docs/UsersManual.rst - third_party/swift-clang - Git at Google

 ============================
 Clang Compiler User's Manual
 ============================

 .. include:: <isonum.txt>

 .. contents::
    :local:

 Introduction
 ============

 The Clang Compiler is an open-source compiler for the C family of
 programming languages, aiming to be the best in class implementation of
 these languages. Clang builds on the LLVM optimizer and code generator,
 allowing it to provide high-quality optimization and code generation
 support for many targets. For more general information, please see the
 `Clang Web Site <http://clang.llvm.org>`_ or the `LLVM Web
 Site <http://llvm.org>`_.

 This document describes important notes about using Clang as a compiler
 for an end-user, documenting the supported features, command line
 options, etc. If you are interested in using Clang to build a tool that
 processes code, please see :doc:`InternalsManual`. If you are interested in the
 `Clang Static Analyzer <http://clang-analyzer.llvm.org>`_, please see its web
 page.

 Clang is one component in a complete toolchain for C family languages.
 A separate document describes the other pieces necessary to
 :doc:`assemble a complete toolchain <Toolchain>`.

 Clang is designed to support the C family of programming languages,
 which includes :ref:`C <c>`, :ref:`Objective-C <objc>`, :ref:`C++ <cxx>`, and
 :ref:`Objective-C++ <objcxx>` as well as many dialects of those. For
 language-specific information, please see the corresponding language
 specific section:

 -  :ref:`C Language <c>`: K&R C, ANSI C89, ISO C90, ISO C94 (C89+AMD1), ISO
    C99 (+TC1, TC2, TC3).
 -  :ref:`Objective-C Language <objc>`: ObjC 1, ObjC 2, ObjC 2.1, plus
    variants depending on base language.
 -  :ref:`C++ Language <cxx>`
 -  :ref:`Objective C++ Language <objcxx>`
 -  :ref:`OpenCL C Language <opencl>`: v1.0, v1.1, v1.2, v2.0.

 In addition to these base languages and their dialects, Clang supports a
 broad variety of language extensions, which are documented in the
 corresponding language section. These extensions are provided to be
 compatible with the GCC, Microsoft, and other popular compilers as well
 as to improve functionality through Clang-specific features. The Clang
 driver and language features are intentionally designed to be as
 compatible with the GNU GCC compiler as reasonably possible, easing
 migration from GCC to Clang. In most cases, code "just works".
 Clang also provides an alternative driver, :ref:`clang-cl`, that is designed
 to be compatible with the Visual C++ compiler, cl.exe.

 In addition to language specific features, Clang has a variety of
 features that depend on what CPU architecture or operating system is
 being compiled for. Please see the :ref:`Target-Specific Features and
 Limitations <target_features>` section for more details.

 The rest of the introduction introduces some basic :ref:`compiler
 terminology <terminology>` that is used throughout this manual and
 contains a basic :ref:`introduction to using Clang <basicusage>` as a
 command line compiler.

 .. _terminology:

 Terminology
 -----------

 Front end, parser, backend, preprocessor, undefined behavior,
 diagnostic, optimizer

 .. _basicusage:

 Basic Usage
 -----------

 Intro to how to use a C compiler for newbies.

 compile + link compile then link debug info enabling optimizations
 picking a language to use, defaults to C11 by default. Autosenses based
 on extension. using a makefile

 Command Line Options
 ====================

 This section is generally an index into other sections. It does not go
 into depth on the ones that are covered by other sections. However, the
 first part introduces the language selection and other high level
 options like :option:`-c`, :option:`-g`, etc.

 Options to Control Error and Warning Messages
 ---------------------------------------------

 .. option:: -Werror

   Turn warnings into errors.

 .. This is in plain monospaced font because it generates the same label as
 .. -Werror, and Sphinx complains.

 ``-Werror=foo``

   Turn warning "foo" into an error.

 .. option:: -Wno-error=foo

   Turn warning "foo" into an warning even if :option:`-Werror` is specified.

 .. option:: -Wfoo

   Enable warning "foo".
   See the :doc:`diagnostics reference <DiagnosticsReference>` for a complete
   list of the warning flags that can be specified in this way.

 .. option:: -Wno-foo

   Disable warning "foo".

 .. option:: -w

   Disable all diagnostics.

 .. option:: -Weverything

   :ref:`Enable all diagnostics. <diagnostics_enable_everything>`

 .. option:: -pedantic

   Warn on language extensions.

 .. option:: -pedantic-errors

   Error on language extensions.

 .. option:: -Wsystem-headers

   Enable warnings from system headers.

 .. option:: -ferror-limit=123

   Stop emitting diagnostics after 123 errors have been produced. The default is
   20, and the error limit can be disabled with `-ferror-limit=0`.

 .. option:: -ftemplate-backtrace-limit=123

   Only emit up to 123 template instantiation notes within the template
   instantiation backtrace for a single warning or error. The default is 10, and
   the limit can be disabled with `-ftemplate-backtrace-limit=0`.

 .. _cl_diag_formatting:

 Formatting of Diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^

 Clang aims to produce beautiful diagnostics by default, particularly for
 new users that first come to Clang. However, different people have
 different preferences, and sometimes Clang is driven not by a human,
 but by a program that wants consistent and easily parsable output. For
 these cases, Clang provides a wide range of options to control the exact
 output format of the diagnostics that it generates.

 .. _opt_fshow-column:

 **-f[no-]show-column**
    Print column number in diagnostic.

    This option, which defaults to on, controls whether or not Clang
    prints the column number of a diagnostic. For example, when this is
    enabled, Clang will print something like:

    ::

          test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
          #endif bad
                 ^
                 //

    When this is disabled, Clang will print "test.c:28: warning..." with
    no column number.

    The printed column numbers count bytes from the beginning of the
    line; take care if your source contains multibyte characters.

 .. _opt_fshow-source-location:

 **-f[no-]show-source-location**
    Print source file/line/column information in diagnostic.

    This option, which defaults to on, controls whether or not Clang
    prints the filename, line number and column number of a diagnostic.
    For example, when this is enabled, Clang will print something like:

    ::

          test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
          #endif bad
                 ^
                 //

    When this is disabled, Clang will not print the "test.c:28:8: "
    part.

 .. _opt_fcaret-diagnostics:

 **-f[no-]caret-diagnostics**
    Print source line and ranges from source code in diagnostic.
    This option, which defaults to on, controls whether or not Clang
    prints the source line, source ranges, and caret when emitting a
    diagnostic. For example, when this is enabled, Clang will print
    something like:

    ::

          test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
          #endif bad
                 ^
                 //

 **-f[no-]color-diagnostics**
    This option, which defaults to on when a color-capable terminal is
    detected, controls whether or not Clang prints diagnostics in color.

    When this option is enabled, Clang will use colors to highlight
    specific parts of the diagnostic, e.g.,

    .. nasty hack to not lose our dignity

    .. raw:: html

        <pre>
          <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>
          #endif bad
                 <span style="color:green">^</span>
                 <span style="color:green">//</span>
        </pre>

    When this is disabled, Clang will just print:

    ::

          test.c:2:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
          #endif bad
                 ^
                 //

 **-fansi-escape-codes**
    Controls whether ANSI escape codes are used instead of the Windows Console
    API to output colored diagnostics. This option is only used on Windows and
    defaults to off.

 .. option:: -fdiagnostics-format=clang/msvc/vi

    Changes diagnostic output format to better match IDEs and command line tools.

    This option controls the output format of the filename, line number,
    and column printed in diagnostic messages. The options, and their
    affect on formatting a simple conversion diagnostic, follow:

    **clang** (default)
        ::

            t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int'

    **msvc**
        ::

            t.c(3,11) : warning: conversion specifies type 'char *' but the argument has type 'int'

    **vi**
        ::

            t.c +3:11: warning: conversion specifies type 'char *' but the argument has type 'int'

 .. _opt_fdiagnostics-show-option:

 **-f[no-]diagnostics-show-option**
    Enable ``[-Woption]`` information in diagnostic line.

    This option, which defaults to on, controls whether or not Clang
    prints the associated :ref:`warning group <cl_diag_warning_groups>`
    option name when outputting a warning diagnostic. For example, in
    this output:

    ::

          test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
          #endif bad
                 ^
                 //

    Passing **-fno-diagnostics-show-option** will prevent Clang from
    printing the [:ref:`-Wextra-tokens <opt_Wextra-tokens>`] information in
    the diagnostic. This information tells you the flag needed to enable
    or disable the diagnostic, either from the command line or through
    :ref:`#pragma GCC diagnostic <pragma_GCC_diagnostic>`.

 .. _opt_fdiagnostics-show-category:

 .. option:: -fdiagnostics-show-category=none/id/name

    Enable printing category information in diagnostic line.

    This option, which defaults to "none", controls whether or not Clang
    prints the category associated with a diagnostic when emitting it.
    Each diagnostic may or many not have an associated category, if it
    has one, it is listed in the diagnostic categorization field of the
    diagnostic line (in the []'s).

    For example, a format string warning will produce these three
    renditions based on the setting of this option:

    ::

          t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int' [-Wformat]
          t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int' [-Wformat,1]
          t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int' [-Wformat,Format String]

    This category can be used by clients that want to group diagnostics
    by category, so it should be a high level category. We want dozens
    of these, not hundreds or thousands of them.

 .. _opt_fsave-optimization-record:

 **-fsave-optimization-record**
    Write optimization remarks to a YAML file.

    This option, which defaults to off, controls whether Clang writes
    optimization reports to a YAML file. By recording diagnostics in a file,
    using a structured YAML format, users can parse or sort the remarks in a
    convenient way.

 .. _opt_foptimization-record-file:

 **-foptimization-record-file**
    Control the file to which optimization reports are written.

    When optimization reports are being output (see
    :ref:`-fsave-optimization-record <opt_fsave-optimization-record>`), this
    option controls the file to which those reports are written.

    If this option is not used, optimization records are output to a file named
    after the primary file being compiled. If that's "foo.c", for example,
    optimization records are output to "foo.opt.yaml".

 .. _opt_fdiagnostics-show-hotness:

 **-f[no-]diagnostics-show-hotness**
    Enable profile hotness information in diagnostic line.

    This option controls whether Clang prints the profile hotness associated
    with diagnostics in the presence of profile-guided optimization information.
    This is currently supported with optimization remarks (see
    :ref:`Options to Emit Optimization Reports <rpass>`). The hotness information
    allows users to focus on the hot optimization remarks that are likely to be
    more relevant for run-time performance.

    For example, in this output, the block containing the callsite of `foo` was
    executed 3000 times according to the profile data:

    ::

          s.c:7:10: remark: foo inlined into bar (hotness: 3000) [-Rpass-analysis=inline]
            sum += foo(x, x - 2);
                   ^

    This option is implied when
    :ref:`-fsave-optimization-record <opt_fsave-optimization-record>` is used.
    Otherwise, it defaults to off.

 .. _opt_fdiagnostics-hotness-threshold:

 **-fdiagnostics-hotness-threshold**
    Prevent optimization remarks from being output if they do not have at least
    this hotness value.

    This option, which defaults to zero, controls the minimum hotness an
    optimization remark would need in order to be output by Clang. This is
    currently supported with optimization remarks (see :ref:`Options to Emit
    Optimization Reports <rpass>`) when profile hotness information in
    diagnostics is enabled (see
    :ref:`-fdiagnostics-show-hotness <opt_fdiagnostics-show-hotness>`).

 .. _opt_fdiagnostics-fixit-info:

 **-f[no-]diagnostics-fixit-info**
    Enable "FixIt" information in the diagnostics output.

    This option, which defaults to on, controls whether or not Clang
    prints the information on how to fix a specific diagnostic
    underneath it when it knows. For example, in this output:

    ::

          test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
          #endif bad
                 ^
                 //

    Passing **-fno-diagnostics-fixit-info** will prevent Clang from
    printing the "//" line at the end of the message. This information
    is useful for users who may not understand what is wrong, but can be
    confusing for machine parsing.

 .. _opt_fdiagnostics-print-source-range-info:

 **-fdiagnostics-print-source-range-info**
    Print machine parsable information about source ranges.
    This option makes Clang print information about source ranges in a machine
    parsable format after the file/line/column number information. The
    information is a simple sequence of brace enclosed ranges, where each range
    lists the start and end line/column locations. For example, in this output:

    ::

        exprs.c:47:15:{47:8-47:14}{47:17-47:24}: error: invalid operands to binary expression ('int *' and '_Complex float')
           P = (P-42) + Gamma*4;
               ~~~~~~ ^ ~~~~~~~

    The {}'s are generated by -fdiagnostics-print-source-range-info.

    The printed column numbers count bytes from the beginning of the
    line; take care if your source contains multibyte characters.

