Reverted r103214.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@103222 91177308-0d34-0410-b5e6-96231b3b80d8
author: mike-m <mikem.llvm@gmail.com> 2010-05-07 00:42:33 +0000
committer: mike-m <mikem.llvm@gmail.com> 2010-05-07 00:42:33 +0000
commit: 3e5d00c2c727e54d96ce984edfc5cca4710eae10 (patch)
tree: 1486fcced3985d138da8a6b426b0d698f53f3f0a /docs/UsersManual.html
parent: 6fb94391dc7cb11fd4bbdb969bbab11b6b48c223 (diff)
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+<html>
+<head>
+<title>Clang Compiler User's Manual</title>
+<link type="text/css" rel="stylesheet" href="../menu.css" />
+<link type="text/css" rel="stylesheet" href="../content.css" />
+<style type="text/css">
+td {
+	vertical-align: top;
+}
+</style>
+</head>
+<body>
+
+<!--#include virtual="../menu.html.incl"-->
+
+<div id="content">
+
+<h1>Clang Compiler User's Manual</h1>
+
+<ul>
+<li><a href="#intro">Introduction</a>
+  <ul>
+  <li><a href="#terminology">Terminology</a></li>
+  <li><a href="#basicusage">Basic Usage</a></li>
+  </ul>
+</li>
+<li><a href="#commandline">Command Line Options</a>
+  <ul>
+  <li><a href="#cl_diagnostics">Options to Control Error and Warning
+      Messages</a></li>
+  </ul>
+</li>
+<li><a href="#general_features">Language and Target-Independent Features</a>
+ <ul>
+  <li><a href="#diagnostics">Controlling Errors and Warnings</a></li>
+   <ul>
+   <li><a href="#diagnostics_display">Controlling How Clang Displays Diagnostics</a></li>
+   <li><a href="#diagnostics_mappings">Diagnostic Mappings</a></li>
+   <li><a href="#diagnostics_commandline">Controlling Diagnostics via Command Line Flags</a></li>
+   <li><a href="#diagnostics_pragmas">Controlling Diagnostics via Pragmas</a></li>
+   </ul>
+  <li><a href="#precompiledheaders">Precompiled Headers</a></li>
+  <li><a href="#codegen">Controlling Code Generation</a></li>
+  </ul>
+</li>
+<li><a href="#c">C Language Features</a>
+  <ul>
+  <li><a href="#c_ext">Extensions supported by clang</a></li>
+  <li><a href="#c_modes">Differences between various standard modes</a></li>
+  <li><a href="#c_unimpl_gcc">GCC extensions not implemented yet</a></li>
+  <li><a href="#c_unsupp_gcc">Intentionally unsupported GCC extensions</a></li>
+  <li><a href="#c_ms">Microsoft extensions</a></li>
+  </ul>
+</li>
+<li><a href="#objc">Objective-C Language Features</a>
+  <ul>
+  <li><a href="#objc_incompatibilities">Intentional Incompatibilities with
+      GCC</a></li>
+  </ul>
+</li>
+<li><a href="#cxx">C++ Language Features</a>
+</li>
+<li><a href="#objcxx">Objective C++ Language Features</a>
+</li>
+<li><a href="#target_features">Target-Specific Features and Limitations</a>
+  <ul>
+  <li><a href="#target_arch">CPU Architectures Features and Limitations</a>
+    <ul>
+    <li><a href="#target_arch_x86">X86</a></li>
+    <li><a href="#target_arch_arm">ARM</a></li>
+    <li><a href="#target_arch_other">Other platforms</a></li>
+    </ul>
+  </li>
+  <li><a href="#target_os">Operating System Features and Limitations</a>
+    <ul>
+    <li><a href="#target_os_darwin">Darwin (Mac OS/X)</a></li>
+    <li>Linux, etc.</li>
+    </ul>
+  
+  </li>
+  </ul>
+</li>
+</ul>
+
+
+<!-- ======================================================================= -->
+<h2 id="intro">Introduction</h2>
+<!-- ======================================================================= -->
+
+<p>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 <a href="http://clang.llvm.org">Clang
+Web Site</a> or the <a href="http://llvm.org">LLVM Web Site</a>.</p>
+
+<p>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 <a href="InternalsManual.html">the Clang Internals Manual</a>.  If you are
+interested in the <a href="http://clang.llvm.org/StaticAnalysis.html">Clang
+Static Analyzer</a>, please see its web page.</p>
+
+<p>Clang is designed to support the C family of programming languages, which
