Profile-guided Optimization

rustc supports doing profile-guided optimization (PGO). This chapter describes what PGO is, what it is good for, and how it can be used.

What Is Profiled-Guided Optimization?

The basic concept of PGO is to collect data about the typical execution of a program (e.g. which branches it is likely to take) and then use this data to inform optimizations such as inlining, machine-code layout, register allocation, etc.

There are different ways of collecting data about a program's execution. One is to run the program inside a profiler (such as perf) and another is to create an instrumented binary, that is, a binary that has data collection built into it, and run that. The latter usually provides more accurate data and it is also what is supported by rustc.

Usage

Generating a PGO-optimized program involves following a workflow with four steps:

Compile the program with instrumentation enabled (e.g. rustc -Cprofile-generate=/tmp/pgo-data main.rs)
Run the instrumented program (e.g. ./main) which generates a default_<id>.profraw file
Convert the .profraw file into a .profdata file using LLVM's llvm-profdata tool
Compile the program again, this time making use of the profiling data (for example rustc -Cprofile-use=merged.profdata main.rs)

An instrumented program will create one or more .profraw files, one for each instrumented binary. E.g. an instrumented executable that loads two instrumented dynamic libraries at runtime will generate three .profraw files. Running an instrumented binary multiple times, on the other hand, will re-use the respective .profraw files, updating them in place.

These .profraw files have to be post-processed before they can be fed back into the compiler. This is done by the llvm-profdata tool. This tool is most easily installed via

rustup component add llvm-tools-preview

Note that installing the llvm-tools-preview component won't add llvm-profdata to the PATH. Rather, the tool can be found in:

~/.rustup/toolchains/<toolchain>/lib/rustlib/<target-triple>/bin/

Alternatively, an llvm-profdata coming with a recent LLVM or Clang version usually works too.

The llvm-profdata tool merges multiple .profraw files into a single .profdata file that can then be fed back into the compiler via -Cprofile-use:

# STEP 1: Compile the binary with instrumentation
rustc -Cprofile-generate=/tmp/pgo-data -O ./main.rs

# STEP 2: Run the binary a few times, maybe with common sets of args.
#         Each run will create or update `.profraw` files in /tmp/pgo-data
./main mydata1.csv
./main mydata2.csv
./main mydata3.csv

# STEP 3: Merge and post-process all the `.profraw` files in /tmp/pgo-data
llvm-profdata merge -o ./merged.profdata /tmp/pgo-data

# STEP 4: Use the merged `.profdata` file during optimization. All `rustc`
#         flags have to be the same.
rustc -Cprofile-use=./merged.profdata -O ./main.rs

A Complete Cargo Workflow

Using this feature with Cargo works very similar to using it with rustc directly. Again, we generate an instrumented binary, run it to produce data, merge the data, and feed it back into the compiler. Some things of note:

We use the RUSTFLAGS environment variable in order to pass the PGO compiler flags to the compilation of all crates in the program.
We pass the --target flag to Cargo, which prevents the RUSTFLAGS arguments to be passed to Cargo build scripts. We don't want the build scripts to generate a bunch of .profraw files.
We pass --release to Cargo because that's where PGO makes the most sense. In theory, PGO can also be done on debug builds but there is little reason to do so.
It is recommended to use absolute paths for the argument of -Cprofile-generate and -Cprofile-use. Cargo can invoke rustc with varying working directories, meaning that rustc will not be able to find the supplied .profdata file. With absolute paths this is not an issue.
It is good practice to make sure that there is no left-over profiling data from previous compilation sessions. Just deleting the directory is a simple way of doing so (see STEP 0 below).

This is what the entire workflow looks like:

# STEP 0: Make sure there is no left-over profiling data from previous runs
rm -rf /tmp/pgo-data

# STEP 1: Build the instrumented binaries
RUSTFLAGS="-Cprofile-generate=/tmp/pgo-data" \
    cargo build --release --target=x86_64-unknown-linux-gnu

# STEP 2: Run the instrumented binaries with some typical data
./target/x86_64-unknown-linux-gnu/release/myprogram mydata1.csv
./target/x86_64-unknown-linux-gnu/release/myprogram mydata2.csv
./target/x86_64-unknown-linux-gnu/release/myprogram mydata3.csv

# STEP 3: Merge the `.profraw` files into a `.profdata` file
llvm-profdata merge -o /tmp/pgo-data/merged.profdata /tmp/pgo-data

# STEP 4: Use the `.profdata` file for guiding optimizations
RUSTFLAGS="-Cprofile-use=/tmp/pgo-data/merged.profdata" \
    cargo build --release --target=x86_64-unknown-linux-gnu

Troubleshooting

It is recommended to pass -Cllvm-args=-pgo-warn-missing-function during the -Cprofile-use phase. LLVM by default does not warn if it cannot find profiling data for a given function. Enabling this warning will make it easier to spot errors in your setup.
There is a known issue in Cargo prior to version 1.39 that will prevent PGO from working correctly. Be sure to use Cargo 1.39 or newer when doing PGO.

Community Maintained Tools

As an alternative to directly using the compiler for Profile-Guided Optimization, you may choose to go with cargo-pgo, which has an intuitive command-line API and saves you the trouble of doing all the manual work. You can read more about it in their repository accessible from this link: https://github.com/Kobzol/cargo-pgo

For the sake of completeness, here are the corresponding steps using cargo-pgo:

# Install if you haven't already
cargo install cargo-pgo

cargo pgo build
cargo pgo optimize

These steps will do the following just as before:

Build an instrumented binary from the source code.
Run the instrumented binary to gather PGO profiles.
Use the gathered PGO profiles from the last step to build an optimized binary.

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