[PATCH] D53598: Add docs+a script for building clang/LLVM with PGO

[George Burgess IV via Phabricator via llvm-commits]

george.burgess.iv added a comment.

> I notice that you're using LLVM_BUILD_INSTRUMENTED=IR, which corresponds to -fprofile-generate (IR-level profiling), instead of -fprofile-instr-generate (clang-level profiling). Did you play around with both and observe that IR-level profiling gave you better results?

IR-level profiling gave me better results for unrelated projects in the past, so not initially, but it's probably a good idea.

A quick experiment shows that `ninja opt` consumes 2.5% fewer user cycles when clang is optimized with an IR-level profile rather than with a frontend one. Building an arbitrary-but-large cpp file (clang/lib/Sema/SemaOverload.cpp) shows a similar 4% win for the IR profile.

> Btw, I tried this out and got a 20% improvement on a self-host with PGO, which is pretty handy :)

Woohoo!


Repository:
  rL LLVM

https://reviews.llvm.org/D53598