Optimizing Clang : A Practical Example of Applying BOLT


BOLT (Binary Optimization and Layout Tool) is designed to improve the application performance by laying out code in a manner that helps CPU better utilize its caching and branch predicting resources.

The most obvious candidates for BOLT optimizations are programs that suffer from many instruction cache and iTLB misses, such as large applications measuring over hundreds of megabytes in size. However, medium-sized programs can benefit too. Clang, one of the most popular open-source C/C++ compilers, is a good example of the latter. Its code size could easily be in the order of tens of megabytes. As we will see, the Clang binary suffers from many instruction cache misses and can be significantly improved with BOLT, even on top of profile-guided and link-time optimizations.

In this tutorial we will first build Clang with PGO and LTO, and then will show steps on how to apply BOLT optimizations to make Clang up to 15% faster. We will also analyze where the compile-time performance gains are coming from, and verify that the speed-ups are sustainable while building other applications.

Building Clang

The process of getting Clang sources and performing the build is very similar to the one described at http://clang.llvm.org/get_started.html. For completeness, we provide the detailed steps on how to obtain and build Clang in Bootstrapping Clang-7 with PGO and LTO section.

The only difference from the standard Clang build is that we require the -Wl,-q flag to be present during the final link. This option saves relocation metadata in the executable file, but does not affect the generated code in any way.

Optimizing Clang with BOLT

We will use the setup described in Bootstrapping Clang-7 with PGO and LTO. Adjust the steps accordingly if you skipped that section. We will also assume that llvm-bolt is present in your $PATH.

Before we can run BOLT optimizations, we need to collect the profile for Clang, and we will use Clang/LLVM sources for that. Collecting accurate profile requires running perf on a hardware that implements taken branch sampling (-b/-j flag). For that reason, it may not be possible to collect the accurate profile in a virtualized environment, e.g. in the cloud. We do support regular sampling profiles, but the performance improvements are expected to be more modest.

$ mkdir ${TOPLEV}/stage3
$ cd ${TOPLEV}/stage3
$ CPATH=${TOPLEV}/stage2-prof-use-lto/install/bin/
$ cmake -G Ninja ${TOPLEV}/llvm -DLLVM_TARGETS_TO_BUILD=X86 -DCMAKE_BUILD_TYPE=Release \
$ perf record -e cycles:u -j any,u -- ninja clang

Once the last command is finished, it will create a perf.data file larger than 10GiB. We will first convert this profile into a more compact aggregated form suitable to be consumed by BOLT:

  $ perf2bolt $CPATH/clang-7 -p perf.data -o clang-7.fdata -w clang-7.yaml

Notice that we are passing clang-7 to perf2bolt which is the real binary that clang and clang++ are symlinking to. The next step will optimize Clang using the generated profile:

$ llvm-bolt $CPATH/clang-7 -o $CPATH/clang-7.bolt -b clang-7.yaml \
    -reorder-blocks=ext-tsp -reorder-functions=hfsort+ -split-functions \
    -split-all-cold -dyno-stats -icf=1 -use-gnu-stack

The output will look similar to the one below:

BOLT-INFO: enabling relocation mode
BOLT-INFO: 11415 functions out of 104526 simple functions (10.9%) have non-empty execution profile.
BOLT-INFO: ICF folded 29144 out of 105177 functions in 8 passes. 82 functions had jump tables.
BOLT-INFO: Removing all identical functions will save 5466.69 KB of code space. Folded functions were called 2131985 times based on profile.
BOLT-INFO: basic block reordering modified layout of 7848 (10.32%) functions
           660155947 : executed forward branches (-2.3%)
            48252553 : taken forward branches (-57.2%)
           129897961 : executed backward branches (+13.8%)
            52389551 : taken backward branches (-19.5%)
            35650038 : executed unconditional branches (-33.2%)
           128338874 : all function calls (=)
            19010563 : indirect calls (=)
             9918250 : PLT calls (=)
          6113398840 : executed instructions (-0.6%)
          1519537463 : executed load instructions (=)
           943321306 : executed store instructions (=)
            20467109 : taken jump table branches (=)
           825703946 : total branches (-2.1%)
           136292142 : taken branches (-41.1%)
           689411804 : non-taken conditional branches (+12.6%)
           100642104 : taken conditional branches (-43.4%)
           790053908 : all conditional branches (=)

The statistics in the output is based on the LBR profile collected with perf, and since we were using the cycles counter, its accuracy is affected. However, the relative improvement in taken conditional branches is a good indication that BOLT was able to straighten out the code even after PGO.

