[InstCombine] Replace call to haveNoCommonBitsSet in visitXor with just the special case that doesn't use computeKnownBits.

topperc · topperc · commit 8bb49218bc9d · 2018-08-13T00:38:27.000Z
Summary: computeKnownBits is expensive. The cases that would be detected by the computeKnownBits portion of haveNoCommonBitsSet were already handled by the earlier call to SimplifyDemandedInstructionBits. Reviewers: spatel, lebedev.ri Reviewed By: lebedev.ri Subscribers: llvm-commits Differential Revision: https://reviews.llvm.org/D50604 llvm-svn: 339531
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
@@ -2507,9 +2507,15 @@ Instruction *InstCombiner::visitXor(BinaryOperator &I) {
   if (Value *V = SimplifyBSwap(I, Builder))
     return replaceInstUsesWith(I, V);
 
-  // A^B --> A|B iff A and B have no bits set in common.
   Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
-  if (haveNoCommonBitsSet(Op0, Op1, DL, &AC, &I, &DT))
+
+  // Fold (X & M) ^ (Y & ~M) -> (X & M) | (Y & ~M)
+  // This it a special case in haveNoCommonBitsSet, but the commputeKnownBits
+  // calls in there are unnecessary as SimplifyDemandedInstructionBits should
+  // have already taken care of those cases.
+  Value *M;
+  if (match(&I, m_c_Xor(m_c_And(m_Not(m_Value(M)), m_Value()),
+                        m_c_And(m_Deferred(M), m_Value()))))
     return BinaryOperator::CreateOr(Op0, Op1);
 
   // Apply DeMorgan's Law for 'nand' / 'nor' logic with an inverted operand.
