[Clang] CS-IR PGO doesn't generate profiles with LTO enabled

[gitoss]

I'm trying to use IR+CS-IR PGO to optimize the SVT-AV1-PSY encoder psy-ex/svt-av1-psy#69 (reply in thread)

A binary compiled with -fcs-profile-generate -fprofile-use= silently fails to generate any profiles if (thin or full) LTO is enabled. This happens on LLVM 17 (Visual Stdío built-in) and LLVM 19 (last official binary available).

This seems to be a problem with other source code, too https://www.reddit.com/r/LLVM/comments/182psjg/comment/lrdktnd/?context=3

This could be a bug or "works as designed" because LTO defers performing optimization until link time https://llvm.org/docs/LinkTimeOptimization.html - but context sensitive PGO performs a post-inline late instrumentation https://clang.llvm.org/docs/UsersManual.html

If whis is "wad" please a) add the information about incompatibility with -fcs-profile-generate and -flto to the documentation, b) add a clang error/warning if this combination is used or c) warn and auto-disable -flto if -fcs-profile-generate is used.

[ilovepi]

@gitoss AFAIK CS-PGO should work with LTO/ThinLTO.

@teresajohnson @aeubanks Are there any incompatibilities we don't document?

[teresajohnson]

We use it with ThinLTO. But you will need to pass the correct option to (Thin)LTO backend handling, presumably via the linker, since CS PGO instrumentation happens there.

@xur-llvm can you provide some guidance on how to invoke CS-PGO for (Thin)LTO (presumably in-process via the linker).