+includes <a href="#c">C</a>, <a href="#objc">Objective-C</a>, <a
+href="#cxx">C++</a>, and <a href="#objcxx">Objective-C++</a> as well as many
+dialects of those.  For language-specific information, please see the
+corresponding language specific section:</p>
+
+<ul>
+<li><a href="#c">C Language</a>: K&amp;R C, ANSI C89, ISO C90, ISO C94
+    (C89+AMD1), ISO C99 (+TC1, TC2, TC3). </li>
+<li><a href="#objc">Objective-C Language</a>: ObjC 1, ObjC 2, ObjC 2.1, plus
+    variants depending on base language.</li>
+<li><a href="#cxx">C++ Language Features</a></li>
+<li><a href="#objcxx">Objective C++ Language</a></li>
+</ul>
+
+<p>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".</p>
+
+<p>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 <a href="#target_features">Target-Specific Features and
+Limitations</a> section for more details.</p>
+
+<p>The rest of the introduction introduces some basic <a
+href="#terminology">compiler terminology</a> that is used throughout this manual
+and contains a basic <a href="#basicusage">introduction to using Clang</a>
+as a command line compiler.</p>
+
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+<h3 id="terminology">Terminology</h3>
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+
+<p>Front end, parser, backend, preprocessor, undefined behavior, diagnostic,
+ optimizer</p>
+
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+<h3 id="basicusage">Basic Usage</h3>
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+
+<p>Intro to how to use a C compiler for newbies.</p>
+<p>
+compile + link
+
+compile then link
+
+debug info
+
+enabling optimizations
+
+picking a language to use, defaults to C99 by default.  Autosenses based on
+extension.
+
+using a makefile
+</p>
+
+
+<!-- ======================================================================= -->
+<h2 id="commandline">Command Line Options</h2>
+<!-- ======================================================================= -->
+
+<p>
+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 -c, -g, etc.
+</p>
+
+
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+<h3 id="cl_diagnostics">Options to Control Error and Warning Messages</h3>
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+
+<p><b>-Werror</b>: Turn warnings into errors.</p>
+<p><b>-Werror=foo</b>: Turn warning "foo" into an error.</p>
+<p><b>-Wno-error=foo</b>: Turn warning "foo" into an warning even if -Werror is
+   specified.</p>
+<p><b>-Wfoo</b>: Enable warning foo</p>
+<p><b>-Wno-foo</b>: Disable warning foo</p>
+<p><b>-w</b>: Disable all warnings.</p>
+<p><b>-pedantic</b>: Warn on language extensions.</p>
+<p><b>-pedantic-errors</b>: Error on language extensions.</p>
+<p><b>-Wsystem-headers</b>: Enable warnings from system headers.</p>
+
+<p><b>-ferror-limit=123</b>: Stop emitting diagnostics after 123 errors have
+   been produced.  The default is 20, and the error limit can be disabled with
+   -ferror-limit=0.</p>
+
+<p><b>-ftemplate-backtrace-limit=123</b>: 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.</p>
+
+<!-- ================================================= -->
+<h4 id="cl_diag_formatting">Formatting of Diagnostics</h4>
+<!-- ================================================= -->
+
+<p>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 by another program that wants to
+parse simple and consistent output, not a person. For these cases, Clang
+provides a wide range of options to control the exact output format of the
+diagnostics that it generates.</p>
+
+<dl>
+
+<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
+<dt id="opt_fshow-column"><b>-f[no-]show-column</b>: Print column number in
+diagnostic.</dt>
+<dd>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:</p>
+
+<pre>
+  test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
+  #endif bad
+         ^
+         //
+</pre>
+
+<p>When this is disabled, Clang will print "test.c:28: warning..." with no