 .. option:: -fdiagnostics-parseable-fixits

    Print Fix-Its in a machine parseable form.

    This option makes Clang print available Fix-Its in a machine
    parseable format at the end of diagnostics. The following example
    illustrates the format:

    ::

         fix-it:"t.cpp":{7:25-7:29}:"Gamma"

    The range printed is a half-open range, so in this example the
    characters at column 25 up to but not including column 29 on line 7
    in t.cpp should be replaced with the string "Gamma". Either the
    range or the replacement string may be empty (representing strict
    insertions and strict erasures, respectively). Both the file name
    and the insertion string escape backslash (as "\\\\"), tabs (as
    "\\t"), newlines (as "\\n"), double quotes(as "\\"") and
    non-printable characters (as octal "\\xxx").

    The printed column numbers count bytes from the beginning of the
    line; take care if your source contains multibyte characters.

 .. option:: -fno-elide-type

    Turns off elision in template type printing.

    The default for template type printing is to elide as many template
    arguments as possible, removing those which are the same in both
    template types, leaving only the differences. Adding this flag will
    print all the template arguments. If supported by the terminal,
    highlighting will still appear on differing arguments.

    Default:

    ::

        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;

    -fno-elide-type:

    ::

        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;

 .. option:: -fdiagnostics-show-template-tree

    Template type diffing prints a text tree.

    For diffing large templated types, this option will cause Clang to
    display the templates as an indented text tree, one argument per
    line, with differences marked inline. This is compatible with
    -fno-elide-type.

    Default:

    ::

        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;

    With :option:`-fdiagnostics-show-template-tree`:

    ::

        t.cc:4:5: note: candidate function not viable: no known conversion for 1st argument;
          vector<
            map<
              [...],
              map<
                [float != double],
                [...]>>>

 .. _cl_diag_warning_groups:

 Individual Warning Groups
 ^^^^^^^^^^^^^^^^^^^^^^^^^

 TODO: Generate this from tblgen. Define one anchor per warning group.

 .. _opt_wextra-tokens:

 .. option:: -Wextra-tokens

    Warn about excess tokens at the end of a preprocessor directive.

    This option, which defaults to on, enables warnings about extra
    tokens at the end of preprocessor directives. For example:

    ::

          test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
          #endif bad
                 ^

    These extra tokens are not strictly conforming, and are usually best
    handled by commenting them out.

 .. option:: -Wambiguous-member-template

    Warn about unqualified uses of a member template whose name resolves to
    another template at the location of the use.

    This option, which defaults to on, enables a warning in the
    following code:

    ::

        template<typename T> struct set{};
        template<typename T> struct trait { typedef const T& type; };
        struct Value {
          template<typename T> void set(typename trait<T>::type value) {}
        };
        void foo() {
          Value v;
          v.set<double>(3.2);
        }

    C++ [basic.lookup.classref] requires this to be an error, but,
    because it's hard to work around, Clang downgrades it to a warning
    as an extension.

 .. option:: -Wbind-to-temporary-copy

    Warn about an unusable copy constructor when binding a reference to a
    temporary.

    This option enables warnings about binding a
    reference to a temporary when the temporary doesn't have a usable
    copy constructor. For example:

    ::

          struct NonCopyable {
            NonCopyable();
          private:
            NonCopyable(const NonCopyable&);
          };
          void foo(const NonCopyable&);
          void bar() {
            foo(NonCopyable());  // Disallowed in C++98; allowed in C++11.
          }

    ::

          struct NonCopyable2 {
            NonCopyable2();
            NonCopyable2(NonCopyable2&);
          };
          void foo(const NonCopyable2&);
          void bar() {
            foo(NonCopyable2());  // Disallowed in C++98; allowed in C++11.
          }

    Note that if ``NonCopyable2::NonCopyable2()`` has a default argument
    whose instantiation produces a compile error, that error will still
    be a hard error in C++98 mode even if this warning is turned off.

 Options to Control Clang Crash Diagnostics
 ------------------------------------------

 As unbelievable as it may sound, Clang does crash from time to time.
 Generally, this only occurs to those living on the `bleeding
 edge <http://llvm.org/releases/download.html#svn>`_. Clang goes to great
 lengths to assist you in filing a bug report. Specifically, Clang
 generates preprocessed source file(s) and associated run script(s) upon
 a crash. These files should be attached to a bug report to ease
 reproducibility of the failure. Below are the command line options to
 control the crash diagnostics.

 .. option:: -fno-crash-diagnostics

   Disable auto-generation of preprocessed source files during a clang crash.

 The -fno-crash-diagnostics flag can be helpful for speeding the process
 of generating a delta reduced test case.

 Clang is also capable of generating preprocessed source file(s) and associated
 run script(s) even without a crash. This is specially useful when trying to
 generate a reproducer for warnings or errors while using modules.

 .. option:: -gen-reproducer

   Generates preprocessed source files, a reproducer script and if relevant, a
   cache containing: built module pcm's and all headers needed to rebuilt the
   same modules.

 .. _rpass:

 Options to Emit Optimization Reports
 ------------------------------------

 Optimization reports trace, at a high-level, all the major decisions
 done by compiler transformations. For instance, when the inliner
 decides to inline function ``foo()`` into ``bar()``, or the loop unroller
 decides to unroll a loop N times, or the vectorizer decides to
 vectorize a loop body.

 Clang offers a family of flags which the optimizers can use to emit
 a diagnostic in three cases:

 1. When the pass makes a transformation (`-Rpass`).

 2. When the pass fails to make a transformation (`-Rpass-missed`).

 3. When the pass determines whether or not to make a transformation
    (`-Rpass-analysis`).

 NOTE: Although the discussion below focuses on `-Rpass`, the exact
 same options apply to `-Rpass-missed` and `-Rpass-analysis`.

 Since there are dozens of passes inside the compiler, each of these flags
 take a regular expression that identifies the name of the pass which should
 emit the associated diagnostic. For example, to get a report from the inliner,
 compile the code with:

 .. code-block:: console

    $ clang -O2 -Rpass=inline code.cc -o code
    code.cc:4:25: remark: foo inlined into bar [-Rpass=inline]
    int bar(int j) { return foo(j, j - 2); }
                            ^

 Note that remarks from the inliner are identified with `[-Rpass=inline]`.
 To request a report from every optimization pass, you should use
 `-Rpass=.*` (in fact, you can use any valid POSIX regular
 expression). However, do not expect a report from every transformation
 made by the compiler. Optimization remarks do not really make sense
 outside of the major transformations (e.g., inlining, vectorization,
 loop optimizations) and not every optimization pass supports this
 feature.

 Note that when using profile-guided optimization information, profile hotness
 information can be included in the remarks (see
 :ref:`-fdiagnostics-show-hotness <opt_fdiagnostics-show-hotness>`).

 Current limitations
 ^^^^^^^^^^^^^^^^^^^

 1. Optimization remarks that refer to function names will display the
    mangled name of the function. Since these remarks are emitted by the
    back end of the compiler, it does not know anything about the input
    language, nor its mangling rules.

 2. Some source locations are not displayed correctly. The front end has
    a more detailed source location tracking than the locations included
    in the debug info (e.g., the front end can locate code inside macro
    expansions). However, the locations used by `-Rpass` are
    translated from debug annotations. That translation can be lossy,
    which results in some remarks having no location information.

 Other Options
 -------------
 Clang options that that don't fit neatly into other categories.

 .. option:: -MV

   When emitting a dependency file, use formatting conventions appropriate
   for NMake or Jom. Ignored unless another option causes Clang to emit a
   dependency file.

 When Clang emits a dependency file (e.g., you supplied the -M option)
 most filenames can be written to the file without any special formatting.
 Different Make tools will treat different sets of characters as "special"
 and use different conventions for telling the Make tool that the character
 is actually part of the filename. Normally Clang uses backslash to "escape"
 a special character, which is the convention used by GNU Make. The -MV
 option tells Clang to put double-quotes around the entire filename, which
 is the convention used by NMake and Jom.


 Language and Target-Independent Features
 ========================================

 Controlling Errors and Warnings
 -------------------------------

 Clang provides a number of ways to control which code constructs cause
 it to emit errors and warning messages, and how they are displayed to
 the console.

 Controlling How Clang Displays Diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

 When Clang emits a diagnostic, it includes rich information in the
 output, and gives you fine-grain control over which information is
 printed. Clang has the ability to print this information, and these are
 the options that control it:

 #. A file/line/column indicator that shows exactly where the diagnostic
    occurs in your code [:ref:`-fshow-column <opt_fshow-column>`,
    :ref:`-fshow-source-location <opt_fshow-source-location>`].
 #. A categorization of the diagnostic as a note, warning, error, or
    fatal error.
 #. A text string that describes what the problem is.
 #. An option that indicates how to control the diagnostic (for
    diagnostics that support it)
    [:ref:`-fdiagnostics-show-option <opt_fdiagnostics-show-option>`].
 #. A :ref:`high-level category <diagnostics_categories>` for the diagnostic
    for clients that want to group diagnostics by class (for diagnostics
    that support it)
    [:ref:`-fdiagnostics-show-category <opt_fdiagnostics-show-category>`].
 #. The line of source code that the issue occurs on, along with a caret
    and ranges that indicate the important locations
    [:ref:`-fcaret-diagnostics <opt_fcaret-diagnostics>`].
 #. "FixIt" information, which is a concise explanation of how to fix the
    problem (when Clang is certain it knows)
    [:ref:`-fdiagnostics-fixit-info <opt_fdiagnostics-fixit-info>`].
 #. A machine-parsable representation of the ranges involved (off by
    default)
    [:ref:`-fdiagnostics-print-source-range-info <opt_fdiagnostics-print-source-range-info>`].

 For more information please see :ref:`Formatting of
 Diagnostics <cl_diag_formatting>`.

 Diagnostic Mappings
 ^^^^^^^^^^^^^^^^^^^

 All diagnostics are mapped into one of these 6 classes:

 -  Ignored
 -  Note
 -  Remark
 -  Warning
 -  Error
 -  Fatal

 .. _diagnostics_categories:

 Diagnostic Categories
 ^^^^^^^^^^^^^^^^^^^^^

 Though not shown by default, diagnostics may each be associated with a
 high-level category. This category is intended to make it possible to
 triage builds that produce a large number of errors or warnings in a
 grouped way.

 Categories are not shown by default, but they can be turned on with the
 :ref:`-fdiagnostics-show-category <opt_fdiagnostics-show-category>` option.
 When set to "``name``", the category is printed textually in the
 diagnostic output. When it is set to "``id``", a category number is
 printed. The mapping of category names to category id's can be obtained
 by running '``clang   --print-diagnostic-categories``'.

 Controlling Diagnostics via Command Line Flags
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

 TODO: -W flags, -pedantic, etc

 .. _pragma_gcc_diagnostic:

 Controlling Diagnostics via Pragmas
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

 Clang can also control what diagnostics are enabled through the use of
 pragmas in the source code. This is useful for turning off specific
 warnings in a section of source code. Clang supports GCC's pragma for
 compatibility with existing source code, as well as several extensions.

 The pragma may control any warning that can be used from the command
 line. Warnings may be set to ignored, warning, error, or fatal. The
 following example code will tell Clang or GCC to ignore the -Wall
 warnings:

 .. code-block:: c

   #pragma GCC diagnostic ignored "-Wall"

 In addition to all of the functionality provided by GCC's pragma, Clang
 also allows you to push and pop the current warning state. This is
 particularly useful when writing a header file that will be compiled by
 other people, because you don't know what warning flags they build with.

 In the below example :option:`-Wextra-tokens` is ignored for only a single line
 of code, after which the diagnostics return to whatever state had previously
 existed.

 .. code-block:: c

   #if foo
   #endif foo // warning: extra tokens at end of #endif directive

   #pragma clang diagnostic push
   #pragma clang diagnostic ignored "-Wextra-tokens"

   #if foo
   #endif foo // no warning

   #pragma clang diagnostic pop

 The push and pop pragmas will save and restore the full diagnostic state
 of the compiler, regardless of how it was set. That means that it is
 possible to use push and pop around GCC compatible diagnostics and Clang
 will push and pop them appropriately, while GCC will ignore the pushes
 and pops as unknown pragmas. It should be noted that while Clang
 supports the GCC pragma, Clang and GCC do not support the exact same set
 of warnings, so even when using GCC compatible #pragmas there is no
 guarantee that they will have identical behaviour on both compilers.

