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If it's safe to speculatively execute load(alloca) the it's safe to execute

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load(gep(alloca)) where the gep is all-zeros. There's more we could do here but
this is a common case.

llvm-svn: 108101
This commit is contained in:
Nick Lewycky 2010-07-11 20:36:29 +00:00
parent 656c2c8700
commit 9abc58b327
1 changed files with 11 additions and 3 deletions
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14
lib/VMCore/Instruction.cpp
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@ -401,12 +401,20 @@ bool Instruction::isSafeToSpeculativelyExecute() const {
return false;
// Note that it is not safe to speculate into a malloc'd region because
// malloc may return null.
if (isa<AllocaInst>(getOperand(0)))
// It's also not safe to follow a bitcast, for example:
// bitcast i8* (alloca i8) to i32*
// would result in a 4-byte load from a 1-byte alloca.
Value *Op0 = getOperand(0);
if (GEPOperator *GEP = dyn_cast<GEPOperator>(Op0)) {
// TODO: it's safe to do this for any GEP with constant indices that
// compute inside the allocated type, but not for any inbounds gep.
if (GEP->hasAllZeroIndices())
Op0 = GEP->getPointerOperand();
}
if (isa<AllocaInst>(Op0))
return true;
if (GlobalVariable *GV = dyn_cast<GlobalVariable>(getOperand(0)))
return !GV->hasExternalWeakLinkage();
// FIXME: Handle cases involving GEPs. We have to be careful because
// a load of a out-of-bounds GEP has undefined behavior.
return false;
}
case Call: