diff options
Diffstat (limited to 'lib/Transforms')
-rw-r--r-- | lib/Transforms/Scalar/InstructionCombining.cpp | 34 |
1 files changed, 33 insertions, 1 deletions
diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp index 92760b6..a5bf60b 100644 --- a/lib/Transforms/Scalar/InstructionCombining.cpp +++ b/lib/Transforms/Scalar/InstructionCombining.cpp @@ -5986,7 +5986,39 @@ Instruction *InstCombiner::commonIntCastTransforms(CastInst &CI) { } Instruction *InstCombiner::visitTrunc(CastInst &CI) { - return commonIntCastTransforms(CI); + if (Instruction *Result = commonIntCastTransforms(CI)) + return Result; + + Value *Src = CI.getOperand(0); + const Type *Ty = CI.getType(); + unsigned DestBitWidth = Ty->getPrimitiveSizeInBits(); + + if (Instruction *SrcI = dyn_cast<Instruction>(Src)) { + switch (SrcI->getOpcode()) { + default: break; + case Instruction::LShr: + // We can shrink lshr to something smaller if we know the bits shifted in + // are already zeros. + if (ConstantInt *ShAmtV = dyn_cast<ConstantInt>(SrcI->getOperand(1))) { + unsigned ShAmt = ShAmtV->getZExtValue(); + + // Get a mask for the bits shifting in. + uint64_t Mask = (~0ULL >> (64-ShAmt)) << DestBitWidth; + if (SrcI->hasOneUse() && MaskedValueIsZero(SrcI->getOperand(0), Mask)) { + if (ShAmt >= DestBitWidth) // All zeros. + return ReplaceInstUsesWith(CI, Constant::getNullValue(Ty)); + + // Okay, we can shrink this. Truncate the input, then return a new + // shift. + Value *V = InsertCastBefore(SrcI->getOperand(0), Ty, CI); + return new ShiftInst(Instruction::LShr, V, SrcI->getOperand(1)); + } + } + break; + } + } + + return 0; } Instruction *InstCombiner::visitZExt(CastInst &CI) { |