From b7363791edfbbc017fd6670c20682aca9f2c543a Mon Sep 17 00:00:00 2001 From: Chris Lattner Date: Mon, 18 Sep 2006 04:31:40 +0000 Subject: [PATCH] Implement Transforms/InstCombine/shift-sra.ll:test0 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@30450 91177308-0d34-0410-b5e6-96231b3b80d8 --- .../Scalar/InstructionCombining.cpp | 20 +++++++++++++++++++ 1 file changed, 20 insertions(+) diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp index 4d7dfe9e707..4ed827944de 100644 --- a/lib/Transforms/Scalar/InstructionCombining.cpp +++ b/lib/Transforms/Scalar/InstructionCombining.cpp @@ -1089,6 +1089,26 @@ bool InstCombiner::SimplifyDemandedBits(Value *V, uint64_t DemandedMask, } 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(I->getOperand(1))) { unsigned ShAmt = SA->getValue(); -- 2.34.1