InstCombine: Add a combine that turns (2^n)-1 ^ x back into (2^n)-1 - x iff x is smaller than 2^n and it fuses with a following add.

This was intended to undo the sub canonicalization in cases where it's not profitable, but it also
finds some cases on it's own.

llvm-svn: 147256

1 files changed, 13 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 604d9c8bb44..f2be01b73be 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -136,6 +136,19 @@ Instruction *InstCombiner::visitAdd(BinaryOperator &I) {
         Value *NewShl = Builder->CreateShl(XorLHS, ShAmt, "sext");
         return BinaryOperator::CreateAShr(NewShl, ShAmt);
       }
+
+      // If this is a xor that was canonicalized from a sub, turn it back into
+      // a sub and fuse this add with it.
+      if (LHS->hasOneUse() && (XorRHS->getValue()+1).isPowerOf2()) {
+        IntegerType *IT = cast<IntegerType>(I.getType());
+        APInt Mask = APInt::getAllOnesValue(IT->getBitWidth());
+        APInt LHSKnownOne(IT->getBitWidth(), 0);
+        APInt LHSKnownZero(IT->getBitWidth(), 0);
+        ComputeMaskedBits(XorLHS, Mask, LHSKnownZero, LHSKnownOne);
+        if ((XorRHS->getValue() | LHSKnownZero).isAllOnesValue())
+          return BinaryOperator::CreateSub(ConstantExpr::getAdd(XorRHS, CI),
+                                           XorLHS);
+      }
     }
   }