Profile-Guided Optimization workflow for Cargo

Setup

• git clone https://github.com/vadimcn/cargo-pgo.git,
• cd cargo-pgo,
• git submodule update --init - this may take a while as LLVM is one of the upstream dependencies. Fortunately, only a small part of it needs to be built.
• cargo build --release,
• Add cargo-pgo/target/release to your PATH.

Usage

Remove any old profiling data

cargo pgo clean

Instrument your binary for profiling

cargo pgo instr build

This will spawn a normal Cargo build (with some extra flags passed to rustc via RUSTFLAGS), so all the usual cargo build flags do apply. Note that cargo-pgo will automatically add the --release flag, since there's little reason to PGO-optimize debug builds.

Run training scenarios

cargo pgo instr run <params1>
cargo pgo instr run <params2>
...  

You can also use cargo pgo instr test or cargo pgo instr bench.
Each execution will create a new raw profile file under target/release/pgo.

Merge profiles

Before using generated profiles, they must be first merged into an 'indexed' format:

cargo pgo merge

The output will be saved in target/release/pgo/merged.profdata.

Build optimized binary

cargo pgo opt build

Run optimized binary

cargo pgo opt run|test|bench

("Why not just 'cargo run'?": Cargo keeps track of the flags it had passed to rustc last time, and automatically rebuilds the target if they change. Thus, cargo run would first revert the binary back to non-optimized state, which probably isn't what you want.)

Do it in less steps

Cargo automatically (re)builds stale binaries before running them, so you may skip both of the build steps above and jump straight to running. In addition to that, cargo pgo opt ... commands will automatically merge raw profiles if needed. All of the above steps may be condensed to just two commands:

cargo pgo instr run
cargo pgo opt run