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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        <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 clang data structures in
93      <a href="https://github.com/llvm/llvm-project/blob/main/clang/utils/ClangDataFormat.py">
94      <tt>clang/utils/ClangDataFormat.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>For Visual Studio 2013 only, put the files into
108    <tt>%USERPROFILE%\Documents\Visual Studio 2013\Visualizers</tt> or
109    create a symbolic link so they update automatically.</p>
110  <p>For later versions of Visual Studio, no installation is required.
111    Note also that later versions of Visual Studio also display better visualizations.</p>
112
113  <!--=====================================================================-->
114  <h2 id="testing">Testing</h2>
115  <!--=====================================================================-->
116
117  <!--=====================================================================-->
118  <h3 id="testingNonWindows">Testing on Unix-like Systems</h3>
119  <!--=====================================================================-->
120
121  <p>Clang includes a basic regression suite in the tree which can be
122  run with <tt>make test</tt> from the top-level clang directory, or
123  just <tt>make</tt> in the <em>test</em> sub-directory.
124  <tt>make VERBOSE=1</tt> can be used to show more detail
125  about what is being run.</p>
126
127  <p>If you built LLVM and Clang using CMake, the test suite can be run
128  with <tt>make clang-test</tt> from the top-level LLVM directory.</p>
129
130  <p>The tests primarily consist of a test runner script running the compiler
131  under test on individual test files grouped in the directories under the
132  test directory.  The individual test files include comments at the
133  beginning indicating the Clang compile options to use, to be read
134  by the test runner. Embedded comments also can do things like telling
135  the test runner that an error is expected at the current line.
136  Any output files produced by the test will be placed under
137  a created Output directory.</p>
138
139  <p>During the run of <tt>make test</tt>, the terminal output will
140  display a line similar to the following:</p>
141
142  <pre>--- Running clang tests for i686-pc-linux-gnu ---</pre>
143
144  <p>followed by a line continually overwritten with the current test
145  file being compiled, and an overall completion percentage.</p>
146
147  <p>After the <tt>make test</tt> run completes, the absence of any
148  <tt>Failing Tests (count):</tt> message indicates that no tests
149  failed unexpectedly.  If any tests did fail, the
150  <tt>Failing Tests (count):</tt> message will be followed by a list
151  of the test source file paths that failed.  For example:</p>
152
153  <pre>
154  Failing Tests (3):
155      /home/john/llvm/tools/clang/test/SemaCXX/member-name-lookup.cpp
156      /home/john/llvm/tools/clang/test/SemaCXX/namespace-alias.cpp
157      /home/john/llvm/tools/clang/test/SemaCXX/using-directive.cpp
158</pre>
159
160  <p>If you used the <tt>make VERBOSE=1</tt> option, the terminal
161  output will reflect the error messages from the compiler and
162  test runner.</p>
163
164  <p>The regression suite can also be run with Valgrind by running
165  <tt>make test VG=1</tt> in the top-level clang directory.</p>
166
167  <p>For more intensive changes, running
168  the <a href="https://llvm.org/docs/TestingGuide.html#quick-start">LLVM
169  Test Suite</a> with clang is recommended. Currently the best way to
170  override LLVMGCC, as in: <tt>make LLVMGCC="clang -std=gnu89"
171  TEST=nightly report</tt> (make sure <tt>clang</tt> is in your PATH or use the
172  full path).</p>
173
174  <!--=====================================================================-->
175  <h3 id="testingWindows">Testing using Visual Studio on Windows</h3>
176  <!--=====================================================================-->
177
178  <p>The Clang test suite can be run from either Visual Studio or
179  the command line.</p>
180
181  <p>Note that the test runner is based on
182  Python, which must be installed.  Find Python at:
183  <a href="https://www.python.org/downloads/">https://www.python.org/downloads/</a>.
184  Download the latest stable version.</p>
185
186  <p>The GnuWin32 tools are also necessary for running the tests.
187  Get them from <a href="http://getgnuwin32.sourceforge.net/">
188  http://getgnuwin32.sourceforge.net/</a>.
189  If the environment variable <tt>%PATH%</tt> does not have GnuWin32,
190  or if other grep(s) supercedes GnuWin32 on <tt>%PATH%,</tt>
191  you should specify <tt>LLVM_LIT_TOOLS_DIR</tt>
192  to CMake explicitly.</p>
193
194  <p>The cmake build tool is set up to create Visual Studio project files
195  for running the tests, "clang-test" being the root.  Therefore, to
196  run the test from Visual Studio, right-click the clang-test project
197  and select "Build".</p>
198
199  <p>
200    Please see also
201    <a href="https://llvm.org/docs/GettingStartedVS.html">Getting Started
202    with the LLVM System using Microsoft Visual Studio</a> and
203    <a href="https://llvm.org/docs/CMake.html">Building LLVM with CMake</a>.