+column number.</p>
+</dd>
+
+<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
+<dt id="opt_fshow-source-location"><b>-f[no-]show-source-location</b>: Print
+source file/line/column information in diagnostic.</dt>
+<dd>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:</p>
+
+<pre>
+  test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
+  #endif bad
+         ^
+         //
+</pre>
+
+<p>When this is disabled, Clang will not print the "test.c:28:8: " part.</p>
+</dd>
+
+<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
+<dt id="opt_fcaret-diagnostics"><b>-f[no-]caret-diagnostics</b>: Print source
+line and ranges from source code in diagnostic.</dt>
+<dd>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:</p>
+
+<pre>
+  test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
+  #endif bad
+         ^
+         //
+</pre>
+
+<p>When this is disabled, Clang will just print:</p>
+
+<pre>
+  test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
+</pre>
+
+</dd>
+
+<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
+<dt id="opt_fdiagnostics-show-option"><b>-f[no-]diagnostics-show-option</b>:
+Enable <tt>[-Woption]</tt> information in diagnostic line.</dt>
+<dd>This option, which defaults to on,
+controls whether or not Clang prints the associated <A
+href="#cl_diag_warning_groups">warning group</a> option name when outputting
+a warning diagnostic.  For example, in this output:</p>
+
+<pre>
+  test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
+  #endif bad
+         ^
+         //
+</pre>
+
+<p>Passing <b>-fno-diagnostics-show-option</b> will prevent Clang from printing
+the [<a href="#opt_Wextra-tokens">-Wextra-tokens</a>] information in the
+diagnostic.  This information tells you the flag needed to enable or disable the
+diagnostic, either from the command line or through <a 
+href="#pragma_GCC_diagnostic">#pragma GCC diagnostic</a>.</dd>
+
+<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
+<dt id="opt_fdiagnostics-show-category"><b>-fdiagnostics-show-category=none/id/name</b>:
+Enable printing category information in diagnostic line.</dt>
+<dd>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).</p>
+
+<p>For example, a format string warning will produce these three renditions
+based on the setting of this option:</p>
+
+<pre>
+  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<b>,1</b>]
+  t.c:3:11: warning: conversion specifies type 'char *' but the argument has type 'int' [-Wformat<b>,Format String</b>]
+</pre>
+
+<p>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.</p>
+</dd>
+
+
+
+<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
+<dt id="opt_fdiagnostics-fixit-info"><b>-f[no-]diagnostics-fixit-info</b>:
+Enable "FixIt" information in the diagnostics output.</dt>
+<dd>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:</p>
+
+<pre>
+  test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
+  #endif bad
+         ^
+         //
+</pre>
+
+<p>Passing <b>-fno-diagnostics-fixit-info</b> 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.</p>
+</dd>
+
+<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
+<dt id="opt_fdiagnostics-print-source-range-info">
+<b>-f[no-]diagnostics-print-source-range-info</b>:
+Print machine parsable information about source ranges.</dt>
+<dd>This option, which defaults to off, controls whether or not Clang prints
+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:</p>
+
+<pre>
+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;
+       ~~~~~~ ^ ~~~~~~~
+</pre>
+
+<p>The {}'s are generated by -fdiagnostics-print-source-range-info.</p>
+</dd>
+
+
+</dl>
+
+ 
+
+
+<!-- ===================================================== -->
+<h4 id="cl_diag_warning_groups">Individual Warning Groups</h4>
+<!-- ===================================================== -->
+
