[PATCH] InstCombine: Move Sub->Xor rule from SimplifyDemanded to InstCombine

[Matthias Braun]

Hi majnemer,

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.

REPOSITORY
  rL LLVM

http://reviews.llvm.org/D9417

Files:
  lib/Transforms/InstCombine/InstCombineAddSub.cpp
  lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp

Index: lib/Transforms/InstCombine/InstCombineAddSub.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1586,6 +1586,19 @@
           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);
+      }
+    }
   }
 
 
Index: lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -630,16 +630,6 @@
     // 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))) {

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