 In addition to controlling warnings and errors generated by the compiler, it is
 possible to generate custom warning and error messages through the following
 pragmas:

 .. code-block:: c

   // The following will produce warning messages
   #pragma message "some diagnostic message"
   #pragma GCC warning "TODO: replace deprecated feature"

   // The following will produce an error message
   #pragma GCC error "Not supported"

 These pragmas operate similarly to the ``#warning`` and ``#error`` preprocessor
 directives, except that they may also be embedded into preprocessor macros via
 the C99 ``_Pragma`` operator, for example:

 .. code-block:: c

   #define STR(X) #X
   #define DEFER(M,...) M(__VA_ARGS__)
   #define CUSTOM_ERROR(X) _Pragma(STR(GCC error(X " at line " DEFER(STR,__LINE__))))

   CUSTOM_ERROR("Feature not available");

 Controlling Diagnostics in System Headers
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

 Warnings are suppressed when they occur in system headers. By default,
 an included file is treated as a system header if it is found in an
 include path specified by ``-isystem``, but this can be overridden in
 several ways.

 The ``system_header`` pragma can be used to mark the current file as
 being a system header. No warnings will be produced from the location of
 the pragma onwards within the same file.

 .. code-block:: c

   #if foo
   #endif foo // warning: extra tokens at end of #endif directive

   #pragma clang system_header

   #if foo
   #endif foo // no warning

 The `--system-header-prefix=` and `--no-system-header-prefix=`
 command-line arguments can be used to override whether subsets of an include
 path are treated as system headers. When the name in a ``#include`` directive
 is found within a header search path and starts with a system prefix, the
 header is treated as a system header. The last prefix on the
 command-line which matches the specified header name takes precedence.
 For instance:

 .. code-block:: console

   $ clang -Ifoo -isystem bar --system-header-prefix=x/ \
       --no-system-header-prefix=x/y/

 Here, ``#include "x/a.h"`` is treated as including a system header, even
 if the header is found in ``foo``, and ``#include "x/y/b.h"`` is treated
 as not including a system header, even if the header is found in
 ``bar``.

 A ``#include`` directive which finds a file relative to the current
 directory is treated as including a system header if the including file
 is treated as a system header.

 .. _diagnostics_enable_everything:

 Enabling All Diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

 In addition to the traditional ``-W`` flags, one can enable **all**
 diagnostics by passing :option:`-Weverything`. This works as expected
 with
 :option:`-Werror`, and also includes the warnings from :option:`-pedantic`.

 Note that when combined with :option:`-w` (which disables all warnings), that
 flag wins.

 Controlling Static Analyzer Diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

 While not strictly part of the compiler, the diagnostics from Clang's
 `static analyzer <http://clang-analyzer.llvm.org>`_ can also be
 influenced by the user via changes to the source code. See the available
 `annotations <http://clang-analyzer.llvm.org/annotations.html>`_ and the
 analyzer's `FAQ
 page <http://clang-analyzer.llvm.org/faq.html#exclude_code>`_ for more
 information.

 .. _usersmanual-precompiled-headers:

 Precompiled Headers
 -------------------

 `Precompiled headers <http://en.wikipedia.org/wiki/Precompiled_header>`__
 are a general approach employed by many compilers to reduce compilation
 time. The underlying motivation of the approach is that it is common for
 the same (and often large) header files to be included by multiple
 source files. Consequently, compile times can often be greatly improved
 by caching some of the (redundant) work done by a compiler to process
 headers. Precompiled header files, which represent one of many ways to
 implement this optimization, are literally files that represent an
 on-disk cache that contains the vital information necessary to reduce
 some of the work needed to process a corresponding header file. While
 details of precompiled headers vary between compilers, precompiled
 headers have been shown to be highly effective at speeding up program
 compilation on systems with very large system headers (e.g., Mac OS X).

 Generating a PCH File
 ^^^^^^^^^^^^^^^^^^^^^

 To generate a PCH file using Clang, one invokes Clang with the
 `-x <language>-header` option. This mirrors the interface in GCC
 for generating PCH files:

 .. code-block:: console

   $ gcc -x c-header test.h -o test.h.gch
   $ clang -x c-header test.h -o test.h.pch

 Using a PCH File
 ^^^^^^^^^^^^^^^^

 A PCH file can then be used as a prefix header when a :option:`-include`
 option is passed to ``clang``:

 .. code-block:: console

   $ clang -include test.h test.c -o test

 The ``clang`` driver will first check if a PCH file for ``test.h`` is
 available; if so, the contents of ``test.h`` (and the files it includes)
 will be processed from the PCH file. Otherwise, Clang falls back to
 directly processing the content of ``test.h``. This mirrors the behavior
 of GCC.

 .. note::

   Clang does *not* automatically use PCH files for headers that are directly
   included within a source file. For example:

   .. code-block:: console

     $ clang -x c-header test.h -o test.h.pch
     $ cat test.c
     #include "test.h"
     $ clang test.c -o test

   In this example, ``clang`` will not automatically use the PCH file for
   ``test.h`` since ``test.h`` was included directly in the source file and not
   specified on the command line using :option:`-include`.

 Relocatable PCH Files
 ^^^^^^^^^^^^^^^^^^^^^

 It is sometimes necessary to build a precompiled header from headers
 that are not yet in their final, installed locations. For example, one
 might build a precompiled header within the build tree that is then
 meant to be installed alongside the headers. Clang permits the creation
 of "relocatable" precompiled headers, which are built with a given path
 (into the build directory) and can later be used from an installed
 location.

 To build a relocatable precompiled header, place your headers into a
 subdirectory whose structure mimics the installed location. For example,
 if you want to build a precompiled header for the header ``mylib.h``
 that will be installed into ``/usr/include``, create a subdirectory
 ``build/usr/include`` and place the header ``mylib.h`` into that
 subdirectory. If ``mylib.h`` depends on other headers, then they can be
 stored within ``build/usr/include`` in a way that mimics the installed
 location.

 Building a relocatable precompiled header requires two additional
 arguments. First, pass the ``--relocatable-pch`` flag to indicate that
 the resulting PCH file should be relocatable. Second, pass
 `-isysroot /path/to/build`, which makes all includes for your library
 relative to the build directory. For example:

 .. code-block:: console

   # clang -x c-header --relocatable-pch -isysroot /path/to/build /path/to/build/mylib.h mylib.h.pch

 When loading the relocatable PCH file, the various headers used in the
 PCH file are found from the system header root. For example, ``mylib.h``
 can be found in ``/usr/include/mylib.h``. If the headers are installed
 in some other system root, the `-isysroot` option can be used provide
 a different system root from which the headers will be based. For
 example, `-isysroot /Developer/SDKs/MacOSX10.4u.sdk` will look for
 ``mylib.h`` in ``/Developer/SDKs/MacOSX10.4u.sdk/usr/include/mylib.h``.

 Relocatable precompiled headers are intended to be used in a limited
 number of cases where the compilation environment is tightly controlled
 and the precompiled header cannot be generated after headers have been
 installed.

 .. _controlling-code-generation:

 Controlling Code Generation
 ---------------------------

 Clang provides a number of ways to control code generation. The options
 are listed below.

 **-f[no-]sanitize=check1,check2,...**
    Turn on runtime checks for various forms of undefined or suspicious
    behavior.

    This option controls whether Clang adds runtime checks for various
    forms of undefined or suspicious behavior, and is disabled by
    default. If a check fails, a diagnostic message is produced at
    runtime explaining the problem. The main checks are:

    -  .. _opt_fsanitize_address:

       ``-fsanitize=address``:
       :doc:`AddressSanitizer`, a memory error
       detector.
    -  .. _opt_fsanitize_thread:

       ``-fsanitize=thread``: :doc:`ThreadSanitizer`, a data race detector.
    -  .. _opt_fsanitize_memory:

       ``-fsanitize=memory``: :doc:`MemorySanitizer`,
       a detector of uninitialized reads. Requires instrumentation of all
       program code.
    -  .. _opt_fsanitize_undefined:

       ``-fsanitize=undefined``: :doc:`UndefinedBehaviorSanitizer`,
       a fast and compatible undefined behavior checker.

    -  ``-fsanitize=dataflow``: :doc:`DataFlowSanitizer`, a general data
       flow analysis.
    -  ``-fsanitize=cfi``: :doc:`control flow integrity <ControlFlowIntegrity>`
       checks. Requires ``-flto``.
    -  ``-fsanitize=safe-stack``: :doc:`safe stack <SafeStack>`
       protection against stack-based memory corruption errors.

    There are more fine-grained checks available: see
    the :ref:`list <ubsan-checks>` of specific kinds of
    undefined behavior that can be detected and the :ref:`list <cfi-schemes>`
    of control flow integrity schemes.

    The ``-fsanitize=`` argument must also be provided when linking, in
    order to link to the appropriate runtime library.

    It is not possible to combine more than one of the ``-fsanitize=address``,
    ``-fsanitize=thread``, and ``-fsanitize=memory`` checkers in the same
    program.

 **-f[no-]sanitize-recover=check1,check2,...**

 **-f[no-]sanitize-recover=all**

    Controls which checks enabled by ``-fsanitize=`` flag are non-fatal.
    If the check is fatal, program will halt after the first error
    of this kind is detected and error report is printed.

    By default, non-fatal checks are those enabled by
    :doc:`UndefinedBehaviorSanitizer`,
    except for ``-fsanitize=return`` and ``-fsanitize=unreachable``. Some
    sanitizers may not support recovery (or not support it by default
    e.g. :doc:`AddressSanitizer`), and always crash the program after the issue
    is detected.

    Note that the ``-fsanitize-trap`` flag has precedence over this flag.
    This means that if a check has been configured to trap elsewhere on the
    command line, or if the check traps by default, this flag will not have
    any effect unless that sanitizer's trapping behavior is disabled with
    ``-fno-sanitize-trap``.

    For example, if a command line contains the flags ``-fsanitize=undefined
    -fsanitize-trap=undefined``, the flag ``-fsanitize-recover=alignment``
    will have no effect on its own; it will need to be accompanied by
    ``-fno-sanitize-trap=alignment``.

 **-f[no-]sanitize-trap=check1,check2,...**

    Controls which checks enabled by the ``-fsanitize=`` flag trap. This
    option is intended for use in cases where the sanitizer runtime cannot
    be used (for instance, when building libc or a kernel module), or where
    the binary size increase caused by the sanitizer runtime is a concern.

    This flag is only compatible with :doc:`control flow integrity
    <ControlFlowIntegrity>` schemes and :doc:`UndefinedBehaviorSanitizer`
    checks other than ``vptr``. If this flag
    is supplied together with ``-fsanitize=undefined``, the ``vptr`` sanitizer
    will be implicitly disabled.

    This flag is enabled by default for sanitizers in the ``cfi`` group.

 .. option:: -fsanitize-blacklist=/path/to/blacklist/file

    Disable or modify sanitizer checks for objects (source files, functions,
    variables, types) listed in the file. See
    :doc:`SanitizerSpecialCaseList` for file format description.

 .. option:: -fno-sanitize-blacklist

    Don't use blacklist file, if it was specified earlier in the command line.

 **-f[no-]sanitize-coverage=[type,features,...]**

    Enable simple code coverage in addition to certain sanitizers.
    See :doc:`SanitizerCoverage` for more details.

 **-f[no-]sanitize-stats**

    Enable simple statistics gathering for the enabled sanitizers.
    See :doc:`SanitizerStats` for more details.

 .. option:: -fsanitize-undefined-trap-on-error

    Deprecated alias for ``-fsanitize-trap=undefined``.

 .. option:: -fsanitize-cfi-cross-dso

    Enable cross-DSO control flow integrity checks. This flag modifies
    the behavior of sanitizers in the ``cfi`` group to allow checking
    of cross-DSO virtual and indirect calls.


 .. option:: -fstrict-vtable-pointers

    Enable optimizations based on the strict rules for overwriting polymorphic
    C++ objects, i.e. the vptr is invariant during an object's lifetime.
    This enables better devirtualization. Turned off by default, because it is
    still experimental.

 .. option:: -ffast-math

    Enable fast-math mode. This defines the ``__FAST_MATH__`` preprocessor
    macro, and lets the compiler make aggressive, potentially-lossy assumptions
    about floating-point math.  These include:

    * Floating-point math obeys regular algebraic rules for real numbers (e.g.
      ``+`` and ``*`` are associative, ``x/y == x * (1/y)``, and
      ``(a + b) * c == a * c + b * c``),
    * operands to floating-point operations are not equal to ``NaN`` and
      ``Inf``, and
    * ``+0`` and ``-0`` are interchangeable.

 .. option:: -fdenormal-fp-math=[values]

    Select which denormal numbers the code is permitted to require.

    Valid values are: ``ieee``, ``preserve-sign``, and ``positive-zero``,
    which correspond to IEEE 754 denormal numbers, the sign of a
    flushed-to-zero number is preserved in the sign of 0, denormals are
    flushed to positive zero, respectively.

 .. option:: -fwhole-program-vtables

    Enable whole-program vtable optimizations, such as single-implementation
    devirtualization and virtual constant propagation, for classes with
    :doc:`hidden LTO visibility <LTOVisibility>`. Requires ``-flto``.

 .. option:: -fno-assume-sane-operator-new

    Don't assume that the C++'s new operator is sane.

    This option tells the compiler to do not assume that C++'s global
    new operator will always return a pointer that does not alias any
    other pointer when the function returns.

 .. option:: -ftrap-function=[name]

    Instruct code generator to emit a function call to the specified
    function name for ``__builtin_trap()``.

    LLVM code generator translates ``__builtin_trap()`` to a trap
    instruction if it is supported by the target ISA. Otherwise, the
    builtin is translated into a call to ``abort``. If this option is
    set, then the code generator will always lower the builtin to a call
    to the specified function regardless of whether the target ISA has a
    trap instruction. This option is useful for environments (e.g.
    deeply embedded) where a trap cannot be properly handled, or when
    some custom behavior is desired.

 .. option:: -ftls-model=[model]

    Select which TLS model to use.

    Valid values are: ``global-dynamic``, ``local-dynamic``,
    ``initial-exec`` and ``local-exec``. The default value is
    ``global-dynamic``. The compiler may use a different model if the
    selected model is not supported by the target, or if a more
    efficient model can be used. The TLS model can be overridden per
    variable using the ``tls_model`` attribute.

 .. option:: -femulated-tls

    Select emulated TLS model, which overrides all -ftls-model choices.

    In emulated TLS mode, all access to TLS variables are converted to
    calls to __emutls_get_address in the runtime library.

 .. option:: -mhwdiv=[values]

    Select the ARM modes (arm or thumb) that support hardware division
    instructions.

    Valid values are: ``arm``, ``thumb`` and ``arm,thumb``.
    This option is used to indicate which mode (arm or thumb) supports
    hardware division instructions. This only applies to the ARM
    architecture.

 .. option:: -m[no-]crc

    Enable or disable CRC instructions.

    This option is used to indicate whether CRC instructions are to
    be generated. This only applies to the ARM architecture.

    CRC instructions are enabled by default on ARMv8.

 .. option:: -mgeneral-regs-only

    Generate code which only uses the general purpose registers.

    This option restricts the generated code to use general registers
    only. This only applies to the AArch64 architecture.

 .. option:: -mcompact-branches=[values]

    Control the usage of compact branches for MIPSR6.

    Valid values are: ``never``, ``optimal`` and ``always``.
    The default value is ``optimal`` which generates compact branches
    when a delay slot cannot be filled. ``never`` disables the usage of
    compact branches and ``always`` generates compact branches whenever
    possible.

 **-f[no-]max-type-align=[number]**
    Instruct the code generator to not enforce a higher alignment than the given
    number (of bytes) when accessing memory via an opaque pointer or reference.
    This cap is ignored when directly accessing a variable or when the pointee
    type has an explicit “aligned” attribute.

    The value should usually be determined by the properties of the system allocator.
    Some builtin types, especially vector types, have very high natural alignments;
    when working with values of those types, Clang usually wants to use instructions
    that take advantage of that alignment.  However, many system allocators do
    not promise to return memory that is more than 8-byte or 16-byte-aligned.  Use
    this option to limit the alignment that the compiler can assume for an arbitrary
    pointer, which may point onto the heap.

    This option does not affect the ABI alignment of types; the layout of structs and
    unions and the value returned by the alignof operator remain the same.

    This option can be overridden on a case-by-case basis by putting an explicit
    “aligned” alignment on a struct, union, or typedef.  For example:

    .. code-block:: console

       #include <immintrin.h>
       // Make an aligned typedef of the AVX-512 16-int vector type.
       typedef __v16si __aligned_v16si __attribute__((aligned(64)));

       void initialize_vector(__aligned_v16si *v) {
         // The compiler may assume that ‘v’ is 64-byte aligned, regardless of the
         // value of -fmax-type-align.
       }


 Profile Guided Optimization
 ---------------------------

 Profile information enables better optimization. For example, knowing that a
 branch is taken very frequently helps the compiler make better decisions when
 ordering basic blocks. Knowing that a function ``foo`` is called more
 frequently than another function ``bar`` helps the inliner.

 Clang supports profile guided optimization with two different kinds of
 profiling. A sampling profiler can generate a profile with very low runtime
 overhead, or you can build an instrumented version of the code that collects
 more detailed profile information. Both kinds of profiles can provide execution
 counts for instructions in the code and information on branches taken and
 function invocation.

 Regardless of which kind of profiling you use, be careful to collect profiles
 by running your code with inputs that are representative of the typical
 behavior. Code that is not exercised in the profile will be optimized as if it
 is unimportant, and the compiler may make poor optimization choices for code
 that is disproportionately used while profiling.

 Differences Between Sampling and Instrumentation
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

 Although both techniques are used for similar purposes, there are important
 differences between the two:

 1. Profile data generated with one cannot be used by the other, and there is no
    conversion tool that can convert one to the other. So, a profile generated
    via ``-fprofile-instr-generate`` must be used with ``-fprofile-instr-use``.
    Similarly, sampling profiles generated by external profilers must be
    converted and used with ``-fprofile-sample-use``.

 2. Instrumentation profile data can be used for code coverage analysis and
    optimization.

 3. Sampling profiles can only be used for optimization. They cannot be used for
    code coverage analysis. Although it would be technically possible to use
    sampling profiles for code coverage, sample-based profiles are too
    coarse-grained for code coverage purposes; it would yield poor results.

 4. Sampling profiles must be generated by an external tool. The profile
    generated by that tool must then be converted into a format that can be read
    by LLVM. The section on sampling profilers describes one of the supported
    sampling profile formats.


 Using Sampling Profilers
 ^^^^^^^^^^^^^^^^^^^^^^^^

 Sampling profilers are used to collect runtime information, such as
 hardware counters, while your application executes. They are typically
 very efficient and do not incur a large runtime overhead. The
 sample data collected by the profiler can be used during compilation
 to determine what the most executed areas of the code are.

 Using the data from a sample profiler requires some changes in the way
 a program is built. Before the compiler can use profiling information,
 the code needs to execute under the profiler. The following is the
 usual build cycle when using sample profilers for optimization:

 1. Build the code with source line table information. You can use all the
    usual build flags that you always build your application with. The only
    requirement is that you add ``-gline-tables-only`` or ``-g`` to the
    command line. This is important for the profiler to be able to map
    instructions back to source line locations.

    .. code-block:: console

      $ clang++ -O2 -gline-tables-only code.cc -o code

 2. Run the executable under a sampling profiler. The specific profiler
    you use does not really matter, as long as its output can be converted
    into the format that the LLVM optimizer understands. Currently, there
    exists a conversion tool for the Linux Perf profiler
    (https://perf.wiki.kernel.org/), so these examples assume that you
    are using Linux Perf to profile your code.

    .. code-block:: console

      $ perf record -b ./code

    Note the use of the ``-b`` flag. This tells Perf to use the Last Branch
    Record (LBR) to record call chains. While this is not strictly required,
    it provides better call information, which improves the accuracy of
    the profile data.

 3. Convert the collected profile data to LLVM's sample profile format.
    This is currently supported via the AutoFDO converter ``create_llvm_prof``.
    It is available at http://github.com/google/autofdo. Once built and
    installed, you can convert the ``perf.data`` file to LLVM using
    the command:

    .. code-block:: console

      $ create_llvm_prof --binary=./code --out=code.prof

    This will read ``perf.data`` and the binary file ``./code`` and emit
    the profile data in ``code.prof``. Note that if you ran ``perf``
    without the ``-b`` flag, you need to use ``--use_lbr=false`` when
    calling ``create_llvm_prof``.

 4. Build the code again using the collected profile. This step feeds
    the profile back to the optimizers. This should result in a binary
    that executes faster than the original one. Note that you are not
    required to build the code with the exact same arguments that you
    used in the first step. The only requirement is that you build the code
    with ``-gline-tables-only`` and ``-fprofile-sample-use``.

    .. code-block:: console

      $ clang++ -O2 -gline-tables-only -fprofile-sample-use=code.prof code.cc -o code


 Sample Profile Formats
 """"""""""""""""""""""

 Since external profilers generate profile data in a variety of custom formats,
 the data generated by the profiler must be converted into a format that can be
 read by the backend. LLVM supports three different sample profile formats:

 1. ASCII text. This is the easiest one to generate. The file is divided into
    sections, which correspond to each of the functions with profile
    information. The format is described below. It can also be generated from
    the binary or gcov formats using the ``llvm-profdata`` tool.

 2. Binary encoding. This uses a more efficient encoding that yields smaller
    profile files. This is the format generated by the ``create_llvm_prof`` tool
    in http://github.com/google/autofdo.

 3. GCC encoding. This is based on the gcov format, which is accepted by GCC. It
    is only interesting in environments where GCC and Clang co-exist. This
    encoding is only generated by the ``create_gcov`` tool in
    http://github.com/google/autofdo. It can be read by LLVM and
    ``llvm-profdata``, but it cannot be generated by either.

 If you are using Linux Perf to generate sampling profiles, you can use the
 conversion tool ``create_llvm_prof`` described in the previous section.
 Otherwise, you will need to write a conversion tool that converts your
 profiler's native format into one of these three.


 Sample Profile Text Format
 """"""""""""""""""""""""""

 This section describes the ASCII text format for sampling profiles. It is,
 arguably, the easiest one to generate. If you are interested in generating any
 of the other two, consult the ``ProfileData`` library in in LLVM's source tree
 (specifically, ``include/llvm/ProfileData/SampleProfReader.h``).

 .. code-block:: console

     function1:total_samples:total_head_samples
      offset1[.discriminator]: number_of_samples [fn1:num fn2:num ... ]
      offset2[.discriminator]: number_of_samples [fn3:num fn4:num ... ]
      ...
      offsetN[.discriminator]: number_of_samples [fn5:num fn6:num ... ]
      offsetA[.discriminator]: fnA:num_of_total_samples
       offsetA1[.discriminator]: number_of_samples [fn7:num fn8:num ... ]
       offsetA1[.discriminator]: number_of_samples [fn9:num fn10:num ... ]
       offsetB[.discriminator]: fnB:num_of_total_samples
        offsetB1[.discriminator]: number_of_samples [fn11:num fn12:num ... ]

 This is a nested tree in which the identation represents the nesting level
 of the inline stack. There are no blank lines in the file. And the spacing
 within a single line is fixed. Additional spaces will result in an error
 while reading the file.

 Any line starting with the '#' character is completely ignored.

 Inlined calls are represented with indentation. The Inline stack is a
 stack of source locations in which the top of the stack represents the
 leaf function, and the bottom of the stack represents the actual
 symbol to which the instruction belongs.

 Function names must be mangled in order for the profile loader to
 match them in the current translation unit. The two numbers in the
 function header specify how many total samples were accumulated in the
 function (first number), and the total number of samples accumulated
 in the prologue of the function (second number). This head sample
 count provides an indicator of how frequently the function is invoked.

 There are two types of lines in the function body.

 -  Sampled line represents the profile information of a source location.
    ``offsetN[.discriminator]: number_of_samples [fn5:num fn6:num ... ]``

 -  Callsite line represents the profile information of an inlined callsite.
    ``offsetA[.discriminator]: fnA:num_of_total_samples``

 Each sampled line may contain several items. Some are optional (marked
 below):

 a. Source line offset. This number represents the line number
    in the function where the sample was collected. The line number is
    always relative to the line where symbol of the function is
    defined. So, if the function has its header at line 280, the offset
    13 is at line 293 in the file.

    Note that this offset should never be a negative number. This could
    happen in cases like macros. The debug machinery will register the
    line number at the point of macro expansion. So, if the macro was
    expanded in a line before the start of the function, the profile
    converter should emit a 0 as the offset (this means that the optimizers
    will not be able to associate a meaningful weight to the instructions
    in the macro).

 b. [OPTIONAL] Discriminator. This is used if the sampled program
    was compiled with DWARF discriminator support
    (http://wiki.dwarfstd.org/index.php?title=Path_Discriminators).
    DWARF discriminators are unsigned integer values that allow the
    compiler to distinguish between multiple execution paths on the
    same source line location.

    For example, consider the line of code ``if (cond) foo(); else bar();``.
    If the predicate ``cond`` is true 80% of the time, then the edge
    into function ``foo`` should be considered to be taken most of the
    time. But both calls to ``foo`` and ``bar`` are at the same source
    line, so a sample count at that line is not sufficient. The
    compiler needs to know which part of that line is taken more
    frequently.

    This is what discriminators provide. In this case, the calls to
    ``foo`` and ``bar`` will be at the same line, but will have
    different discriminator values. This allows the compiler to correctly
    set edge weights into ``foo`` and ``bar``.

 c. Number of samples. This is an integer quantity representing the
    number of samples collected by the profiler at this source
    location.

 d. [OPTIONAL] Potential call targets and samples. If present, this
    line contains a call instruction. This models both direct and
    number of samples. For example,

    .. code-block:: console

      130: 7  foo:3  bar:2  baz:7

    The above means that at relative line offset 130 there is a call
    instruction that calls one of ``foo()``, ``bar()`` and ``baz()``,
    with ``baz()`` being the relatively more frequently called target.

 As an example, consider a program with the call chain ``main -> foo -> bar``.
 When built with optimizations enabled, the compiler may inline the
 calls to ``bar`` and ``foo`` inside ``main``. The generated profile
 could then be something like this:

 .. code-block:: console

     main:35504:0
     1: _Z3foov:35504
       2: _Z32bari:31977
       1.1: 31977
     2: 0

 This profile indicates that there were a total of 35,504 samples
 collected in main. All of those were at line 1 (the call to ``foo``).
 Of those, 31,977 were spent inside the body of ``bar``. The last line
 of the profile (``2: 0``) corresponds to line 2 inside ``main``. No
 samples were collected there.

 Profiling with Instrumentation
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

 Clang also supports profiling via instrumentation. This requires building a
 special instrumented version of the code and has some runtime
 overhead during the profiling, but it provides more detailed results than a
 sampling profiler. It also provides reproducible results, at least to the
 extent that the code behaves consistently across runs.

 Here are the steps for using profile guided optimization with
 instrumentation:

 1. Build an instrumented version of the code by compiling and linking with the
    ``-fprofile-instr-generate`` option.

    .. code-block:: console

      $ clang++ -O2 -fprofile-instr-generate code.cc -o code

 2. Run the instrumented executable with inputs that reflect the typical usage.
    By default, the profile data will be written to a ``default.profraw`` file
    in the current directory. You can override that default by using option
    ``-fprofile-instr-generate=`` or by setting the ``LLVM_PROFILE_FILE``
    environment variable to specify an alternate file. If non-default file name
    is specified by both the environment variable and the command line option,
    the environment variable takes precedence. The file name pattern specified
    can include different modifiers: ``%p``, ``%h``, and ``%m``.

    Any instance of ``%p`` in that file name will be replaced by the process
    ID, so that you can easily distinguish the profile output from multiple
    runs.

    .. code-block:: console

      $ LLVM_PROFILE_FILE="code-%p.profraw" ./code

    The modifier ``%h`` can be used in scenarios where the same instrumented
    binary is run in multiple different host machines dumping profile data
    to a shared network based storage. The ``%h`` specifier will be substituted
    with the hostname so that profiles collected from different hosts do not
    clobber each other.

    While the use of ``%p`` specifier can reduce the likelihood for the profiles
    dumped from different processes to clobber each other, such clobbering can still
    happen because of the ``pid`` re-use by the OS. Another side-effect of using
    ``%p`` is that the storage requirement for raw profile data files is greatly
    increased.  To avoid issues like this, the ``%m`` specifier can used in the profile
    name.  When this specifier is used, the profiler runtime will substitute ``%m``
    with a unique integer identifier associated with the instrumented binary. Additionally,
    multiple raw profiles dumped from different processes that share a file system (can be
    on different hosts) will be automatically merged by the profiler runtime during the
    dumping. If the program links in multiple instrumented shared libraries, each library
    will dump the profile data into its own profile data file (with its unique integer
    id embedded in the profile name). Note that the merging enabled by ``%m`` is for raw
    profile data generated by profiler runtime. The resulting merged "raw" profile data
    file still needs to be converted to a different format expected by the compiler (
    see step 3 below).

    .. code-block:: console

      $ LLVM_PROFILE_FILE="code-%m.profraw" ./code


 3. Combine profiles from multiple runs and convert the "raw" profile format to
    the input expected by clang. Use the ``merge`` command of the
    ``llvm-profdata`` tool to do this.

    .. code-block:: console

      $ llvm-profdata merge -output=code.profdata code-*.profraw

    Note that this step is necessary even when there is only one "raw" profile,
    since the merge operation also changes the file format.

 4. Build the code again using the ``-fprofile-instr-use`` option to specify the
    collected profile data.

    .. code-block:: console

      $ clang++ -O2 -fprofile-instr-use=code.profdata code.cc -o code

    You can repeat step 4 as often as you like without regenerating the
    profile. As you make changes to your code, clang may no longer be able to
    use the profile data. It will warn you when this happens.

 Profile generation using an alternative instrumentation method can be
 controlled by the GCC-compatible flags ``-fprofile-generate`` and
 ``-fprofile-use``. Although these flags are semantically equivalent to
 their GCC counterparts, they *do not* handle GCC-compatible profiles.
 They are only meant to implement GCC's semantics with respect to
 profile creation and use.

 .. option:: -fprofile-generate[=<dirname>]

   The ``-fprofile-generate`` and ``-fprofile-generate=`` flags will use
   an alterantive instrumentation method for profile generation. When
   given a directory name, it generates the profile file
   ``default_%m.profraw`` in the directory named ``dirname`` if specified.
   If ``dirname`` does not exist, it will be created at runtime. ``%m`` specifier
   will be substibuted with a unique id documented in step 2 above. In other words,
   with ``-fprofile-generate[=<dirname>]`` option, the "raw" profile data automatic
   merging is turned on by default, so there will no longer any risk of profile
   clobbering from different running processes.  For example,

   .. code-block:: console

     $ clang++ -O2 -fprofile-generate=yyy/zzz code.cc -o code

   When ``code`` is executed, the profile will be written to the file
   ``yyy/zzz/default_xxxx.profraw``.

   To generate the profile data file with the compiler readable format, the
   ``llvm-profdata`` tool can be used with the profile directory as the input:

    .. code-block:: console

      $ llvm-profdata merge -output=code.profdata yyy/zzz/

  If the user wants to turn off the auto-merging feature, or simply override the
  the profile dumping path specified at command line, the environment variable
  ``LLVM_PROFILE_FILE`` can still be used to override
  the directory and filename for the profile file at runtime.

 .. option:: -fprofile-use[=<pathname>]

   Without any other arguments, ``-fprofile-use`` behaves identically to
   ``-fprofile-instr-use``. Otherwise, if ``pathname`` is the full path to a
   profile file, it reads from that file. If ``pathname`` is a directory name,
   it reads from ``pathname/default.profdata``.

 Disabling Instrumentation
 ^^^^^^^^^^^^^^^^^^^^^^^^^

 In certain situations, it may be useful to disable profile generation or use
 for specific files in a build, without affecting the main compilation flags
 used for the other files in the project.

 In these cases, you can use the flag ``-fno-profile-instr-generate`` (or
 ``-fno-profile-generate``) to disable profile generation, and
 ``-fno-profile-instr-use`` (or ``-fno-profile-use``) to disable profile use.

 Note that these flags should appear after the corresponding profile
 flags to have an effect.

 Controlling Debug Information
 -----------------------------

 Controlling Size of Debug Information
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

 Debug info kind generated by Clang can be set by one of the flags listed
 below. If multiple flags are present, the last one is used.

 .. option:: -g0

   Don't generate any debug info (default).

 .. option:: -gline-tables-only

   Generate line number tables only.

   This kind of debug info allows to obtain stack traces with function names,
   file names and line numbers (by such tools as ``gdb`` or ``addr2line``).  It
   doesn't contain any other data (e.g. description of local variables or
   function parameters).

 .. option:: -fstandalone-debug

   Clang supports a number of optimizations to reduce the size of debug
   information in the binary. They work based on the assumption that
   the debug type information can be spread out over multiple
   compilation units.  For instance, Clang will not emit type
   definitions for types that are not needed by a module and could be
   replaced with a forward declaration.  Further, Clang will only emit
   type info for a dynamic C++ class in the module that contains the
   vtable for the class.

   The **-fstandalone-debug** option turns off these optimizations.
   This is useful when working with 3rd-party libraries that don't come
   with debug information.  Note that Clang will never emit type
   information for types that are not referenced at all by the program.

 .. option:: -fno-standalone-debug

    On Darwin **-fstandalone-debug** is enabled by default. The
    **-fno-standalone-debug** option can be used to get to turn on the
    vtable-based optimization described above.

 .. option:: -g

   Generate complete debug info.

 Controlling Macro Debug Info Generation
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

 Debug info for C preprocessor macros increases the size of debug information in
 the binary. Macro debug info generated by Clang can be controlled by the flags
 listed below.

 .. option:: -fdebug-macro

   Generate debug info for preprocessor macros. This flag is discarded when
   **-g0** is enabled.

 .. option:: -fno-debug-macro

   Do not generate debug info for preprocessor macros (default).

 Controlling Debugger "Tuning"
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

 While Clang generally emits standard DWARF debug info (http://dwarfstd.org),
 different debuggers may know how to take advantage of different specific DWARF
 features. You can "tune" the debug info for one of several different debuggers.

 .. option:: -ggdb, -glldb, -gsce

   Tune the debug info for the ``gdb``, ``lldb``, or Sony PlayStation\ |reg|
   debugger, respectively. Each of these options implies **-g**. (Therefore, if
   you want both **-gline-tables-only** and debugger tuning, the tuning option
   must come first.)


 Comment Parsing Options
 -----------------------

 Clang parses Doxygen and non-Doxygen style documentation comments and attaches
 them to the appropriate declaration nodes.  By default, it only parses
 Doxygen-style comments and ignores ordinary comments starting with ``//`` and
 ``/*``.

 .. option:: -Wdocumentation

   Emit warnings about use of documentation comments.  This warning group is off
   by default.

   This includes checking that ``\param`` commands name parameters that actually
   present in the function signature, checking that ``\returns`` is used only on
   functions that actually return a value etc.

 .. option:: -Wno-documentation-unknown-command

   Don't warn when encountering an unknown Doxygen command.

 .. option:: -fparse-all-comments

   Parse all comments as documentation comments (including ordinary comments
   starting with ``//`` and ``/*``).

 .. option:: -fcomment-block-commands=[commands]

   Define custom documentation commands as block commands.  This allows Clang to
   construct the correct AST for these custom commands, and silences warnings
   about unknown commands.  Several commands must be separated by a comma
   *without trailing space*; e.g. ``-fcomment-block-commands=foo,bar`` defines
   custom commands ``\foo`` and ``\bar``.

   It is also possible to use ``-fcomment-block-commands`` several times; e.g.
   ``-fcomment-block-commands=foo -fcomment-block-commands=bar`` does the same
   as above.

 .. _c:

 C Language Features
 ===================

 The support for standard C in clang is feature-complete except for the
 C99 floating-point pragmas.

 Extensions supported by clang
 -----------------------------

 See :doc:`LanguageExtensions`.

 Differences between various standard modes
 ------------------------------------------

 clang supports the -std option, which changes what language mode clang
 uses. The supported modes for C are c89, gnu89, c94, c99, gnu99, c11,
 gnu11, and various aliases for those modes. If no -std option is
 specified, clang defaults to gnu11 mode. Many C99 and C11 features are
 supported in earlier modes as a conforming extension, with a warning. Use
 ``-pedantic-errors`` to request an error if a feature from a later standard
 revision is used in an earlier mode.

 Differences between all ``c*`` and ``gnu*`` modes:

 -  ``c*`` modes define "``__STRICT_ANSI__``".
 -  Target-specific defines not prefixed by underscores, like "linux",
    are defined in ``gnu*`` modes.
 -  Trigraphs default to being off in ``gnu*`` modes; they can be enabled by
    the -trigraphs option.
 -  The parser recognizes "asm" and "typeof" as keywords in ``gnu*`` modes;
    the variants "``__asm__``" and "``__typeof__``" are recognized in all
    modes.
 -  The Apple "blocks" extension is recognized by default in ``gnu*`` modes
    on some platforms; it can be enabled in any mode with the "-fblocks"
    option.
 -  Arrays that are VLA's according to the standard, but which can be
    constant folded by the frontend are treated as fixed size arrays.
    This occurs for things like "int X[(1, 2)];", which is technically a
    VLA. ``c*`` modes are strictly compliant and treat these as VLAs.

 Differences between ``*89`` and ``*99`` modes:

 -  The ``*99`` modes default to implementing "inline" as specified in C99,
    while the ``*89`` modes implement the GNU version. This can be
    overridden for individual functions with the ``__gnu_inline__``
    attribute.
 -  Digraphs are not recognized in c89 mode.
 -  The scope of names defined inside a "for", "if", "switch", "while",
    or "do" statement is different. (example: "``if ((struct x {int
    x;}*)0) {}``".)
 -  ``__STDC_VERSION__`` is not defined in ``*89`` modes.
 -  "inline" is not recognized as a keyword in c89 mode.
 -  "restrict" is not recognized as a keyword in ``*89`` modes.
 -  Commas are allowed in integer constant expressions in ``*99`` modes.
 -  Arrays which are not lvalues are not implicitly promoted to pointers
    in ``*89`` modes.
 -  Some warnings are different.

