If it's safe to speculatively execute load(alloca) the it's safe to execute
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
diff --git a/llvm/lib/VMCore/Instruction.cpp b/llvm/lib/VMCore/Instruction.cpp
index e8d2814..d3f62d0 100644
--- a/llvm/lib/VMCore/Instruction.cpp
+++ b/llvm/lib/VMCore/Instruction.cpp
@@ -401,12 +401,20 @@
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: