1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" 2 "http://www.w3.org/TR/html4/strict.dtd"> 3<!-- Material used from: HTML 4.01 specs: http://www.w3.org/TR/html401/ --> 4<html> 5<head> 6 <META http-equiv="Content-Type" content="text/html; charset=utf-8"> 7 <title>Hacking on clang</title> 8 <link type="text/css" rel="stylesheet" href="menu.css"> 9 <link type="text/css" rel="stylesheet" href="content.css"> 10 <style type="text/css"> 11 pre { margin-left: 1.5em; } 12 </style> 13</head> 14<body> 15<!--#include virtual="menu.html.incl"--> 16<div id="content"> 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 <li><a href="#testingLibc++">Testing changes affecting libc++</a></li> 34 </ul> 35 </li> 36 <li><a href="#patches">Creating Patch Files</a></li> 37 <li><a href="#irgen">LLVM IR Generation</a></li> 38 </ul> 39 40 <!--=====================================================================--> 41 <h2 id="style">Coding Standards</h2> 42 <!--=====================================================================--> 43 44 <p>Clang follows the 45 LLVM <a href="https://llvm.org/docs/CodingStandards.html">Coding 46 Standards</a>. When submitting patches, please take care to follow these standards 47 and to match the style of the code to that present in Clang (for example, in 48 terms of indentation, bracing, and statement spacing).</p> 49 50 <p>Clang has a few additional coding standards:</p> 51 <ul> 52 <li><i>cstdio is forbidden</i>: library code should not output diagnostics 53 or other information using <tt>cstdio</tt>; debugging routines should 54 use <tt>llvm::errs()</tt>. Other uses of <tt>cstdio</tt> impose behavior 55 upon clients and block integrating Clang as a library. Libraries should 56 support <tt>raw_ostream</tt> based interfaces for textual 57 output. See <a href="https://llvm.org/docs/CodingStandards.html#use-raw-ostream">Coding 58 Standards</a>.</li> 59 </ul> 60 61 <!--=====================================================================--> 62 <h2 id="docs">Developer Documentation</h2> 63 <!--=====================================================================--> 64 65 <p>Both Clang and LLVM use doxygen to provide API documentation. Their 66 respective web pages (generated nightly) are here:</p> 67 <ul> 68 <li><a href="https://clang.llvm.org/doxygen">Clang</a></li> 69 <li><a href="https://llvm.org/doxygen">LLVM</a></li> 70 </ul> 71 72 <p>For work on the LLVM IR generation, the LLVM assembly language 73 <a href="https://llvm.org/docs/LangRef.html">reference manual</a> is 74 also useful.</p> 75 76 <!--=====================================================================--> 77 <h2 id="debugging">Debugging</h2> 78 <!--=====================================================================--> 79 80 <p>Inspecting data structures in a debugger:</p> 81 <ul> 82 <li>Many LLVM and Clang data structures provide 83 a <tt>dump()</tt> method which will print a description of the 84 data structure to <tt>stderr</tt>.</li> 85 <li>The <a href="docs/InternalsManual.html#QualType"><tt>QualType</tt></a> 86 structure is used pervasively. This is a simple value class for 87 wrapping types with qualifiers; you can use 88 the <tt>isConstQualified()</tt>, for example, to get one of the 89 qualifiers, and the <tt>getTypePtr()</tt> method to get the 90 wrapped <tt>Type*</tt> which you can then dump.</li> 91 <li>For <a href="https://lldb.llvm.org"> <tt>LLDB</tt></a> users there are 92 data formatters for LLVM data structures in 93 <a href="https://github.com/llvm/llvm-project/blob/main/llvm/utils/lldbDataFormatters.py"> 94 <tt>llvm/utils/lldbDataFormatters.py</tt></a>.</li> 95 </ul> 96 97 <!--=====================================================================--> 98 <h3 id="debuggingVisualStudio">Debugging using Visual Studio</h3> 99 <!