+<p>TODO: Generate this from tblgen.  Define one anchor per warning group.</p>
+
+
+<dl>
+
+
+<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
+<dt id="opt_Wextra-tokens"><b>-Wextra-tokens</b>: Warn about excess tokens at
+    the end of a preprocessor directive.</dt>
+<dd>This option, which defaults to on, enables warnings about extra tokens at
+the end of preprocessor directives.  For example:</p>
+
+<pre>
+  test.c:28:8: warning: extra tokens at end of #endif directive [-Wextra-tokens]
+  #endif bad
+         ^
+</pre>
+
+<p>These extra tokens are not strictly conforming, and are usually best handled
+by commenting them out.</p>
+
+<p>This option is also enabled by <a href="">-Wfoo</a>, <a href="">-Wbar</a>,
+ and <a href="">-Wbaz</a>.</p>
+</dd>
+
+</dl>
+
+<!-- ======================================================================= -->
+<h2 id="general_features">Language and Target-Independent Features</h2>
+<!-- ======================================================================= -->
+
+
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+<h3 id="diagnostics">Controlling Errors and Warnings</h3>
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+
+<p>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.</p>
+
+<h4 id="diagnostics_display">Controlling How Clang Displays Diagnostics</h4>
+
+<p>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:</p>
+
+<p>
+<ol>
+<li>A file/line/column indicator that shows exactly where the diagnostic occurs
+    in your code [<a href="#opt_fshow-column">-fshow-column</a>, <a
+    href="#opt_fshow-source-location">-fshow-source-location</a>].</li>
+<li>A categorization of the diagnostic as a note, warning, error, or fatal
+    error.</li>
+<li>A text string that describes what the problem is.</li>
+<li>An option that indicates how to control the diagnostic (for diagnostics that
+    support it) [<a 
+   href="#opt_fdiagnostics-show-option">-fdiagnostics-show-option</a>].</li>
+<li>A high-level category for the diagnostic for clients that want to group
+    diagnostics by class (for diagnostics that
+    support it) [<a 
+   href="#opt_fdiagnostics-show-category">-fdiagnostics-show-category</a>].</li>
+<li>The line of source code that the issue occurs on, along with a caret and
+    ranges that indicate the important locations [<a
+    href="opt_fcaret-diagnostics">-fcaret-diagnostics</a>].</li>
+<li>"FixIt" information, which is a concise explanation of how to fix the
+    problem (when Clang is certain it knows) [<a
+    href="opt_fdiagnostics-fixit-info">-fdiagnostics-fixit-info</a>].</li>
+<li>A machine-parsable representation of the ranges involved (off by
+    default) [<a
+      href="opt_fdiagnostics-print-source-range-info">-fdiagnostics-print-source-range-info</a>].</li>
+</ol></p>
+
+<p>For more information please see <a href="#cl_diag_formatting">Formatting of
+Diagnostics</a>.</p>
+
+<h4 id="diagnostics_mappings">Diagnostic Mappings</h4>
+
+<p>All diagnostics are mapped into one of these 5 classes:</p>
+
+<p>
+<ul>
+<li>Ignored</li>
+<li>Note</li>
+<li>Warning</li>
+<li>Error</li>
+<li>Fatal</li>
+</ul></p>
+
+<h4 id="diagnostics_commandline">Controlling Diagnostics via Command Line Flags</h4>
+
+<p>-W flags, -pedantic, etc</p>
+
+<h4 id="diagnostics_pragmas">Controlling Diagnostics via Pragmas</h4>
+
+<p>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. </p>
+
+<p>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:</p>
+
+<pre>
+#pragma GCC diagnostic ignored "-Wall"
+</pre>
+
+<p>In addition to all of the functionality of 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.</p>
+
+<p>In the below example
+-Wmultichar is ignored for only a single line of code, after which the
+diagnostics return to whatever state had previously existed.</p>
+
+<pre>
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wmultichar"
+
+char b = 'df'; // no warning.
