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   <!--*********************************************************************-->
   <h1>Hacking on Clang</h1>
   <!--*********************************************************************-->

   <p>This document provides some hints for how to get started hacking
   on Clang for developers who are new to the Clang and/or LLVM
   codebases.</p>
     <ul>
       <li><a href="#style">Coding Standards</a></li>
       <li><a href="#docs">Developer Documentation</a></li>
       <li><a href="#debugging">Debugging</a></li>
       <li><a href="#testing">Testing</a>
       <ul>
         <li><a href="#testingNonWindows">Testing on Unix-like Systems</a></li>
         <li><a href="#testingWindows">Testing using Visual Studio on Windows</a></li>
         <li><a href="#testingCommands">Testing on the Command Line</a></li>
         <li><a href="#testingLibc++">Testing changes affecting libc++</a></li>
       </ul>
       </li>
       <li><a href="#patches">Creating Patch Files</a></li>
       <li><a href="#irgen">LLVM IR Generation</a></li>
     </ul>

   <!--=====================================================================-->
   <h2 id="style">Coding Standards</h2>
   <!--=====================================================================-->

   <p>Clang follows the
   LLVM <a href="https://llvm.org/docs/CodingStandards.html">Coding
   Standards</a>. When submitting patches, please take care to follow these standards
   and to match the style of the code to that present in Clang (for example, in
   terms of indentation, bracing, and statement spacing).</p>

   <p>Clang has a few additional coding standards:</p>
   <ul>
     <li><i>cstdio is forbidden</i>: library code should not output diagnostics
       or other information using <tt>cstdio</tt>; debugging routines should
       use <tt>llvm::errs()</tt>. Other uses of <tt>cstdio</tt> impose behavior
       upon clients and block integrating Clang as a library. Libraries should
       support <tt>raw_ostream</tt> based interfaces for textual
       output. See <a href="https://llvm.org/docs/CodingStandards.html#use-raw-ostream">Coding
       Standards</a>.</li>
   </ul>

   <!--=====================================================================-->
   <h2 id="docs">Developer Documentation</h2>
   <!--=====================================================================-->

   <p>Both Clang and LLVM use doxygen to provide API documentation. Their
   respective web pages (generated nightly) are here:</p>
     <ul>
       <li><a href="https://clang.llvm.org/doxygen">Clang</a></li>
       <li><a href="https://llvm.org/doxygen">LLVM</a></li>
     </ul>

   <p>For work on the LLVM IR generation, the LLVM assembly language
   <a href="https://llvm.org/docs/LangRef.html">reference manual</a> is
   also useful.</p>

   <!--=====================================================================-->
   <h2 id="debugging">Debugging</h2>
   <!--=====================================================================-->

   <p>Inspecting data structures in a debugger:</p>
     <ul>
       <li>Many LLVM and Clang data structures provide
         a <tt>dump()</tt> method which will print a description of the
         data structure to <tt>stderr</tt>.</li>
       <li>The <a href="docs/InternalsManual.html#QualType"><tt>QualType</tt></a>
       structure is used pervasively. This is a simple value class for
       wrapping types with qualifiers; you can use
       the <tt>isConstQualified()</tt>, for example, to get one of the
       qualifiers, and the <tt>getTypePtr()</tt> method to get the
       wrapped <tt>Type*</tt> which you can then dump.</li>
       <li>For <a href="https://lldb.llvm.org"> <tt>LLDB</tt></a> users there are
       data formatters for clang data structures in
       <a href="https://github.com/llvm/llvm-project/blob/main/clang/utils/ClangDataFormat.py">
       <tt>clang/utils/ClangDataFormat.py</tt></a>.</li>
     </ul>

   <!--=====================================================================-->
   <h3 id="debuggingVisualStudio">Debugging using Visual Studio</h3>
   <!--=====================================================================-->

