X86 cmp lowering is looking past truncate on the condition node. It should only
do so when the high bits are known zero. This caused a subtle miscompilation.
rdar://12027825
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161451 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/X86/X86ISelLowering.cpp b/lib/Target/X86/X86ISelLowering.cpp
index 8125b4c..d70cdb8 100644
--- a/lib/Target/X86/X86ISelLowering.cpp
+++ b/lib/Target/X86/X86ISelLowering.cpp
@@ -8741,6 +8741,16 @@
return C && C->isAllOnesValue();
}
+static bool isTruncWithZeroHighBitsInput(SDValue V, SelectionDAG &DAG) {
+ if (V.getOpcode() != ISD::TRUNCATE)
+ return false;
+
+ SDValue VOp0 = V.getOperand(0);
+ unsigned InBits = VOp0.getValueSizeInBits();
+ unsigned Bits = V.getValueSizeInBits();
+ return DAG.MaskedValueIsZero(VOp0, APInt::getHighBitsSet(InBits,InBits-Bits));
+}
+
SDValue X86TargetLowering::LowerSELECT(SDValue Op, SelectionDAG &DAG) const {
bool addTest = true;
SDValue Cond = Op.getOperand(0);
@@ -8910,9 +8920,9 @@
}
if (addTest) {
- // Look pass the truncate.
- if (Cond.getOpcode() == ISD::TRUNCATE)
- Cond = Cond.getOperand(0);
+ // Look pass the truncate if the high bits are known zero.
+ if (isTruncWithZeroHighBitsInput(Cond, DAG))
+ Cond = Cond.getOperand(0);
// We know the result of AND is compared against zero. Try to match
// it to BT.
@@ -9219,9 +9229,9 @@
}
if (addTest) {
- // Look pass the truncate.
- if (Cond.getOpcode() == ISD::TRUNCATE)
- Cond = Cond.getOperand(0);
+ // Look pass the truncate if the high bits are known zero.
+ if (isTruncWithZeroHighBitsInput(Cond, DAG))
+ Cond = Cond.getOperand(0);
// We know the result of AND is compared against zero. Try to match
// it to BT.