+
+#pragma clang diagnostic pop
+</pre>
+
+<p>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. </p>
+
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+<h3 id="precompiledheaders">Precompiled Headers</h3>
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+
+<p><a href="http://en.wikipedia.org/wiki/Precompiled_header">Precompiled
+headers</a> 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 a
+highly effective at speeding up program compilation on systems with very large
+system headers (e.g., Mac OS/X).</p>
+
+<h4>Generating a PCH File</h4>
+
+<p>To generate a PCH file using Clang, one invokes Clang with
+the <b><tt>-x <i>&lt;language&gt;</i>-header</tt></b> option. This mirrors the
+interface in GCC for generating PCH files:</p>
+
+<pre>
+  $ gcc -x c-header test.h -o test.h.gch
+  $ clang -x c-header test.h -o test.h.pch
+</pre>
+
+<h4>Using a PCH File</h4>
+
+<p>A PCH file can then be used as a prefix header when a
+<b><tt>-include</tt></b> option is passed to <tt>clang</tt>:</p>
+
+<pre>
+  $ clang -include test.h test.c -o test
+</pre>
+
+<p>The <tt>clang</tt> driver will first check if a PCH file for <tt>test.h</tt>
+is available; if so, the contents of <tt>test.h</tt> (and the files it includes)
+will be processed from the PCH file. Otherwise, Clang falls back to
+directly processing the content of <tt>test.h</tt>. This mirrors the behavior of
+GCC.</p>
+
+<p><b>NOTE:</b> Clang does <em>not</em> automatically use PCH files
+for headers that are directly included within a source file. For example:</p>
+
+<pre>
+  $ clang -x c-header test.h -o test.h.pch
+  $ cat test.c
+  #include "test.h"
+  $ clang test.c -o test
+</pre>
+
+<p>In this example, <tt>clang</tt> will not automatically use the PCH file for
+<tt>test.h</tt> since <tt>test.h</tt> was included directly in the source file
+and not specified on the command line using <tt>-include</tt>.</p>
+
+<h4>Relocatable PCH Files</h4>
+<p>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.</p>
+
+<p>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 <code>mylib.h</code> that
+will be installed into <code>/usr/include</code>, create a subdirectory 
+<code>build/usr/include</code> and place the header <code>mylib.h</code> into
+that subdirectory. If <code>mylib.h</code> depends on other headers, then 
+they can be stored within <code>build/usr/include</code> in a way that mimics
+the installed location.</p>
+
+<p>Building a relocatable precompiled header requires two additional arguments.
+First, pass the <code>--relocatable-pch</code> flag to indicate that the
+resulting PCH file should be relocatable. Second, pass 
+<code>-isysroot /path/to/build</code>, which makes all includes for your
+library relative to the build directory. For example:</p>
+
+<pre>
+  # clang -x c-header --relocatable-pch -isysroot /path/to/build /path/to/build/mylib.h mylib.h.pch
+</pre>
+
+<p>When loading the relocatable PCH file, the various headers used in the PCH
+file are found from the system header root. For example, <code>mylib.h</code>
+can be found in <code>/usr/include/mylib.h</code>. If the headers are installed
+in some other system root, the <code>-isysroot</code> option can be used provide
+a different system root from which the headers will be based. For example,
+<code>-isysroot /Developer/SDKs/MacOSX10.4u.sdk</code> will look for 
+<code>mylib.h</code> in 
+<code>/Developer/SDKs/MacOSX10.4u.sdk/usr/include/mylib.h</code>.</p>
+
+<p>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. 
+Relocatable precompiled headers also have some performance impact, because
+the difference in location between the header locations at PCH build time vs. 