   <p>The files
     <a href="https://github.com/llvm/llvm-project/blob/main/llvm/utils/LLVMVisualizers/llvm.natvis">
       <tt>llvm/utils/LLVMVisualizers/llvm.natvis</tt></a> and
     <a href="https://github.com/llvm/llvm-project/blob/main/clang/utils/ClangVisualizers/clang.natvis">
       <tt>clang/utils/ClangVisualizers/clang.natvis</tt></a> provide debugger visualizers
       that make debugging of more complex data types much easier.</p>
   <p>Depending on how you configure the project, Visual Studio may automatically
   use these visualizers when debugging or you may be required to put the files
   into <tt>%USERPROFILE%\Documents\Visual Studio &lt;version&gt;\Visualizers</tt>
   or create a symbolic link so they update automatically. See
   <a href="https://learn.microsoft.com/en-us/visualstudio/debugger/create-custom-views-of-native-objects">
   Microsoft's documentation</a> for more details on use of NATVIS.</p>

   <!--=====================================================================-->
   <h2 id="testing">Testing</h2>
   <!--=====================================================================-->

   <!--=====================================================================-->
   <h3 id="testingNonWindows">Testing on Unix-like Systems</h3>
   <!--=====================================================================-->

   <p>Clang includes a basic regression suite in the tree which can be
   run with <tt>make test</tt> from the top-level clang directory, or
   just <tt>make</tt> in the <em>test</em> sub-directory.
   <tt>make VERBOSE=1</tt> can be used to show more detail
   about what is being run.</p>

   <p>If you built LLVM and Clang using CMake, the test suite can be run
   with <tt>make check-clang</tt> from the top-level LLVM directory.</p>

   <p>The tests primarily consist of a test runner script running the compiler
   under test on individual test files grouped in the directories under the
   test directory.  The individual test files include comments at the
   beginning indicating the Clang compile options to use, to be read
   by the test runner. Embedded comments also can do things like telling
   the test runner that an error is expected at the current line.
   Any output files produced by the test will be placed under
   a created Output directory.</p>

   <p>During the run of <tt>make test</tt>, the terminal output will
   display a line similar to the following:</p>

   <pre>--- Running clang tests for i686-pc-linux-gnu ---</pre>

   <p>followed by a line continually overwritten with the current test
   file being compiled, and an overall completion percentage.</p>

   <p>After the <tt>make test</tt> run completes, the absence of any
   <tt>Failing Tests (count):</tt> message indicates that no tests
   failed unexpectedly.  If any tests did fail, the
   <tt>Failing Tests (count):</tt> message will be followed by a list
   of the test source file paths that failed.  For example:</p>

   <pre>
   Failing Tests (3):
       /home/john/llvm/tools/clang/test/SemaCXX/member-name-lookup.cpp
       /home/john/llvm/tools/clang/test/SemaCXX/namespace-alias.cpp
       /home/john/llvm/tools/clang/test/SemaCXX/using-directive.cpp
 </pre>

   <p>If you used the <tt>make VERBOSE=1</tt> option, the terminal
   output will reflect the error messages from the compiler and
   test runner.</p>

   <p>The regression suite can also be run with Valgrind by running
   <tt>make test VG=1</tt> in the top-level clang directory.</p>

   <p>For more intensive changes, running
   the <a href="https://llvm.org/docs/TestingGuide.html#quick-start">LLVM
   Test Suite</a> with clang is recommended. Currently the best way to
   override LLVMGCC, as in: <tt>make LLVMGCC="clang -std=gnu89"
   TEST=nightly report</tt> (make sure <tt>clang</tt> is in your PATH or use the
   full path).</p>

   <!--=====================================================================-->
   <h3 id="testingWindows">Testing using Visual Studio on Windows</h3>
   <!--=====================================================================-->

   <p>The Clang test suite can be run from either Visual Studio or
   the command line.</p>

   <p>Note that the test runner is based on
   Python, which must be installed.  Find Python at:
   <a href="https://www.python.org/downloads/">https://www.python.org/downloads/</a>.
   Download the latest stable version.</p>

   <p>The GnuWin32 tools are also necessary for running the tests.
   Get them from <a href="http://getgnuwin32.sourceforge.net/">
   http://getgnuwin32.sourceforge.net/</a>.
   If the environment variable <tt>%PATH%</tt> does not have GnuWin32,
   or if other grep(s) supercedes GnuWin32 on <tt>%PATH%,</tt>
   you should specify <tt>LLVM_LIT_TOOLS_DIR</tt>
   to CMake explicitly.</p>

