InstCombine: Canonicalize (2^n)-1 - x into (2^n)-1 ^ x iff x is known to be smaller than 2^n.
This has the obvious advantage of being commutable and is always a win on x86 because
const - x wastes a register there. On less weird architectures this may lead to
a regression because other arithmetic doesn't fuse with it anymore. I'll address that
problem in a followup.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147254 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
index 5cd9a4b..4c72020 100644
--- a/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ b/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -567,9 +567,20 @@
LHSKnownZero, LHSKnownOne, Depth+1))
return I;
}
+
// Otherwise just hand the sub off to ComputeMaskedBits to fill in
// the known zeros and ones.
ComputeMaskedBits(V, DemandedMask, KnownZero, KnownOne, Depth);
+
+ // 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))) {