The srem -> urem transform is not safe for any divisor that's not a power of two.
E.g. -5 % 5 is 0 with srem and 1 with urem.
Also addresses Frits van Bommel's comments.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@120049 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/InstCombine/InstCombineShifts.cpp b/lib/Transforms/InstCombine/InstCombineShifts.cpp
index 7969dbf..e52f201 100644
--- a/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -54,19 +54,17 @@
if (Instruction *Res = FoldShiftByConstant(Op0, CUI, I))
return Res;
- // X shift (A srem B) -> X shift (A urem B) iff B is positive.
+ // X shift (A srem B) -> X shift (A and B-1) iff B is a power of 2.
// Because shifts by negative values are undefined.
if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Op1))
- if (BO->getOpcode() == Instruction::SRem && BO->getType()->isIntegerTy()) {
- // Make sure the divisor's sign bit is zero.
- APInt Mask = APInt::getSignBit(BO->getType()->getPrimitiveSizeInBits());
- if (MaskedValueIsZero(BO->getOperand(1), Mask)) {
- Value *URem = Builder->CreateURem(BO->getOperand(0), BO->getOperand(1),
- BO->getName());
- I.setOperand(1, URem);
- return &I;
- }
- }
+ if (BO->hasOneUse() && BO->getOpcode() == Instruction::SRem)
+ if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1)))
+ if (CI->getValue().isPowerOf2()) {
+ Constant *C = ConstantInt::get(BO->getType(), CI->getValue()-1);
+ Value *Rem = Builder->CreateAnd(BO->getOperand(0), C, BO->getName());
+ I.setOperand(1, Rem);
+ return &I;
+ }
return 0;
}