--=====================================================================--> 100 101 <p>The files 102 <a href="https://github.com/llvm/llvm-project/blob/main/llvm/utils/LLVMVisualizers/llvm.natvis"> 103 <tt>llvm/utils/LLVMVisualizers/llvm.natvis</tt></a> and 104 <a href="https://github.com/llvm/llvm-project/blob/main/clang/utils/ClangVisualizers/clang.natvis"> 105 <tt>clang/utils/ClangVisualizers/clang.natvis</tt></a> provide debugger visualizers 106 that make debugging of more complex data types much easier.</p> 107 <p>Depending on how you configure the project, Visual Studio may automatically 108 use these visualizers when debugging or you may be required to put the files 109 into <tt>%USERPROFILE%\Documents\Visual Studio <version>\Visualizers</tt> 110 or create a symbolic link so they update automatically. See 111 <a href="https://learn.microsoft.com/en-us/visualstudio/debugger/create-custom-views-of-native-objects"> 112 Microsoft's documentation</a> for more details on use of NATVIS.</p> 113 114 <!--=====================================================================--> 115 <h2 id="testing">Testing</h2> 116 <!--=====================================================================--> 117 118 <!--=====================================================================--> 119 <h3 id="testingNonWindows">Testing on Unix-like Systems</h3> 120 <!--=====================================================================--> 121 122 <p>Clang includes a basic regression suite in the tree which can be 123 run with <tt>make test</tt> from the top-level clang directory, or 124 just <tt>make</tt> in the <em>test</em> sub-directory. 125 <tt>make VERBOSE=1</tt> can be used to show more detail 126 about what is being run.</p> 127 128 <p>If you built LLVM and Clang using CMake, the test suite can be run 129 with <tt>make check-clang</tt> from the top-level LLVM directory.</p> 130 131 <p>The tests primarily consist of a test runner script running the compiler 132 under test on individual test files grouped in the directories under the 133 test directory. The individual test files include comments at the 134 beginning indicating the Clang compile options to use, to be read 135 by the test runner. Embedded comments also can do things like telling 136 the test runner that an error is expected at the current line. 137 Any output files produced by the test will be placed under 138 a created Output directory.</p> 139 140 <p>During the run of <tt>make test</tt>, the terminal output will 141 display a line similar to the following:</p> 142 143 <pre>--- Running clang tests for i686-pc-linux-gnu ---</pre> 144 145 <p>followed by a line continually overwritten with the current test 146 file being compiled, and an overall completion percentage.</p> 147 148 <p>After the <tt>make test</tt> run completes, the absence of any 149 <tt>Failing Tests (count):</tt> message indicates that no tests 150 failed unexpectedly. If any tests did fail, the 151 <tt>Failing Tests (count):</tt> message will be followed by a list 152 of the test source file paths that failed. For example:</p> 153 154 <pre> 155 Failing Tests (3): 156 /home/john/llvm/tools/clang/test/SemaCXX/member-name-lookup.cpp 157 /home/john/llvm/tools/clang/test/SemaCXX/namespace-alias.cpp 158 /home/john/llvm/tools/clang/test/SemaCXX/using-directive.cpp 159</pre> 160 161 <p>If you used the <tt>make VERBOSE=1</tt> option, the terminal 162 output will reflect the error messages from the compiler and 163 test runner.</p> 164 165 <p>The regression suite can also be run with Valgrind by running 166 <tt>make test VG=1</tt> in the top-level clang directory.</p> 167 168 <p>For more intensive changes, running 169 the <a href="https://llvm.org/docs/TestingGuide.html#quick-start">LLVM 170 Test Suite</a> with clang is recommended. Currently the best way to 171 override LLVMGCC, as in: <tt>make LLVMGCC="clang -std=gnu89" 172 TEST=nightly report</tt> (make sure <tt>clang</tt> is in your PATH or use the 173 full path).