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 - ``-fsanitize=integer``: Enables checks for undefined or
956 suspicious integer behavior.
957 - .. _opt_fsanitize_thread:
959 ``-fsanitize=thread``: :doc:`ThreadSanitizer`, a data race detector.
960 - .. _opt_fsanitize_memory:
962 ``-fsanitize=memory``: :doc:`MemorySanitizer`,
963 an *experimental* detector of uninitialized reads. Not ready for
965 - .. _opt_fsanitize_undefined:
967 ``-fsanitize=undefined``: Fast and compatible undefined behavior
968 checker. Enables the undefined behavior checks that have small
969 runtime cost and no impact on address space layout or ABI. This
970 includes all of the checks listed below other than
971 ``unsigned-integer-overflow``.
973 - ``-fsanitize=undefined-trap``: This is a deprecated alias for
974 ``-fsanitize=undefined``.
976 - ``-fsanitize=dataflow``: :doc:`DataFlowSanitizer`, a general data
978 - ``-fsanitize=cfi``: :doc:`control flow integrity <ControlFlowIntegrity>`
979 checks. Requires ``-flto``.
980 - ``-fsanitize=safe-stack``: :doc:`safe stack <SafeStack>`
981 protection against stack-based memory corruption errors.
983 The following more fine-grained checks are also available:
985 - ``-fsanitize=alignment``: Use of a misaligned pointer or creation
986 of a misaligned reference.
987 - ``-fsanitize=bool``: Load of a ``bool`` value which is neither
988 ``true`` nor ``false``.
989 - ``-fsanitize=bounds``: Out of bounds array indexing, in cases
990 where the array bound can be statically determined.
991 - ``-fsanitize=cfi-cast-strict``: Enables :ref:`strict cast checks
993 - ``-fsanitize=cfi-derived-cast``: Base-to-derived cast to the wrong
994 dynamic type. Requires ``-flto``.
995 - ``-fsanitize=cfi-unrelated-cast``: Cast from ``void*`` or another
996 unrelated type to the wrong dynamic type. Requires ``-flto``.
997 - ``-fsanitize=cfi-nvcall``: Non-virtual call via an object whose vptr is of
998 the wrong dynamic type. Requires ``-flto``.
999 - ``-fsanitize=cfi-vcall``: Virtual call via an object whose vptr is of the
1000 wrong dynamic type. Requires ``-flto``.
1001 - ``-fsanitize=enum``: Load of a value of an enumerated type which
1002 is not in the range of representable values for that enumerated
1004 - ``-fsanitize=float-cast-overflow``: Conversion to, from, or
1005 between floating-point types which would overflow the
1007 - ``-fsanitize=float-divide-by-zero``: Floating point division by
1009 - ``-fsanitize=function``: Indirect call of a function through a
1010 function pointer of the wrong type (Linux, C++ and x86/x86_64 only).
1011 - ``-fsanitize=integer-divide-by-zero``: Integer division by zero.
1012 - ``-fsanitize=nonnull-attribute``: Passing null pointer as a function
1013 parameter which is declared to never be null.
1014 - ``-fsanitize=null``: Use of a null pointer or creation of a null
1016 - ``-fsanitize=object-size``: An attempt to use bytes which the
1017 optimizer can determine are not part of the object being
1018 accessed. The sizes of objects are determined using
1019 ``__builtin_object_size``, and consequently may be able to detect
1020 more problems at higher optimization levels.
1021 - ``-fsanitize=return``: In C++, reaching the end of a
1022 value-returning function without returning a value.
1023 - ``-fsanitize=returns-nonnull-attribute``: Returning null pointer
1024 from a function which is declared to never return null.
1025 - ``-fsanitize=shift``: Shift operators where the amount shifted is
1026 greater or equal to the promoted bit-width of the left hand side
1027 or less than zero, or where the left hand side is negative. For a
1028 signed left shift, also checks for signed overflow in C, and for
1029 unsigned overflow in C++. You can use ``-fsanitize=shift-base`` or
1030 ``-fsanitize=shift-exponent`` to check only left-hand side or
1031 right-hand side of shift operation, respectively.
1032 - ``-fsanitize=signed-integer-overflow``: Signed integer overflow,
1033 including all the checks added by ``-ftrapv``, and checking for
1034 overflow in signed division (``INT_MIN / -1``).
1035 - ``-fsanitize=unreachable``: If control flow reaches
1036 ``__builtin_unreachable``.
1037 - ``-fsanitize=unsigned-integer-overflow``: Unsigned integer
1039 - ``-fsanitize=vla-bound``: A variable-length array whose bound
1040 does not evaluate to a positive value.
1041 - ``-fsanitize=vptr``: Use of an object whose vptr indicates that
1042 it is of the wrong dynamic type, or that its lifetime has not
1043 begun or has ended. Incompatible with ``-fno-rtti``.
1045 You can turn off or modify checks for certain source files, functions
1046 or even variables by providing a special file:
1048 - ``-fsanitize-blacklist=/path/to/blacklist/file``: disable or modify
1049 sanitizer checks for objects listed in the file. See
1050 :doc:`SanitizerSpecialCaseList` for file format description.
1051 - ``-fno-sanitize-blacklist``: don't use blacklist file, if it was
1052 specified earlier in the command line.
1054 Extra features of MemorySanitizer (require explicit
1055 ``-fsanitize=memory``):
1057 - ``-fsanitize-memory-track-origins[=level]``: Enables origin tracking in
1058 MemorySanitizer. Adds a second section to MemorySanitizer
1059 reports pointing to the heap or stack allocation the
1060 uninitialized bits came from. Slows down execution by additional
1063 Possible values for level are 0 (off), 1, 2 (default). Level 2
1064 adds more sections to MemorySanitizer reports describing the
1065 order of memory stores the uninitialized value went
1066 through. This mode may use extra memory in programs that copy
1067 uninitialized memory a lot.
