Reapply r88830 with a bugfix: this transform only applies to icmp eq/ne. This

fixes part of PR5438.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@89639 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 30 insertions, 0 deletions

diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp
index 6f74902e1b..99555ea9b3 100644
--- a/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/lib/Transforms/Scalar/InstructionCombining.cpp
@@ -8539,6 +8539,36 @@ Instruction *InstCombiner::transformZExtICmp(ICmpInst *ICI, Instruction &CI,
     }
   }
 
+  // icmp ne A, B is equal to xor A, B when A and B only really have one bit.
+  // It is also profitable to transform icmp eq into not(xor(A, B)) because that
+  // may lead to additional simplifications.
+  if (ICI->isEquality() && CI.getType() == ICI->getOperand(0)->getType()) {
+    if (const IntegerType *ITy = dyn_cast<IntegerType>(CI.getType())) {
+      uint32_t BitWidth = ITy->getBitWidth();
+      if (BitWidth > 1) {
+        Value *LHS = ICI->getOperand(0);
+        Value *RHS = ICI->getOperand(1);
+
+        APInt KnownZeroLHS(BitWidth, 0), KnownOneLHS(BitWidth, 0);
+        APInt KnownZeroRHS(BitWidth, 0), KnownOneRHS(BitWidth, 0);
+        APInt TypeMask(APInt::getHighBitsSet(BitWidth, BitWidth-1));
+        ComputeMaskedBits(LHS, TypeMask, KnownZeroLHS, KnownOneLHS);
+        ComputeMaskedBits(RHS, TypeMask, KnownZeroRHS, KnownOneRHS);
+
+        if (KnownZeroLHS.countLeadingOnes() == BitWidth-1 &&
+            KnownZeroRHS.countLeadingOnes() == BitWidth-1) {
+          if (!DoXform) return ICI;
+
+          Value *Xor = Builder->CreateXor(LHS, RHS);
+          if (ICI->getPredicate() == ICmpInst::ICMP_EQ)
+            Xor = Builder->CreateXor(Xor, ConstantInt::get(ITy, 1));
+          Xor->takeName(ICI);
+          return ReplaceInstUsesWith(CI, Xor);
+        }
+      }
+    }
+  }
+
   return 0;
 }