[PATCH] D86193: [CSSPGO] Pseudo probe instrumentation for basic blocks.

[Wei Mi via Phabricator via llvm-commits]

wmi added a comment.

> There are some optimizations such as if-convert, tail call elimination, that were initially blocked by the pseudo probe intrinsic but is now unblocked by fixes included in this change. With the current change we do not see perf degradation out of SPEC and one of our internal large services.
> The main optimizations left blocked intentionally are those that merge blocks for smaller code size, such as tail merge which is the opposite of jump threading. We believe that those optimizations are not very beneficial for performance and AutoFDO.

If the optimizations are not very beneficial for performance and AutoFDO and should be blocked, it may be better to block them in a more general way and not depend on pseudo probe, because blocking them may also be beneficial for debug info based AutoFDO.

Another reason is that pseudo probe looks pretty much like debug information to me. They are used to annotate the IR but shouldn't affect the transformation. Binaries built w/wo debug information are required to be identical in LLVM. I think that requirement could be applied on pseudo probe as well. It is even better to have some test to enforce it so that no change in the future could break the requirement.


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  https://reviews.llvm.org/D86193/new/

https://reviews.llvm.org/D86193