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<title>Hacking on clang</title>
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<div id="content">
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<!--*********************************************************************-->
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<h1>Hacking on Clang</h1>
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<!--*********************************************************************-->
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<p>This document provides some hints for how to get started hacking
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on Clang for developers who are new to the Clang and/or LLVM
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codebases.</p>
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<ul>
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<li><a href="#style">Coding Standards</a></li>
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<li><a href="#docs">Developer Documentation</a></li>
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<li><a href="#debugging">Debugging</a></li>
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<li><a href="#testing">Testing</a>
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<ul>
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<li><a href="#testingNonWindows">Testing on Unix-like Systems</a></li>
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<li><a href="#testingWindows">Testing using Visual Studio on Windows</a></li>
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<li><a href="#testingCommands">Testing on the Command Line</a></li>
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</ul>
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</li>
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<li><a href="#patches">Creating Patch Files</a></li>
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<li><a href="#irgen">LLVM IR Generation</a></li>
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</ul>
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<!--=====================================================================-->
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<h2 id="style">Coding Standards</h2>
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<!--=====================================================================-->
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<p>Clang follows the
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LLVM <a href="http://llvm.org/docs/CodingStandards.html">Coding
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Standards</a>. When submitting patches, please take care to follow these standards
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and to match the style of the code to that present in Clang (for example, in
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terms of indentation, bracing, and statement spacing).</p>
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<p>Clang has a few additional coding standards:</p>
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<ul>
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<li><i>cstdio is forbidden</i>: library code should not output diagnostics
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or other information using <tt>cstdio</tt>; debugging routines should
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use <tt>llvm::errs()</tt>. Other uses of <tt>cstdio</tt> impose behavior
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upon clients and block integrating Clang as a library. Libraries should
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support <tt>raw_ostream</tt> based interfaces for textual
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output. See <a href="http://llvm.org/docs/CodingStandards.html#ll_raw_ostream">Coding
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Standards</a>.</li>
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</ul>
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<!--=====================================================================-->
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<h2 id="docs">Developer Documentation</h2>
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<!--=====================================================================-->
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<p>Both Clang and LLVM use doxygen to provide API documentation. Their
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respective web pages (generated nightly) are here:</p>
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<ul>
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<li><a href="http://clang.llvm.org/doxygen">Clang</a></li>
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<li><a href="http://llvm.org/doxygen">LLVM</a></li>
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</ul>
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<p>For work on the LLVM IR generation, the LLVM assembly language
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<a href="http://llvm.org/docs/LangRef.html">reference manual</a> is
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also useful.</p>
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<!--=====================================================================-->
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<h2 id="debugging">Debugging</h2>
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<!--=====================================================================-->
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<p>Inspecting data structures in a debugger:</p>
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<ul>
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<li>Many LLVM and Clang data structures provide
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a <tt>dump()</tt> method which will print a description of the
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data structure to <tt>stderr</tt>.</li>
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<li>The <a href="docs/InternalsManual.html#QualType"><tt>QualType</tt></a>
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structure is used pervasively. This is a simple value class for
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wrapping types with qualifiers; you can use
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the <tt>isConstQualified()</tt>, for example, to get one of the
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qualifiers, and the <tt>getTypePtr()</tt> method to get the
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wrapped <tt>Type*</tt> which you can then dump.</li>
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<li>For <a href="http://lldb.llvm.org"> <tt>LLDB</tt></a> users there are
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data formatters for clang data structures in
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<a href="http://llvm.org/svn/llvm-project/cfe/trunk/utils/ClangDataFormat.py">
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<tt>utils/ClangDataFormat.py</tt></a>.</li>
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</ul>
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<!--=====================================================================-->
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<h3 id="debuggingVisualStudio">Debugging using Visual Studio</h3>
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<!--=====================================================================-->
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<p>The files
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<a href="http://llvm.org/svn/llvm-project/llvm/trunk/utils/LLVMVisualizers/llvm.natvis">
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<tt>utils/LLVMVisualizers/llvm.natvis</tt></a> and
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<a href="http://llvm.org/svn/llvm-project/cfe/trunk/utils/ClangVisualizers/clang.natvis">
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<tt>utils/ClangVisualizers/clang.natvis</tt></a> provide debugger visualizers
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that make debugging of more complex data types much easier.</p>
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<p>For Visual Studio 2013 only, put the files into
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<tt>%USERPROFILE%\Documents\Visual Studio 2013\Visualizers</tt> or
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create a symbolic link so they update automatically.</p>
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<p>For later versions of Visual Studio, no installation is required.