 Differences between ``*99`` and ``*11`` modes:

 -  Warnings for use of C11 features are disabled.
 -  ``__STDC_VERSION__`` is defined to ``201112L`` rather than ``199901L``.

 c94 mode is identical to c89 mode except that digraphs are enabled in
 c94 mode (FIXME: And ``__STDC_VERSION__`` should be defined!).

 GCC extensions not implemented yet
 ----------------------------------

 clang tries to be compatible with gcc as much as possible, but some gcc
 extensions are not implemented yet:

 -  clang does not support decimal floating point types (``_Decimal32`` and
    friends) or fixed-point types (``_Fract`` and friends); nobody has
    expressed interest in these features yet, so it's hard to say when
    they will be implemented.
 -  clang does not support nested functions; this is a complex feature
    which is infrequently used, so it is unlikely to be implemented
    anytime soon. In C++11 it can be emulated by assigning lambda
    functions to local variables, e.g:

    .. code-block:: cpp

      auto const local_function = [&](int parameter) {
        // Do something
      };
      ...
      local_function(1);

 -  clang only supports global register variables when the register specified
    is non-allocatable (e.g. the stack pointer). Support for general global
    register variables is unlikely to be implemented soon because it requires
    additional LLVM backend support.
 -  clang does not support static initialization of flexible array
    members. This appears to be a rarely used extension, but could be
    implemented pending user demand.
 -  clang does not support
    ``__builtin_va_arg_pack``/``__builtin_va_arg_pack_len``. This is
    used rarely, but in some potentially interesting places, like the
    glibc headers, so it may be implemented pending user demand. Note
    that because clang pretends to be like GCC 4.2, and this extension
    was introduced in 4.3, the glibc headers will not try to use this
    extension with clang at the moment.
 -  clang does not support the gcc extension for forward-declaring
    function parameters; this has not shown up in any real-world code
    yet, though, so it might never be implemented.

 This is not a complete list; if you find an unsupported extension
 missing from this list, please send an e-mail to cfe-dev. This list
 currently excludes C++; see :ref:`C++ Language Features <cxx>`. Also, this
 list does not include bugs in mostly-implemented features; please see
 the `bug
 tracker <https://bugs.llvm.org/buglist.cgi?quicksearch=product%3Aclang+component%3A-New%2BBugs%2CAST%2CBasic%2CDriver%2CHeaders%2CLLVM%2BCodeGen%2Cparser%2Cpreprocessor%2CSemantic%2BAnalyzer>`_
 for known existing bugs (FIXME: Is there a section for bug-reporting
 guidelines somewhere?).

 Intentionally unsupported GCC extensions
 ----------------------------------------

 -  clang does not support the gcc extension that allows variable-length
    arrays in structures. This is for a few reasons: one, it is tricky to
    implement, two, the extension is completely undocumented, and three,
    the extension appears to be rarely used. Note that clang *does*
    support flexible array members (arrays with a zero or unspecified
    size at the end of a structure).
 -  clang does not have an equivalent to gcc's "fold"; this means that
    clang doesn't accept some constructs gcc might accept in contexts
    where a constant expression is required, like "x-x" where x is a
    variable.
 -  clang does not support ``__builtin_apply`` and friends; this extension
    is extremely obscure and difficult to implement reliably.

 .. _c_ms:

 Microsoft extensions
 --------------------

 clang has support for many extensions from Microsoft Visual C++. To enable these
 extensions, use the ``-fms-extensions`` command-line option. This is the default
 for Windows targets. Clang does not implement every pragma or declspec provided
 by MSVC, but the popular ones, such as ``__declspec(dllexport)`` and ``#pragma
 comment(lib)`` are well supported.

 clang has a ``-fms-compatibility`` flag that makes clang accept enough
 invalid C++ to be able to parse most Microsoft headers. For example, it
 allows `unqualified lookup of dependent base class members
 <http://clang.llvm.org/compatibility.html#dep_lookup_bases>`_, which is
 a common compatibility issue with clang. This flag is enabled by default
 for Windows targets.

 ``-fdelayed-template-parsing`` lets clang delay parsing of function template
 definitions until the end of a translation unit. This flag is enabled by
 default for Windows targets.

 For compatibility with existing code that compiles with MSVC, clang defines the
 ``_MSC_VER`` and ``_MSC_FULL_VER`` macros. These default to the values of 1800
 and 180000000 respectively, making clang look like an early release of Visual
 C++ 2013. The ``-fms-compatibility-version=`` flag overrides these values.  It
 accepts a dotted version tuple, such as 19.00.23506. Changing the MSVC
 compatibility version makes clang behave more like that version of MSVC. For
 example, ``-fms-compatibility-version=19`` will enable C++14 features and define
 ``char16_t`` and ``char32_t`` as builtin types.

 .. _cxx:

 C++ Language Features
 =====================

 clang fully implements all of standard C++98 except for exported
 templates (which were removed in C++11), and all of standard C++11
 and the current draft standard for C++1y.

 Controlling implementation limits
 ---------------------------------

 .. option:: -fbracket-depth=N

   Sets the limit for nested parentheses, brackets, and braces to N.  The
   default is 256.

 .. option:: -fconstexpr-depth=N

   Sets the limit for recursive constexpr function invocations to N.  The
   default is 512.

 .. option:: -ftemplate-depth=N

   Sets the limit for recursively nested template instantiations to N.  The
   default is 256.

 .. option:: -foperator-arrow-depth=N

   Sets the limit for iterative calls to 'operator->' functions to N.  The
   default is 256.

 .. _objc:

 Objective-C Language Features
 =============================

 .. _objcxx:

 Objective-C++ Language Features
 ===============================

 .. _openmp:

 OpenMP Features
 ===============

 Clang supports all OpenMP 3.1 directives and clauses.  In addition, some
 features of OpenMP 4.0 are supported.  For example, ``#pragma omp simd``,
 ``#pragma omp for simd``, ``#pragma omp parallel for simd`` directives, extended
 set of atomic constructs, ``proc_bind`` clause for all parallel-based
 directives, ``depend`` clause for ``#pragma omp task`` directive (except for
 array sections), ``#pragma omp cancel`` and ``#pragma omp cancellation point``
 directives, and ``#pragma omp taskgroup`` directive.

 Use `-fopenmp` to enable OpenMP. Support for OpenMP can be disabled with
 `-fno-openmp`.

 Controlling implementation limits
 ---------------------------------

 .. option:: -fopenmp-use-tls

  Controls code generation for OpenMP threadprivate variables. In presence of
  this option all threadprivate variables are generated the same way as thread
  local variables, using TLS support. If `-fno-openmp-use-tls`
  is provided or target does not support TLS, code generation for threadprivate
  variables relies on OpenMP runtime library.

 .. _opencl:

 OpenCL Features
 ===============

 Clang can be used to compile OpenCL kernels for execution on a device
 (e.g. GPU). It is possible to compile the kernel into a binary (e.g. for AMD or
 Nvidia targets) that can be uploaded to run directly on a device (e.g. using
 `clCreateProgramWithBinary
 <https://www.khronos.org/registry/OpenCL/specs/opencl-1.1.pdf#111>`_) or
 into generic bitcode files loadable into other toolchains.

 Compiling to a binary using the default target from the installation can be done
 as follows:

    .. code-block:: console

      $ echo "kernel void k(){}" > test.cl
      $ clang test.cl

 Compiling for a specific target can be done by specifying the triple corresponding
 to the target, for example:

    .. code-block:: console

      $ clang -target nvptx64-unknown-unknown test.cl
      $ clang -target amdgcn-amd-amdhsa-opencl test.cl

 Compiling to bitcode can be done as follows:

    .. code-block:: console

      $ clang -c -emit-llvm test.cl

 This will produce a generic test.bc file that can be used in vendor toolchains
 to perform machine code generation.

 Clang currently supports OpenCL C language standards up to v2.0.

 OpenCL Specific Options
 -----------------------

 Most of the OpenCL build options from `the specification v2.0 section 5.8.4
 <https://www.khronos.org/registry/cl/specs/opencl-2.0.pdf#200>`_ are available.

 Examples:

    .. code-block:: console

      $ clang -cl-std=CL2.0 -cl-single-precision-constant test.cl

 Some extra options are available to support special OpenCL features.

 .. option:: -finclude-default-header

 Loads standard includes during compilations. By default OpenCL headers are not
 loaded and therefore standard library includes are not available. To load them
 automatically a flag has been added to the frontend (see also :ref:`the section
 on the OpenCL Header <opencl_header>`):

    .. code-block:: console

      $ clang -Xclang -finclude-default-header test.cl

 Alternatively ``-include`` or ``-I`` followed by the path to the header location
 can be given manually.

    .. code-block:: console

      $ clang -I<path to clang>/lib/Headers/opencl-c.h test.cl

 In this case the kernel code should contain ``#include <opencl-c.h>`` just as a
 regular C include.

 .. _opencl_cl_ext:

 .. option:: -cl-ext

 Disables support of OpenCL extensions. All OpenCL targets provide a list
 of extensions that they support. Clang allows to amend this using the ``-cl-ext``
 flag with a comma-separated list of extensions prefixed with ``'+'`` or ``'-'``.
 The syntax: ``-cl-ext=<(['-'|'+']<extension>[,])+>``,  where extensions
 can be either one of `the OpenCL specification extensions
 <https://www.khronos.org/registry/cl/sdk/2.0/docs/man/xhtml/EXTENSION.html>`_
 or any known vendor extension. Alternatively, ``'all'`` can be used to enable
 or disable all known extensions.
 Example disabling double support for the 64-bit SPIR target:

    .. code-block:: console

      $ clang -cc1 -triple spir64-unknown-unknown -cl-ext=-cl_khr_fp64 test.cl

 Enabling all extensions except double support in R600 AMD GPU can be done using:

    .. code-block:: console

      $ clang -cc1 -triple r600-unknown-unknown -cl-ext=-all,+cl_khr_fp16 test.cl

 .. _opencl_fake_address_space_map:

 .. option:: -ffake-address-space-map

 Overrides the target address space map with a fake map.
 This allows adding explicit address space IDs to the bitcode for non-segmented
 memory architectures that don't have separate IDs for each of the OpenCL
 logical address spaces by default. Passing ``-ffake-address-space-map`` will
 add/override address spaces of the target compiled for with the following values:
 ``1-global``, ``2-constant``, ``3-local``, ``4-generic``. The private address
 space is represented by the absence of an address space attribute in the IR (see
 also :ref:`the section on the address space attribute <opencl_addrsp>`).

    .. code-block:: console

      $ clang -ffake-address-space-map test.cl

 Some other flags used for the compilation for C can also be passed while
 compiling for OpenCL, examples: ``-c``, ``-O<1-4|s>``, ``-o``, ``-emit-llvm``, etc.

 OpenCL Targets
 --------------

 OpenCL targets are derived from the regular Clang target classes. The OpenCL
 specific parts of the target representation provide address space mapping as
 well as a set of supported extensions.

 Specific Targets
 ^^^^^^^^^^^^^^^^

 There is a set of concrete HW architectures that OpenCL can be compiled for.

 - For AMD target:

    .. code-block:: console

      $ clang -target amdgcn-amd-amdhsa-opencl test.cl

 - For Nvidia architectures:

    .. code-block:: console

      $ clang -target nvptx64-unknown-unknown test.cl


 Generic Targets
 ^^^^^^^^^^^^^^^

 - SPIR is available as a generic target to allow portable bitcode to be produced
   that can be used across GPU toolchains. The implementation follows `the SPIR
   specification <https://www.khronos.org/spir>`_. There are two flavors
   available for 32 and 64 bits.

    .. code-block:: console

     $ clang -target spir-unknown-unknown test.cl
     $ clang -target spir64-unknown-unknown test.cl

   All known OpenCL extensions are supported in the SPIR targets. Clang will
   generate SPIR v1.2 compatible IR for OpenCL versions up to 2.0 and SPIR v2.0
   for OpenCL v2.0.

 - x86 is used by some implementations that are x86 compatible and currently
   remains for backwards compatibility (with older implementations prior to
   SPIR target support). For "non-SPMD" targets which cannot spawn multiple
   work-items on the fly using hardware, which covers practically all non-GPU
   devices such as CPUs and DSPs, additional processing is needed for the kernels
   to support multiple work-item execution. For this, a 3rd party toolchain,
   such as for example `POCL <http://portablecl.org/>`_, can be used.