   <p>The cmake build tool is set up to create Visual Studio project files
   for running the tests, "check-clang" being the root.  Therefore, to
   run the test from Visual Studio, right-click the check-clang project
   and select "Build".</p>

   <p>
     Please see also
     <a href="https://llvm.org/docs/GettingStartedVS.html">Getting Started
     with the LLVM System using Microsoft Visual Studio</a> and
     <a href="https://llvm.org/docs/CMake.html">Building LLVM with CMake</a>.
   </p>

   <!--=====================================================================-->
   <h3 id="testingCommands">Testing on the Command Line</h3>
   <!--=====================================================================-->

   <p>If you want more control over how the tests are run, it may
   be convenient to run the test harness on the command-line directly. Before
   running tests from the command line, you will need to ensure that
   <tt>lit.site.cfg</tt> files have been created for your build.  You can do
   this by running the tests as described in the previous sections. Once the
   tests have started running, you can stop them with control+C, as the
   files are generated before running any tests.</p>

   <p>Once that is done, to run all the tests from the command line,
   execute a command like the following:</p>

   <pre>
   python (path to llvm)\llvm\utils\lit\lit.py -sv
   --param=build_mode=Win32 --param=build_config=Debug
   --param=clang_site_config=(build dir)\tools\clang\test\lit.site.cfg
  (path to llvm)\llvm\tools\clang\test
 </pre>

   <p>For CMake builds e.g. on Windows with Visual Studio, you will need
   to specify your build configuration (Debug, Release, etc.) via
   <tt>--param=build_config=(build config)</tt>.  You may also need to specify
   the build mode (Win32, etc) via <tt>--param=build_mode=(build mode)</tt>.</p>

   <p>Additionally, you will need to specify the lit site configuration which
   lives in (build dir)\tools\clang\test, via
   <tt>--param=clang_site_config=(build dir)\tools\clang\test\lit.site.cfg</tt>.
   </p>

   <p>To run a single test:</p>

   <pre>
   python (path to llvm)\llvm\utils\lit\lit.py -sv
   --param=build_mode=Win32 --param=build_config=Debug
   --param=clang_site_config=(build dir)\tools\clang\test\lit.site.cfg
   (path to llvm)\llvm\tools\clang\test\(dir)\(test)
 </pre>

   <p>For example:</p>

   <pre>
   python C:\Tools\llvm\utils\lit\lit.py -sv
   --param=build_mode=Win32 --param=build_config=Debug
   --param=clang_site_config=C:\Tools\build\tools\clang\test\lit.site.cfg
   C:\Tools\llvm\tools\clang\test\Sema\wchar.c
 </pre>

   <p>The -sv option above tells the runner to show the test output if
   any tests failed, to help you determine the cause of failure.</p>

   <p>You can also pass in the --no-progress-bar option if you wish to disable
   progress indications while the tests are running.</p>

   <p>Your output might look something like this:</p>

   <pre>lit.py: lit.cfg:152: note: using clang: 'C:\Tools\llvm\bin\Release\clang.EXE'
 -- Testing: Testing: 2534 tests, 4 threads --
 Testing: 0 .. 10.. 20.. 30.. 40.. 50.. 60.. 70.. 80.. 90..
 Testing Time: 81.52s
   Passed           : 2503
   Expectedly Failed:   28
   Unsupported      :    3
 </pre>

   <p>The statistic, "Failed" (not shown if all tests pass), is the important one.</p>

   <!--=====================================================================-->
   <h3 id="testingLibc++">Testing changes affecting libc++</h3>
   <!--=====================================================================-->

   <p>Some changes in Clang affect <a href="https://libcxx.llvm.org">libc++</a>,
   for example:</p>
   <ul>
       <li>Changing the output of Clang's diagnostics.</li>
       <li>Changing compiler builtins, especially the builtins used for type traits
       or replacements of library functions like <tt>std::move</tt> or
       <tt>std::forward</tt>.</li>
   </ul>

   <p>After adjusting libc++ to work with the changes, the next revision will be
   tested by libc++'s
   <a href="https://buildkite.com/llvm-project/libcxx-ci">pre-commit CI</a>.

