Changes in directory llvm/lib/Transforms/Scalar:
InstructionCombining.cpp updated: 1.506 -> 1.507
---
Log message:
Implement Transforms/InstCombine/shift-sra.ll:test0
---
Diffs of the changes: (+20 -0)
InstructionCombining.cpp | 20 ++++++++++++++++++++
1 files changed, 20 insertions(+)
Index: llvm/lib/Transforms/Scalar/InstructionCombining.cpp
diff -u llvm/lib/Transforms/Scalar/InstructionCombining.cpp:1.506
llvm/lib/Transforms/Scalar/InstructionCombining.cpp:1.507
--- llvm/lib/Transforms/Scalar/InstructionCombining.cpp:1.506 Sun Sep 17
23:22:48 2006
+++ llvm/lib/Transforms/Scalar/InstructionCombining.cpp Sun Sep 17 23:31:40 2006
@@ -1089,6 +1089,26 @@
}
break;
case Instruction::Shr:
+ // If this is an arithmetic shift right and only the low-bit is set, we can
+ // always convert this into a logical shr, even if the shift amount is
+ // variable. The low bit of the shift cannot be an input sign bit unless
+ // the shift amount is >= the size of the datatype, which is undefined.
+ if (DemandedMask == 1 && I->getType()->isSigned()) {
+ // Convert the input to unsigned.
+ Instruction *NewVal = new CastInst(I->getOperand(0),
+ I->getType()->getUnsignedVersion(),
+ I->getOperand(0)->getName());
+ InsertNewInstBefore(NewVal, *I);
+ // Perform the unsigned shift right.
+ NewVal = new ShiftInst(Instruction::Shr, NewVal, I->getOperand(1),
+ I->getName());
+ InsertNewInstBefore(NewVal, *I);
+ // Then cast that to the destination type.
+ NewVal = new CastInst(NewVal, I->getType(), I->getName());
+ InsertNewInstBefore(NewVal, *I);
+ return UpdateValueUsesWith(I, NewVal);
+ }
+
if (ConstantUInt *SA = dyn_cast<ConstantUInt>(I->getOperand(1))) {
unsigned ShAmt = SA->getValue();
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