Measuring Compile-time Improvement

clang-7.bolt can be used as a replacement for PGO+LTO Clang:

$ mv $CPATH/clang-7 $CPATH/clang-7.org
$ ln -fs $CPATH/clang-7.bolt $CPATH/clang-7

Doing a new build of Clang using the new binary shows a significant overall build time reduction on a 48-core Haswell system:

$ ln -fs $CPATH/clang-7.org $CPATH/clang-7
$ ninja clean && /bin/time -f %e ninja clang -j48
$ ln -fs $CPATH/clang-7.bolt $CPATH/clang-7
$ ninja clean && /bin/time -f %e ninja clang -j48

That's 22.61 seconds (or 12%) faster compared to the PGO+LTO build. Notice that we are measuring an improvement of the total build time, which includes the time spent in the linker. Compilation time improvements for individual files differ, and speedups over 15% are not uncommon. If we run BOLT on a Clang binary compiled without PGO+LTO (in which case the build is finished in 253.32 seconds), the gains we see are over 50 seconds (25%), but, as expected, the result is still slower than PGO+LTO+BOLT build.

Source of the Wins

We mentioned that Clang suffers from considerable instruction cache misses. This can be measured with perf:

$ ln -fs $CPATH/clang-7.org $CPATH/clang-7
$ ninja clean && perf stat -e instructions,L1-icache-misses -- ninja clang -j48
   16,366,101,626,647      instructions
      359,996,216,537      L1-icache-misses

That‘s about 22 instruction cache misses per thousand instructions. As a rule of thumb, if the application has over 10 misses per thousand instructions, it is a good indication that it will be improved by BOLT. Now let’s see how many misses are in the BOLTed binary:

$ ln -fs $CPATH/clang-7.bolt $CPATH/clang-7
$ ninja clean && perf stat -e instructions,L1-icache-misses -- ninja clang -j48
  16,319,818,488,769      instructions
     244,888,677,972      L1-icache-misses

The number of misses per thousand instructions went down from 22 to 15, significantly reducing the number of stalls in the CPU front-end. Notice how the number of executed instructions stayed roughly the same. That‘s because we didn’t run any optimizations beyond the ones affecting the code layout. Other than instruction cache misses, BOLT also improves branch mispredictions, iTLB misses, and misses in L2 and L3.

Using Clang for Other Applications

We have collected profile for Clang using its own source code. Would it be enough to speed up the compilation of other projects? We picked mysqld, an open-source database, to do the test.

On our 48-core Haswell system using the PGO+LTO Clang, the build finished in 136.06 seconds, while using the PGO+LTO+BOLT Clang, 126.10 seconds. That's a noticeable improvement, but not as significant as the one we saw on Clang itself. This is partially because the number of instruction cache misses is slightly lower on this scenario : 19 vs 22. Another reason is that Clang is run with a different set of options while building mysqld compared to the training run.

Different options exercise different code paths, and if we trained without a specific option, we may have misplaced parts of the code responsible for handling it. To test this theory, we have collected another perf profile while building mysqld, and merged it with an existing profile using the merge-fdata utility that comes with BOLT. Optimized with that profile, the PGO+LTO+BOLT Clang was able to perform the mysqld build in 124.74 seconds, i.e. 11 seconds or 9% faster compared to PGO+LGO Clang. The merged profile didn't make the original Clang compilation slower either, while the number of profiled functions in Clang increased from 11,415 to 14,025.

Ideally, the profile run has to be done with a superset of all commonly used options. However, the main improvement is expected with just the basic set.


In this tutorial we demonstrated how to use BOLT to improve the performance of the Clang compiler. Similarly, BOLT could be used to improve the performance of GCC, or any other application suffering from a high number of instruction cache misses.


