[llvm] [InstCombine] Change (add x, c) to (xor x, c) (PR #75129)

[Nikita Popov via llvm-commits]

================
@@ -552,6 +552,14 @@ Value *InstCombinerImpl::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
     if (DemandedFromOps.isSubsetOf(LHSKnown.Zero))
       return I->getOperand(1);
 
+    // (add X, C) --> (xor X, C) IFF C is equal to the top bit of the DemandMask
+    {
+      const APInt *C;
+      if (match(I->getOperand(1), m_APInt(C)) &&
+          C->isOneBitSet(DemandedMask.getActiveBits() - 1))
+        return Builder.CreateXor(I->getOperand(0), ConstantInt::get(VTy, *C));
----------------
nikic wrote:

Ah, sorry for missing that. As this is in SimplifyDemanded we should explicitly set the IRBuilder InsertPoint.

https://github.com/llvm/llvm-project/pull/75129