   This target does not support multiple memory segments and, therefore, the fake
   address space map can be added using the :ref:`-ffake-address-space-map
   <opencl_fake_address_space_map>` flag.

 .. _opencl_header:

 OpenCL Header
 -------------

 By default Clang will not include standard headers and therefore OpenCL builtin
 functions and some types (i.e. vectors) are unknown. The default CL header is,
 however, provided in the Clang installation and can be enabled by passing the
 ``-finclude-default-header`` flag to the Clang frontend.

    .. code-block:: console

      $ echo "bool is_wg_uniform(int i){return get_enqueued_local_size(i)==get_local_size(i);}" > test.cl
      $ clang -Xclang -finclude-default-header -cl-std=CL2.0 test.cl

 Because the header is very large and long to parse, PCH (:doc:`PCHInternals`)
 and modules (:doc:`Modules`) are used internally to improve the compilation
 speed.

 To enable modules for OpenCL:

    .. code-block:: console

      $ clang -target spir-unknown-unknown -c -emit-llvm -Xclang -finclude-default-header -fmodules -fimplicit-module-maps -fmodules-cache-path=<path to the generated module> test.cl

 OpenCL Extensions
 -----------------

 All of the ``cl_khr_*`` extensions from `the official OpenCL specification
 <https://www.khronos.org/registry/OpenCL/sdk/2.0/docs/man/xhtml/EXTENSION.html>`_
 up to and including version 2.0 are available and set per target depending on the
 support available in the specific architecture.

 It is possible to alter the default extensions setting per target using
 ``-cl-ext`` flag. (See :ref:`flags description <opencl_cl_ext>` for more details).

 Vendor extensions can be added flexibly by declaring the list of types and
 functions associated with each extensions enclosed within the following
 compiler pragma directives:

   .. code-block:: c

        #pragma OPENCL EXTENSION the_new_extension_name : begin
        // declare types and functions associated with the extension here
        #pragma OPENCL EXTENSION the_new_extension_name : end

 For example, parsing the following code adds ``my_t`` type and ``my_func``
 function to the custom ``my_ext`` extension.

   .. code-block:: c

        #pragma OPENCL EXTENSION my_ext : begin
        typedef struct{
          int a;
        }my_t;
        void my_func(my_t);
        #pragma OPENCL EXTENSION my_ext : end

 Declaring the same types in different vendor extensions is disallowed.

 OpenCL Metadata
 ---------------

 Clang uses metadata to provide additional OpenCL semantics in IR needed for
 backends and OpenCL runtime.

 Each kernel will have function metadata attached to it, specifying the arguments.
 Kernel argument metadata is used to provide source level information for querying
 at runtime, for example using the `clGetKernelArgInfo
 <https://www.khronos.org/registry/OpenCL/specs/opencl-1.2.pdf#167>`_
 call.

 Note that ``-cl-kernel-arg-info`` enables more information about the original CL
 code to be added e.g. kernel parameter names will appear in the OpenCL metadata
 along with other information.

 The IDs used to encode the OpenCL's logical address spaces in the argument info
 metadata follows the SPIR address space mapping as defined in the SPIR
 specification `section 2.2
 <https://www.khronos.org/registry/spir/specs/spir_spec-2.0.pdf#18>`_

 OpenCL-Specific Attributes
 --------------------------

 OpenCL support in Clang contains a set of attribute taken directly from the
 specification as well as additional attributes.

 See also :doc:`AttributeReference`.

 nosvm
 ^^^^^

 Clang supports this attribute to comply to OpenCL v2.0 conformance, but it
 does not have any effect on the IR. For more details reffer to the specification
 `section 6.7.2
 <https://www.khronos.org/registry/cl/specs/opencl-2.0-openclc.pdf#49>`_


 opencl_unroll_hint
 ^^^^^^^^^^^^^^^^^^

 The implementation of this feature mirrors the unroll hint for C.
 More details on the syntax can be found in the specification
 `section 6.11.5
 <https://www.khronos.org/registry/cl/specs/opencl-2.0-openclc.pdf#61>`_

 convergent
 ^^^^^^^^^^

 To make sure no invalid optimizations occur for single program multiple data
 (SPMD) / single instruction multiple thread (SIMT) Clang provides attributes that
 can be used for special functions that have cross work item semantics.
 An example is the subgroup operations such as `intel_sub_group_shuffle
 <https://www.khronos.org/registry/cl/extensions/intel/cl_intel_subgroups.txt>`_

    .. code-block:: c

      // Define custom my_sub_group_shuffle(data, c)
      // that makes use of intel_sub_group_shuffle
      r1 = ...
      if (r0) r1 = computeA();
      // Shuffle data from r1 into r3
      // of threads id r2.
      r3 = my_sub_group_shuffle(r1, r2);
      if (r0) r3 = computeB();

 with non-SPMD semantics this is optimized to the following equivalent code:

    .. code-block:: c

      r1 = ...
      if (!r0)
        // Incorrect functionality! The data in r1
        // have not been computed by all threads yet.
        r3 = my_sub_group_shuffle(r1, r2);
      else {
        r1 = computeA();
        r3 = my_sub_group_shuffle(r1, r2);
        r3 = computeB();
      }

 Declaring the function ``my_sub_group_shuffle`` with the convergent attribute
 would prevent this:

    .. code-block:: c

      my_sub_group_shuffle() __attribute__((convergent));

 Using ``convergent`` guarantees correct execution by keeping CFG equivalence
 wrt operations marked as ``convergent``. CFG ``G´`` is equivalent to ``G`` wrt
 node ``Ni`` : ``iff ∀ Nj (i≠j)`` domination and post-domination relations with
 respect to ``Ni`` remain the same in both ``G`` and ``G´``.

 noduplicate
 ^^^^^^^^^^^

 ``noduplicate`` is more restrictive with respect to optimizations than
 ``convergent`` because a convergent function only preserves CFG equivalence.
 This allows some optimizations to happen as long as the control flow remains
 unmodified.

    .. code-block:: c

      for (int i=0; i<4; i++)
        my_sub_group_shuffle()

 can be modified to:

    .. code-block:: c

      my_sub_group_shuffle();
      my_sub_group_shuffle();
      my_sub_group_shuffle();
      my_sub_group_shuffle();

 while using ``noduplicate`` would disallow this. Also ``noduplicate`` doesn't
 have the same safe semantics of CFG as ``convergent`` and can cause changes in
 CFG that modify semantics of the original program.

 ``noduplicate`` is kept for backwards compatibility only and it considered to be
 deprecated for future uses.

 .. _opencl_addrsp:

 address_space
 ^^^^^^^^^^^^^

 Clang has arbitrary address space support using the ``address_space(N)``
 attribute, where ``N`` is an integer number in the range ``0`` to ``16777215``
 (``0xffffffu``).

 An OpenCL implementation provides a list of standard address spaces using
 keywords: ``private``, ``local``, ``global``, and ``generic``. In the AST and
 in the IR local, global, or generic will be represented by the address space
 attribute with the corresponding unique number. Note that private does not have
 any corresponding attribute added and, therefore, is represented by the absence
 of an address space number. The specific IDs for an address space do not have to
 match between the AST and the IR. Typically in the AST address space numbers
 represent logical segments while in the IR they represent physical segments.
 Therefore, machines with flat memory segments can map all AST address space
 numbers to the same physical segment ID or skip address space attribute
 completely while generating the IR. However, if the address space information
 is needed by the IR passes e.g. to improve alias analysis, it is recommended
 to keep it and only lower to reflect physical memory segments in the late
 machine passes.

 OpenCL builtins
 ---------------

 There are some standard OpenCL functions that are implemented as Clang builtins:

 - All pipe functions from `section 6.13.16.2/6.13.16.3
   <https://www.khronos.org/registry/cl/specs/opencl-2.0-openclc.pdf#160>`_ of
   the OpenCL v2.0 kernel language specification. `

 - Address space qualifier conversion functions ``to_global``/``to_local``/``to_private``
   from `section 6.13.9
   <https://www.khronos.org/registry/cl/specs/opencl-2.0-openclc.pdf#101>`_.

 - All the ``enqueue_kernel`` functions from `section 6.13.17.1
   <https://www.khronos.org/registry/cl/specs/opencl-2.0-openclc.pdf#164>`_ and
   enqueue query functions from `section 6.13.17.5
   <https://www.khronos.org/registry/cl/specs/opencl-2.0-openclc.pdf#171>`_.

 .. _target_features:

 Target-Specific Features and Limitations
 ========================================

 CPU Architectures Features and Limitations
 ------------------------------------------

 X86
 ^^^

 The support for X86 (both 32-bit and 64-bit) is considered stable on
 Darwin (Mac OS X), Linux, FreeBSD, and Dragonfly BSD: it has been tested
 to correctly compile many large C, C++, Objective-C, and Objective-C++
 codebases.

 On ``x86_64-mingw32``, passing i128(by value) is incompatible with the
 Microsoft x64 calling convention. You might need to tweak
 ``WinX86_64ABIInfo::classify()`` in lib/CodeGen/TargetInfo.cpp.

 For the X86 target, clang supports the `-m16` command line
 argument which enables 16-bit code output. This is broadly similar to
 using ``asm(".code16gcc")`` with the GNU toolchain. The generated code
 and the ABI remains 32-bit but the assembler emits instructions
 appropriate for a CPU running in 16-bit mode, with address-size and
 operand-size prefixes to enable 32-bit addressing and operations.

 ARM
 ^^^

 The support for ARM (specifically ARMv6 and ARMv7) is considered stable
 on Darwin (iOS): it has been tested to correctly compile many large C,
 C++, Objective-C, and Objective-C++ codebases. Clang only supports a
 limited number of ARM architectures. It does not yet fully support
 ARMv5, for example.

 PowerPC
 ^^^^^^^

 The support for PowerPC (especially PowerPC64) is considered stable
 on Linux and FreeBSD: it has been tested to correctly compile many
 large C and C++ codebases. PowerPC (32bit) is still missing certain
 features (e.g. PIC code on ELF platforms).

 Other platforms
 ^^^^^^^^^^^^^^^

 clang currently contains some support for other architectures (e.g. Sparc);
 however, significant pieces of code generation are still missing, and they
 haven't undergone significant testing.

 clang contains limited support for the MSP430 embedded processor, but
 both the clang support and the LLVM backend support are highly
 experimental.

 Other platforms are completely unsupported at the moment. Adding the
 minimal support needed for parsing and semantic analysis on a new
 platform is quite easy; see ``lib/Basic/Targets.cpp`` in the clang source
 tree. This level of support is also sufficient for conversion to LLVM IR
 for simple programs. Proper support for conversion to LLVM IR requires
 adding code to ``lib/CodeGen/CGCall.cpp`` at the moment; this is likely to
 change soon, though. Generating assembly requires a suitable LLVM
 backend.

 Operating System Features and Limitations
 -----------------------------------------

 Darwin (Mac OS X)
 ^^^^^^^^^^^^^^^^^

 Thread Sanitizer is not supported.

 Windows
 ^^^^^^^

 Clang has experimental support for targeting "Cygming" (Cygwin / MinGW)
 platforms.

 See also :ref:`Microsoft Extensions <c_ms>`.

 Cygwin
 """"""

 Clang works on Cygwin-1.7.

 MinGW32
 """""""

 Clang works on some mingw32 distributions. Clang assumes directories as
 below;

 -  ``C:/mingw/include``
 -  ``C:/mingw/lib``
 -  ``C:/mingw/lib/gcc/mingw32/4.[3-5].0/include/c++``

 On MSYS, a few tests might fail.

 MinGW-w64
 """""""""

 For 32-bit (i686-w64-mingw32), and 64-bit (x86\_64-w64-mingw32), Clang
 assumes as below;

 -  ``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)``
 -  ``some_directory/bin/gcc.exe``
 -  ``some_directory/bin/clang.exe``
 -  ``some_directory/bin/clang++.exe``
 -  ``some_directory/bin/../include/c++/GCC_version``
 -  ``some_directory/bin/../include/c++/GCC_version/x86_64-w64-mingw32``
 -  ``some_directory/bin/../include/c++/GCC_version/i686-w64-mingw32``
 -  ``some_directory/bin/../include/c++/GCC_version/backward``
 -  ``some_directory/bin/../x86_64-w64-mingw32/include``
 -  ``some_directory/bin/../i686-w64-mingw32/include``
 -  ``some_directory/bin/../include``

 This directory layout is standard for any toolchain you will find on the
 official `MinGW-w64 website <http://mingw-w64.sourceforge.net>`_.

 Clang expects the GCC executable "gcc.exe" compiled for
 ``i686-w64-mingw32`` (or ``x86_64-w64-mingw32``) to be present on PATH.

 `Some tests might fail <https://bugs.llvm.org/show_bug.cgi?id=9072>`_ on
 ``x86_64-w64-mingw32``.