1069 The ``-fsanitize=`` argument must also be provided when linking, in
1070 order to link to the appropriate runtime library. When using
1071 ``-fsanitize=vptr`` (or a group that includes it, such as
1072 ``-fsanitize=undefined``) with a C++ program, the link must be
1073 performed by ``clang++``, not ``clang``, in order to link against the
1074 C++-specific parts of the runtime library.
1076 It is not possible to combine more than one of the ``-fsanitize=address``,
1077 ``-fsanitize=thread``, and ``-fsanitize=memory`` checkers in the same
1078 program. The ``-fsanitize=undefined`` checks can only be combined with
1079 ``-fsanitize=address``.
1081 **-f[no-]sanitize-recover=check1,check2,...**
1083 Controls which checks enabled by ``-fsanitize=`` flag are non-fatal.
1084 If the check is fatal, program will halt after the first error
1085 of this kind is detected and error report is printed.
1087 By default, non-fatal checks are those enabled by UndefinedBehaviorSanitizer,
1088 except for ``-fsanitize=return`` and ``-fsanitize=unreachable``. Some
1089 sanitizers (e.g. :doc:`AddressSanitizer`) may not support recovery,
1090 and always crash the program after the issue is detected.
1092 Note that the ``-fsanitize-trap`` flag has precedence over this flag.
1093 This means that if a check has been configured to trap elsewhere on the
1094 command line, or if the check traps by default, this flag will not have
1095 any effect unless that sanitizer's trapping behavior is disabled with
1096 ``-fno-sanitize-trap``.
1098 For example, if a command line contains the flags ``-fsanitize=undefined
1099 -fsanitize-trap=undefined``, the flag ``-fsanitize-recover=alignment``
1100 will have no effect on its own; it will need to be accompanied by
1101 ``-fno-sanitize-trap=alignment``.
1103 **-f[no-]sanitize-trap=check1,check2,...**
1105 Controls which checks enabled by the ``-fsanitize=`` flag trap. This
1106 option is intended for use in cases where the sanitizer runtime cannot
1107 be used (for instance, when building libc or a kernel module), or where
1108 the binary size increase caused by the sanitizer runtime is a concern.
1110 This flag is only compatible with ``local-bounds``,
1111 ``unsigned-integer-overflow``, sanitizers in the ``cfi`` group and
1112 sanitizers in the ``undefined`` group other than ``vptr``. If this flag
1113 is supplied together with ``-fsanitize=undefined``, the ``vptr`` sanitizer
1114 will be implicitly disabled.
1116 This flag is enabled by default for sanitizers in the ``cfi`` group.
1118 **-f[no-]sanitize-coverage=[type,features,...]**
1120 Enable simple code coverage in addition to certain sanitizers.
1121 See :doc:`SanitizerCoverage` for more details.
1123 .. option:: -fsanitize-undefined-trap-on-error
1125 Deprecated alias for ``-fsanitize-trap=undefined``.
1127 .. option:: -fno-assume-sane-operator-new
1129 Don't assume that the C++'s new operator is sane.
1131 This option tells the compiler to do not assume that C++'s global
1132 new operator will always return a pointer that does not alias any
1133 other pointer when the function returns.
1135 .. option:: -ftrap-function=[name]
1137 Instruct code generator to emit a function call to the specified
1138 function name for ``__builtin_trap()``.
1140 LLVM code generator translates ``__builtin_trap()`` to a trap
1141 instruction if it is supported by the target ISA. Otherwise, the
1142 builtin is translated into a call to ``abort``. If this option is
1143 set, then the code generator will always lower the builtin to a call
1144 to the specified function regardless of whether the target ISA has a
1145 trap instruction. This option is useful for environments (e.g.
1146 deeply embedded) where a trap cannot be properly handled, or when
1147 some custom behavior is desired.
1149 .. option:: -ftls-model=[model]
1151 Select which TLS model to use.
1153 Valid values are: ``global-dynamic``, ``local-dynamic``,
1154 ``initial-exec`` and ``local-exec``. The default value is
1155 ``global-dynamic``. The compiler may use a different model if the
1156 selected model is not supported by the target, or if a more
1157 efficient model can be used. The TLS model can be overridden per
1158 variable using the ``tls_model`` attribute.
1160 .. option:: -femulated-tls
1162 Select emulated TLS model, which overrides all -ftls-model choices.
1164 In emulated TLS mode, all access to TLS variables are converted to
1165 calls to __emutls_get_address in the runtime library.
1167 .. option:: -mhwdiv=[values]
1169 Select the ARM modes (arm or thumb) that support hardware division
1172 Valid values are: ``arm``, ``thumb`` and ``arm,thumb``.
1173 This option is used to indicate which mode (arm or thumb) supports
1174 hardware division instructions. This only applies to the ARM
1177 .. option:: -m[no-]crc
1179 Enable or disable CRC instructions.
1181 This option is used to indicate whether CRC instructions are to
1182 be generated. This only applies to the ARM architecture.
1184 CRC instructions are enabled by default on ARMv8.
1186 .. option:: -mgeneral-regs-only
1188 Generate code which only uses the general purpose registers.
1190 This option restricts the generated code to use general registers
1191 only. This only applies to the AArch64 architecture.
1193 **-f[no-]max-unknown-pointer-align=[number]**
1194 Instruct the code generator to not enforce a higher alignment than the given
1195 number (of bytes) when accessing memory via an opaque pointer or reference.
1196 This cap is ignored when directly accessing a variable or when the pointee
1197 type has an explicit “aligned” attribute.
1199 The value should usually be determined by the properties of the system allocator.
1200 Some builtin types, especially vector types, have very high natural alignments;
1201 when working with values of those types, Clang usually wants to use instructions
1202 that take advantage of that alignment. However, many system allocators do
1203 not promise to return memory that is more than 8-byte or 16-byte-aligned. Use
1204 this option to limit the alignment that the compiler can assume for an arbitrary
1205 pointer, which may point onto the heap.
