[X86] Keep dependencies when constructing loads in combineStore

Summary:
Preserve chain dependecies between old and new loads constructed to
prevent loads from reordering below later stores.

Fixes PR34088.

Reviewers: craig.topper, spatel, RKSimon, efriedma

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D36528

llvm-svn: 310604

1 files changed, 6 insertions, 5 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index a4a6562e33a..b37704946a0 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -7339,22 +7339,23 @@ void SelectionDAG::TransferDbgValues(SDValue From, SDValue To) {
     AddDbgValue(I, ToNode, false);
 }
 
-void SelectionDAG::makeEquivalentMemoryOrdering(LoadSDNode *OldLoad,
-                                                SDValue NewMemOp) {
+SDValue SelectionDAG::makeEquivalentMemoryOrdering(LoadSDNode *OldLoad,
+                                                   SDValue NewMemOp) {
   assert(isa<MemSDNode>(NewMemOp.getNode()) && "Expected a memop node");
-  if (!OldLoad->hasAnyUseOfValue(1))
-    return;
-
   // The new memory operation must have the same position as the old load in
   // terms of memory dependency. Create a TokenFactor for the old load and new
   // memory operation and update uses of the old load's output chain to use that
   // TokenFactor.
   SDValue OldChain = SDValue(OldLoad, 1);
   SDValue NewChain = SDValue(NewMemOp.getNode(), 1);
+  if (!OldLoad->hasAnyUseOfValue(1))
+    return NewChain;
+
   SDValue TokenFactor =
       getNode(ISD::TokenFactor, SDLoc(OldLoad), MVT::Other, OldChain, NewChain);
   ReplaceAllUsesOfValueWith(OldChain, TokenFactor);
   UpdateNodeOperands(TokenFactor.getNode(), OldChain, NewChain);
+  return TokenFactor;
 }
 
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