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Overview

In this demo, we look at how to:

  • collect a training corpus for training a -Oz inliner policy
  • perform that training
  • use the trained policy to build a 'release' clang which embeds the policy

For the first two steps, we use a 'development' mode clang which depends on TFLite and allows swapping policies from the command line - which is necessary to enable the reinforcement training loop iterate and improve over policies.

The 'release' mode compiler does not have this option. The policy is fixed and embedded in the compiler, and a user opts in to using it via a command line flag (-mllvm -enable-ml-inliner=release). Also the compiler has no runtime tensorflow dependencies.

For corpus extraction, any project that is built with clang and uses a build system that produces a compilation database json file (like clang -DCMAKE_EXPORT_COMPILE_COMMANDS) would work. The larger the number of modules, the better (for training). For this demo, we're using the Fuchsia project.

Preliminaries

We assume all repositories are installed under $HOME, e.g. you should have (after the next step) a ~/fuchsia, ~/ml-compiler-opt, etc.

Get repositories

LLVM

Follow the instructions available at https://llvm.org/docs/GettingStarted.html. In most cases, it should be as simple as:

cd ~ && git clone https://github.com/llvm/llvm-project.git

Typical prerequisites:

sudo apt-get install cmake ninja-build lld
export LLVM_SRCDIR=~/llvm-project
export LLVM_INSTALLDIR=~/llvm-install

Fuchsia

See instructions at https://fuchsia.dev/fuchsia-src/get-started/get_fuchsia_source. Make sure PATH is set up appropriately.

export IDK_DIR=~/fuchsia-idk
export SYSROOT_DIR=~/fuchsia-sysroot
export FUCHSIA_SRCDIR=~/fuchsia

We also need:

This should amount to:

cipd install fuchsia/sdk/core/linux-amd64 latest -root ${IDK_DIR}
cipd install fuchsia/sysroot/linux-arm64 latest -root ${SYSROOT_DIR}/linux-arm64
cipd install fuchsia/sysroot/linux-amd64 latest -root ${SYSROOT_DIR}/linux-x64

Note: If your shell can't find the cipd command, it's likely your $PATH variable doesn't contain the path to .jiri_root/bin. To add this to the path, run the following command:

export PATH=$PATH:$(realpath $FUCHSIA_SRCDIR/.jiri_root/bin)

Set up the correct package versions

We need to make sure the git revision of llvm is one that works with the version of the Fuchsia tree.

To get the git hash at which we know this version of Fuchsia will build, do:

cd ${FUCHSIA_SRCDIR}
jiri package 'fuchsia/third_party/clang/${platform}'

The output will contain a git_revision line, for example fa4c3f70ff0768a270b0620dc6d158ed1205ec4e. Copy that hash and then (using the example):

cd ${LLVM_SRCDIR}
git checkout fa4c3f70ff0768a270b0620dc6d158ed1205ec4e

ML Training

This is this repository.

Tensorflow dependencies

We need to install all of the Python dependencies for this repository, setup some environment variables for the AOT compiler, and also build the TFLite dependency to compile LLVM in MLGO development mode.

See also the build bot script

cd ~/ml-compiler-opt
sudo apt-get install python3-pip
pip3 install pipenv
./versioned_pipenv sync --system

TF_PIP=$(python3 -m pip show tensorflow | grep Location | cut -d ' ' -f 2)

export TENSORFLOW_AOT_PATH="${TF_PIP}/tensorflow"

export TFLITE_PATH=~/tflite
mkdir ${TFLITE_PATH}
cd ${TFLITE_PATH}
~/ml-compiler-opt/buildbot/build_tflite.sh

Build LLVM

Build LLVM with the 'development' ML mode, and additional Fuchsia-specific settings:

cd ${LLVM_SRCDIR}
mkdir build
cd build
cmake -G Ninja \
  -DLLVM_ENABLE_LTO=OFF \
  -DLINUX_x86_64-unknown-linux-gnu_SYSROOT=${SYSROOT_DIR}/linux-x64 \
  -DLINUX_aarch64-unknown-linux-gnu_SYSROOT=${SYSROOT_DIR}/linux-arm64 \
  -DFUCHSIA_SDK=${IDK_DIR} \
  -DCMAKE_INSTALL_PREFIX= \
  -DCMAKE_INSTALL_RPATH_USE_LINK_PATH=On \
  -C ${LLVM_SRCDIR}/clang/cmake/caches/Fuchsia-stage2.cmake \
  -C ${TFLITE_PATH}/tflite.cmake \
  ${LLVM_SRCDIR}/llvm

ninja toolchain-distribution
DESTDIR=${LLVM_INSTALLDIR} ninja install-toolchain-distribution-stripped
cd ${FUCHSIA_SRCDIR}
python3 scripts/clang/generate_runtimes.py --clang-prefix=$LLVM_INSTALLDIR --sdk-dir=$IDK_DIR --build-id-dir=$LLVM_INSTALLDIR/lib/.build-id > $LLVM_INSTALLDIR/lib/runtime.json

NOTE 1: The only flag specific to MLGO is -C ${TFLITE_PATH}/tflite.cmake.

