InstCombine: Move Sub->Xor rule from SimplifyDemanded to InstCombine

The rule that turns a sub to xor if the LHS is 2^n-1 and the remaining bits
are known zero, does not use the demanded bits at all: Move it to the
normal InstCombine code path.

Differential Revision: http://reviews.llvm.org/D9417

llvm-svn: 236268

2 files changed, 13 insertions, 10 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index c608f84bc7b..5f8746e7a15 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1586,6 +1586,19 @@ Instruction *InstCombiner::visitSub(BinaryOperator &I) {
           CI->getValue() == I.getType()->getPrimitiveSizeInBits() - 1)
         return BinaryOperator::CreateLShr(X, CI);
     }
+
+    // Turn this into a xor if LHS is 2^n-1 and the remaining bits are known
+    // zero.
+    APInt IntVal = C->getValue();
+    if ((IntVal + 1).isPowerOf2()) {
+      unsigned BitWidth = I.getType()->getScalarSizeInBits();
+      APInt KnownZero(BitWidth, 0);
+      APInt KnownOne(BitWidth, 0);
+      computeKnownBits(&I, KnownZero, KnownOne, 0, &I);
+      if ((IntVal | KnownZero).isAllOnesValue()) {
+        return BinaryOperator::CreateXor(Op1, C);
+      }
+    }
   }
 
 
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
index 3dbb1b190be..ac921331a01 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -630,16 +630,6 @@ Value *InstCombiner::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
     // Otherwise just hand the sub off to computeKnownBits to fill in
     // the known zeros and ones.
     computeKnownBits(V, KnownZero, KnownOne, Depth, CxtI);
-
-    // Turn this into a xor if LHS is 2^n-1 and the remaining bits are known
-    // zero.
-    if (ConstantInt *C0 = dyn_cast<ConstantInt>(I->getOperand(0))) {
-      APInt I0 = C0->getValue();
-      if ((I0 + 1).isPowerOf2() && (I0 | KnownZero).isAllOnesValue()) {
-        Instruction *Xor = BinaryOperator::CreateXor(I->getOperand(1), C0);
-        return InsertNewInstWith(Xor, *I);
-      }
-    }
     break;
   case Instruction::Shl:
     if (ConstantInt *SA = dyn_cast<ConstantInt>(I->getOperand(1))) {