+at the time of PCH use requires one of the PCH optimizations,
+<code>stat()</code> caching, to be disabled. However, this change is only 
+likely to affect PCH files that reference a large number of headers.</p>
+
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+<h3 id="codegen">Controlling Code Generation</h3>
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+
+<p>Clang provides a number of ways to control code generation.  The options are listed below.</p>
+
+<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
+<dt id="opt_fcatch-undefined-behavior"><b>-fcatch-undefined-behavior</b>: Turn
+on runtime code generation to check for undefined behavior.</dt>
+
+<dd>This option, which defaults to off, controls whether or not Clang
+adds runtime checks for undefined runtime behavior.  If the check fails,
+<tt>__builtin_trap()</tt> is used to indicate failure.
+The checks are:
+<p>
+<li>Subscripting where the static type of one operand is variable
+    which is decayed from an array type and the other operand is
+    greater than the size of the array or less than zero.</li>
+<li>Shift operators where the amount shifted is greater or equal to the
+    promoted bit-width of the left-hand-side or less than zero.</li>
+<li>If control flow reaches __builtin_unreachable.
+<li>When llvm implements more __builtin_object_size support, reads and
+    writes for objects that __builtin_object_size indicates we aren't
+    accessing valid memory.  Bit-fields and vectors are not yet checked.
+</p>
+</dd>
+
+<dt id="opt_fno-assume-sane-operator-new"><b>-fno-assume-sane-operator-new</b>:
+Don't assume that the C++'s new operator is sane.</dt>
+<dd>This option tells the compiler to do not assume that C++'s global new
+operator will always return a pointer that do not
+alias any other pointer when the function returns.</dd>
+
+<!-- ======================================================================= -->
+<h2 id="c">C Language Features</h2>
+<!-- ======================================================================= -->
+
+<p>The support for standard C in clang is feature-complete except for the C99
+floating-point pragmas.</p>
+
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+<h3 id="c_ext">Extensions supported by clang</h3>
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+
+<p>See <a href="LanguageExtensions.html">clang language extensions</a>.</p>
+
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+<h3 id="c_modes">Differences between various standard modes</h3>
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+
+<p>clang supports the -std option, which changes what language mode clang uses.
+The supported modes for C are c89, gnu89, c94, c99, gnu99 and various aliases
+for those modes.  If no -std option is specified, clang defaults to gnu99 mode.
+</p>
+
+<p>Differences between all c* and gnu* modes:</p>
+<ul>
+<li>c* modes define "__STRICT_ANSI__".</li>
+<li>Target-specific defines not prefixed by underscores, like "linux", are
+defined in gnu* modes.</li>
+<li>Trigraphs default to being off in gnu* modes; they can be enabled by the
+-trigraphs option.</li>
+<li>The parser recognizes "asm" and "typeof" as keywords in gnu* modes; the
+variants "__asm__" and "__typeof__" are recognized in all modes.</li>
+<li>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.</li>
+</ul>
+
+<p>Differences between *89 and *99 modes:</p>
+<ul>
+<li>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.</li>
+<li>Digraphs are not recognized in c89 mode.</li>
+<li>The scope of names defined inside a "for", "if", "switch", "while", or "do"
+statement is different. (example: "if ((struct x {int x;}*)0) {}".)</li>
+<li>__STDC_VERSION__ is not defined in *89 modes.</li>
+<li>"inline" is not recognized as a keyword in c89 mode.</li>
+<li>"restrict" is not recognized as a keyword in *89 modes.</li>
+<li>Commas are allowed in integer constant expressions in *99 modes.</li>
+<li>Arrays which are not lvalues are not implicitly promoted to pointers in
+*89 modes.</li>
+<li>Some warnings are different.</li>
+</ul>
+
+<p>c94 mode is identical to c89 mode except that digraphs are enabled in
+c94 mode (FIXME: And __STDC_VERSION__ should be defined!).</p>
+
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+<h3 id="c_unimpl_gcc">GCC extensions not implemented yet</h3>
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+
+<p>clang tries to be compatible with gcc as much as possible, but some gcc
+extensions are not implemented yet:</p>
+
+<ul>
+<li>clang does not support __label__
+(<a href="http://llvm.org/bugs/show_bug.cgi?id=3429">bug 3429</a>). This is
+a relatively small feature, so it is likely to be implemented relatively
+soon.</li>
+
+<li>clang does not support attributes on function pointers
+(<a href="http://llvm.org/bugs/show_bug.cgi?id=2461">bug 2461</a>). This is
+a relatively important feature, so it is likely to be implemented relatively
+soon.</li>
+
+<li>clang does not support #pragma weak
+(<a href="http://llvm.org/bugs/show_bug.cgi?id=3679">bug 3679</a>). Due to
+the uses described in the bug, this is likely to be implemented at some
+point, at least partially.</li>
+
+<li>clang does not support #pragma align
+(<a href="http://llvm.org/bugs/show_bug.cgi?id=3811">bug 3811</a>). This is a
+relatively small feature, so it is likely to be implemented relatively
+soon.</li>
+
+<li>clang does not support code generation for local variables pinned to
+registers (<a href="http://llvm.org/bugs/show_bug.cgi?id=3933">bug 3933</a>).
