "/flag:foo")`` feature
1808 for adding linker flags to COFF object files. The user is responsible for
1809 ensuring that the linker understands the flags.
1810 - clang defaults to C++11 for Windows targets.
1814 C++ Language Features
1815 =====================
1817 clang fully implements all of standard C++98 except for exported
1818 templates (which were removed in C++11), and all of standard C++11
1819 and the current draft standard for C++1y.
1821 Controlling implementation limits
1822 ---------------------------------
1824 .. option:: -fbracket-depth=N
1826 Sets the limit for nested parentheses, brackets, and braces to N. The
1829 .. option:: -fconstexpr-depth=N
1831 Sets the limit for recursive constexpr function invocations to N. The
1834 .. option:: -ftemplate-depth=N
1836 Sets the limit for recursively nested template instantiations to N. The
1839 .. option:: -foperator-arrow-depth=N
1841 Sets the limit for iterative calls to 'operator->' functions to N. The
1846 Objective-C Language Features
1847 =============================
1851 Objective-C++ Language Features
1852 ===============================
1859 Clang supports all OpenMP 3.1 directives and clauses. In addition, some
1860 features of OpenMP 4.0 are supported. For example, ``#pragma omp simd``,
1861 ``#pragma omp for simd``, ``#pragma omp parallel for simd`` directives, extended
1862 set of atomic constructs, ``proc_bind`` clause for all parallel-based
1863 directives, ``depend`` clause for ``#pragma omp task`` directive (except for
1864 array sections), ``#pragma omp cancel`` and ``#pragma omp cancellation point``
1865 directives, and ``#pragma omp taskgroup`` directive.
1867 OpenMP support is disabled by default. Use :option:`-fopenmp=libomp` to enable
1868 it. Support for OpenMP can be disabled with :option:`-fno-openmp`.
1870 Controlling implementation limits
1871 ---------------------------------
1873 .. option:: -fopenmp-use-tls
1875 Controls code generation for OpenMP threadprivate variables. In presence of
1876 this option all threadprivate variables are generated the same way as thread
1877 local variables, using TLS support. If :option:`-fno-openmp-use-tls`
1878 is provided or target does not support TLS, code generation for threadprivate
1879 variables relies on OpenMP runtime library.
1881 .. _target_features:
1883 Target-Specific Features and Limitations
1884 ========================================
1886 CPU Architectures Features and Limitations
1887 ------------------------------------------
1892 The support for X86 (both 32-bit and 64-bit) is considered stable on
1893 Darwin (Mac OS X), Linux, FreeBSD, and Dragonfly BSD: it has been tested
1894 to correctly compile many large C, C++, Objective-C, and Objective-C++
1897 On ``x86_64-mingw32``, passing i128(by value) is incompatible with the
1898 Microsoft x64 calling convention. You might need to tweak
1899 ``WinX86_64ABIInfo::classify()`` in lib/CodeGen/TargetInfo.cpp.
1901 For the X86 target, clang supports the :option:`-m16` command line
1902 argument which enables 16-bit code output. This is broadly similar to
1903 using ``asm(".code16gcc")`` with the GNU toolchain. The generated code
1904 and the ABI remains 32-bit but the assembler emits instructions
1905 appropriate for a CPU running in 16-bit mode, with address-size and
1906 operand-size prefixes to enable 32-bit addressing and operations.
1911 The support for ARM (specifically ARMv6 and ARMv7) is considered stable
1912 on Darwin (iOS): it has been tested to correctly compile many large C,
1913 C++, Objective-C, and Objective-C++ codebases. Clang only supports a
1914 limited number of ARM architectures. It does not yet fully support
1920 The support for PowerPC (especially PowerPC64) is considered stable
1921 on Linux and FreeBSD: it has been tested to correctly compile many
1922 large C and C++ codebases. PowerPC (32bit) is still missing certain
1923 features (e.g. PIC code on ELF platforms).
1928 clang currently contains some support for other architectures (e.g. Sparc);
1929 however, significant pieces of code generation are still missing, and they
1930 haven't undergone significant testing.
