cmd/compile: consider giving a warning or an error if a PGO profile looks mismatched

[mvdan]

For example, if I run go build on a Go codebase with a default.pgo file, and the PGO logic can see that the majority of the symbols mentioned in the profile are not present in the source code at all, it could give a warning or an error to the user because one of two things might have happened:

1. The profile was obtained for a different program with a fairly different codebase.
2. The profile is old and the codebase has changed enough where the profile is no longer representative.

In both cases, it's possible that PGO may still give some minor benefit, but it's hard to say. Most importantly, I think we should surface this to the user so that they can grab a new profile.

(this idea came up during a brief PGO discussion at the Go contributor summit at golab.io)

[gabyhelp]

Related Issues

• cmd/compile: profile-guided optimization #55022 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[seankhliao]

cc @golang/compiler

[prattmic]

Thanks for filing an issue. We've definitely discussed having some kind of profile staleness/match metric, though I can't find an existing issue for it.

[seankhliao]

#65615 was a bad failure for a mismatched profile vs build. Maybe it should still fail but with a better error?