   <p>For most configurations, the pre-commit CI uses a recent
   <a href="https://apt.llvm.org/">nightly build</a> of Clang from LLVM's main
   branch. These configurations do <em>not</em> use the Clang changes in the
   patch. They only use the libc++ changes.</p>

   <p>The &quot;Bootstrapping build&quot; builds Clang and uses it to build and
   test libc++. This build <em>does</em> use the Clang changes in the patch.</p>

   <p>Libc++ supports multiple versions of Clang. Therefore when a patch changes
   the diagnostics it might be required to use a regex in the
   &quot;expected&quot; tests to make it pass the CI.</p>

   <p>Libc++ has more
   <a href="https://libcxx.llvm.org/Contributing.html#pre-commit-ci">
   documentation</a> about the pre-commit CI. For questions regarding
   libc++, the best place to ask is the <tt>#libcxx</tt> channel on
   <a href="https://discord.gg/jzUbyP26tQ">LLVM's Discord server</a>.</p>

   <!--=====================================================================-->
   <h2 id="patches">Creating Patch Files</h2>
   <!--=====================================================================-->

   <p>To contribute changes to Clang see
     <a href="https://llvm.org/docs/GettingStarted.html#sending-patches">LLVM's Getting Started page</a></p>

   <!--=====================================================================-->
   <h2 id="irgen">LLVM IR Generation</h2>
   <!--=====================================================================-->

   <p>The LLVM IR generation part of clang handles conversion of the
     AST nodes output by the Sema module to the LLVM Intermediate
     Representation (IR). Historically, this was referred to as
     "codegen", and the Clang code for this lives
     in <tt>lib/CodeGen</tt>.</p>

   <p>The output is most easily inspected using the <tt>-emit-llvm</tt>
     option to clang (possibly in conjunction with <tt>-o -</tt>). You
     can also use <tt>-emit-llvm-bc</tt> to write an LLVM bitcode file
     which can be processed by the suite of LLVM tools
     like <tt>llvm-dis</tt>, <tt>llvm-nm</tt>, etc. See the LLVM
     <a href="https://llvm.org/docs/CommandGuide/">Command Guide</a>
     for more information.</p>

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	<!--#include virtual="menu.html.incl"-->
	<div id="content">
	<!--*********************************************************************-->
	<h1>Hacking on Clang</h1>
	<!--*********************************************************************-->

	<p>This document provides some hints for how to get started hacking
	on Clang for developers who are new to the Clang and/or LLVM
	codebases.</p>
	<ul>
	<li><a href="#style">Coding Standards</a></li>
	<li><a href="#docs">Developer Documentation</a></li>
	<li><a href="#debugging">Debugging</a></li>
	<li><a href="#testing">Testing</a>
	<ul>
	<li><a href="#testingNonWindows">Testing on Unix-like Systems</a></li>
	<li><a href="#testingWindows">Testing using Visual Studio on Windows</a></li>
	<li><a href="#testingCommands">Testing on the Command Line</a></li>
	<li><a href="#testingLibc++">Testing changes affecting libc++</a></li>
	</ul>
	</li>
	<li><a href="#patches">Creating Patch Files</a></li>
	<li><a href="#irgen">LLVM IR Generation</a></li>
	</ul>

	<!--=====================================================================-->
	<h2 id="style">Coding Standards</h2>
	<!--=====================================================================-->

	<p>Clang follows the
	LLVM <a href="https://llvm.org/docs/CodingStandards.html">Coding
	Standards</a>. When submitting patches, please take care to follow these standards
	and to match the style of the code to that present in Clang (for example, in
	terms of indentation, bracing, and statement spacing).</p>

	<p>Clang has a few additional coding standards:</p>
	<ul>
	<li><i>cstdio is forbidden</i>: library code should not output diagnostics
	or other information using <tt>cstdio</tt>; debugging routines should
	use <tt>llvm::errs()</tt>. Other uses of <tt>cstdio</tt> impose behavior
	upon clients and block integrating Clang as a library. Libraries should
	support <tt>raw_ostream</tt> based interfaces for textual
	output. See <a href="https://llvm.org/docs/CodingStandards.html#use-raw-ostream">Coding
	Standards</a>.</li>
	</ul>

