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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
<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></li>
<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="#patches">Creating Patch Files</a></li>
<li><a href="#irgen">LLVM IR Generation</a></li>
<h2 id="docs">Coding Standards</h2>
<p>Clang follows the
LLVM <a href="">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>
<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="">Coding
<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>
<li><a href="">Clang</a></li>
<li><a href="">LLVM</a></li>
<p>For work on the LLVM IR generation, the LLVM assembly language
<a href="">reference manual</a> is
also useful.</p>
<h2 id="debugging">Debugging</h2>
<p>Inspecting data structures in a debugger:</p>
<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>
<h2 id="testing">Testing</h2>
<p><i>[Note: The test running mechanism is currently under revision, so the
following might change shortly.]</i></p>
<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>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>
<ul><tt>--- Running clang tests for i686-pc-linux-gnu ---</tt></ul>
<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>
Failing Tests (3):
<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="">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=""></a>.
Download the latest stable version (2.6.2 at the time of this writing).</p>
<p>The GnuWin32 tools are also necessary for running the tests.
(Note that the grep from MSYS or Cygwin doesn't work with the tests
because of embedded double-quotes in the search strings. The GNU
grep does work in this case.)
Get them from <a href=""></a>.</p>
<p>The cmake build tool is set up to create Visual Studio project files
for running the tests, "clang-test" being the root. Therefore, to
run the test from Visual Studio, right-click the clang-test project
and select "Build".</p>
<p>To run all the tests from the command line, execute a command like
the following:</p>
python (path to llvm)/llvm/utils/lit/ -sv --no-progress-bar
(path to llvm)/llvm/tools/clang/test
<p>To run a single test:</p>
python (path to llvm)/llvm/utils/lit/ -sv --no-progress-bar
(path to llvm)/llvm/tools/clang/test/(dir)/(test)
<p>For example:</p>
python C:/Tools/llvm/utils/lit/ -sv --no-progress-bar
<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>Note that a few tests currently fail on Windows. We are working to
correct this. Therefore your output might look something like this:</p>
<tt><pre> lit.cfg:152: note: using clang: 'C:/Tools/llvm/bin/Debug\\clang.EXE'
-- Testing: 1723 tests, 2 threads --
FAIL: Clang::(test path) (659 of 1723)
******************** TEST 'Clang::(test path)' FAILED ********************
(commands run)
Command Output (stdout):
(output here)
Command Output (stderr):
(output here)
Testing Time: 83.66s
Failing Tests (1):
Clang::(test path)
Expected Passes : 1704
Expected Failures : 18
Unexpected Failures: 1
<p>The last statistic, "Unexpected Failures", is the important one.</p>
<h2 id="patches">Creating Patch Files</h2>
<p>To return changes to the Clang team, unless you have checkin
privileges, the prefered way is to send patch files to the
cfe-commits mailing list, with an explanation of what the patch is for.
Or, if you have questions, or want to have a wider discussion of what
you are doing, such as if you are new to Clang development, you can use
the cfe-dev mailing list also.
<p>To create these patch files, change directory
to the llvm/tools/clang root and run:</p>
<ul><tt>svn diff (relative path) >(patch file name)</tt></ul>
<p>For example, for getting the diffs of all of clang:</p>
<ul><tt>svn diff . >~/mypatchfile.patch</tt></ul>
<p>For example, for getting the diffs of a single file:</p>
<ul><tt>svn diff lib/Parse/ParseDeclCXX.cpp >~/ParseDeclCXX.patch</tt></ul>
<p>Note that the paths embedded in the patch depend on where you run it,
so changing directory to the llvm/tools/clang directory is recommended.</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="">Command Guide</a>
for more information.</p>