Evaluate using Profile-Guided Optimization (PGO) and LLVM BOLT

Hi!

Recently I checked Profile-Guided Optimization (PGO) improvements on multiple projects. The results are here. E.g. PGO results for LLVM-related tooling are here. According to the tests, PGO usually helps with the compiler and compiler-like workloads (like static analysis) - e.g. Clang gets +20% compilation speed with PGO. That's why I think trying to optimize grcov with PGO can be a good idea.

I already did some benchmarks and want to share my results.

Test environment

Fedora 38
Linux kernel 6.5.6
AMD Ryzen 9 5900x
48 Gib RAM
SSD Samsung 980 Pro 2 Tib
Compiler - Rustc 1.73
grcov version: the latest for now from the master branch on commit 322fc39acacd75aca0ff1c0a1ec2a3e91f04011c
Disabled Turbo boost

Benchmark

For benchmark purposes, I use grcov benchmarks via cargo +nightly bench. For PGO optimization I use cargo-pgo tool. The same benchmark suite was used for the PGO training phase via cargo +nightly pgo bench. PGO optimized results I got with cargo +nightly pgo optimize bench.

I met a small issue with benchmarks https://github.com/mozilla/grcov/issues/1127 but quickly fixed it by using precision = 2 (found a similar parameter somewhere in the repository).

Results

I got the following results:

Release: https://gist.github.com/zamazan4ik/b35115ae404d9583c57fce19ce5a3821
Release + PGO optimized: https://gist.github.com/zamazan4ik/121853dcf1baa3bf486717c042b68042
(just for reference) Release + PGO training: https://gist.github.com/zamazan4ik/bf6321e490337c6d51a61c4306fc5e07

At least according to the provided by grcov project benchmarks, PGO helps with optimizing performance.

Further steps

I can suggest the following action points:

Perform more PGO benchmarks on grcov. And if it shows improvements - add a note about possible improvements in grcov's tool performance with PGO.
Providing an easier way (e.g. a build option) to build scripts with PGO can be helpful for the end-users and maintainers since they will be able to optimize grcov according to their own workloads.
Optimize pre-built binaries

Testing Post-Link Optimization techniques (like LLVM BOLT) would be interesting too (Clang and Rustc already use BOLT as an addition to PGO) but I recommend starting from the usual PGO.

Here are some examples of how PGO optimization is integrated in other projects:

Rustc: a CI script for the multi-stage build
GCC:
- Official docs, section "Building with profile feedback" (even AutoFDO build is supported)
- A part in a "wonderful" configure script
Clang: Docs
Python:
- CPython: README
- Pyston: README
Go: Bash script
V8: Bazel flag
ChakraCore: Scripts
Chromium: Script
Firefox: Docs
- Thunderbird has PGO support too
PHP - Makefile command and old Centminmod scripts
MySQL: CMake script
YugabyteDB: GitHub commit
FoundationDB: Script
Zstd: Makefile
Foot: Scripts
Windows Terminal: GitHub PR
Pydantic-core: GitHub PR
file.d: GitHub PR
OceanBase: CMake flag

mozilla / grcov