1932 clang contains limited support for the MSP430 embedded processor, but
1933 both the clang support and the LLVM backend support are highly
1936 Other platforms are completely unsupported at the moment. Adding the
1937 minimal support needed for parsing and semantic analysis on a new
1938 platform is quite easy; see ``lib/Basic/Targets.cpp`` in the clang source
1939 tree. This level of support is also sufficient for conversion to LLVM IR
1940 for simple programs. Proper support for conversion to LLVM IR requires
1941 adding code to ``lib/CodeGen/CGCall.cpp`` at the moment; this is likely to
1942 change soon, though. Generating assembly requires a suitable LLVM
1945 Operating System Features and Limitations
1946 -----------------------------------------
1951 Thread Sanitizer is not supported.
1956 Clang has experimental support for targeting "Cygming" (Cygwin / MinGW)
1959 See also :ref:`Microsoft Extensions <c_ms>`.
1964 Clang works on Cygwin-1.7.
1969 Clang works on some mingw32 distributions. Clang assumes directories as
1972 - ``C:/mingw/include``
1974 - ``C:/mingw/lib/gcc/mingw32/4.[3-5].0/include/c++``
1976 On MSYS, a few tests might fail.
1981 For 32-bit (i686-w64-mingw32), and 64-bit (x86\_64-w64-mingw32), Clang
1984 - ``GCC versions 4.5.0 to 4.5.3, 4.6.0 to 4.6.2, or 4.7.0 (for the C++ header search path)``
1985 - ``some_directory/bin/gcc.exe``
1986 - ``some_directory/bin/clang.exe``
1987 - ``some_directory/bin/clang++.exe``
1988 - ``some_directory/bin/../include/c++/GCC_version``
1989 - ``some_directory/bin/../include/c++/GCC_version/x86_64-w64-mingw32``
1990 - ``some_directory/bin/../include/c++/GCC_version/i686-w64-mingw32``
1991 - ``some_directory/bin/../include/c++/GCC_version/backward``
1992 - ``some_directory/bin/../x86_64-w64-mingw32/include``
1993 - ``some_directory/bin/../i686-w64-mingw32/include``
1994 - ``some_directory/bin/../include``
1996 This directory layout is standard for any toolchain you will find on the
1997 official `MinGW-w64 website <http://mingw-w64.sourceforge.net>`_.
1999 Clang expects the GCC executable "gcc.exe" compiled for
2000 ``i686-w64-mingw32`` (or ``x86_64-w64-mingw32``) to be present on PATH.
2002 `Some tests might fail <http://llvm.org/bugs/show_bug.cgi?id=9072>`_ on
2003 ``x86_64-w64-mingw32``.
2010 clang-cl is an alternative command-line interface to Clang driver, designed for
2011 compatibility with the Visual C++ compiler, cl.exe.
2013 To enable clang-cl to find system headers, libraries, and the linker when run
2014 from the command-line, it should be executed inside a Visual Studio Native Tools
2015 Command Prompt or a regular Command Prompt where the environment has been set
2016 up using e.g. `vcvars32.bat <http://msdn.microsoft.com/en-us/library/f2ccy3wt.aspx>`_.
2018 clang-cl can also be used from inside Visual Studio by using an LLVM Platform
2021 Command-Line Options
2022 --------------------
2024 To be compatible with cl.exe, clang-cl supports most of the same command-line
2025 options. Those options can start with either ``/`` or ``-``. It also supports
2026 some of Clang's core options, such as the ``-W`` options.
2028 Options that are known to clang-cl, but not currently supported, are ignored
2029 with a warning. For example:
2033 clang-cl.exe: warning: argument unused during compilation: '/AI'
2035 To suppress warnings about unused arguments, use the ``-Qunused-arguments`` option.
2037 Options that are not known to clang-cl will cause errors. If they are spelled with a
2038 leading ``/``, they will be mistaken for a filename:
2042 clang-cl.exe: error: no such file or directory: '/foobar'
2044 Please `file a bug <http://llvm.org/bugs/enter_bug.cgi?product=clang&component=Driver>`_
2045 for any valid cl.exe flags that clang-cl does not understand.
2047 Execute ``clang-cl /?`` to see a list of supported options:
2051 CL.EXE COMPATIBILITY OPTIONS:
2052 /? Display available options
2053 /arch:<value> Set architecture for code generation
2054 /C Don't discard comments when preprocessing
2056 /D <macro[=value]> Define macro
2057 /EH<value> Exception handling model
2058 /EP Disable linemarker output and preprocess to stdout
2059 /E Preprocess to stdout
2060 /fallback Fall back to cl.exe if clang-cl fails to compile
2061 /FA Output assembly code file during compilation
2062 /Fa<file or directory> Output assembly code to this file during compilation (with /FA)
2063 /Fe<file or directory> Set output executable file or directory (ends in / or \)
2064 /FI <value> Include file before parsing
2065 /Fi<file> Set preprocess output file name (with /P)
2066 /Fo<file or directory> Set output object file, or directory (ends in / or \) (with /c)
2072 /GA Assume thread-local variables are defined in the executable
2073 /GF- Disable string pooling
2074 /GR- Disable emission of RTTI data
2075 /GR Enable emission of RTTI data
2076 /Gs<value> Set stack probe size
2077 /Gw- Don't put each data item in its own section
2078 /Gw Put each data item in its own section