+This is a relatively small feature, so it is likely to be implemented
+relatively soon.</li>
+
+<li>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.</li>
+
+<li>clang does not support nested functions; this is a complex feature which
+is infrequently used, so it is unlikely to be implemented anytime soon.</li>
+
+<li>clang does not support global register variables, this is unlikely
+to be implemented soon because it requires additional LLVM backend support.
+</li>
+
+<li>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.</li>
+
+<li>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.</li>
+
+<li>clang does not support the gcc extension for forward-declaring function
+parameters; this has not showed up in any real-world code yet, though, so it
+might never be implemented.</li>
+
+</ul>
+
+<p>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 <a href="#cxx">C++ Language Features</a>.
+Also, this list does not include bugs in mostly-implemented features; please
+see the <a href="http://llvm.org/bugs/buglist.cgi?quicksearch=product%3Aclang+component%3A-New%2BBugs%2CAST%2CBasic%2CDriver%2CHeaders%2CLLVM%2BCodeGen%2Cparser%2Cpreprocessor%2CSemantic%2BAnalyzer">
+bug tracker</a> for known existing bugs (FIXME: Is there a section for
+bug-reporting guidelines somewhere?).</p>
+
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+<h3 id="c_unsupp_gcc">Intentionally unsupported GCC extensions</h3>
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+
+<ul>
+
+<li>clang does not support the gcc extension that allows variable-length arrays
+in structures.  This is for a few of reasons: one, it is tricky
+to implement, two, the extension is completely undocumented, and three, the
+extension appears to be rarely used.</li>
+
+<li>clang does not support duplicate definitions of a function where one is
+inline. This complicates clients of the AST which normally can expect there is
+at most one definition for each function. Source code using this feature should
+be changed to define the inline and out-of-line definitions in separate
+translation units.</li>
+
+<li>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, or calls
+to C library functions like strlen.</li>
+
+<li>clang does not support multiple alternative constraints in inline asm; this
+is an extremely obscure feature which would be complicated to implement
+correctly.</li>
+
+<li>clang does not support __builtin_apply and friends; this extension is
+extremely obscure and difficult to implement reliably.</li>
+
+</ul>
+
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+<h3 id="c_ms">Microsoft extensions</h3>
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+
+<p>clang has some experimental support for extensions from
+Microsoft Visual C++; to enable it, use the -fms-extensions command-line
+option.  This is the default for Windows targets.  Note that the
+support is incomplete; enabling Microsoft extensions will silently drop
+certain constructs (including __declspec and Microsoft-style asm statements).
