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Merging r221318:

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r221318 | majnemer | 2014-11-04 15:49:08 -0800 (Tue, 04 Nov 2014) | 10 lines

Analysis: Make isSafeToSpeculativelyExecute fire less for divides

Divides and remainder operations do not behave like other operations
when they are given poison: they turn into undefined behavior.

It's really hard to know if the operands going into a div are or are not
poison.  Because of this, we should only choose to speculate if there
are constant operands which we can easily reason about.

This fixes PR21412.
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_35@223647 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
David Majnemer
2014-12-08 09:16:46 +00:00
parent 62e4903904
commit 7c5cb20b30
2 changed files with 27 additions and 21 deletions
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38
lib/Analysis/ValueTracking.cpp
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@@ -1987,23 +1987,31 @@ bool llvm::isSafeToSpeculativelyExecute(const Value *V,
default:
return true;
case Instruction::UDiv:
case Instruction::URem:
// x / y is undefined if y == 0, but calculations like x / 3 are safe.
return isKnownNonZero(Inst->getOperand(1), TD);
case Instruction::URem: {
// x / y is undefined if y == 0.
const APInt *V;
if (match(Inst->getOperand(1), m_APInt(V)))
return *V != 0;
return false;
}
case Instruction::SDiv:
case Instruction::SRem: {
Value *Op = Inst->getOperand(1);
// x / y is undefined if y == 0
if (!isKnownNonZero(Op, TD))
return false;
// x / y might be undefined if y == -1
unsigned BitWidth = getBitWidth(Op->getType(), TD);
if (BitWidth == 0)
return false;
APInt KnownZero(BitWidth, 0);
APInt KnownOne(BitWidth, 0);
computeKnownBits(Op, KnownZero, KnownOne, TD);
return !!KnownZero;
// x / y is undefined if y == 0 or x == INT_MIN and y == -1
const APInt *X, *Y;
if (match(Inst->getOperand(1), m_APInt(Y))) {
if (*Y != 0) {
if (*Y == -1) {
// The numerator can't be MinSignedValue if the denominator is -1.
if (match(Inst->getOperand(0), m_APInt(X)))
return !Y->isMinSignedValue();
// The numerator *might* be MinSignedValue.
return false;
}
// The denominator is not 0 or -1, it's safe to proceed.
return true;
}
}
return false;
}
case Instruction::Load: {
const LoadInst *LI = cast<LoadInst>(Inst);