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.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147256 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 604d9c8..f2be01b 100644
--- a/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -136,6 +136,19 @@
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);
+ }
}
}