	<!--=====================================================================-->
	<h2 id="docs">Developer Documentation</h2>
	<!--=====================================================================-->

	<p>Both Clang and LLVM use doxygen to provide API documentation. Their
	respective web pages (generated nightly) are here:</p>
	<ul>
	<li><a href="https://clang.llvm.org/doxygen">Clang</a></li>
	<li><a href="https://llvm.org/doxygen">LLVM</a></li>
	</ul>

	<p>For work on the LLVM IR generation, the LLVM assembly language
	<a href="https://llvm.org/docs/LangRef.html">reference manual</a> is
	also useful.</p>

	<!--=====================================================================-->
	<h2 id="debugging">Debugging</h2>
	<!--=====================================================================-->

	<p>Inspecting data structures in a debugger:</p>
	<ul>
	<li>Many LLVM and Clang data structures provide
	a <tt>dump()</tt> method which will print a description of the
	data structure to <tt>stderr</tt>.</li>
	<li>The <a href="docs/InternalsManual.html#QualType"><tt>QualType</tt></a>
	structure is used pervasively. This is a simple value class for
	wrapping types with qualifiers; you can use
	the <tt>isConstQualified()</tt>, for example, to get one of the
	qualifiers, and the <tt>getTypePtr()</tt> method to get the
	wrapped <tt>Type*</tt> which you can then dump.</li>
	<li>For <a href="https://lldb.llvm.org"> <tt>LLDB</tt></a> users there are
	data formatters for clang data structures in
	<a href="https://github.com/llvm/llvm-project/blob/main/clang/utils/ClangDataFormat.py">
	<tt>clang/utils/ClangDataFormat.py</tt></a>.</li>
	</ul>

	<!--=====================================================================-->
	<h3 id="debuggingVisualStudio">Debugging using Visual Studio</h3>
	<!--=====================================================================-->

	<p>The files
	<a href="https://github.com/llvm/llvm-project/blob/main/llvm/utils/LLVMVisualizers/llvm.natvis">
	<tt>llvm/utils/LLVMVisualizers/llvm.natvis</tt></a> and
	<a href="https://github.com/llvm/llvm-project/blob/main/clang/utils/ClangVisualizers/clang.natvis">
	<tt>clang/utils/ClangVisualizers/clang.natvis</tt></a> provide debugger visualizers
	that make debugging of more complex data types much easier.</p>
	<p>Depending on how you configure the project, Visual Studio may automatically
	use these visualizers when debugging or you may be required to put the files
	into <tt>%USERPROFILE%\Documents\Visual Studio <version>\Visualizers</tt>
	or create a symbolic link so they update automatically. See
	<a href="https://learn.microsoft.com/en-us/visualstudio/debugger/create-custom-views-of-native-objects">
	Microsoft's documentation</a> for more details on use of NATVIS.</p>

	<!--=====================================================================-->
	<h2 id="testing">Testing</h2>
	<!--=====================================================================-->

	<!--=====================================================================-->
	<h3 id="testingNonWindows">Testing on Unix-like Systems</h3>
	<!--=====================================================================-->

	<p>Clang includes a basic regression suite in the tree which can be
	run with <tt>make test</tt> from the top-level clang directory, or
	just <tt>make</tt> in the <em>test</em> sub-directory.
	<tt>make VERBOSE=1</tt> can be used to show more detail
	about what is being run.</p>

	<p>If you built LLVM and Clang using CMake, the test suite can be run
	with <tt>make check-clang</tt> from the top-level LLVM directory.</p>

	<p>The tests primarily consist of a test runner script running the compiler
	under test on individual test files grouped in the directories under the
	test directory. The individual test files include comments at the
	beginning indicating the Clang compile options to use, to be read
	by the test runner. Embedded comments also can do things like telling
	the test runner that an error is expected at the current line.
	Any output files produced by the test will be placed under
	a created Output directory.</p>

	<p>During the run of <tt>make test</tt>, the terminal output will
	display a line similar to the following:</p>

	<pre>--- Running clang tests for i686-pc-linux-gnu ---</pre>

	<p>followed by a line continually overwritten with the current test
	file being compiled, and an overall completion percentage.</p>

