clang/www/hacking.html

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  17   <!--*********************************************************************-->
  18   <h1>Hacking on Clang</h1>
  19   <!--*********************************************************************-->
  20
  21   <p>This document provides some hints for how to get started hacking
  22   on Clang for developers who are new to the Clang and/or LLVM
  23   codebases.</p>
  24     <ul>
  25       <li><a href="#style">Coding Standards</a></li>
  26       <li><a href="#docs">Developer Documentation</a></li>
  27       <li><a href="#debugging">Debugging</a></li>
  28       <li><a href="#testing">Testing</a>
  29       <ul>
  30         <li><a href="#testingNonWindows">Testing on Unix-like Systems</a></li>
  31         <li><a href="#testingWindows">Testing using Visual Studio on Windows</a></li>
  32         <li><a href="#testingCommands">Testing on the Command Line</a></li>
  33       </ul>
  34       </li>
  35       <li><a href="#patches">Creating Patch Files</a></li>
  36       <li><a href="#irgen">LLVM IR Generation</a></li>
  37     </ul>
  38
  39   <!--=====================================================================-->
  40   <h2 id="style">Coding Standards</h2>
  41   <!--=====================================================================-->
  42
  43   <p>Clang follows the
  44   LLVM <a href="https://llvm.org/docs/CodingStandards.html">Coding
  45   Standards</a>. When submitting patches, please take care to follow these standards
  46   and to match the style of the code to that present in Clang (for example, in
  47   terms of indentation, bracing, and statement spacing).</p>
  48
  49   <p>Clang has a few additional coding standards:</p>
  50   <ul>
  51     <li><i>cstdio is forbidden</i>: library code should not output diagnostics
  52       or other information using <tt>cstdio</tt>; debugging routines should
  53       use <tt>llvm::errs()</tt>. Other uses of <tt>cstdio</tt> impose behavior
  54       upon clients and block integrating Clang as a library. Libraries should
  55       support <tt>raw_ostream</tt> based interfaces for textual
  56       output. See <a href="https://llvm.org/docs/CodingStandards.html#use-raw-ostream">Coding
  57       Standards</a>.</li>
  58   </ul>
  59
  60   <!--=====================================================================-->
  61   <h2 id="docs">Developer Documentation</h2>
  62   <!--=====================================================================-->
  63
  64   <p>Both Clang and LLVM use doxygen to provide API documentation. Their
  65   respective web pages (generated nightly) are here:</p>
  66     <ul>
  67       <li><a href="https://clang.llvm.org/doxygen">Clang</a></li>
  68       <li><a href="https://llvm.org/doxygen">LLVM</a></li>
  69     </ul>
  70
  71   <p>For work on the LLVM IR generation, the LLVM assembly language
  72   <a href="https://llvm.org/docs/LangRef.html">reference manual</a> is
  73   also useful.</p>
  74
  75   <!--=====================================================================-->
  76   <h2 id="debugging">Debugging</h2>
  77   <!--=====================================================================-->
  78
  79   <p>Inspecting data structures in a debugger:</p>
  80     <ul>
  81       <li>Many LLVM and Clang data structures provide
  82         a <tt>dump()</tt> method which will print a description of the
  83         data structure to <tt>stderr</tt>.</li>
  84       <li>The <a href="docs/InternalsManual.html#QualType"><tt>QualType</tt></a>
  85       structure is used pervasively. This is a simple value class for
  86       wrapping types with qualifiers; you can use
  87       the <tt>isConstQualified()</tt>, for example, to get one of the
  88       qualifiers, and the <tt>getTypePtr()</tt> method to get the
  89       wrapped <tt>Type*</tt> which you can then dump.</li>
  90       <li>For <a href="https://lldb.llvm.org"> <tt>LLDB</tt></a> users there are
  91       data formatters for clang data structures in
  92       <a href="https://github.com/llvm/llvm-project/blob/main/clang/utils/ClangDataFormat.py">
  93       <tt>clang/utils/ClangDataFormat.py</tt></a>.</li>
  94     </ul>
  95
  96   <!--=====================================================================-->
  97   <h3 id="debuggingVisualStudio">Debugging using Visual Studio</h3>
  98   <!--=====================================================================-->
  99
 100   <p>The files
 101     <a href="https://github.com/llvm/llvm-project/blob/main/llvm/utils/LLVMVisualizers/llvm.natvis">
 102       <tt>llvm/utils/LLVMVisualizers/llvm.natvis</tt></a> and
 103     <a href="https://github.com/llvm/llvm-project/blob/main/clang/utils/ClangVisualizers/clang.natvis">
 104       <tt>clang/utils/ClangVisualizers/clang.natvis</tt></a> provide debugger visualizers
 105       that make debugging of more complex data types much easier.</p>
 106   <p>For Visual Studio 2013 only, put the files into
 107     <tt>%USERPROFILE%\Documents\Visual Studio 2013\Visualizers</tt> or
 108     create a symbolic link so they update automatically.</p>
 109   <p>For later versions of Visual Studio, no installation is required.
