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InstCombine: Check for zero shift amounts before subtracting one causing integer overflow.

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PR17026. Also avoid undefined shifts and shift amounts larger than 64 bits
(those are always undef because we can't represent integer types that large).

llvm-svn: 189672
This commit is contained in:
Benjamin Kramer
2013-08-30 14:35:35 +00:00
parent 1790ea000b
commit 010f108382
2 changed files with 51 additions and 10 deletions
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25
llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
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@@ -845,21 +845,26 @@ Value *InstCombiner::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
Value *InstCombiner::SimplifyShrShlDemandedBits(Instruction *Shr,
Instruction *Shl, APInt DemandedMask, APInt &KnownZero, APInt &KnownOne) {
unsigned ShlAmt = cast<ConstantInt>(Shl->getOperand(1))->getZExtValue();
unsigned ShrAmt = cast<ConstantInt>(Shr->getOperand(1))->getZExtValue();
const APInt &ShlOp1 = cast<ConstantInt>(Shl->getOperand(1))->getValue();
const APInt &ShrOp1 = cast<ConstantInt>(Shr->getOperand(1))->getValue();
if (!ShlOp1 || !ShrOp1)
return 0; // Noop.
Value *VarX = Shr->getOperand(0);
Type *Ty = VarX->getType();
unsigned BitWidth = Ty->getIntegerBitWidth();
if (ShlOp1.uge(BitWidth) || ShrOp1.uge(BitWidth))
return 0; // Undef.
unsigned ShlAmt = ShlOp1.getZExtValue();
unsigned ShrAmt = ShrOp1.getZExtValue();
KnownOne.clearAllBits();
KnownZero = APInt::getBitsSet(KnownZero.getBitWidth(), 0, ShlAmt-1);
KnownZero &= DemandedMask;
if (ShlAmt == 0 || ShrAmt == 0)
return 0;
Value *VarX = Shr->getOperand(0);
Type *Ty = VarX->getType();
APInt BitMask1(APInt::getAllOnesValue(Ty->getIntegerBitWidth()));
APInt BitMask2(APInt::getAllOnesValue(Ty->getIntegerBitWidth()));
APInt BitMask1(APInt::getAllOnesValue(BitWidth));
APInt BitMask2(APInt::getAllOnesValue(BitWidth));
bool isLshr = (Shr->getOpcode() == Instruction::LShr);
BitMask1 = isLshr ? (BitMask1.lshr(ShrAmt) << ShlAmt) :