[ValueTracking] Have sub and xor in KnownNonZero take the same exact path

[AZero13]

If x - y == 0, then x ^ y == 0. Therefore, we can do the exact same checks.

https://alive2.llvm.org/ce/z/MtBRoj

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

@llvm/pr-subscribers-llvm-analysis

Author: AZero13 (AZero13)

Changes

It goes without saying that for x ^ y to be 0, x and y must be equal.

---

Full diff: https://github.com/llvm/llvm-project/pull/146975.diff

1 Files Affected:

• (modified) llvm/lib/Analysis/ValueTracking.cpp (+4)

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index e576f4899810a..ba8ae3a975729 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -3043,6 +3043,10 @@ static bool isKnownNonZeroFromOperator(const Operator *I,
     // (X ^ (X != 0)) is non zero
     if (matchOpWithOpEqZero(I->getOperand(0), I->getOperand(1)))
       return true;
+    // X ^ Y != 0 if X != Y.
+    if (isKnownNonEqual(I->getOperand(0), I->getOperand(1), DemandedElts, Q,
+                        Depth))
+      return true;
     break;
   case Instruction::Or:
     // (X | (X != 0)) is non zero

[AZero13]

I really cannot find a case for this, but this is just too obvious and a fast path.

[AZero13]

@dtcxzyw Can I see if it affects opt at all so I can use it to make some tests.

[AZero13]

Done! @nikic

[nikic]

Please also add a test showing an improvement (you can take inspiration from the llvm-opt-benchmark results).

[AZero13]

Please also add a test showing an improvement (you can take inspiration from the llvm-opt-benchmark results).

Done!

[AZero13]

Actually, I have a better idea!

[AZero13]

@nikic I am going to have XOR and sub take the same path:

https://alive2.llvm.org/ce/z/MtBRoj

[nikic]

LGTM

[AZero13]

Thanks. I don't have merge permissions.

[AZero13]

@nikic