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

[zamazan4ik]

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). That's why I think trying to optimize Chapel tooling with PGO can be a good idea.

I can suggest the following action points:

• Perform PGO benchmarks on Chapel (compiler and related tooling). And if it shows improvements - add a note about possible improvements in Chapel's compiler 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 Chapel tooling according to their own workloads.
• Optimize pre-built binaries

Maybe testing Post-Link Optimization techniques (like LLVM BOLT) would be interesting too 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

By the way, does Chapel compiler support compiling a program with PGO (like Clang, GCC, Rustc)? If Chapel has an Ahead-of-Time compilation model, it would be a nice feature to have.

[mppf]

By the way, does Chapel compiler support compiling a program with PGO (like Clang, GCC, Rustc)? If Chapel has an Ahead-of-Time compilation model, it would be a nice feature to have.

Sometimes, Chapel can work with flags passed to clang with --ccflags <clang flag> and these will work with the LLVM backend. You can also use Chapel with a C compiler of your choice.

With either of these modes, I think it'd need some experimentation to decide what does and doesn't work. I don't know of any experiences trying to use PGO with Chapel.

According to the tests, PGO usually helps with the compiler and compiler-like workloads (like static analysis). That's why I think trying to optimize Chapel tooling with PGO can be a good idea.

It's an interesting question. Today, the compiler itself is building from C++ code using cmake. I don't know of any experiments in this area so far, though.