NOTE 2: Fuchsia's clang/cmake/caches/Fuchsia-stage2.cmake enables the new pass manager by default. This allows us to not need to require it explicitly at compile time, but it is a requirement for the MLGO project (all our work assumes the new pass manager)

NOTE 3: The python executable should be explicitly set if there are multiple (particularly, newer) Python executables on the system with -DPython3_EXECUTABLE=/path/to/compatible/python

Build Fuchsia

cd ${FUCHSIA_SRCDIR}
fx set core.x64 \
  --args='clang_prefix="${LLVM_INSTALLDIR}/bin"' \
  --args=clang_embed_bitcode=true \
  --args='optimize="size"' \
  --args='clang_ml_inliner=false'
fx build

We set clang_ml_inliner to false here because by default it is set to true, but that wouldn't yet work since we don't have a model to embed.

Fuchsia build conveniently generates a size report. Let's copy it for reference.

Note The clang_prefix is the absolute path of $LLVM_INSTALLDIR/bin(replace it by yours). The --args=clang_embed_bitcode=true option above adds the compilation flag -Xclang=-fembed-bitcode=all. This can be seen in the compilation database. The effect of this is that the object files have the llvm bytecode produced by clang, before the optimization passes, and the clang command line, captured in the .llvmbc and .llvmcmd sections, respectively. This is the mechanism by which we extract our corpus.

Naturally, the effect of this is that the object files, and the linked binaries, are larger. Fuchsia strips the final object; but, more importantly, the size report captures multiple dimensions, beside the final file size - including, for example, the size of the text section.

cp out/default/elf_sizes.json /tmp/orig_sizes.json

ML training

Extract the corpus

cd ${FUCHSIA_SRCDIR}
fx compdb

This produces a compile_commands.json compilation database, akin cmake's.

Install the corpus extraction utilities:

pip3 install mlgo-utils

and then run the extract_ir script to extract the corpus:

export CORPUS=$HOME/corpus
cd ~/ml-compiler-opt
extract_ir \
  --cmd_filter="^-O2|-Os|-Oz$" \
  --input=$FUCHSIA_SRCDIR/out/default/compile_commands.json \
  --input_type=json \
  --llvm_objcopy_path=$LLVM_INSTALLDIR/bin/llvm-objcopy \
  --output_dir=$CORPUS

If you get an error saying the extract_ir script cannot be found, make sure the local binary directory that Python installs scripts to is in your $PATH. In most cases this is ~/.local/bin.

Collect Trace and Generate Vocab

export DEFAULT_TRACE=$HOME/default_trace
export DEFAULT_VOCAB=compiler_opt/rl/inlining/vocab

Collect traces from the default heuristic, to kick off the training process.

NOTE the double and single quotes for the --gin-bindings - this is because the last value must appear, syntactically, as a python string.

rm -rf $DEFAULT_TRACE &&
  PYTHONPATH=$PYTHONPATH:. python3 \
    compiler_opt/tools/generate_default_trace.py \
    --data_path=$CORPUS \
    --output_path=$DEFAULT_TRACE \
    --gin_files=compiler_opt/rl/inlining/gin_configs/common.gin \
    --gin_bindings=config_registry.get_configuration.implementation=@configs.InliningConfig \
    --gin_bindings=clang_path="'$LLVM_INSTALLDIR/bin/clang'" \
    --gin_bindings=llvm_size_path="'$LLVM_INSTALLDIR/bin/llvm-size'" \
    --sampling_rate=0.2

Generate vocab for the generated trace. This is an optional step and should be triggered if the set of features or the distribution of features in the trace changes.

rm -rf $DEFAULT_VOCAB &&
  PYTHONPATH=$PYTHONPATH:. python3 \
    compiler_opt/tools/generate_vocab.py \
    --gin_files=compiler_opt/rl/inlining/gin_configs/common.gin \
    --input=$DEFAULT_TRACE \
    --output_dir=$DEFAULT_VOCAB

Note The generate_vocab.py tool optionally accepts two more additional flags --sampling_fraction and --parallelism. sampling_fraction downsamples input features and parallelism controls the degree of parallelism. These flags can be tuned to reduce memory footprint and improve execution speed of the vocab generator.

Train a new model

export WARMSTART_OUTPUT_DIR=$HOME/warmstart
export OUTPUT_DIR=$HOME/model

Train a behavioral cloning model based on the above trace, that mimics default inlining behavior. This is the 'warmstart' model.

rm -rf $WARMSTART_OUTPUT_DIR && \
  PYTHONPATH=$PYTHONPATH:. python3 \
  compiler_opt/rl/train_bc.py \
  --root_dir=$WARMSTART_OUTPUT_DIR \
  --data_path=$DEFAULT_TRACE \
  --gin_files=compiler_opt/rl/inlining/gin_configs/behavioral_cloning_nn_agent.gin

