teach instcombine to delete sign extending shift pairs (sra(shl X, C), C) when
the input is already sign extended.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@93019 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/InstCombine/InstCombineShifts.cpp b/lib/Transforms/InstCombine/InstCombineShifts.cpp
index 404fcec..1c75e55 100644
--- a/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -414,17 +414,28 @@
if (Instruction *R = commonShiftTransforms(I))
return R;
- Value *Op0 = I.getOperand(0);
+ Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
- // ashr int -1, X = -1 (for any arithmetic shift rights of ~0)
- if (ConstantInt *CSI = dyn_cast<ConstantInt>(Op0))
+ if (ConstantInt *CSI = dyn_cast<ConstantInt>(Op0)) {
+ // ashr int -1, X = -1 (for any arithmetic shift rights of ~0)
if (CSI->isAllOnesValue())
return ReplaceInstUsesWith(I, CSI);
+ }
+
+ if (ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
+ // If the input is a SHL by the same constant (ashr (shl X, C), C), then we
+ // have a sign-extend idiom. If the input value is known to already be sign
+ // extended enough, delete the extension.
+ Value *X;
+ if (match(Op0, m_Shl(m_Value(X), m_Specific(Op1))) &&
+ ComputeNumSignBits(X) > Op1C->getZExtValue())
+ return ReplaceInstUsesWith(I, X);
+ }
// See if we can turn a signed shr into an unsigned shr.
if (MaskedValueIsZero(Op0,
APInt::getSignBit(I.getType()->getScalarSizeInBits())))
- return BinaryOperator::CreateLShr(Op0, I.getOperand(1));
+ return BinaryOperator::CreateLShr(Op0, Op1);
// Arithmetic shifting an all-sign-bit value is a no-op.
unsigned NumSignBits = ComputeNumSignBits(Op0);