1207 This option does not affect the ABI alignment of types; the layout of structs and
1208 unions and the value returned by the alignof operator remain the same.
1210 This option can be overridden on a case-by-case basis by putting an explicit
1211 “aligned” alignment on a struct, union, or typedef. For example:
1213 .. code-block:: console
1215 #include <immintrin.h>
1216 // Make an aligned typedef of the AVX-512 16-int vector type.
1217 typedef __v16si __aligned_v16si __attribute__((aligned(64)));
1219 void initialize_vector(__aligned_v16si *v) {
1220 // The compiler may assume that ‘v’ is 64-byte aligned, regardless of the
1221 // value of -fmax-unknown-pointer-align.
1225 Profile Guided Optimization
1226 ---------------------------
1228 Profile information enables better optimization. For example, knowing that a
1229 branch is taken very frequently helps the compiler make better decisions when
1230 ordering basic blocks. Knowing that a function ``foo`` is called more
1231 frequently than another function ``bar`` helps the inliner.
1233 Clang supports profile guided optimization with two different kinds of
1234 profiling. A sampling profiler can generate a profile with very low runtime
1235 overhead, or you can build an instrumented version of the code that collects
1236 more detailed profile information. Both kinds of profiles can provide execution
1237 counts for instructions in the code and information on branches taken and
1238 function invocation.
1240 Regardless of which kind of profiling you use, be careful to collect profiles
1241 by running your code with inputs that are representative of the typical
1242 behavior. Code that is not exercised in the profile will be optimized as if it
1243 is unimportant, and the compiler may make poor optimization choices for code
1244 that is disproportionately used while profiling.
1246 Differences Between Sampling and Instrumentation
1247 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1249 Although both techniques are used for similar purposes, there are important
1250 differences between the two:
1252 1. Profile data generated with one cannot be used by the other, and there is no
1253 conversion tool that can convert one to the other. So, a profile generated
1254 via ``-fprofile-instr-generate`` must be used with ``-fprofile-instr-use``.
1255 Similarly, sampling profiles generated by external profilers must be
1256 converted and used with ``-fprofile-sample-use``.
1258 2. Instrumentation profile data can be used for code coverage analysis and
1261 3. Sampling profiles can only be used for optimization. They cannot be used for
1262 code coverage analysis. Although it would be technically possible to use
1263 sampling profiles for code coverage, sample-based profiles are too
1264 coarse-grained for code coverage purposes; it would yield poor results.
1266 4. Sampling profiles must be generated by an external tool. The profile
1267 generated by that tool must then be converted into a format that can be read
1268 by LLVM. The section on sampling profilers describes one of the supported
1269 sampling profile formats.
1272 Using Sampling Profilers
1273 ^^^^^^^^^^^^^^^^^^^^^^^^
1275 Sampling profilers are used to collect runtime information, such as
1276 hardware counters, while your application executes. They are typically
1277 very efficient and do not incur a large runtime overhead. The
1278 sample data collected by the profiler can be used during compilation
1279 to determine what the most executed areas of the code are.
1281 Using the data from a sample profiler requires some changes in the way
1282 a program is built. Before the compiler can use profiling information,
1283 the code needs to execute under the profiler. The following is the
1284 usual build cycle when using sample profilers for optimization:
1286 1. Build the code with source line table information. You can use all the
1287 usual build flags that you always build your application with. The only
1288 requirement is that you add ``-gline-tables-only`` or ``-g`` to the
1289 command line. This is important for the profiler to be able to map
1290 instructions back to source line locations.
1292 .. code-block:: console
1294 $ clang++ -O2 -gline-tables-only code.cc -o code
1296 2. Run the executable under a sampling profiler. The specific profiler
1297 you use does not really matter, as long as its output can be converted
1298 into the format that the LLVM optimizer understands. Currently, there
1299 exists a conversion tool for the Linux Perf profiler
1300 (https://perf.wiki.kernel.org/), so these examples assume that you
1301 are using Linux Perf to profile your code.
1303 .. code-block:: console
1305 $ perf record -b ./code
1307 Note the use of the ``-b`` flag. This tells Perf to use the Last Branch
1308 Record (LBR) to record call chains. While this is not strictly required,
1309 it provides better call information, which improves the accuracy of
1312 3. Convert the collected profile data to LLVM's sample profile format.
1313 This is currently supported via the AutoFDO converter ``create_llvm_prof``.
1314 It is available at http://github.com/google/autofdo. Once built and
1315 installed, you can convert the ``perf.data`` file to LLVM using
1318 .. code-block:: console
1320 $ create_llvm_prof --binary=./code --out=code.prof
1322 This will read ``perf.data`` and the binary file ``./code`` and emit
1323 the profile data in ``code.prof``. Note that if you ran ``perf``
1324 without the ``-b`` flag, you need to use ``--use_lbr=false`` when
1325 calling ``create_llvm_prof``.
1327 4. Build the code again using the collected profile. This step feeds
1328 the profile back to the optimizers. This should result in a binary
1329 that executes faster than the original one. Note that you are not
1330 required to build the code with the exact same arguments that you
1331 used in the first step. The only requirement is that you build the code
1332 with ``-gline-tables-only`` and ``-fprofile-sample-use``.
1334 .. code-block:: console
1336 $ clang++ -O2 -gline-tables-only -fprofile-sample-use=code.prof code.cc -o code
1339 Sample Profile Formats
1340 """"""""""""""""""""""
1342 Since external profilers generate profile data in a variety of custom formats,
1343 the data generated by the profiler must be converted into a format that can be
1344 read by the backend. LLVM supports three different sample profile formats:
1346 1. ASCII text. This is the easiest one to generate. The file is divided into
1347 sections, which correspond to each of the functions with profile
1348 information. The format is described below.
1350 2. Binary encoding. This uses a more efficient encoding that yields smaller
1351 profile files, which may be useful when generating large profiles. It can be
1352 generated from the text format using the ``llvm-profdata`` tool.