 110     Note also that later versions of Visual Studio also display better visualizations.</p>
 111
 112   <!--=====================================================================-->
 113   <h2 id="testing">Testing</h2>
 114   <!--=====================================================================-->
 115
 116   <!--=====================================================================-->
 117   <h3 id="testingNonWindows">Testing on Unix-like Systems</h3>
 118   <!--=====================================================================-->
 119
 120   <p>Clang includes a basic regression suite in the tree which can be
 121   run with <tt>make test</tt> from the top-level clang directory, or
 122   just <tt>make</tt> in the <em>test</em> sub-directory.
 123   <tt>make VERBOSE=1</tt> can be used to show more detail
 124   about what is being run.</p>
 125
 126   <p>If you built LLVM and Clang using CMake, the test suite can be run
 127   with <tt>make clang-test</tt> from the top-level LLVM directory.</p>
 128
 129   <p>The tests primarily consist of a test runner script running the compiler
 130   under test on individual test files grouped in the directories under the
 131   test directory.  The individual test files include comments at the
 132   beginning indicating the Clang compile options to use, to be read
 133   by the test runner. Embedded comments also can do things like telling
 134   the test runner that an error is expected at the current line.
 135   Any output files produced by the test will be placed under
 136   a created Output directory.</p>
 137
 138   <p>During the run of <tt>make test</tt>, the terminal output will
 139   display a line similar to the following:</p>
 140
 141   <pre>--- Running clang tests for i686-pc-linux-gnu ---</pre>
 142
 143   <p>followed by a line continually overwritten with the current test
 144   file being compiled, and an overall completion percentage.</p>
 145
 146   <p>After the <tt>make test</tt> run completes, the absence of any
 147   <tt>Failing Tests (count):</tt> message indicates that no tests
 148   failed unexpectedly.  If any tests did fail, the
 149   <tt>Failing Tests (count):</tt> message will be followed by a list
 150   of the test source file paths that failed.  For example:</p>
 151
 152   <pre>
 153   Failing Tests (3):
 154       /home/john/llvm/tools/clang/test/SemaCXX/member-name-lookup.cpp
 155       /home/john/llvm/tools/clang/test/SemaCXX/namespace-alias.cpp
 156       /home/john/llvm/tools/clang/test/SemaCXX/using-directive.cpp
 157 </pre>
 158
 159   <p>If you used the <tt>make VERBOSE=1</tt> option, the terminal
 160   output will reflect the error messages from the compiler and
 161   test runner.</p>
 162
 163   <p>The regression suite can also be run with Valgrind by running
 164   <tt>make test VG=1</tt> in the top-level clang directory.</p>
 165
 166   <p>For more intensive changes, running
 167   the <a href="https://llvm.org/docs/TestingGuide.html#quick-start">LLVM
 168   Test Suite</a> with clang is recommended. Currently the best way to
 169   override LLVMGCC, as in: <tt>make LLVMGCC="clang -std=gnu89"
 170   TEST=nightly report</tt> (make sure <tt>clang</tt> is in your PATH or use the
 171   full path).</p>
 172
 173   <!--=====================================================================-->
 174   <h3 id="testingWindows">Testing using Visual Studio on Windows</h3>
 175   <!--=====================================================================-->
 176
 177   <p>The Clang test suite can be run from either Visual Studio or
 178   the command line.</p>
 179
 180   <p>Note that the test runner is based on
 181   Python, which must be installed.  Find Python at:
 182   <a href="https://www.python.org/downloads/">https://www.python.org/downloads/</a>.
 183   Download the latest stable version.</p>
 184
 185   <p>The GnuWin32 tools are also necessary for running the tests.
 186   Get them from <a href="http://getgnuwin32.sourceforge.net/">
 187   http://getgnuwin32.sourceforge.net/</a>.
 188   If the environment variable <tt>%PATH%</tt> does not have GnuWin32,
 189   or if other grep(s) supercedes GnuWin32 on <tt>%PATH%,</tt>
 190   you should specify <tt>LLVM_LIT_TOOLS_DIR</tt>
 191   to CMake explicitly.</p>
 192
 193   <p>The cmake build tool is set up to create Visual Studio project files
 194   for running the tests, "clang-test" being the root.  Therefore, to
 195   run the test from Visual Studio, right-click the clang-test project
 196   and select "Build".</p>
 197
 198   <p>
 199     Please see also
 200     <a href="https://llvm.org/docs/GettingStartedVS.html">Getting Started
 201     with the LLVM System using Microsoft Visual Studio</a> and
 202     <a href="https://llvm.org/docs/CMake.html">Building LLVM with CMake</a>.