Starting from the warmstart model, train the optimized model - this will take about half a day

rm -rf $OUTPUT_DIR && \
  PYTHONPATH=$PYTHONPATH:. python3 \
  compiler_opt/rl/train_locally.py \
  --root_dir=$OUTPUT_DIR \
  --data_path=$CORPUS \
  --gin_bindings=clang_path="'$LLVM_INSTALLDIR/bin/clang'" \
  --gin_bindings=llvm_size_path="'$LLVM_INSTALLDIR/bin/llvm-size'" \
  --gin_files=compiler_opt/rl/inlining/gin_configs/ppo_nn_agent.gin \
  --gin_bindings=train_eval.warmstart_policy_dir=\"$WARMSTART_OUTPUT_DIR/saved_policy\"

You can resume training from a previously saved checkpoint by specifying the directory path containing ckpt-*.index files as the root_dir. Typically, this would be the $OUTPUT_DIR. So, if $OUTPUT_DIR has previously saved checkpoints, running the above command would restore the latest checkpoint and resume training.

You may also start a tensorboard to monitor the training process with

tensorboard --logdir=$OUTPUT_DIR

Mainly check the reward_distribution section for the model performance. It includes the average reward and the percentile of the reward distributions during training. Positive reward means an improvement against the heuristic, and negative reward means a regression.

Evaluate trained policy on a corpus (Optional)

Optionally, if you are interested in seeing how the trained policy ($OUTPUT_DIR/saved_policy) performs on a given corpus (take the training corpus $CORPUS as an example), the following command line generates a csv-format report with 4 columns: module_name, identifier (default in inlining case), size under heuristic, size under the trained policy at $OUTPUT_PERFORMANCE_PATH.

export OUTPUT_PERFORMANCE_PATH=$HOME/performance_report && \
PYTHONPATH=$PYTHONPATH:. python3 \
compiler_opt/tools/generate_default_trace.py \
  --data_path=$CORPUS \
  --policy_path=$OUTPUT_DIR/saved_policy \
  --output_performance_path=$OUTPUT_PERFORMANCE_PATH \
  --gin_files=compiler_opt/rl/inlining/gin_configs/common.gin \
  --gin_bindings=clang_path="'$LLVM_INSTALLDIR/bin/clang'" \
  --gin_bindings=llvm_size_path="'$LLVM_INSTALLDIR/bin/llvm-size'" \
  --sampling_rate=0.2

Deploying and using the new policy

We need to build the 'release' mode of the compiler. Currently, that means overwriting the model in llvm/lib/Analysis/models/inliner.

cd $LLVM_SRCDIR
rm -rf llvm/lib/Analysis/models/inliner/*
cp -rf $OUTPUT_DIR/saved_policy/* llvm/lib/Analysis/models/inliner/

Setup the release build:

mkdir build-release
cd build-release
cmake -G Ninja \
  -DLLVM_ENABLE_LTO=OFF \
  -DLINUX_x86_64-unknown-linux-gnu_SYSROOT=${SYSROOT_DIR}/linux-x64 \
  -DLINUX_aarch64-unknown-linux-gnu_SYSROOT=${SYSROOT_DIR}/linux-arm64 \
  -DFUCHSIA_SDK=${IDK_DIR} \
  -DCMAKE_INSTALL_PREFIX= \
  -DTENSORFLOW_AOT_PATH=${TENSORFLOW_AOT_PATH} \
  -C ${LLVM_SRCDIR}/clang/cmake/caches/Fuchsia-stage2.cmake \
  ${LLVM_SRCDIR}/llvm

export LLVM_INSTALLDIR_RELEASE=$LLVM_INSTALLDIR-release
ninja toolchain-distribution
DESTDIR=${LLVM_INSTALLDIR_RELEASE} ninja install-toolchain-distribution-stripped
cd ${FUCHSIA_SRCDIR}
python3 scripts/clang/generate_runtimes.py \
  --clang-prefix=$LLVM_INSTALLDIR_RELEASE \
  --sdk-dir=$IDK_DIR \
  --build-id-dir=$LLVM_INSTALLDIR_RELEASE/lib/.build-id > $LLVM_INSTALLDIR_RELEASE/lib/runtime.json

NOTE 1: If you are using LLVM-at-head instead of an exact repro, there is an additional flag -DLLVM_INLINER_MODEL_PATH= that you need to set to the path to your model. If you set the flag to download, then the latest compatible model release from github will be downloaded.

NOTE 2: The only flag specific to MLGO is TENSORFLOW_AOT_PATH, which replaces -C ${TFLITE_PATH}/tflite.cmake used earlier.

cd ${FUCHSIA_SRCDIR}
fx set core.x64 \
  --args='clang_prefix="${LLVM_INSTALLDIR_RELEASE}/bin"' \
  --args='optimize="size"' \
  --args=clang_ml_inliner=true
fx build

Fuchsia has a nice utility for comparing the sizes of binaries' text section, however, it currently does so indiscriminately for all targets - including those compiled with -O3 (which are unchanged). We can filter them out to get a -Oz effect:

python3 -m pip install --user tabulate
scripts/compare_elf_sizes.py \
  /tmp/orig_sizes.json \
  out/default/elf_sizes.json \
  --field code