1354 3. GCC encoding. This is based on the gcov format, which is accepted by GCC. It
1355 is only interesting in environments where GCC and Clang co-exist. Similarly
1356 to the binary encoding, it can be generated using the ``llvm-profdata`` tool.
1358 If you are using Linux Perf to generate sampling profiles, you can use the
1359 conversion tool ``create_llvm_prof`` described in the previous section.
1360 Otherwise, you will need to write a conversion tool that converts your
1361 profiler's native format into one of these three.
1364 Sample Profile Text Format
1365 """"""""""""""""""""""""""
1367 This section describes the ASCII text format for sampling profiles. It is,
1368 arguably, the easiest one to generate. If you are interested in generating any
1369 of the other two, consult the ``ProfileData`` library in in LLVM's source tree
1370 (specifically, ``llvm/lib/ProfileData/SampleProfWriter.cpp``).
1372 .. code-block:: console
1374 function1:total_samples:total_head_samples
1375 offset1[.discriminator]: number_of_samples [fn1:num fn2:num ... ]
1376 offset2[.discriminator]: number_of_samples [fn3:num fn4:num ... ]
1378 offsetN[.discriminator]: number_of_samples [fn5:num fn6:num ... ]
1380 The file may contain blank lines between sections and within a
1381 section. However, the spacing within a single line is fixed. Additional
1382 spaces will result in an error while reading the file.
1384 Function names must be mangled in order for the profile loader to
1385 match them in the current translation unit. The two numbers in the
1386 function header specify how many total samples were accumulated in the
1387 function (first number), and the total number of samples accumulated
1388 in the prologue of the function (second number). This head sample
1389 count provides an indicator of how frequently the function is invoked.
1391 Each sampled line may contain several items. Some are optional (marked
1394 a. Source line offset. This number represents the line number
1395 in the function where the sample was collected. The line number is
1396 always relative to the line where symbol of the function is
1397 defined. So, if the function has its header at line 280, the offset
1398 13 is at line 293 in the file.
1400 Note that this offset should never be a negative number. This could
1401 happen in cases like macros. The debug machinery will register the
1402 line number at the point of macro expansion. So, if the macro was
1403 expanded in a line before the start of the function, the profile
1404 converter should emit a 0 as the offset (this means that the optimizers
1405 will not be able to associate a meaningful weight to the instructions
1408 b. [OPTIONAL] Discriminator. This is used if the sampled program
1409 was compiled with DWARF discriminator support
1410 (http://wiki.dwarfstd.org/index.php?title=Path_Discriminators).
1411 DWARF discriminators are unsigned integer values that allow the
1412 compiler to distinguish between multiple execution paths on the
1413 same source line location.
1415 For example, consider the line of code ``if (cond) foo(); else bar();``.
1416 If the predicate ``cond`` is true 80% of the time, then the edge
1417 into function ``foo`` should be considered to be taken most of the
1418 time. But both calls to ``foo`` and ``bar`` are at the same source
1419 line, so a sample count at that line is not sufficient. The
1420 compiler needs to know which part of that line is taken more
1423 This is what discriminators provide. In this case, the calls to
1424 ``foo`` and ``bar`` will be at the same line, but will have
1425 different discriminator values. This allows the compiler to correctly
1426 set edge weights into ``foo`` and ``bar``.
1428 c. Number of samples. This is an integer quantity representing the
1429 number of samples collected by the profiler at this source
1432 d. [OPTIONAL] Potential call targets and samples. If present, this
1433 line contains a call instruction. This models both direct and
1434 number of samples. For example,
1436 .. code-block:: console
1438 130: 7 foo:3 bar:2 baz:7
1440 The above means that at relative line offset 130 there is a call
1441 instruction that calls one of ``foo()``, ``bar()`` and ``baz()``,
1442 with ``baz()`` being the relatively more frequently called target.
1445 Profiling with Instrumentation
1446 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1448 Clang also supports profiling via instrumentation. This requires building a
1449 special instrumented version of the code and has some runtime
1450 overhead during the profiling, but it provides more detailed results than a
1451 sampling profiler. It also provides reproducible results, at least to the
1452 extent that the code behaves consistently across runs.
1454 Here are the steps for using profile guided optimization with
1457 1. Build an instrumented version of the code by compiling and linking with the
1458 ``-fprofile-instr-generate`` option.
1460 .. code-block:: console
1462 $ clang++ -O2 -fprofile-instr-generate code.cc -o code
1464 2. Run the instrumented executable with inputs that reflect the typical usage.
1465 By default, the profile data will be written to a ``default.profraw`` file
1466 in the current directory. You can override that default by setting the
1467 ``LLVM_PROFILE_FILE`` environment variable to specify an alternate file.
1468 Any instance of ``%p`` in that file name will be replaced by the process
1469 ID, so that you can easily distinguish the profile output from multiple
1472 .. code-block:: console
1474 $ LLVM_PROFILE_FILE="code-%p.profraw" ./code
1476 3. Combine profiles from multiple runs and convert the "raw" profile format to
1477 the input expected by clang. Use the ``merge`` command of the
1478 ``llvm-profdata`` tool to do this.
1480 .. code-block:: console
1482 $ llvm-profdata merge -output=code.profdata code-*.profraw
1484 Note that this step is necessary even when there is only one "raw" profile,
1485 since the merge operation also changes the file format.
1487 4. Build the code again using the ``-fprofile-instr-use`` option to specify the
1488 collected profile data.
1490 .. code-block:: console
1492 $ clang++ -O2 -fprofile-instr-use=code.profdata code.cc -o code
1494 You can repeat step 4 as often as you like without regenerating the
1495 profile. As you make changes to your code, clang may no longer be able to
1496 use the profile data. It will warn you when this happens.
1498 Profile generation and use can also be controlled by the GCC-compatible flags
1499 ``-fprofile-generate`` and ``-fprofile-use``. Although these flags are
1500 semantically equivalent to their GCC counterparts, they *do not* handle
1501 GCC-compatible profiles. They are only meant to implement GCC's semantics
1502 with respect to profile creation and use.