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Note also that later versions of Visual Studio also display better visualizations.</p>
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<!--=====================================================================-->
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<h2 id="testing">Testing</h2>
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<!--=====================================================================-->
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<!--=====================================================================-->
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<h3 id="testingNonWindows">Testing on Unix-like Systems</h3>
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<!--=====================================================================-->
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<p>Clang includes a basic regression suite in the tree which can be
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run with <tt>make test</tt> from the top-level clang directory, or
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just <tt>make</tt> in the <em>test</em> sub-directory.
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<tt>make VERBOSE=1</tt> can be used to show more detail
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about what is being run.</p>
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<p>If you built LLVM and Clang using CMake, the test suite can be run
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with <tt>make clang-test</tt> from the top-level LLVM directory.</p>
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<p>The tests primarily consist of a test runner script running the compiler
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under test on individual test files grouped in the directories under the
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test directory. The individual test files include comments at the
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beginning indicating the Clang compile options to use, to be read
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by the test runner. Embedded comments also can do things like telling
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the test runner that an error is expected at the current line.
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Any output files produced by the test will be placed under
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a created Output directory.</p>
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<p>During the run of <tt>make test</tt>, the terminal output will
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display a line similar to the following:</p>
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<pre>--- Running clang tests for i686-pc-linux-gnu ---</pre>
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<p>followed by a line continually overwritten with the current test
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file being compiled, and an overall completion percentage.</p>
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<p>After the <tt>make test</tt> run completes, the absence of any
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<tt>Failing Tests (count):</tt> message indicates that no tests
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failed unexpectedly. If any tests did fail, the
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<tt>Failing Tests (count):</tt> message will be followed by a list
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of the test source file paths that failed. For example:</p>
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<pre>
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Failing Tests (3):
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/home/john/llvm/tools/clang/test/SemaCXX/member-name-lookup.cpp
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/home/john/llvm/tools/clang/test/SemaCXX/namespace-alias.cpp
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/home/john/llvm/tools/clang/test/SemaCXX/using-directive.cpp
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</pre>
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<p>If you used the <tt>make VERBOSE=1</tt> option, the terminal
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output will reflect the error messages from the compiler and
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test runner.</p>
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<p>The regression suite can also be run with Valgrind by running
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<tt>make test VG=1</tt> in the top-level clang directory.</p>
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<p>For more intensive changes, running
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the <a href="http://llvm.org/docs/TestingGuide.html#testsuiterun">LLVM
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Test Suite</a> with clang is recommended. Currently the best way to
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override LLVMGCC, as in: <tt>make LLVMGCC="clang -std=gnu89"
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TEST=nightly report</tt> (make sure <tt>clang</tt> is in your PATH or use the
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full path).</p>
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<!--=====================================================================-->
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<h3 id="testingWindows">Testing using Visual Studio on Windows</h3>
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<!--=====================================================================-->
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<p>The Clang test suite can be run from either Visual Studio or
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the command line.</p>
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<p>Note that the test runner is based on
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Python, which must be installed. Find Python at:
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<a href="http://www.python.org/download/">http://www.python.org/download/</a>.
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Download the latest stable version.</p>
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<p>The GnuWin32 tools are also necessary for running the tests.
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Get them from <a href="http://getgnuwin32.sourceforge.net/">
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http://getgnuwin32.sourceforge.net/</a>.
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If the environment variable <tt>%PATH%</tt> does not have GnuWin32,
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or if other grep(s) supercedes GnuWin32 on <tt>%PATH%,</tt>
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you should specify <tt>LLVM_LIT_TOOLS_DIR</tt>
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to CMake explicitly.</p>
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<p>The cmake build tool is set up to create Visual Studio project files
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for running the tests, "clang-test" being the root. Therefore, to
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run the test from Visual Studio, right-click the clang-test project
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and select "Build".</p>
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<p>
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Please see also
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<a href="http://llvm.org/docs/GettingStartedVS.html">Getting Started
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with the LLVM System using Microsoft Visual Studio</a> and
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<a href="http://llvm.org/docs/CMake.html">Building LLVM with CMake</a>.
