ConstantInt *C2 =
dyn_cast<ConstantInt>(GEP2->getOperand(GEP2->getNumOperands() - 1));
- // If the last (struct) indices aren't constants, we can't say anything.
- // If they're identical, the other indices might be also be dynamically
- // equal, so the GEPs can alias.
- if (!C1 || !C2 || C1 == C2)
+ // If the last (struct) indices are constants and are equal, the other indices
+ // might be also be dynamically equal, so the GEPs can alias.
+ if (C1 && C2 && C1 == C2)
return MayAlias;
// Find the last-indexed type of the GEP, i.e., the type you'd get if
IntermediateIndices.push_back(GEP1->getOperand(i + 1));
}
- StructType *LastIndexedStruct =
- dyn_cast<StructType>(GetElementPtrInst::getIndexedType(
- GEP1->getSourceElementType(), IntermediateIndices));
+ auto *Ty = GetElementPtrInst::getIndexedType(
+ GEP1->getSourceElementType(), IntermediateIndices);
+ StructType *LastIndexedStruct = dyn_cast<StructType>(Ty);
- if (!LastIndexedStruct)
+ if (isa<SequentialType>(Ty)) {
+ // We know that:
+ // - both GEPs begin indexing from the exact same pointer;
+ // - the last indices in both GEPs are constants, indexing into a sequential
+ // type (array or pointer);
+ // - both GEPs only index through arrays prior to that.
+ //
+ // Because array indices greater than the number of elements are valid in
+ // GEPs, unless we know the intermediate indices are identical between
+ // GEP1 and GEP2 we cannot guarantee that the last indexed arrays don't
+ // partially overlap.
+ for (unsigned i = 0, e = GEP1->getNumIndices() - 1; i != e; ++i)
+ if (GEP1->getOperand(i + 1) != GEP2->getOperand(i + 1))
+ return MayAlias;
+
+ // Now we know that the array/pointer that GEP1 indexes into and that
+ // that GEP2 indexes into must either precisely overlap or be disjoint.
+ // Because they cannot partially overlap and because fields in an array
+ // cannot overlap, if we can prove the final indices are different between
+ // GEP1 and GEP2, we can conclude GEP1 and GEP2 don't alias.
+
+ // If the last indices are constants, we've already checked they don't
+ // equal each other so we can exit early.
+ if (C1 && C2)
+ return NoAlias;
+ if (isKnownNonEqual(GEP1->getOperand(GEP1->getNumOperands() - 1),
+ GEP2->getOperand(GEP2->getNumOperands() - 1),
+ DL))
+ return NoAlias;
+ return MayAlias;
+ } else if (!LastIndexedStruct || !C1 || !C2) {
return MayAlias;
+ }
// We know that:
// - both GEPs begin indexing from the exact same pointer;