1815220 - Investigate relevancy of using PGO order file

Status: RESOLVED DUPLICATE of bug 1839832

(Firefox Build System :: General, enhancement)

Description

[[:sergesanspaille]]

On windows only, we currently use an order file, as specified in https://searchfox.org/mozilla-central/source/toolkit/library/build/Makefile.in . The input file is not kept up-to-date and it turns out chromium as dropped using that setup as /call-graph-profile-sort can now directly use PGO profile information.

Comment 1

[[:sergesanspaille]]

According to this discussion: https://bugs.chromium.org/p/chromium/issues/detail?id=1113282 we should be able to rely on the linker flag -call-graph-profile-sort which in turns uses PGO information instead of relying on a manually generated file. I still need to investigate the setup but this looks promising both from performance and maintenance point of view.

Comment 2

[[:sergesanspaille]]

And after some experiments & digging, this option is on by default when using PGO, which means using the orderfile is probably counter-productive.

Comment 3

[[:sergesanspaille]]

Attachment: Bug 1815220 - get rid of orderfile

These files were only used under PGO configuration and were manually
generated.

It turns out that clang's PGO already sorts function based on their
hotness through -call-graph-profile-sort which is ON by default for

thin lto (since https://github.com/llvm/llvm-project/commit/cc18f8aa0f3ca35295419e1f4f0322a6cbe3c70b)
full lto (since https://github.com/llvm/llvm-project/commit/325e7e8b87423f83ba9fc53b9486ce67329d486c)