2079 /Gy- Don't put each function in its own section
2080 /Gy Put each function in its own section
2081 /help Display available options
2082 /I <dir> Add directory to include search path
2083 /J Make char type unsigned
2084 /LDd Create debug DLL
2086 /link <options> Forward options to the linker
2087 /MDd Use DLL debug run-time
2088 /MD Use DLL run-time
2089 /MTd Use static debug run-time
2090 /MT Use static run-time
2091 /Ob0 Disable inlining
2092 /Od Disable optimization
2093 /Oi- Disable use of builtin functions
2094 /Oi Enable use of builtin functions
2095 /Os Optimize for size
2096 /Ot Optimize for speed
2097 /Oy- Disable frame pointer omission
2098 /Oy Enable frame pointer omission
2099 /O<value> Optimization level
2100 /o <file or directory> Set output file or directory (ends in / or \)
2101 /P Preprocess to file
2102 /Qvec- Disable the loop vectorization passes
2103 /Qvec Enable the loop vectorization passes
2104 /showIncludes Print info about included files to stderr
2105 /TC Treat all source files as C
2106 /Tc <filename> Specify a C source file
2107 /TP Treat all source files as C++
2108 /Tp <filename> Specify a C++ source file
2109 /U <macro> Undefine macro
2110 /vd<value> Control vtordisp placement
2111 /vmb Use a best-case representation method for member pointers
2112 /vmg Use a most-general representation for member pointers
2113 /vmm Set the default most-general representation to multiple inheritance
2114 /vms Set the default most-general representation to single inheritance
2115 /vmv Set the default most-general representation to virtual inheritance
2116 /volatile:iso Volatile loads and stores have standard semantics
2117 /volatile:ms Volatile loads and stores have acquire and release semantics
2118 /W0 Disable all warnings
2124 /WX- Do not treat warnings as errors
2125 /WX Treat warnings as errors
2126 /w Disable all warnings
2127 /Z7 Enable CodeView debug information in object files
2128 /Zc:sizedDealloc- Disable C++14 sized global deallocation functions
2129 /Zc:sizedDealloc Enable C++14 sized global deallocation functions
2130 /Zc:strictStrings Treat string literals as const
2131 /Zc:threadSafeInit- Disable thread-safe initialization of static variables
2132 /Zc:threadSafeInit Enable thread-safe initialization of static variables
2133 /Zc:trigraphs- Disable trigraphs (default)
2134 /Zc:trigraphs Enable trigraphs
2135 /Zi Alias for /Z7. Does not produce PDBs.
2136 /Zl Don't mention any default libraries in the object file
2137 /Zp Set the default maximum struct packing alignment to 1
2138 /Zp<value> Specify the default maximum struct packing alignment
2139 /Zs Syntax-check only
2142 -### Print (but do not run) the commands to run for this compilation
2143 --analyze Run the static analyzer
2144 -fansi-escape-codes Use ANSI escape codes for diagnostics
2145 -fcolor-diagnostics Use colors in diagnostics
2146 -fdiagnostics-parseable-fixits
2147 Print fix-its in machine parseable form
2148 -fms-compatibility-version=<value>
2149 Dot-separated value representing the Microsoft compiler version
2150 number to report in _MSC_VER (0 = don't define it (default))
2151 -fmsc-version=<value> Microsoft compiler version number to report in _MSC_VER (0 = don't
2152 define it (default))
2153 -fno-sanitize-coverage=<value>
2154 Disable specified features of coverage instrumentation for Sanitizers
2155 -fno-sanitize-recover=<value>
2156 Disable recovery for specified sanitizers
2157 -fno-sanitize-trap=<value>
2158 Disable trapping for specified sanitizers
2159 -fsanitize-blacklist=<value>
2160 Path to blacklist file for sanitizers
2161 -fsanitize-coverage=<value>
2162 Specify the type of coverage instrumentation for Sanitizers
2163 -fsanitize-recover=<value>
2164 Enable recovery for specified sanitizers
2165 -fsanitize-trap=<value> Enable trapping for specified sanitizers
2166 -fsanitize=<check> Turn on runtime checks for various forms of undefined or suspicious
2167 behavior. See user manual for available checks
2168 -gcodeview Generate CodeView debug information
2169 -mllvm <value> Additional arguments to forward to LLVM's option processing
2170 -Qunused-arguments Don't emit warning for unused driver arguments
2171 -R<remark> Enable the specified remark
2172 --target=<value> Generate code for the given target
2173 -v Show commands to run and use verbose output
2174 -W<warning> Enable the specified warning
2175 -Xclang <arg> Pass <arg> to the clang compiler
2177 The /fallback Option
2178 ^^^^^^^^^^^^^^^^^^^^
2180 When clang-cl is run with the ``/fallback`` option, it will first try to
2181 compile files itself. For any file that it fails to compile, it will fall back
2182 and try to compile the file by invoking cl.exe.
2184 This option is intended to be used as a temporary means to build projects where
2185 clang-cl cannot successfully compile all the files. clang-cl may fail to compile
2186 a file either because it cannot generate code for some C++ feature, or because
2187 it cannot parse some Microsoft language extension.