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1=============================================================2How To Build Clang and LLVM with Profile-Guided Optimizations3=============================================================4 5Introduction6============7 8PGO (Profile-Guided Optimization) allows your compiler to better optimize code9for how it actually runs. Users report that applying this to Clang and LLVM can10decrease overall compile time by 20%.11 12This guide walks you through how to build Clang with PGO, though it also applies13to other subprojects, such as LLD.14 15If you want to build other software with PGO, see the `end-user documentation16for PGO <https://clang.llvm.org/docs/UsersManual.html#profile-guided-optimization>`_.17 18 19Using preconfigured CMake caches20================================21 22See https://llvm.org/docs/AdvancedBuilds.html#multi-stage-pgo23 24Using the script25================26 27We have a script at ``utils/collect_and_build_with_pgo.py``. This script is28tested on a few Linux flavors, and requires a checkout of LLVM, Clang, and29compiler-rt. Despite the name, it performs four clean builds of Clang, so it30can take a while to run to completion. Please see the script's ``--help`` for31more information on how to run it, and the different options available to you.32If you want to get the most out of PGO for a particular use-case (e.g. compiling33a specific large piece of software), please do read the section below on34'benchmark' selection.35 36Please note that this script is only tested on a few Linux distros. Patches to37add support for other platforms, as always, are highly appreciated. :)38 39This script also supports a ``--dry-run`` option, which causes it to print40important commands instead of running them.41 42 43Selecting 'benchmarks'44======================45 46PGO does best when the profiles gathered represent how the user plans to use the47compiler. Notably, highly accurate profiles of llc building x86_64 code aren't48incredibly helpful if you're going to be targeting ARM.49 50By default, the script above does two things to get solid coverage. It:51 52- runs all of Clang and LLVM's lit tests, and53- uses the instrumented Clang to build Clang, LLVM, and all of the other54 LLVM subprojects available to it.55 56Together, these should give you:57 58- solid coverage of building C++,59- good coverage of building C,60- great coverage of running optimizations,61- great coverage of the backend for your host's architecture, and62- some coverage of other architectures (if other arches are supported backends).63 64Altogether, this should cover a diverse set of uses for Clang and LLVM. If you65have very specific needs (e.g. your compiler is meant to compile a large browser66for four different platforms, or similar), you may want to do something else.67This is configurable in the script itself.68 69 70Building Clang with PGO71=======================72 73If you prefer to not use the script or the cmake cache, this briefly goes over74how to build Clang/LLVM with PGO.75 76First, you should have at least LLVM, Clang, and compiler-rt checked out77locally.78 79Next, at a high level, you're going to need to do the following:80 811. Build a standard Release Clang and the relevant libclang_rt.profile library822. Build Clang using the Clang you built above, but with instrumentation833. Use the instrumented Clang to generate profiles, which consists of two steps:84 85 - Running the instrumented Clang/LLVM/lld/etc. on tasks that represent how86 users will use said tools.87 - Using a tool to convert the "raw" profiles generated above into a single,88 final PGO profile.89 904. Build a final release Clang (along with whatever other binaries you need)91 using the profile collected from your benchmark92 93In more detailed steps:94 951. Configure a Clang build as you normally would. It's highly recommended that96 you use the Release configuration for this, since it will be used to build97 another Clang. Because you need Clang and supporting libraries, you'll want98 to build the ``all`` target (e.g. ``ninja all`` or ``make -j4 all``).99 1002. Configure a Clang build as above, but add the following CMake args:101 102 - ``-DLLVM_BUILD_INSTRUMENTED=IR`` -- This causes us to build everything103 with instrumentation.104 - ``-DLLVM_BUILD_RUNTIME=No`` -- A few projects have bad interactions when105 built with profiling, and aren't necessary to build. This flag turns them106 off.107 - ``-DCMAKE_C_COMPILER=/path/to/stage1/clang`` - Use the Clang we built in108 step 1.109 - ``-DCMAKE_CXX_COMPILER=/path/to/stage1/clang++`` - Same as above.110 111 In this build directory, you simply need to build the ``clang`` target (and112 whatever supporting tooling your benchmark requires).113 1143. As mentioned above, this has two steps: gathering profile data, and then115 massaging it into a useful form:116 117 a. Build your benchmark using the Clang generated in step 2. The 'standard'118 benchmark recommended is to run ``check-clang`` and ``check-llvm`` in your119 instrumented Clang's build directory, and to do a full build of Clang/LLVM120 using your instrumented Clang. So, create yet another build directory,121 with the following CMake arguments:122 123 - ``-DCMAKE_C_COMPILER=/path/to/stage2/clang`` - Use the Clang we built in124 step 2.125 - ``-DCMAKE_CXX_COMPILER=/path/to/stage2/clang++`` - Same as above.126 127 If your users are fans of debug info, you may want to consider using128 ``-DCMAKE_BUILD_TYPE=RelWithDebInfo`` instead of129 ``-DCMAKE_BUILD_TYPE=Release``. This will grant better coverage of130 debug info pieces of clang, but will take longer to complete and will131 result in a much larger build directory.132 133 It's recommended to build the ``all`` target with your instrumented Clang,134 since more coverage is often better.135 136 b. You should now have a few ``*.profraw`` files in137 ``path/to/stage2/profiles/``. You need to merge these using138 ``llvm-profdata`` (even if you only have one! The profile merge transforms139 profraw into actual profile data, as well). This can be done with140 ``/path/to/stage1/llvm-profdata merge141 -output=/path/to/output/profdata.prof path/to/stage2/profiles/*.profraw``.142 1434. Now, build your final, PGO-optimized Clang. To do this, you'll want to pass144 the following additional arguments to CMake.145 146 - ``-DLLVM_PROFDATA_FILE=/path/to/output/profdata.prof`` - Use the PGO147 profile from the previous step.148 - ``-DCMAKE_C_COMPILER=/path/to/stage1/clang`` - Use the Clang we built in149 step 1.150 - ``-DCMAKE_CXX_COMPILER=/path/to/stage1/clang++`` - Same as above.151 152 From here, you can build whatever targets you need.153 154 .. note::155 You may see warnings about a mismatched profile in the build output. These156 are generally harmless. To silence them, you can add157 ``-DCMAKE_C_FLAGS='-Wno-backend-plugin'158 -DCMAKE_CXX_FLAGS='-Wno-backend-plugin'`` to your CMake invocation.159 160 161Congrats! You now have a Clang built with profile-guided optimizations, and you162can delete all but the final build directory if you'd like.163 164If this worked well for you and you plan on doing it often, there's a slight165optimization that can be made: LLVM and Clang have a tool called tblgen that's166built and run during the build process. While it's potentially nice to build167this for coverage as part of step 3, none of your other builds should benefit168from building it. You can pass the CMake option169``-DLLVM_NATIVE_TOOL_DIR=/path/to/stage1/bin``170to steps 2 and onward to avoid these useless rebuilds.171