+</p>
+
+<li>clang does not support the Microsoft extension where anonymous
+record members can be declared using user defined typedefs.</li>
+
+<li>clang supports the Microsoft "#pragma pack" feature for
+controlling record layout. GCC also contains support for this feature,
+however where MSVC and GCC are incompatible clang follows the MSVC
+definition.</li>
+
+<!-- ======================================================================= -->
+<h2 id="objc">Objective-C Language Features</h2>
+<!-- ======================================================================= -->
+
+
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+<h3 id="objc_incompatibilities">Intentional Incompatibilities with GCC</h3>
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+
+<p>No cast of super, no lvalue casts.</p>
+
+
+
+<!-- ======================================================================= -->
+<h2 id="cxx">C++ Language Features</h2>
+<!-- ======================================================================= -->
+
+<p>At this point, Clang C++ is not production-quality and is not recommended for use beyond experimentation.  However, Clang C++ support
+is under active development and is progressing rapidly.  Please see the <a
+href="http://clang.llvm.org/cxx_status.html">C++ Status</a> page for details or
+ask on the mailing list about how you can help.</p>
+
+<p>Note that released Clang compilers will refuse to even try to use clang to compile C++ code unless you pass the <tt>-ccc-clang-cxx</tt> option to the driver. To turn on Clang's C++ support, please pass that flag. Clang compilers built from the Subversion trunk enable C++ support by default, and do not require the <tt>-ccc-clang-cxx</tt> flag.</p>
+ 
+<p>Clang strives to strictly conform to the C++ standard.  That means
+it will reject invalid C++ code that another compiler may accept.  If
+Clang reports errors in your code, please check
+the <a href="http://clang.llvm.org/cxx_compatibility.html">C++
+Compatibility</a> page to see whether they are C++-conformance bugs
+and how you can fix them.</p>
+
+<!-- ======================================================================= -->
+<h2 id="objcxx">Objective C++ Language Features</h2>
+<!-- ======================================================================= -->
+
+<p>At this point, Clang C++ support is not generally useful (and therefore,
+neither is Objective-C++).  Please see the <a href="#cxx">C++ section</a> for
+more information.</p>
+
+<!-- ======================================================================= -->
+<h2 id="target_features">Target-Specific Features and Limitations</h2>
+<!-- ======================================================================= -->
+
+
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+<h3 id="target_arch">CPU Architectures Features and Limitations</h3>
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+
+<!-- ======================== -->
+<h4 id="target_arch_x86">X86</h4>
+<!-- ======================== -->
+<p>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 large C and Objective-C codebases. (FIXME: Anything specific
+we want to say here? Possibly mention some LLVM x86 limitations?)
+
+<!-- ======================== -->
+<h4 id="target_arch_arm">ARM</h4>
+<!-- ======================== -->
+ARM support is mostly feature-complete, but still experimental; it hasn't
+undergone significant testing.
+
+<!-- ======================== -->
+<h4 id="target_arch_other">Other platforms</h4>
+<!-- ======================== -->
+clang currently contains some support for PPC and Sparc; however, significant
+pieces of code generation are still missing, and they haven't undergone
+significant testing.
+
+<p>clang contains some support for the embedded PIC16 processor
+(FIXME: I haven't been keeping track of this; what should this say?).
+
+<p>clang contains limited support for the MSP430 embedded processor, but both
+the clang support and the LLVM backend support are highly experimental.
+
+<p>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. 
+
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+<h3 id="target_os">Operating System Features and Limitations</h3>
+<!-- = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = -->
+
+<!-- ======================================= -->
+<h4 id="target_os_darwin">Darwin (Mac OS/X)</h4>
+<!-- ======================================= -->
+
+<p>No __thread support, 64-bit ObjC support requires SL tools.</p>
+
+</div>
+</body>
+</html>
author	mike-m <mikem.llvm@gmail.com>	2010-05-07 00:42:33 +0000
committer	mike-m <mikem.llvm@gmail.com>	2010-05-07 00:42:33 +0000
commit	3e5d00c2c727e54d96ce984edfc5cca4710eae10 (patch)
tree	1486fcced3985d138da8a6b426b0d698f53f3f0a /docs/UsersManual.html
parent	6fb94391dc7cb11fd4bbdb969bbab11b6b48c223 (diff)