[DAG] Improve Aliasing of operations to static alloca

Memory accesses offset from frame indices may alias, e.g., we
may merge write from function arguments passed on the stack when they
are contiguous. As a result, when checking aliasing, we consider the
underlying frame index's offset from the stack pointer.

Static allocs are realized as stack objects in SelectionDAG, but its
offset is not set until post-DAG causing DAGCombiner's alias check to
consider access to static allocas to frequently alias. Modify isAlias
to consider access between static allocas and access from other frame
objects to be considered aliasing.

Many test changes are included here. Most are fixes for tests which
indirectly relied on our aliasing ability and needed to be modified to
preserve their original intent.

The remaining tests have minor improvements due to relaxed
ordering. The exception is CodeGen/X86/2011-10-19-widen_vselect.ll
which has a minor degradation dispite though the pre-legalized DAG is
improved.

Reviewers: rnk, mkuper, jonpa, hfinkel, uweigand

Reviewed By: rnk

Subscribers: sdardis, nemanjai, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D33345

llvm-svn: 307546

1 files changed, 14 insertions, 6 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 31458ab1925..70ce2fcd33c 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -11373,12 +11373,8 @@ SDValue DAGCombiner::visitLOAD(SDNode *N) {
       SDValue Token = DAG.getNode(ISD::TokenFactor, SDLoc(N),
                                   MVT::Other, Chain, ReplLoad.getValue(1));
 
-      // Make sure the new and old chains are cleaned up.
-      AddToWorklist(Token.getNode());
-
-      // Replace uses with load result and token factor. Don't add users
-      // to work list.
-      return CombineTo(N, ReplLoad.getValue(0), Token, false);
+      // Replace uses with load result and token factor
+      return CombineTo(N, ReplLoad.getValue(0), Token);
     }
   }
 
@@ -16701,6 +16697,18 @@ bool DAGCombiner::isAlias(LSBaseSDNode *Op0, LSBaseSDNode *Op1) const {
   if (BasePtr0.equalBaseIndex(BasePtr1, DAG, PtrDiff))
     return !((NumBytes0 <= PtrDiff) || (PtrDiff + NumBytes1 <= 0));
 
+  // If both BasePtr0 and BasePtr1 are FrameIndexes, we will not be
+  // able to calculate their relative offset if at least one arises
+  // from an alloca. However, these allocas cannot overlap and we
+  // can infer there is no alias.
+  if (auto *A = dyn_cast<FrameIndexSDNode>(BasePtr0.getBase()))
+    if (auto *B = dyn_cast<FrameIndexSDNode>(BasePtr1.getBase())) {
+      MachineFrameInfo &MFI = DAG.getMachineFunction().getFrameInfo();
+      if (!MFI.isFixedObjectIndex(A->getIndex()) ||
+          !MFI.isFixedObjectIndex(B->getIndex()))
+        return false;
+    }
+
   // FIXME: findBaseOffset and ConstantValue/GlobalValue/FrameIndex analysis
   // modified to use BaseIndexOffset.