[DAGCombiner] Bugfix for lost chain depenedency.

When MergeConsecutiveStores() combines two loads and two stores into
wider loads and stores, the chain users of both of the original loads
must be transfered to the new load, because it may be that a chain
user only depends on one of the loads.

New test case: test/CodeGen/SystemZ/dag-combine-01.ll

Reviewed by James Y Knight.

Bugzilla: https://llvm.org/bugs/show_bug.cgi?id=25310#c6
llvm-svn: 253779

1 files changed, 7 insertions, 13 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index af73b62546e..1cf5c4c913c 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -11529,7 +11529,7 @@ bool DAGCombiner::MergeConsecutiveStores(StoreSDNode* St) {
   StartAddress = LoadNodes[0].OffsetFromBase;
   SDValue FirstChain = FirstLoad->getChain();
   for (unsigned i = 1; i < LoadNodes.size(); ++i) {
-    // All loads much share the same chain.
+    // All loads must share the same chain.
     if (LoadNodes[i].MemNode->getChain() != FirstChain)
       break;
 
@@ -11621,8 +11621,8 @@ bool DAGCombiner::MergeConsecutiveStores(StoreSDNode* St) {
   SDLoc LoadDL(LoadNodes[0].MemNode);
   SDLoc StoreDL(StoreNodes[0].MemNode);
 
-  // The merged loads are required to have the same chain, so using the first's
-  // chain is acceptable.
+  // The merged loads are required to have the same incoming chain, so
+  // using the first's chain is acceptable.
   SDValue NewLoad = DAG.getLoad(
       JointMemOpVT, LoadDL, FirstLoad->getChain(), FirstLoad->getBasePtr(),
       FirstLoad->getPointerInfo(), false, false, false, FirstLoadAlign);
@@ -11634,17 +11634,11 @@ bool DAGCombiner::MergeConsecutiveStores(StoreSDNode* St) {
     NewStoreChain, StoreDL, NewLoad, FirstInChain->getBasePtr(),
       FirstInChain->getPointerInfo(), false, false, FirstStoreAlign);
 
-  // Replace one of the loads with the new load.
-  LoadSDNode *Ld = cast<LoadSDNode>(LoadNodes[0].MemNode);
-  DAG.ReplaceAllUsesOfValueWith(SDValue(Ld, 1),
-                                SDValue(NewLoad.getNode(), 1));
-
-  // Remove the rest of the load chains.
-  for (unsigned i = 1; i < NumElem ; ++i) {
-    // Replace all chain users of the old load nodes with the chain of the new
-    // load node.
+  // Transfer chain users from old loads to the new load.
+  for (unsigned i = 0; i < NumElem; ++i) {
     LoadSDNode *Ld = cast<LoadSDNode>(LoadNodes[i].MemNode);
-    DAG.ReplaceAllUsesOfValueWith(SDValue(Ld, 1), Ld->getChain());
+    DAG.ReplaceAllUsesOfValueWith(SDValue(Ld, 1),
+                                  SDValue(NewLoad.getNode(), 1));
   }
 
   // Replace the last store with the new store.