1504 .. option:: -fprofile-generate[=<dirname>]
1506 Without any other arguments, ``-fprofile-generate`` behaves identically to
1507 ``-fprofile-instr-generate``. When given a directory name, it generates the
1508 profile file ``default.profraw`` in the directory named ``dirname``. If
1509 ``dirname`` does not exist, it will be created at runtime. The environment
1510 variable ``LLVM_PROFILE_FILE`` can be used to override the directory and
1511 filename for the profile file at runtime. For example,
1513 .. code-block:: console
1515 $ clang++ -O2 -fprofile-generate=yyy/zzz code.cc -o code
1517 When ``code`` is executed, the profile will be written to the file
1518 ``yyy/zzz/default.profraw``. This can be altered at runtime via the
1519 ``LLVM_PROFILE_FILE`` environment variable:
1521 .. code-block:: console
1523 $ LLVM_PROFILE_FILE=/tmp/myprofile/code.profraw ./code
1525 The above invocation will produce the profile file
1526 ``/tmp/myprofile/code.profraw`` instead of ``yyy/zzz/default.profraw``.
1527 Notice that ``LLVM_PROFILE_FILE`` overrides the directory *and* the file
1528 name for the profile file.
1530 .. option:: -fprofile-use[=<pathname>]
1532 Without any other arguments, ``-fprofile-use`` behaves identically to
1533 ``-fprofile-instr-use``. Otherwise, if ``pathname`` is the full path to a
1534 profile file, it reads from that file. If ``pathname`` is a directory name,
1535 it reads from ``pathname/default.profdata``.
1538 Controlling Size of Debug Information
1539 -------------------------------------
1541 Debug info kind generated by Clang can be set by one of the flags listed
1542 below. If multiple flags are present, the last one is used.
1546 Don't generate any debug info (default).
1548 .. option:: -gline-tables-only
1550 Generate line number tables only.
1552 This kind of debug info allows to obtain stack traces with function names,
1553 file names and line numbers (by such tools as ``gdb`` or ``addr2line``). It
1554 doesn't contain any other data (e.g. description of local variables or
1555 function parameters).
1557 .. option:: -fstandalone-debug
1559 Clang supports a number of optimizations to reduce the size of debug
1560 information in the binary. They work based on the assumption that
1561 the debug type information can be spread out over multiple
1562 compilation units. For instance, Clang will not emit type
1563 definitions for types that are not needed by a module and could be
1564 replaced with a forward declaration. Further, Clang will only emit
1565 type info for a dynamic C++ class in the module that contains the
1566 vtable for the class.
1568 The **-fstandalone-debug** option turns off these optimizations.
1569 This is useful when working with 3rd-party libraries that don't come
1570 with debug information. Note that Clang will never emit type
1571 information for types that are not referenced at all by the program.
1573 .. option:: -fno-standalone-debug
1575 On Darwin **-fstandalone-debug** is enabled by default. The
1576 **-fno-standalone-debug** option can be used to get to turn on the
1577 vtable-based optimization described above.
1581 Generate complete debug info.
1583 Comment Parsing Options
1584 -----------------------
1586 Clang parses Doxygen and non-Doxygen style documentation comments and attaches
1587 them to the appropriate declaration nodes. By default, it only parses
1588 Doxygen-style comments and ignores ordinary comments starting with ``//`` and
1591 .. option:: -Wdocumentation
1593 Emit warnings about use of documentation comments. This warning group is off
1596 This includes checking that ``\param`` commands name parameters that actually
1597 present in the function signature, checking that ``\returns`` is used only on
1598 functions that actually return a value etc.
1600 .. option:: -Wno-documentation-unknown-command
1602 Don't warn when encountering an unknown Doxygen command.
1604 .. option:: -fparse-all-comments
1606 Parse all comments as documentation comments (including ordinary comments
1607 starting with ``//`` and ``/*``).
1609 .. option:: -fcomment-block-commands=[commands]
1611 Define custom documentation commands as block commands. This allows Clang to
1612 construct the correct AST for these custom commands, and silences warnings
1613 about unknown commands. Several commands must be separated by a comma
1614 *without trailing space*; e.g. ``-fcomment-block-commands=foo,bar`` defines
1615 custom commands ``\foo`` and ``\bar``.
1617 It is also possible to use ``-fcomment-block-commands`` several times; e.g.
1618 ``-fcomment-block-commands=foo -fcomment-block-commands=bar`` does the same
1626 The support for standard C in clang is feature-complete except for the
1627 C99 floating-point pragmas.
1629 Extensions supported by clang
1630 -----------------------------
1632 See :doc:`LanguageExtensions`.
1634 Differences between various standard modes
1635 ------------------------------------------
1637 clang supports the -std option, which changes what language mode clang
1638 uses. The supported modes for C are c89, gnu89, c94, c99, gnu99, c11,
1639 gnu11, and various aliases for those modes. If no -std option is
1640 specified, clang defaults to gnu11 mode. Many C99 and C11 features are
1641 supported in earlier modes as a conforming extension, with a warning. Use
1642 ``-pedantic-errors`` to request an error if a feature from a later standard
1643 revision is used in an earlier mode.
1645 Differences between all ``c*`` and ``gnu*`` modes:
1647 - ``c*`` modes define "``__STRICT_ANSI__``".
1648 - Target-specific defines not prefixed by underscores, like "linux",
1649 are defined in ``gnu*`` modes.
1650 - Trigraphs default to being off in ``gnu*`` modes; they can be enabled by
1651 the -trigraphs option.
1652 - The parser recognizes "asm" and "typeof" as keywords in ``gnu*`` modes;
1653 the variants "``__asm__``" and "``__typeof__``" are recognized in all
1655 - The Apple "blocks" extension is recognized by default in ``gnu*`` modes
1656 on some platforms; it can be enabled in any mode with the "-fblocks"
1658 - Arrays that are VLA's according to the standard, but which can be
1659 constant folded by the frontend are treated as fixed size arrays.