 .. _clang-cl:

 clang-cl
 ========

 clang-cl is an alternative command-line interface to Clang, designed for
 compatibility with the Visual C++ compiler, cl.exe.

 To enable clang-cl to find system headers, libraries, and the linker when run
 from the command-line, it should be executed inside a Visual Studio Native Tools
 Command Prompt or a regular Command Prompt where the environment has been set
 up using e.g. `vcvars32.bat <http://msdn.microsoft.com/en-us/library/f2ccy3wt.aspx>`_.

 clang-cl can also be used from inside Visual Studio by using an LLVM Platform
 Toolset.

 Command-Line Options
 --------------------

 To be compatible with cl.exe, clang-cl supports most of the same command-line
 options. Those options can start with either ``/`` or ``-``. It also supports
 some of Clang's core options, such as the ``-W`` options.

 Options that are known to clang-cl, but not currently supported, are ignored
 with a warning. For example:

   ::

     clang-cl.exe: warning: argument unused during compilation: '/AI'

 To suppress warnings about unused arguments, use the ``-Qunused-arguments`` option.

 Options that are not known to clang-cl will be ignored by default. Use the
 ``-Werror=unknown-argument`` option in order to treat them as errors. If these
 options are spelled with a leading ``/``, they will be mistaken for a filename:

   ::

     clang-cl.exe: error: no such file or directory: '/foobar'

 Please `file a bug <https://bugs.llvm.org/enter_bug.cgi?product=clang&component=Driver>`_
 for any valid cl.exe flags that clang-cl does not understand.

 Execute ``clang-cl /?`` to see a list of supported options:

   ::

     CL.EXE COMPATIBILITY OPTIONS:
       /?                      Display available options
       /arch:<value>           Set architecture for code generation
       /Brepro-                Emit an object file which cannot be reproduced over time
       /Brepro                 Emit an object file which can be reproduced over time
       /C                      Don't discard comments when preprocessing
       /c                      Compile only
       /d1reportAllClassLayout Dump record layout information
       /diagnostics:caret      Enable caret and column diagnostics (on by default)
       /diagnostics:classic    Disable column and caret diagnostics
       /diagnostics:column     Disable caret diagnostics but keep column info
       /D <macro[=value]>      Define macro
       /EH<value>              Exception handling model
       /EP                     Disable linemarker output and preprocess to stdout
       /execution-charset:<value>
                               Runtime encoding, supports only UTF-8
       /E                      Preprocess to stdout
       /fallback               Fall back to cl.exe if clang-cl fails to compile
       /FA                     Output assembly code file during compilation
       /Fa<file or directory>  Output assembly code to this file during compilation (with /FA)
       /Fe<file or directory>  Set output executable file or directory (ends in / or \)
       /FI <value>             Include file before parsing
       /Fi<file>               Set preprocess output file name (with /P)
       /Fo<file or directory>  Set output object file, or directory (ends in / or \) (with /c)
       /fp:except-
       /fp:except
       /fp:fast
       /fp:precise
       /fp:strict
       /Fp<filename>           Set pch filename (with /Yc and /Yu)
       /GA                     Assume thread-local variables are defined in the executable
       /Gd                     Set __cdecl as a default calling convention
       /GF-                    Disable string pooling
       /GR-                    Disable emission of RTTI data
       /GR                     Enable emission of RTTI data
       /Gr                     Set __fastcall as a default calling convention
       /GS-                    Disable buffer security check
       /GS                     Enable buffer security check
       /Gs<value>              Set stack probe size
       /Gv                     Set __vectorcall as a default calling convention
       /Gw-                    Don't put each data item in its own section
       /Gw                     Put each data item in its own section
       /GX-                    Enable exception handling
       /GX                     Enable exception handling
       /Gy-                    Don't put each function in its own section
       /Gy                     Put each function in its own section
       /Gz                     Set __stdcall as a default calling convention
       /help                   Display available options
       /imsvc <dir>            Add directory to system include search path, as if part of %INCLUDE%
       /I <dir>                Add directory to include search path
       /J                      Make char type unsigned
       /LDd                    Create debug DLL
       /LD                     Create DLL
       /link <options>         Forward options to the linker
       /MDd                    Use DLL debug run-time
       /MD                     Use DLL run-time
       /MTd                    Use static debug run-time
       /MT                     Use static run-time
       /Od                     Disable optimization
       /Oi-                    Disable use of builtin functions
       /Oi                     Enable use of builtin functions
       /Os                     Optimize for size
       /Ot                     Optimize for speed
       /O<value>               Optimization level
       /o <file or directory>  Set output file or directory (ends in / or \)
       /P                      Preprocess to file
       /Qvec-                  Disable the loop vectorization passes
       /Qvec                   Enable the loop vectorization passes
       /showIncludes           Print info about included files to stderr
       /source-charset:<value> Source encoding, supports only UTF-8
       /std:<value>            Language standard to compile for
       /TC                     Treat all source files as C
       /Tc <filename>          Specify a C source file
       /TP                     Treat all source files as C++
       /Tp <filename>          Specify a C++ source file
       /utf-8                  Set source and runtime encoding to UTF-8 (default)
       /U <macro>              Undefine macro
       /vd<value>              Control vtordisp placement
       /vmb                    Use a best-case representation method for member pointers
       /vmg                    Use a most-general representation for member pointers
       /vmm                    Set the default most-general representation to multiple inheritance
       /vms                    Set the default most-general representation to single inheritance
       /vmv                    Set the default most-general representation to virtual inheritance
       /volatile:iso           Volatile loads and stores have standard semantics
       /volatile:ms            Volatile loads and stores have acquire and release semantics
       /W0                     Disable all warnings
       /W1                     Enable -Wall
       /W2                     Enable -Wall
       /W3                     Enable -Wall
       /W4                     Enable -Wall and -Wextra
       /Wall                   Enable -Wall and -Wextra
       /WX-                    Do not treat warnings as errors
       /WX                     Treat warnings as errors
       /w                      Disable all warnings
       /Y-                     Disable precompiled headers, overrides /Yc and /Yu
       /Yc<filename>           Generate a pch file for all code up to and including <filename>
       /Yu<filename>           Load a pch file and use it instead of all code up to and including <filename>
       /Z7                     Enable CodeView debug information in object files
       /Zc:sizedDealloc-       Disable C++14 sized global deallocation functions
       /Zc:sizedDealloc        Enable C++14 sized global deallocation functions
       /Zc:strictStrings       Treat string literals as const
       /Zc:threadSafeInit-     Disable thread-safe initialization of static variables
       /Zc:threadSafeInit      Enable thread-safe initialization of static variables
       /Zc:trigraphs-          Disable trigraphs (default)
       /Zc:trigraphs           Enable trigraphs
       /Zc:twoPhase-           Disable two-phase name lookup in templates
       /Zc:twoPhase            Enable two-phase name lookup in templates
       /Zd                     Emit debug line number tables only
       /Zi                     Alias for /Z7. Does not produce PDBs.
       /Zl                     Don't mention any default libraries in the object file
       /Zp                     Set the default maximum struct packing alignment to 1
       /Zp<value>              Specify the default maximum struct packing alignment
       /Zs                     Syntax-check only

     OPTIONS:
       -###                    Print (but do not run) the commands to run for this compilation
       --analyze               Run the static analyzer
       -fansi-escape-codes     Use ANSI escape codes for diagnostics
       -fcolor-diagnostics     Use colors in diagnostics
       -fdebug-macro           Emit macro debug information
       -fdelayed-template-parsing
                               Parse templated function definitions at the end of the translation unit
       -fdiagnostics-absolute-paths
                               Print absolute paths in diagnostics
       -fdiagnostics-parseable-fixits
                               Print fix-its in machine parseable form
       -flto=<value>           Set LTO mode to either 'full' or 'thin'
       -flto                   Enable LTO in 'full' mode
       -fms-compatibility-version=<value>
                               Dot-separated value representing the Microsoft compiler version
                               number to report in _MSC_VER (0 = don't define it (default))
       -fms-compatibility      Enable full Microsoft Visual C++ compatibility
       -fms-extensions         Accept some non-standard constructs supported by the Microsoft compiler
       -fmsc-version=<value>   Microsoft compiler version number to report in _MSC_VER
                               (0 = don't define it (default))
       -fno-debug-macro        Do not emit macro debug information
       -fno-delayed-template-parsing
                               Disable delayed template parsing
       -fno-sanitize-address-use-after-scope
                               Disable use-after-scope detection in AddressSanitizer
       -fno-sanitize-blacklist Don't use blacklist file for sanitizers
       -fno-sanitize-cfi-cross-dso
                               Disable control flow integrity (CFI) checks for cross-DSO calls.
       -fno-sanitize-coverage=<value>
                               Disable specified features of coverage instrumentation for Sanitizers
       -fno-sanitize-memory-track-origins
                               Disable origins tracking in MemorySanitizer
       -fno-sanitize-recover=<value>
                               Disable recovery for specified sanitizers
       -fno-sanitize-stats     Disable sanitizer statistics gathering.
       -fno-sanitize-thread-atomics
                               Disable atomic operations instrumentation in ThreadSanitizer
       -fno-sanitize-thread-func-entry-exit
                               Disable function entry/exit instrumentation in ThreadSanitizer
       -fno-sanitize-thread-memory-access
                               Disable memory access instrumentation in ThreadSanitizer
       -fno-sanitize-trap=<value>
                               Disable trapping for specified sanitizers
       -fno-standalone-debug   Limit debug information produced to reduce size of debug binary
       -fprofile-instr-generate=<file>
                               Generate instrumented code to collect execution counts into <file>
                               (overridden by LLVM_PROFILE_FILE env var)
       -fprofile-instr-generate
                               Generate instrumented code to collect execution counts into default.profraw file
                               (overridden by '=' form of option or LLVM_PROFILE_FILE env var)
       -fprofile-instr-use=<value>
                               Use instrumentation data for profile-guided optimization
       -fsanitize-address-field-padding=<value>
                               Level of field padding for AddressSanitizer
       -fsanitize-address-globals-dead-stripping
                               Enable linker dead stripping of globals in AddressSanitizer
       -fsanitize-address-use-after-scope
                               Enable use-after-scope detection in AddressSanitizer
       -fsanitize-blacklist=<value>
                               Path to blacklist file for sanitizers
       -fsanitize-cfi-cross-dso
                               Enable control flow integrity (CFI) checks for cross-DSO calls.
       -fsanitize-coverage=<value>
                               Specify the type of coverage instrumentation for Sanitizers
       -fsanitize-memory-track-origins=<value>
                               Enable origins tracking in MemorySanitizer
       -fsanitize-memory-track-origins
                               Enable origins tracking in MemorySanitizer
       -fsanitize-memory-use-after-dtor
                               Enable use-after-destroy detection in MemorySanitizer
       -fsanitize-recover=<value>
                               Enable recovery for specified sanitizers
       -fsanitize-stats        Enable sanitizer statistics gathering.
       -fsanitize-thread-atomics
                               Enable atomic operations instrumentation in ThreadSanitizer (default)
       -fsanitize-thread-func-entry-exit
                               Enable function entry/exit instrumentation in ThreadSanitizer (default)
       -fsanitize-thread-memory-access
                               Enable memory access instrumentation in ThreadSanitizer (default)
       -fsanitize-trap=<value> Enable trapping for specified sanitizers
       -fsanitize-undefined-strip-path-components=<number>
                               Strip (or keep only, if negative) a given number of path components when emitting check metadata.
       -fsanitize=<check>      Turn on runtime checks for various forms of undefined or suspicious
                               behavior. See user manual for available checks
       -fstandalone-debug      Emit full debug info for all types used by the program
       -gcodeview              Generate CodeView debug information
       -gline-tables-only      Emit debug line number tables only
       -miamcu                 Use Intel MCU ABI
       -mllvm <value>          Additional arguments to forward to LLVM's option processing
       -nobuiltininc           Disable builtin #include directories
       -Qunused-arguments      Don't emit warning for unused driver arguments
       -R<remark>              Enable the specified remark
       --target=<value>        Generate code for the given target
       -v                      Show commands to run and use verbose output
       -W<warning>             Enable the specified warning
       -Xclang <arg>           Pass <arg> to the clang compiler

 The /fallback Option
 ^^^^^^^^^^^^^^^^^^^^

 When clang-cl is run with the ``/fallback`` option, it will first try to
 compile files itself. For any file that it fails to compile, it will fall back
 and try to compile the file by invoking cl.exe.

 This option is intended to be used as a temporary means to build projects where
 clang-cl cannot successfully compile all the files. clang-cl may fail to compile
 a file either because it cannot generate code for some C++ feature, or because
 it cannot parse some Microsoft language extension.