Bootstrapping Clang-7 with PGO and LTO

Below we describe detailed steps to build Clang, and make it ready for BOLT optimizations. If you already have the build setup, you can skip this section, except for the last step that adds -Wl,-q linker flag to the final build.

Getting Clang-7 Sources

Set $TOPLEV to the directory of your preference where you would like to do builds. E.g. TOPLEV=~/clang-7/. Follow with commands to clone the release_70 branch of LLVM monorepo:

$ mkdir ${TOPLEV}
$ cd ${TOPLEV}
$ git clone --branch=release/7.x https://github.com/llvm/llvm-project.git

Building Stage 1 Compiler

Stage 1 will be the first build we are going to do, and we will be using the default system compiler to build Clang. If your system lacks a compiler, use your distribution package manager to install one that supports C++11. In this example we are going to use GCC. In addition to the compiler, you will need the cmake and ninja packages. Note that we disable the build of certain compiler-rt components that are known to cause build issues at release/7.x.

$ mkdir ${TOPLEV}/stage1
$ cd ${TOPLEV}/stage1
$ cmake -G Ninja ${TOPLEV}/llvm-project/llvm -DLLVM_TARGETS_TO_BUILD=X86 \
      -DCMAKE_BUILD_TYPE=Release \
      -DLLVM_ENABLE_PROJECTS="clang;lld" \
      -DLLVM_ENABLE_RUNTIMES="compiler-rt" \
$ ninja install

Building Stage 2 Compiler With Instrumentation

Using the freshly-baked stage 1 Clang compiler, we are going to build Clang with profile generation capabilities:

$ mkdir ${TOPLEV}/stage2-prof-gen
$ cd ${TOPLEV}/stage2-prof-gen
$ CPATH=${TOPLEV}/stage1/install/bin/
$ cmake -G Ninja ${TOPLEV}/llvm-project/llvm -DLLVM_TARGETS_TO_BUILD=X86 \
    -DCMAKE_BUILD_TYPE=Release \
    -DLLVM_ENABLE_PROJECTS="clang;lld" \
$ ninja install

Generating Profile for PGO

While there are many ways to obtain the profile data, we are going to use the source code already at our disposal, i.e. we are going to collect the profile while building Clang itself:

$ mkdir ${TOPLEV}/stage3-train
$ cd ${TOPLEV}/stage3-train
$ CPATH=${TOPLEV}/stage2-prof-gen/install/bin
$ cmake -G Ninja ${TOPLEV}/llvm-project/llvm -DLLVM_TARGETS_TO_BUILD=X86 \
    -DCMAKE_BUILD_TYPE=Release \
    -DLLVM_USE_LINKER=lld -DCMAKE_INSTALL_PREFIX=${TOPLEV}/stage3-train/install
$ ninja clang

Once the build is completed, the profile files will be saved under ${TOPLEV}/stage2-prof-gen/profiles. We will merge them before they can be passed back into Clang:

$ cd ${TOPLEV}/stage2-prof-gen/profiles
$ ${TOPLEV}/stage1/install/bin/llvm-profdata merge -output=clang.profdata *

Building Clang with PGO and LTO

Now the profile can be used to guide optimizations to produce better code for our scenario, i.e. building Clang. We will also enable link-time optimizations to allow cross-module inlining and other optimizations. Finally, we are going to add one extra step that is useful for BOLT: a linker flag instructing it to preserve relocations in the output binary. Note that this flag does not affect the generated code or data used at runtime, it only writes metadata to the file on disk:

$ mkdir ${TOPLEV}/stage2-prof-use-lto
$ cd ${TOPLEV}/stage2-prof-use-lto
$ CPATH=${TOPLEV}/stage1/install/bin/
$ export LDFLAGS="-Wl,-q"
$ cmake -G Ninja ${TOPLEV}/llvm-project/llvm -DLLVM_TARGETS_TO_BUILD=X86 \
    -DCMAKE_BUILD_TYPE=Release \
    -DLLVM_ENABLE_PROJECTS="clang;lld" \
    -DLLVM_PROFDATA_FILE=${TOPLEV}/stage2-prof-gen/profiles/clang.profdata \
$ ninja install

Now we have a Clang compiler that can build itself much faster. As we will see, it builds other applications faster as well, and, with BOLT, the compile time can be improved even further.