[InstCombine] Replace undef with poison in a test

[xarkenz]

This is intended as part of the effort to deprecate undef. The additional wrap-flags are due to the fact that the add can no longer overflow with poison, but could overflow with undef.

Alive2 proof: https://alive2.llvm.org/ce/z/qiRYcQ

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[llvmorg-github-actions]

@llvm/pr-subscribers-llvm-transforms

Author: Sean Clarke (xarkenz)

Changes

This is intended as part of the effort to deprecate undef. PoisonValue inherits from UndefValue, so testing with poison in this case should imply that it works with undef as well.

Checks were regenerated using UTC.

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Full diff: https://github.com/llvm/llvm-project/pull/199757.diff

1 Files Affected:

• (modified) llvm/test/Transforms/InstCombine/sub-xor.ll (+5-5)

diff --git a/llvm/test/Transforms/InstCombine/sub-xor.ll b/llvm/test/Transforms/InstCombine/sub-xor.ll
index 48e6d830c41f6..b3c35d90e64b3 100644
--- a/llvm/test/Transforms/InstCombine/sub-xor.ll
+++ b/llvm/test/Transforms/InstCombine/sub-xor.ll
@@ -145,15 +145,15 @@ define <2 x i8> @xor_add_splat(<2 x i8> %x) {
   ret <2 x i8> %add
 }
 
-define <2 x i8> @xor_add_splat_undef(<2 x i8> %x) {
-; CHECK-LABEL: @xor_add_splat_undef(
+define <2 x i8> @xor_add_splat_poison(<2 x i8> %x) {
+; CHECK-LABEL: @xor_add_splat_poison(
 ; CHECK-NEXT:    [[AND:%.*]] = and <2 x i8> [[X:%.*]], splat (i8 24)
-; CHECK-NEXT:    [[XOR:%.*]] = xor <2 x i8> [[AND]], <i8 63, i8 undef>
-; CHECK-NEXT:    [[ADD:%.*]] = add <2 x i8> [[XOR]], splat (i8 42)
+; CHECK-NEXT:    [[XOR:%.*]] = xor <2 x i8> [[AND]], <i8 63, i8 poison>
+; CHECK-NEXT:    [[ADD:%.*]] = add nuw nsw <2 x i8> [[XOR]], splat (i8 42)
 ; CHECK-NEXT:    ret <2 x i8> [[ADD]]
 ;
   %and = and <2 x i8> %x, <i8 24, i8 24>
-  %xor = xor <2 x i8> %and, <i8 63, i8 undef>
+  %xor = xor <2 x i8> %and, <i8 63, i8 poison>
   %add = add <2 x i8> %xor, <i8 42, i8 42>
   ret <2 x i8> %add
 }

[artagnon]

PoisonValue inherits from UndefValue, so testing with poison in this case should imply that it works with undef as well.

Ah, but the wrap flags are different, and this is a non-trivial proof? EDIT: I took the liberty to edit your commit message.

[artagnon]

Found the code responsible for adding wrap-flags in InstCombineAddSub's visitAdd:

  bool Changed = false;
  if (!I.hasNoSignedWrap() && willNotOverflowSignedAdd(LHSCache, RHSCache, I)) {
    Changed = true;
    I.setHasNoSignedWrap(true);
  }
  if (!I.hasNoUnsignedWrap() &&
      willNotOverflowUnsignedAdd(LHSCache, RHSCache, I)) {
    Changed = true;
    I.setHasNoUnsignedWrap(true);
  }

I think willNotOverflowSignedAdd is doing the right thing on poison, and falls over on undef.

[github-actions]

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