204  </p>
205
206  <!--=====================================================================-->
207  <h3 id="testingCommands">Testing on the Command Line</h3>
208  <!--=====================================================================-->
209
210  <p>If you want more control over how the tests are run, it may
211  be convenient to run the test harness on the command-line directly. Before
212  running tests from the command line, you will need to ensure that
213  <tt>lit.site.cfg</tt> files have been created for your build.  You can do
214  this by running the tests as described in the previous sections. Once the
215  tests have started running, you can stop them with control+C, as the
216  files are generated before running any tests.</p>
217
218  <p>Once that is done, to run all the tests from the command line,
219  execute a command like the following:</p>
220
221  <pre>
222  python (path to llvm)\llvm\utils\lit\lit.py -sv
223  --param=build_mode=Win32 --param=build_config=Debug
224  --param=clang_site_config=(build dir)\tools\clang\test\lit.site.cfg
225 (path to llvm)\llvm\tools\clang\test
226</pre>
227
228  <p>For CMake builds e.g. on Windows with Visual Studio, you will need
229  to specify your build configuration (Debug, Release, etc.) via
230  <tt>--param=build_config=(build config)</tt>.  You may also need to specify
231  the build mode (Win32, etc) via <tt>--param=build_mode=(build mode)</tt>.</p>
232
233  <p>Additionally, you will need to specify the lit site configuration which
234  lives in (build dir)\tools\clang\test, via
235  <tt>--param=clang_site_config=(build dir)\tools\clang\test\lit.site.cfg</tt>.
236  </p>
237
238  <p>To run a single test:</p>
239
240  <pre>
241  python (path to llvm)\llvm\utils\lit\lit.py -sv
242  --param=build_mode=Win32 --param=build_config=Debug
243  --param=clang_site_config=(build dir)\tools\clang\test\lit.site.cfg
244  (path to llvm)\llvm\tools\clang\test\(dir)\(test)
245</pre>
246
247  <p>For example:</p>
248
249  <pre>
250  python C:\Tools\llvm\utils\lit\lit.py -sv
251  --param=build_mode=Win32 --param=build_config=Debug
252  --param=clang_site_config=C:\Tools\build\tools\clang\test\lit.site.cfg
253  C:\Tools\llvm\tools\clang\test\Sema\wchar.c
254</pre>
255
256  <p>The -sv option above tells the runner to show the test output if
257  any tests failed, to help you determine the cause of failure.</p>
258
259  <p>You can also pass in the --no-progress-bar option if you wish to disable
260  progress indications while the tests are running.</p>
261
262  <p>Your output might look something like this:</p>
263
264  <pre>lit.py: lit.cfg:152: note: using clang: 'C:\Tools\llvm\bin\Release\clang.EXE'
265-- Testing: Testing: 2534 tests, 4 threads --
266Testing: 0 .. 10.. 20.. 30.. 40.. 50.. 60.. 70.. 80.. 90..
267Testing Time: 81.52s
268  Passed           : 2503
269  Expectedly Failed:   28
270  Unsupported      :    3
271</pre>
272
273  <p>The statistic, "Failed" (not shown if all tests pass), is the important one.</p>
274
275  <!--=====================================================================-->
276  <h3 id="testingLibc++">Testing changes affecting libc++</h3>
277  <!--=====================================================================-->
278
279  <p>Some changes in Clang affect <a href="https://libcxx.llvm.org">libc++</a>,
280  for example:</p>
281  <ul>
282      <li>Changing the output of Clang's diagnostics.</li>
283      <li>Changing compiler builtins, especially the builtins used for type traits
284      or replacements of library functions like <tt>std::move</tt> or
285      <tt>std::forward</tt>.</li>
286  </ul>
287
288  <p>After adjusting libc++ to work with the changes, the next revision will be
289  tested by libc++'s
290  <a href="https://buildkite.com/llvm-project/libcxx-ci">pre-commit CI</a>.
291
292  <p>For most configurations, the pre-commit CI uses a recent
293  <a href="https://apt.llvm.org/">nightly build</a> of Clang from LLVM's main
294  branch. These configurations do <em>not</em> use the Clang changes in the
295  patch. They only use the libc++ changes.</p>
296
297  <p>The &quot;Bootstrapping build&quot; builds Clang and uses it to build and
298  test libc++. This build <em>does</em> use the Clang changes in the patch.</p>
299
300  <p>Libc++ supports multiple versions of Clang. Therefore when a patch changes
301  the diagnostics it might be required to use a regex in the
302  &quot;expected&quot; tests to make it pass the CI.</p>
303
304  <p>Libc++ has more
305  <a href="https://libcxx.llvm.org/Contributing.html#pre-commit-ci">
306  documentation</a> about the pre-commit CI. For questions regarding
307  libc++, the best place to ask is the <tt>#libcxx</tt> channel on
308  <a href="https://discord.gg/jzUbyP26tQ">LLVM's Discord server</a>.</p>
309
310  <!--=====================================================================-->
311  <h2 id="patches">Creating Patch Files</h2>
312  <!--=====================================================================-->
313
314  <p>To contribute changes to Clang see
315    <a href="https://llvm.org/docs/GettingStarted.html#sending-patches">LLVM's Getting Started page</a></p>
316
317  <!--=====================================================================-->
318  <h2 id="irgen">LLVM IR Generation</h2>
319  <!--=====================================================================-->
320
321  <p>The LLVM IR generation part of clang handles conversion of the
322    AST nodes output by the Sema module to the LLVM Intermediate
323    Representation (IR). Historically, this was referred to as
324    "codegen", and the Clang code for this lives
325    in <tt>lib/CodeGen</tt>.</p>
326
327  <p>The output is most easily inspected using the <tt>-emit-llvm</tt>
328    option to clang (possibly in conjunction with <tt>-o -</tt>). You
329    can also use <tt>-emit-llvm-bc</tt> to write an LLVM bitcode file
330    which can be processed by the suite of LLVM tools
331    like <tt>llvm-dis</tt>, <tt>llvm-nm</tt>, etc. See the LLVM
332    <a href="https://llvm.org/docs/CommandGuide/">Command Guide</a>
333    for more information.</p>
334
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