Conservative fix for PR17827 - don't optimize a shift + and + compare sequence where the shift is logical unless the comparison is unsigned
llvm-svn: 196129
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 226126b..9bb65ef 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1198,11 +1198,16 @@
Type *AndTy = AndCST->getType(); // Type of the and.
// We can fold this as long as we can't shift unknown bits
- // into the mask. This can only happen with signed shift
- // rights, as they sign-extend.
+ // into the mask. This can happen with signed shift
+ // rights, as they sign-extend. With logical shifts,
+ // we must still make sure the comparison is not signed
+ // because we are effectively changing the
+ // position of the sign bit (PR17827).
+ // TODO: We can relax these constraints a bit more.
if (ShAmt) {
- bool CanFold = Shift->isLogicalShift();
- if (!CanFold) {
+ bool CanFold = false;
+ unsigned ShiftOpcode = Shift->getOpcode();
+ if (ShiftOpcode == Instruction::AShr) {
// To test for the bad case of the signed shr, see if any
// of the bits shifted in could be tested after the mask.
uint32_t TyBits = Ty->getPrimitiveSizeInBits();
@@ -1212,6 +1217,9 @@
if ((APInt::getHighBitsSet(BitWidth, BitWidth-ShAmtVal) &
AndCST->getValue()) == 0)
CanFold = true;
+ } else if (ShiftOpcode == Instruction::Shl ||
+ ShiftOpcode == Instruction::LShr) {
+ CanFold = !ICI.isSigned();
}
if (CanFold) {