[PATCH] D117907: [NewGVN] do phi(undef, x) -> x only if x is not poison

Wed Jan 26 06:12:42 PST 2022

nlopes added a comment.

In D117907#3271869 <https://reviews.llvm.org/D117907#3271869>, @nlopes wrote:

> In D117907#3271648 <https://reviews.llvm.org/D117907#3271648>, @nikic wrote:
>
>> It looks like the current behavior matches what GVN does: https://llvm.godbolt.org/z/eTYzMjKf7
>>
>> If an issue affects both GVN and NewGVN, please always fix it in GVN first. Otherwise the fix will remain untested, and this seems like a situation where it can cause a real regression in optimization quality (because we still treat uninit loads as undef, not poison).
>
> The bug is not in GVN itself, it's in InstSimplify (which is called by GVNPass::processInstruction).
> I do have a patch for InstSimplify as well, but given the potential to regress performance, I would rather delay that until after the LLVM 14 branch.

@nikic do you agree with this plan (commit this patch now and fix InstSimplify after 14 branches in a few weeks)?

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D117907/new/

https://reviews.llvm.org/D117907