	<p>After the <tt>make test</tt> run completes, the absence of any
	<tt>Failing Tests (count):</tt> message indicates that no tests
	failed unexpectedly. If any tests did fail, the
	<tt>Failing Tests (count):</tt> message will be followed by a list
	of the test source file paths that failed. For example:</p>

	<pre>
	Failing Tests (3):
	/home/john/llvm/tools/clang/test/SemaCXX/member-name-lookup.cpp
	/home/john/llvm/tools/clang/test/SemaCXX/namespace-alias.cpp
	/home/john/llvm/tools/clang/test/SemaCXX/using-directive.cpp
	</pre>

	<p>If you used the <tt>make VERBOSE=1</tt> option, the terminal
	output will reflect the error messages from the compiler and
	test runner.</p>

	<p>The regression suite can also be run with Valgrind by running
	<tt>make test VG=1</tt> in the top-level clang directory.</p>

	<p>For more intensive changes, running
	the <a href="https://llvm.org/docs/TestingGuide.html#quick-start">LLVM
	Test Suite</a> with clang is recommended. Currently the best way to
	override LLVMGCC, as in: <tt>make LLVMGCC="clang -std=gnu89"
	TEST=nightly report</tt> (make sure <tt>clang</tt> is in your PATH or use the
	full path).</p>

	<!--=====================================================================-->
	<h3 id="testingWindows">Testing using Visual Studio on Windows</h3>
	<!--=====================================================================-->

	<p>The Clang test suite can be run from either Visual Studio or
	the command line.</p>

	<p>Note that the test runner is based on
	Python, which must be installed. Find Python at:
	<a href="https://www.python.org/downloads/">https://www.python.org/downloads/</a>.
	Download the latest stable version.</p>

	<p>The GnuWin32 tools are also necessary for running the tests.
	Get them from <a href="http://getgnuwin32.sourceforge.net/">
	http://getgnuwin32.sourceforge.net/</a>.
	If the environment variable <tt>%PATH%</tt> does not have GnuWin32,
	or if other grep(s) supercedes GnuWin32 on <tt>%PATH%,</tt>
	you should specify <tt>LLVM_LIT_TOOLS_DIR</tt>
	to CMake explicitly.</p>

	<p>The cmake build tool is set up to create Visual Studio project files
	for running the tests, "check-clang" being the root. Therefore, to
	run the test from Visual Studio, right-click the check-clang project
	and select "Build".</p>

	<p>
	Please see also
	<a href="https://llvm.org/docs/GettingStartedVS.html">Getting Started
	with the LLVM System using Microsoft Visual Studio</a> and
	<a href="https://llvm.org/docs/CMake.html">Building LLVM with CMake</a>.
	</p>

	<!--=====================================================================-->
	<h3 id="testingCommands">Testing on the Command Line</h3>
	<!--=====================================================================-->

	<p>If you want more control over how the tests are run, it may
	be convenient to run the test harness on the command-line directly. Before
	running tests from the command line, you will need to ensure that
	<tt>lit.site.cfg</tt> files have been created for your build. You can do
	this by running the tests as described in the previous sections. Once the
	tests have started running, you can stop them with control+C, as the
	files are generated before running any tests.</p>

	<p>Once that is done, to run all the tests from the command line,
	execute a command like the following:</p>

	<pre>
	python (path to llvm)\llvm\utils\lit\lit.py -sv
	--param=build_mode=Win32 --param=build_config=Debug
	--param=clang_site_config=(build dir)\tools\clang\test\lit.site.cfg
	(path to llvm)\llvm\tools\clang\test
	</pre>

	<p>For CMake builds e.g. on Windows with Visual Studio, you will need
	to specify your build configuration (Debug, Release, etc.) via
	<tt>--param=build_config=(build config)</tt>. You may also need to specify
	the build mode (Win32, etc) via <tt>--param=build_mode=(build mode)</tt>.</p>

	<p>Additionally, you will need to specify the lit site configuration which
	lives in (build dir)\tools\clang\test, via
	<tt>--param=clang_site_config=(build dir)\tools\clang\test\lit.site.cfg</tt>.
	</p>