</p> 174 175 <!--=====================================================================--> 176 <h3 id="testingWindows">Testing using Visual Studio on Windows</h3> 177 <!--=====================================================================--> 178 179 <p>The Clang test suite can be run from either Visual Studio or 180 the command line.</p> 181 182 <p>Note that the test runner is based on 183 Python, which must be installed. Find Python at: 184 <a href="https://www.python.org/downloads/">https://www.python.org/downloads/</a>. 185 Download the latest stable version.</p> 186 187 <p>The GnuWin32 tools are also necessary for running the tests. 188 Get them from <a href="http://getgnuwin32.sourceforge.net/"> 189 http://getgnuwin32.sourceforge.net/</a>. 190 If the environment variable <tt>%PATH%</tt> does not have GnuWin32, 191 or if other grep(s) supercedes GnuWin32 on <tt>%PATH%,</tt> 192 you should specify <tt>LLVM_LIT_TOOLS_DIR</tt> 193 to CMake explicitly.</p> 194 195 <p>The cmake build tool is set up to create Visual Studio project files 196 for running the tests, "check-clang" being the root. Therefore, to 197 run the test from Visual Studio, right-click the check-clang project 198 and select "Build".</p> 199 200 <p> 201 Please see also 202 <a href="https://llvm.org/docs/GettingStartedVS.html">Getting Started 203 with the LLVM System using Microsoft Visual Studio</a> and 204 <a href="https://llvm.org/docs/CMake.html">Building LLVM with CMake</a>. 205 </p> 206 207 <!--=====================================================================--> 208 <h3 id="testingCommands">Testing on the Command Line</h3> 209 <!--=====================================================================--> 210 211 <p>If you want more control over how the tests are run, it may 212 be convenient to run the test harness on the command-line directly. Before 213 running tests from the command line, you will need to ensure that 214 <tt>lit.site.cfg</tt> files have been created for your build. You can do 215 this by running the tests as described in the previous sections. Once the 216 tests have started running, you can stop them with control+C, as the 217 files are generated before running any tests.</p> 218 219 <p>Once that is done, to run all the tests from the command line, 220 execute a command like the following:</p> 221 222 <pre> 223 python (path to llvm)\llvm\utils\lit\lit.py -sv 224 --param=build_mode=Win32 --param=build_config=Debug 225 --param=clang_site_config=(build dir)\tools\clang\test\lit.site.cfg 226 (path to llvm)\llvm\tools\clang\test 227</pre> 228 229 <p>For CMake builds e.g. on Windows with Visual Studio, you will need 230 to specify your build configuration (Debug, Release, etc.) via 231 <tt>--param=build_config=(build config)</tt>. You may also need to specify 232 the build mode (Win32, etc) via <tt>--param=build_mode=(build mode)</tt>.</p> 233 234 <p>Additionally, you will need to specify the lit site configuration which 235 lives in (build dir)\tools\clang\test, via 236 <tt>--param=clang_site_config=(build dir)\tools\clang\test\lit.site.cfg</tt>. 237 </p> 238 239 <p>To run a single test:</p> 240 241 <pre> 242 python (path to llvm)\llvm\utils\lit\lit.py -sv 243 --param=build_mode=Win32 --param=build_config=Debug 244 --param=clang_site_config=(build dir)\tools\clang\test\lit.site.cfg 245 (path to llvm)\llvm\tools\clang\test\(dir)\(test) 246</pre> 247 248 <p>For example:</p> 249 250 <pre> 251 python C:\Tools\llvm\utils\lit\lit.py -sv 252 --param=build_mode=Win32 --param=build_config=Debug 253 --param=clang_site_config=C:\Tools\build\tools\clang\test\lit.site.cfg 254 C:\Tools\llvm\tools\clang\test\Sema\wchar.c 255</pre> 256 257 <p>The -sv option above tells the runner to show the test output if 258 any tests failed, to help you determine the cause of failure.