Fix a fixme by making memdep's handling of allocations more logical.
If we see that a load depends on the allocation of its memory with no
intervening stores, we now return a 'None' depedency instead of "Normal".
This tweaks GVN to do its optimization with the new result.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60267 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Analysis/MemoryDependenceAnalysis.cpp b/lib/Analysis/MemoryDependenceAnalysis.cpp
index 94a3d1b..e135f11 100644
--- a/lib/Analysis/MemoryDependenceAnalysis.cpp
+++ b/lib/Analysis/MemoryDependenceAnalysis.cpp
@@ -159,29 +159,19 @@
continue;
return DepResultTy(Inst, Normal);
}
-
- // FIXME: This claims that an access depends on the allocation. This may
- // make sense, but is dubious at best. It would be better to fix GVN to
- // handle a 'None' Query.
+
+ // If this is an allocation, and if we know that the accessed pointer is to
+ // the allocation, return None. This means that there is no dependence and
+ // the access can be optimized based on that. For example, a load could
+ // turn into undef.
if (AllocationInst *AI = dyn_cast<AllocationInst>(Inst)) {
- Value *Pointer = AI;
- uint64_t PointerSize;
- if (ConstantInt *C = dyn_cast<ConstantInt>(AI->getArraySize()))
- // Use ABI size (size between elements), not store size (size of one
- // element without padding).
- PointerSize = C->getZExtValue() *
- TD.getABITypeSize(AI->getAllocatedType());
- else
- PointerSize = ~0UL;
+ Value *AccessPtr = MemPtr->getUnderlyingObject();
- AliasAnalysis::AliasResult R =
- AA.alias(Pointer, PointerSize, MemPtr, MemSize);
-
- if (R == AliasAnalysis::NoAlias)
- continue;
- return DepResultTy(Inst, Normal);
+ if (AccessPtr == AI ||
+ AA.alias(AI, 1, AccessPtr, 1) == AliasAnalysis::MustAlias)
+ return DepResultTy(0, None);
+ continue;
}
-
// See if this instruction mod/ref's the pointer.
AliasAnalysis::ModRefResult MRR = AA.getModRefInfo(Inst, MemPtr, MemSize);