 203   </p>
 204
 205   <!--=====================================================================-->
 206   <h3 id="testingCommands">Testing on the Command Line</h3>
 207   <!--=====================================================================-->
 208
 209   <p>If you want more control over how the tests are run, it may
 210   be convenient to run the test harness on the command-line directly. Before
 211   running tests from the command line, you will need to ensure that
 212   <tt>lit.site.cfg</tt> files have been created for your build.  You can do
 213   this by running the tests as described in the previous sections. Once the
 214   tests have started running, you can stop them with control+C, as the
 215   files are generated before running any tests.</p>
 216
 217   <p>Once that is done, to run all the tests from the command line,
 218   execute a command like the following:</p>
 219
 220   <pre>
 221   python (path to llvm)\llvm\utils\lit\lit.py -sv
 222   --param=build_mode=Win32 --param=build_config=Debug
 223   --param=clang_site_config=(build dir)\tools\clang\test\lit.site.cfg
 224  (path to llvm)\llvm\tools\clang\test
 225 </pre>
 226
 227   <p>For CMake builds e.g. on Windows with Visual Studio, you will need
 228   to specify your build configuration (Debug, Release, etc.) via
 229   <tt>--param=build_config=(build config)</tt>.  You may also need to specify
 230   the build mode (Win32, etc) via <tt>--param=build_mode=(build mode)</tt>.</p>
 231
 232   <p>Additionally, you will need to specify the lit site configuration which
 233   lives in (build dir)\tools\clang\test, via
 234   <tt>--param=clang_site_config=(build dir)\tools\clang\test\lit.site.cfg</tt>.
 235   </p>
 236
 237   <p>To run a single test:</p>
 238
 239   <pre>
 240   python (path to llvm)\llvm\utils\lit\lit.py -sv
 241   --param=build_mode=Win32 --param=build_config=Debug
 242   --param=clang_site_config=(build dir)\tools\clang\test\lit.site.cfg
 243   (path to llvm)\llvm\tools\clang\test\(dir)\(test)
 244 </pre>
 245
 246   <p>For example:</p>
 247
 248   <pre>
 249   python C:\Tools\llvm\utils\lit\lit.py -sv
 250   --param=build_mode=Win32 --param=build_config=Debug
 251   --param=clang_site_config=C:\Tools\build\tools\clang\test\lit.site.cfg
 252   C:\Tools\llvm\tools\clang\test\Sema\wchar.c
 253 </pre>
 254
 255   <p>The -sv option above tells the runner to show the test output if
 256   any tests failed, to help you determine the cause of failure.</p>
 257
 258   <p>You can also pass in the --no-progress-bar option if you wish to disable
 259   progress indications while the tests are running.</p>
 260
 261   <p>Your output might look something like this:</p>
 262
 263   <pre>lit.py: lit.cfg:152: note: using clang: 'C:\Tools\llvm\bin\Release\clang.EXE'
 264 -- Testing: Testing: 2534 tests, 4 threads --
 265 Testing: 0 .. 10.. 20.. 30.. 40.. 50.. 60.. 70.. 80.. 90..
 266 Testing Time: 81.52s
 267   Passed           : 2503
 268   Expectedly Failed:   28
 269   Unsupported      :    3
 270 </pre>
 271
 272   <p>The statistic, "Failed" (not shown if all tests pass), is the important one.</p>
 273
 274   <!--=====================================================================-->
 275   <h2 id="patches">Creating Patch Files</h2>
 276   <!--=====================================================================-->
 277
 278   <p>To contribute changes to Clang see
 279     <a href="https://llvm.org/docs/GettingStarted.html#sending-patches">LLVM's Getting Started page</a></p>
 280
 281   <!--=====================================================================-->
 282   <h2 id="irgen">LLVM IR Generation</h2>
 283   <!--=====================================================================-->
 284
 285   <p>The LLVM IR generation part of clang handles conversion of the
 286     AST nodes output by the Sema module to the LLVM Intermediate
 287     Representation (IR). Historically, this was referred to as
 288     "codegen", and the Clang code for this lives
 289     in <tt>lib/CodeGen</tt>.</p>
 290
 291   <p>The output is most easily inspected using the <tt>-emit-llvm</tt>
 292     option to clang (possibly in conjunction with <tt>-o -</tt>). You
 293     can also use <tt>-emit-llvm-bc</tt> to write an LLVM bitcode file
 294     which can be processed by the suite of LLVM tools
 295     like <tt>llvm-dis</tt>, <tt>llvm-nm</tt>, etc. See the LLVM
 296     <a href="https://llvm.org/docs/CommandGuide/">Command Guide</a>
 297     for more information.</p>
 298
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