</p> 259 260 <p>You can also pass in the --no-progress-bar option if you wish to disable 261 progress indications while the tests are running.</p> 262 263 <p>Your output might look something like this:</p> 264 265 <pre>lit.py: lit.cfg:152: note: using clang: 'C:\Tools\llvm\bin\Release\clang.EXE' 266-- Testing: Testing: 2534 tests, 4 threads -- 267Testing: 0 .. 10.. 20.. 30.. 40.. 50.. 60.. 70.. 80.. 90.. 268Testing Time: 81.52s 269 Passed : 2503 270 Expectedly Failed: 28 271 Unsupported : 3 272</pre> 273 274 <p>The statistic, "Failed" (not shown if all tests pass), is the important one.</p> 275 276 <!--=====================================================================--> 277 <h3 id="testingLibc++">Testing changes affecting libc++</h3> 278 <!--=====================================================================--> 279 280 <p>Some changes in Clang affect <a href="https://libcxx.llvm.org">libc++</a>, 281 for example:</p> 282 <ul> 283 <li>Changing the output of Clang's diagnostics.</li> 284 <li>Changing compiler builtins, especially the builtins used for type traits 285 or replacements of library functions like <tt>std::move</tt> or 286 <tt>std::forward</tt>.</li> 287 </ul> 288 289 <p>After adjusting libc++ to work with the changes, the next revision will be 290 tested by libc++'s 291 <a href="https://buildkite.com/llvm-project/libcxx-ci">pre-commit CI</a>. 292 293 <p>For most configurations, the pre-commit CI uses a recent 294 <a href="https://apt.llvm.org/">nightly build</a> of Clang from LLVM's main 295 branch. These configurations do <em>not</em> use the Clang changes in the 296 patch. They only use the libc++ changes.</p> 297 298 <p>The "Bootstrapping build" builds Clang and uses it to build and 299 test libc++. This build <em>does</em> use the Clang changes in the patch.</p> 300 301 <p>Libc++ supports multiple versions of Clang. Therefore when a patch changes 302 the diagnostics it might be required to use a regex in the 303 "expected" tests to make it pass the CI.</p> 304 305 <p>Libc++ has more 306 <a href="https://libcxx.llvm.org/Contributing.html#pre-commit-ci"> 307 documentation</a> about the pre-commit CI. For questions regarding 308 libc++, the best place to ask is the <tt>#libcxx</tt> channel on 309 <a href="https://discord.gg/jzUbyP26tQ">LLVM's Discord server</a>.</p> 310 311 <!--=====================================================================--> 312 <h2 id="patches">Creating Patch Files</h2> 313 <!--=====================================================================--> 314 315 <p>To contribute changes to Clang see 316 <a href="https://llvm.org/docs/GettingStarted.html#sending-patches">LLVM's Getting Started page</a></p> 317 318 <!--=====================================================================--> 319 <h2 id="irgen">LLVM IR Generation</h2> 320 <!--=====================================================================--> 321 322 <p>The LLVM IR generation part of clang handles conversion of the 323 AST nodes output by the Sema module to the LLVM Intermediate 324 Representation (IR). Historically, this was referred to as 325 "codegen", and the Clang code for this lives 326 in <tt>lib/CodeGen</tt>.</p> 327 328 <p>The output is most easily inspected using the <tt>-emit-llvm</tt> 329 option to clang (possibly in conjunction with <tt>-o -</tt>). You 330 can also use <tt>-emit-llvm-bc</tt> to write an LLVM bitcode file 331 which can be processed by the suite of LLVM tools 332 like <tt>llvm-dis</tt>, <tt>llvm-nm</tt>, etc. See the LLVM 333 <a href="https://llvm.org/docs/CommandGuide/">Command Guide</a> 334 for more information.</p> 335 336</div> 337</body> 338</html> 339