1660 This occurs for things like "int X[(1, 2)];", which is technically a
1661 VLA. ``c*`` modes are strictly compliant and treat these as VLAs.
1663 Differences between ``*89`` and ``*99`` modes:
1665 - The ``*99`` modes default to implementing "inline" as specified in C99,
1666 while the ``*89`` modes implement the GNU version. This can be
1667 overridden for individual functions with the ``__gnu_inline__``
1669 - Digraphs are not recognized in c89 mode.
1670 - The scope of names defined inside a "for", "if", "switch", "while",
1671 or "do" statement is different. (example: "``if ((struct x {int
1673 - ``__STDC_VERSION__`` is not defined in ``*89`` modes.
1674 - "inline" is not recognized as a keyword in c89 mode.
1675 - "restrict" is not recognized as a keyword in ``*89`` modes.
1676 - Commas are allowed in integer constant expressions in ``*99`` modes.
1677 - Arrays which are not lvalues are not implicitly promoted to pointers
1679 - Some warnings are different.
1681 Differences between ``*99`` and ``*11`` modes:
1683 - Warnings for use of C11 features are disabled.
1684 - ``__STDC_VERSION__`` is defined to ``201112L`` rather than ``199901L``.
1686 c94 mode is identical to c89 mode except that digraphs are enabled in
1687 c94 mode (FIXME: And ``__STDC_VERSION__`` should be defined!).
1689 GCC extensions not implemented yet
1690 ----------------------------------
1692 clang tries to be compatible with gcc as much as possible, but some gcc
1693 extensions are not implemented yet:
1695 - clang does not support #pragma weak (`bug
1696 3679 <http://llvm.org/bugs/show_bug.cgi?id=3679>`_). Due to the uses
1697 described in the bug, this is likely to be implemented at some point,
1699 - clang does not support decimal floating point types (``_Decimal32`` and
1700 friends) or fixed-point types (``_Fract`` and friends); nobody has
1701 expressed interest in these features yet, so it's hard to say when
1702 they will be implemented.
1703 - clang does not support nested functions; this is a complex feature
1704 which is infrequently used, so it is unlikely to be implemented
1705 anytime soon. In C++11 it can be emulated by assigning lambda
1706 functions to local variables, e.g:
1710 auto const local_function = [&](int parameter) {
1716 - clang does not support global register variables; this is unlikely to
1717 be implemented soon because it requires additional LLVM backend
1719 - clang does not support static initialization of flexible array
1720 members. This appears to be a rarely used extension, but could be
1721 implemented pending user demand.
1722 - clang does not support
1723 ``__builtin_va_arg_pack``/``__builtin_va_arg_pack_len``. This is
1724 used rarely, but in some potentially interesting places, like the
1725 glibc headers, so it may be implemented pending user demand. Note
1726 that because clang pretends to be like GCC 4.2, and this extension
1727 was introduced in 4.3, the glibc headers will not try to use this
1728 extension with clang at the moment.
1729 - clang does not support the gcc extension for forward-declaring
1730 function parameters; this has not shown up in any real-world code
1731 yet, though, so it might never be implemented.
1733 This is not a complete list; if you find an unsupported extension
1734 missing from this list, please send an e-mail to cfe-dev. This list
1735 currently excludes C++; see :ref:`C++ Language Features <cxx>`. Also, this
1736 list does not include bugs in mostly-implemented features; please see
1738 tracker <http://llvm.org/bugs/buglist.cgi?quicksearch=product%3Aclang+component%3A-New%2BBugs%2CAST%2CBasic%2CDriver%2CHeaders%2CLLVM%2BCodeGen%2Cparser%2Cpreprocessor%2CSemantic%2BAnalyzer>`_
1739 for known existing bugs (FIXME: Is there a section for bug-reporting
1740 guidelines somewhere?).
1742 Intentionally unsupported GCC extensions
1743 ----------------------------------------
1745 - clang does not support the gcc extension that allows variable-length
1746 arrays in structures. This is for a few reasons: one, it is tricky to
1747 implement, two, the extension is completely undocumented, and three,
1748 the extension appears to be rarely used. Note that clang *does*
1749 support flexible array members (arrays with a zero or unspecified
1750 size at the end of a structure).
1751 - clang does not have an equivalent to gcc's "fold"; this means that
1752 clang doesn't accept some constructs gcc might accept in contexts
1753 where a constant expression is required, like "x-x" where x is a
1755 - clang does not support ``__builtin_apply`` and friends; this extension
1756 is extremely obscure and difficult to implement reliably.
1760 Microsoft extensions
1761 --------------------
1763 clang has some experimental support for extensions from Microsoft Visual
1764 C++; to enable it, use the ``-fms-extensions`` command-line option. This is
1765 the default for Windows targets. Note that the support is incomplete.
1766 Some constructs such as ``dllexport`` on classes are ignored with a warning,
1767 and others such as `Microsoft IDL annotations
1768 <http://msdn.microsoft.com/en-us/library/8tesw2eh.aspx>`_ are silently
1771 clang has a ``-fms-compatibility`` flag that makes clang accept enough
1772 invalid C++ to be able to parse most Microsoft headers. For example, it
1773 allows `unqualified lookup of dependent base class members
1774 <http://clang.llvm.org/compatibility.html#dep_lookup_bases>`_, which is
1775 a common compatibility issue with clang. This flag is enabled by default
1776 for Windows targets.
1778 ``-fdelayed-template-parsing`` lets clang delay parsing of function template
1779 definitions until the end of a translation unit. This flag is enabled by
1780 default for Windows targets.
1782 - clang allows setting ``_MSC_VER`` with ``-fmsc-version=``. It defaults to
1783 1700 which is the same as Visual C/C++ 2012. Any number is supported
1784 and can greatly affect what Windows SDK and c++stdlib headers clang
1786 - clang does not support the Microsoft extension where anonymous record
1787 members can be declared using user defined typedefs.
