[PATCH] D100213: [PassManager][PhaseOrdering] lower expects before running simplifyCFG

Fri Apr 9 09:32:25 PDT 2021

spatel created this revision.
spatel added reviewers: lebedev.ri, Carrot, craig.topper.
Herald added subscribers: wenlei, steven_wu, hiraditya, mcrosier.
spatel requested review of this revision.
Herald added a project: LLVM.

If we run passes before lowering llvm.expect intrinsics to metadata, then those passes have no way to act on the hints provided by llvm.expect. SimplifyCFG is the known offender, and we made it smarter about profile metadata in D98898 <https://reviews.llvm.org/D98898>.

In the motivating example from https://llvm.org/PR49336 , this means we were ignoring the recommended method for a programmer to tell the compiler that a compare+branch is expensive. This change appears to solve that case - the metadata survives to the backend, the compare order is as expected in IR, and the backend does not do anything to reverse it.

I don't know if we have a policy about old (legacy) pass manager changes at this point. We could make the same change there to keep things synchronized, or we let that rot. A potential advantage of not changing it: we have an easy way to regress test this change in the wild by running with `-fno-experimental-new-pass-manager`.

https://reviews.llvm.org/D100213

Files:
  llvm/lib/Passes/PassBuilder.cpp
  llvm/test/Other/new-pm-defaults.ll
  llvm/test/Other/new-pm-pgo.ll
  llvm/test/Other/new-pm-thinlto-defaults.ll
  llvm/test/Other/new-pm-thinlto-postlink-pgo-defaults.ll
  llvm/test/Other/new-pm-thinlto-postlink-samplepgo-defaults.ll
  llvm/test/Other/new-pm-thinlto-prelink-pgo-defaults.ll
  llvm/test/Other/new-pm-thinlto-prelink-samplepgo-defaults.ll
  llvm/test/Transforms/PhaseOrdering/expect.ll

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