[PATCH] D110867: X86InstrInfo: Support immediates that are +1/-1 different in optimizeCompareInstr

Wed Nov 3 08:56:35 PDT 2021

hans added a comment.

This seems to have caused a miscompile of Chromium, see https://bugs.chromium.org/p/chromium/issues/detail?id=1265339 (there's even a screenshot).

It appears that in a switch statement, one of the cases gets lost.

See https://bugs.chromium.org/p/chromium/issues/detail?id=1265339#c38 for the attached IR, and how the codegen changes with this patch. Sadly it's not reduced, but at least it's stand-alone.

What we've got is something like:

  cmp $21, reg
  jle foo

  foo:
  cmp $20, reg
  jle bar

  bar:
  je baz

I suspect what's happening is that this transformation figures the second cmp is redundant if it changes "jle bar" to "jl bar", but it doesn't take into account that the "je baz" was also depending on that second cmp.

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D110867/new/

https://reviews.llvm.org/D110867