1788 - clang supports the Microsoft ``#pragma pack`` feature for controlling
1789 record layout. GCC also contains support for this feature, however
1790 where MSVC and GCC are incompatible clang follows the MSVC
1792 - clang supports the Microsoft ``#pragma comment(lib, "foo.lib")`` feature for
1793 automatically linking against the specified library. Currently this feature
1794 only works with the Visual C++ linker.
1795 - clang supports the Microsoft ``#pragma comment(linker, "/flag:foo")`` feature
1796 for adding linker flags to COFF object files. The user is responsible for
1797 ensuring that the linker understands the flags.
1798 - clang defaults to C++11 for Windows targets.
1802 C++ Language Features
1803 =====================
1805 clang fully implements all of standard C++98 except for exported
1806 templates (which were removed in C++11), and all of standard C++11
1807 and the current draft standard for C++1y.
1809 Controlling implementation limits
1810 ---------------------------------
1812 .. option:: -fbracket-depth=N
1814 Sets the limit for nested parentheses, brackets, and braces to N. The
1817 .. option:: -fconstexpr-depth=N
1819 Sets the limit for recursive constexpr function invocations to N. The
1822 .. option:: -ftemplate-depth=N
1824 Sets the limit for recursively nested template instantiations to N. The
1827 .. option:: -foperator-arrow-depth=N
1829 Sets the limit for iterative calls to 'operator->' functions to N. The
1834 Objective-C Language Features
1835 =============================
1839 Objective-C++ Language Features
1840 ===============================
1843 .. _target_features:
1845 Target-Specific Features and Limitations
1846 ========================================
1848 CPU Architectures Features and Limitations
1849 ------------------------------------------
1854 The support for X86 (both 32-bit and 64-bit) is considered stable on
1855 Darwin (Mac OS X), Linux, FreeBSD, and Dragonfly BSD: it has been tested
1856 to correctly compile many large C, C++, Objective-C, and Objective-C++
1859 On ``x86_64-mingw32``, passing i128(by value) is incompatible with the
1860 Microsoft x64 calling convention. You might need to tweak
1861 ``WinX86_64ABIInfo::classify()`` in lib/CodeGen/TargetInfo.cpp.
1863 For the X86 target, clang supports the :option:`-m16` command line
1864 argument which enables 16-bit code output. This is broadly similar to
1865 using ``asm(".code16gcc")`` with the GNU toolchain. The generated code
1866 and the ABI remains 32-bit but the assembler emits instructions
1867 appropriate for a CPU running in 16-bit mode, with address-size and
1868 operand-size prefixes to enable 32-bit addressing and operations.
1873 The support for ARM (specifically ARMv6 and ARMv7) is considered stable
1874 on Darwin (iOS): it has been tested to correctly compile many large C,
1875 C++, Objective-C, and Objective-C++ codebases. Clang only supports a
1876 limited number of ARM architectures. It does not yet fully support
1882 The support for PowerPC (especially PowerPC64) is considered stable
1883 on Linux and FreeBSD: it has been tested to correctly compile many
1884 large C and C++ codebases. PowerPC (32bit) is still missing certain
1885 features (e.g. PIC code on ELF platforms).
1890 clang currently contains some support for other architectures (e.g. Sparc);
1891 however, significant pieces of code generation are still missing, and they
1892 haven't undergone significant testing.
1894 clang contains limited support for the MSP430 embedded processor, but
1895 both the clang support and the LLVM backend support are highly
1898 Other platforms are completely unsupported at the moment. Adding the
1899 minimal support needed for parsing and semantic analysis on a new
1900 platform is quite easy; see ``lib/Basic/Targets.cpp`` in the clang source
1901 tree. This level of support is also sufficient for conversion to LLVM IR
1902 for simple programs. Proper support for conversion to LLVM IR requires
1903 adding code to ``lib/CodeGen/CGCall.cpp`` at the moment; this is likely to
1904 change soon, though. Generating assembly requires a suitable LLVM
1907 Operating System Features and Limitations
1908 -----------------------------------------
1913 Thread Sanitizer is not supported.
1918 Clang has experimental support for targeting "Cygming" (Cygwin / MinGW)
1921 See also :ref:`Microsoft Extensions <c_ms>`.
1926 Clang works on Cygwin-1.7.
1931 Clang works on some mingw32 distributions. Clang assumes directories as
1934 - ``C:/mingw/include``
1936 - ``C:/mingw/lib/gcc/mingw32/4.[3-5].0/include/c++``
1938 On MSYS, a few tests might fail.
1943 For 32-bit (i686-w64-mingw32), and 64-bit (x86\_64-w64-mingw32), Clang
1946 - ``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)``
1947 - ``some_directory/bin/gcc.exe``
1948 - ``some_directory/bin/clang.exe``
1949 - ``some_directory/bin/clang++.exe``
1950 - ``some_directory/bin/../include/c++/GCC_version``
1951 - ``some_directory/bin/../include/c++/GCC_version/x86_64-w64-mingw32``
1952 - ``some_directory/bin/../include/c++/GCC_version/i686-w64-mingw32``
1953 - ``some_directory/bin/../include/c++/GCC_version/backward``
1954 - ``some_directory/bin/../x86_64-w64-mingw32/include``
1955 - ``some_directory/bin/../i686-w64-mingw32/include``
1956 - ``some_directory/bin/../include``
1958 This directory layout is standard for any toolchain you will find on the
1959 official `MinGW-w64 website <http://mingw-w64.sourceforge.net>`_.
1961 Clang expects the GCC executable "gcc.exe" compiled for
1962 ``i686-w64-mingw32`` (or ``x86_64-w64-mingw32``) to be present on PATH.
1964 `Some tests might fail <http://llvm.org/bugs/show_bug.cgi?id=9072>`_ on
1965 ``x86_64-w64-mingw32``.