	<p>To run a single test:</p>

	<pre>
	python (path to llvm)\llvm\utils\lit\lit.py -sv
	--param=build_mode=Win32 --param=build_config=Debug
	--param=clang_site_config=(build dir)\tools\clang\test\lit.site.cfg
	(path to llvm)\llvm\tools\clang\test\(dir)\(test)
	</pre>

	<p>For example:</p>

	<pre>
	python C:\Tools\llvm\utils\lit\lit.py -sv
	--param=build_mode=Win32 --param=build_config=Debug
	--param=clang_site_config=C:\Tools\build\tools\clang\test\lit.site.cfg
	C:\Tools\llvm\tools\clang\test\Sema\wchar.c
	</pre>

	<p>The -sv option above tells the runner to show the test output if
	any tests failed, to help you determine the cause of failure.</p>

	<p>You can also pass in the --no-progress-bar option if you wish to disable
	progress indications while the tests are running.</p>

	<p>Your output might look something like this:</p>

	<pre>lit.py: lit.cfg:152: note: using clang: 'C:\Tools\llvm\bin\Release\clang.EXE'
	-- Testing: Testing: 2534 tests, 4 threads --
	Testing: 0 .. 10.. 20.. 30.. 40.. 50.. 60.. 70.. 80.. 90..
	Testing Time: 81.52s
	Passed : 2503
	Expectedly Failed: 28
	Unsupported : 3
	</pre>

	<p>The statistic, "Failed" (not shown if all tests pass), is the important one.</p>

	<!--=====================================================================-->
	<h3 id="testingLibc++">Testing changes affecting libc++</h3>
	<!--=====================================================================-->

	<p>Some changes in Clang affect <a href="https://libcxx.llvm.org">libc++</a>,
	for example:</p>
	<ul>
	<li>Changing the output of Clang's diagnostics.</li>
	<li>Changing compiler builtins, especially the builtins used for type traits
	or replacements of library functions like <tt>std::move</tt> or
	<tt>std::forward</tt>.</li>
	</ul>

	<p>After adjusting libc++ to work with the changes, the next revision will be
	tested by libc++'s
	<a href="https://buildkite.com/llvm-project/libcxx-ci">pre-commit CI</a>.

	<p>For most configurations, the pre-commit CI uses a recent
	<a href="https://apt.llvm.org/">nightly build</a> of Clang from LLVM's main
	branch. These configurations do <em>not</em> use the Clang changes in the
	patch. They only use the libc++ changes.</p>

	<p>The "Bootstrapping build" builds Clang and uses it to build and
	test libc++. This build <em>does</em> use the Clang changes in the patch.</p>

	<p>Libc++ supports multiple versions of Clang. Therefore when a patch changes
	the diagnostics it might be required to use a regex in the
	"expected" tests to make it pass the CI.</p>

	<p>Libc++ has more
	<a href="https://libcxx.llvm.org/Contributing.html#pre-commit-ci">
	documentation</a> about the pre-commit CI. For questions regarding
	libc++, the best place to ask is the <tt>#libcxx</tt> channel on
	<a href="https://discord.gg/jzUbyP26tQ">LLVM's Discord server</a>.</p>

	<!--=====================================================================-->
	<h2 id="patches">Creating Patch Files</h2>
	<!--=====================================================================-->

	<p>To contribute changes to Clang see
	<a href="https://llvm.org/docs/GettingStarted.html#sending-patches">LLVM's Getting Started page</a></p>

	<!--=====================================================================-->
	<h2 id="irgen">LLVM IR Generation</h2>
	<!--=====================================================================-->

	<p>The LLVM IR generation part of clang handles conversion of the
	AST nodes output by the Sema module to the LLVM Intermediate
	Representation (IR). Historically, this was referred to as
	"codegen", and the Clang code for this lives
	in <tt>lib/CodeGen</tt>.</p>

	<p>The output is most easily inspected using the <tt>-emit-llvm</tt>
	option to clang (possibly in conjunction with <tt>-o -</tt>). You
	can also use <tt>-emit-llvm-bc</tt> to write an LLVM bitcode file
	which can be processed by the suite of LLVM tools
	like <tt>llvm-dis</tt>, <tt>llvm-nm</tt>, etc. See the LLVM
	<a href="https://llvm.org/docs/CommandGuide/">Command Guide</a>
	for more information.</p>

	</div>
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