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</p>
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<!--=====================================================================-->
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<h3 id="testingCommands">Testing on the Command Line</h3>
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<!--=====================================================================-->
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<p>If you want more control over how the tests are run, it may
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be convenient to run the test harness on the command-line directly. Before
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running tests from the command line, you will need to ensure that
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<tt>lit.site.cfg</tt> files have been created for your build. You can do
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this by running the tests as described in the previous sections. Once the
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tests have started running, you can stop them with control+C, as the
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files are generated before running any tests.</p>
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<p>Once that is done, to run all the tests from the command line,
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execute a command like the following:</p>
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<pre>
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python (path to llvm)\llvm\utils\lit\lit.py -sv
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--param=build_mode=Win32 --param=build_config=Debug
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--param=clang_site_config=(build dir)\tools\clang\test\lit.site.cfg
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(path to llvm)\llvm\tools\clang\test
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</pre>
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<p>For CMake builds e.g. on Windows with Visual Studio, you will need
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to specify your build configuration (Debug, Release, etc.) via
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<tt>--param=build_config=(build config)</tt>. You may also need to specify
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the build mode (Win32, etc) via <tt>--param=build_mode=(build mode)</tt>.</p>
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<p>Additionally, you will need to specify the lit site configuration which
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lives in (build dir)\tools\clang\test, via
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<tt>--param=clang_site_config=(build dir)\tools\clang\test\lit.site.cfg</tt>.
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</p>
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<p>To run a single test:</p>
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<pre>
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python (path to llvm)\llvm\utils\lit\lit.py -sv
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--param=build_mode=Win32 --param=build_config=Debug
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--param=clang_site_config=(build dir)\tools\clang\test\lit.site.cfg
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(path to llvm)\llvm\tools\clang\test\(dir)\(test)
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</pre>
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<p>For example:</p>
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<pre>
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python C:\Tools\llvm\utils\lit\lit.py -sv
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--param=build_mode=Win32 --param=build_config=Debug
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--param=clang_site_config=C:\Tools\build\tools\clang\test\lit.site.cfg
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C:\Tools\llvm\tools\clang\test\Sema\wchar.c
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</pre>
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<p>The -sv option above tells the runner to show the test output if
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any tests failed, to help you determine the cause of failure.</p>
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<p>You can also pass in the --no-progress-bar option if you wish to disable
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progress indications while the tests are running.</p>
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<p>Your output might look something like this:</p>
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<pre>lit.py: lit.cfg:152: note: using clang: 'C:\Tools\llvm\bin\Release\clang.EXE'
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-- Testing: Testing: 2534 tests, 4 threads --
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Testing: 0 .. 10.. 20.. 30.. 40.. 50.. 60.. 70.. 80.. 90..
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Testing Time: 81.52s
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Expected Passes : 2503
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Expected Failures : 28
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Unsupported Tests : 3
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</pre>
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<p>The statistic, "Unexpected Failures" (not shown if all tests pass), is the important one.</p>
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<!--=====================================================================-->
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<h2 id="patches">Creating Patch Files</h2>
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<!--=====================================================================-->
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<p>To return changes to the Clang team, unless you have checkin
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privileges, the preferred way is to send patch files to the
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cfe-commits mailing list, with an explanation of what the patch is
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for. clang follows <a
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href="http://llvm.org/docs/DeveloperPolicy.html">LLVM's developer policy</a>.
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If your patch requires a wider discussion (for example, because it is an
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architectural change), you can use the cfe-dev mailing list.</p>
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<p>To create these patch files, change directory
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to the llvm/tools/clang root and run:</p>
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<pre>svn diff (relative path) >(patch file name)</pre>
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<p>For example, for getting the diffs of all of clang:</p>
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<pre>svn diff . >~/mypatchfile.patch</pre>
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<p>For example, for getting the diffs of a single file:</p>
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<pre>svn diff lib/Parse/ParseDeclCXX.cpp >~/ParseDeclCXX.patch</pre>
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<p>Note that the paths embedded in the patch depend on where you run it,
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so changing directory to the llvm/tools/clang directory is recommended.</p>
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<!--=====================================================================-->
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<h2 id="irgen">LLVM IR Generation</h2>
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<!--=====================================================================-->
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<p>The LLVM IR generation part of clang handles conversion of the
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AST nodes output by the Sema module to the LLVM Intermediate
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Representation (IR). Historically, this was referred to as
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"codegen", and the Clang code for this lives
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in <tt>lib/CodeGen</tt>.</p>
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<p>The output is most easily inspected using the <tt>-emit-llvm</tt>
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option to clang (possibly in conjunction with <tt>-o -</tt>). You
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can also use <tt>-emit-llvm-bc</tt> to write an LLVM bitcode file
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which can be processed by the suite of LLVM tools
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like <tt>llvm-dis</tt>, <tt>llvm-nm</tt>, etc. See the LLVM
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<a href="http://llvm.org/docs/CommandGuide/">Command Guide</a>
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for more information.</p>
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</div>
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