1972 clang-cl is an alternative command-line interface to Clang driver, designed for
1973 compatibility with the Visual C++ compiler, cl.exe.
1975 To enable clang-cl to find system headers, libraries, and the linker when run
1976 from the command-line, it should be executed inside a Visual Studio Native Tools
1977 Command Prompt or a regular Command Prompt where the environment has been set
1978 up using e.g. `vcvars32.bat <http://msdn.microsoft.com/en-us/library/f2ccy3wt.aspx>`_.
1980 clang-cl can also be used from inside Visual Studio by using an LLVM Platform
1983 Command-Line Options
1984 --------------------
1986 To be compatible with cl.exe, clang-cl supports most of the same command-line
1987 options. Those options can start with either ``/`` or ``-``. It also supports
1988 some of Clang's core options, such as the ``-W`` options.
1990 Options that are known to clang-cl, but not currently supported, are ignored
1991 with a warning. For example:
1995 clang-cl.exe: warning: argument unused during compilation: '/Zi'
1997 To suppress warnings about unused arguments, use the ``-Qunused-arguments`` option.
1999 Options that are not known to clang-cl will cause errors. If they are spelled with a
2000 leading ``/``, they will be mistaken for a filename:
2004 clang-cl.exe: error: no such file or directory: '/foobar'
2006 Please `file a bug <http://llvm.org/bugs/enter_bug.cgi?product=clang&component=Driver>`_
2007 for any valid cl.exe flags that clang-cl does not understand.
2009 Execute ``clang-cl /?`` to see a list of supported options:
2013 CL.EXE COMPATIBILITY OPTIONS:
2014 /? Display available options
2015 /arch:<value> Set architecture for code generation
2016 /C Don't discard comments when preprocessing
2018 /D <macro[=value]> Define macro
2019 /EH<value> Exception handling model
2020 /EP Disable linemarker output and preprocess to stdout
2021 /E Preprocess to stdout
2022 /fallback Fall back to cl.exe if clang-cl fails to compile
2023 /FA Output assembly code file during compilation
2024 /Fa<file or directory> Output assembly code to this file during compilation
2025 /Fe<file or directory> Set output executable file or directory (ends in / or \)
2026 /FI <value> Include file before parsing
2027 /Fi<file> Set preprocess output file name
2028 /Fo<file or directory> Set output object file, or directory (ends in / or \)
2029 /GF- Disable string pooling
2030 /GR- Disable emission of RTTI data
2031 /GR Enable emission of RTTI data
2032 /Gw- Don't put each data item in its own section
2033 /Gw Put each data item in its own section
2034 /Gy- Don't put each function in its own section
2035 /Gy Put each function in its own section
2036 /help Display available options
2037 /I <dir> Add directory to include search path
2038 /J Make char type unsigned
2039 /LDd Create debug DLL
2041 /link <options> Forward options to the linker
2042 /MDd Use DLL debug run-time
2043 /MD Use DLL run-time
2044 /MTd Use static debug run-time
2045 /MT Use static run-time
2046 /Ob0 Disable inlining
2047 /Od Disable optimization
2048 /Oi- Disable use of builtin functions
2049 /Oi Enable use of builtin functions
2050 /Os Optimize for size
2051 /Ot Optimize for speed
2052 /Ox Maximum optimization
2053 /Oy- Disable frame pointer omission
2054 /Oy Enable frame pointer omission
2055 /O<n> Optimization level
2056 /P Preprocess to file
2057 /showIncludes Print info about included files to stderr
2058 /TC Treat all source files as C
2059 /Tc <filename> Specify a C source file
2060 /TP Treat all source files as C++
2061 /Tp <filename> Specify a C++ source file
2062 /U <macro> Undefine macro
2063 /vd<value> Control vtordisp placement
2064 /vmb Use a best-case representation method for member pointers
2065 /vmg Use a most-general representation for member pointers
2066 /vmm Set the default most-general representation to multiple inheritance
2067 /vms Set the default most-general representation to single inheritance
2068 /vmv Set the default most-general representation to virtual inheritance
2069 /W0 Disable all warnings
2075 /WX- Do not treat warnings as errors
2076 /WX Treat warnings as errors
2077 /w Disable all warnings
2078 /Zi Enable debug information
2079 /Zp Set the default maximum struct packing alignment to 1
2080 /Zp<value> Specify the default maximum struct packing alignment
2081 /Zs Syntax-check only
2084 -### Print (but do not run) the commands to run for this compilation
2085 -fms-compatibility-version=<value>
2086 Dot-separated value representing the Microsoft compiler version
2087 number to report in _MSC_VER (0 = don't define it (default))
2088 -fmsc-version=<value> Microsoft compiler version number to report in _MSC_VER (0 = don't
2089 define it (default))
2090 -fsanitize-blacklist=<value>
2091 Path to blacklist file for sanitizers
2092 -fsanitize=<check> Enable runtime instrumentation for bug detection: address (memory
2093 errors) | thread (race detection) | undefined (miscellaneous
2095 -mllvm <value> Additional arguments to forward to LLVM's option processing
2096 -Qunused-arguments Don't emit warning for unused driver arguments
2097 --target=<value> Generate code for the given target
2098 -v Show commands to run and use verbose output
2099 -W<warning> Enable the specified warning
2100 -Xclang <arg> Pass <arg> to the clang compiler
2102 The /fallback Option
2103 ^^^^^^^^^^^^^^^^^^^^
2105 When clang-cl is run with the ``/fallback`` option, it will first try to
2106 compile files itself. For any file that it fails to compile, it will fall back
2107 and try to compile the file by invoking cl.exe.
2109 This option is intended to be used as a temporary means to build projects where
2110 clang-cl cannot successfully compile all the files. clang-cl may fail to compile
2111 a file either because it cannot generate code for some C++ feature, or because
2112 it cannot parse some Microsoft language extension.