[CodeGen] Replace the reused stores' chain for extractelt expansion.

This fixes a subtle issue that was introduced in r205153.

When reusing a store for the extractelement expansion (to load directly
from it, inserting of going through the stack), later stores to the
same location might have overwritten the data we were expecting to
extract from.

To fix that, we need to explicitly replace the chain going out of the
reused store, so that later stores also have an explicit dependency on
the generated element-extracting loads, and can't clobber them.

rdar://20066785
Differential Revision: http://reviews.llvm.org/D8180

llvm-svn: 231721

1 files changed, 20 insertions, 6 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
index 3dd73155488..ece38f33a8c 100644
--- a/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
@@ -1442,13 +1442,27 @@ SDValue SelectionDAGLegalize::ExpandExtractFromVectorThroughStack(SDValue Op) {
   Idx = DAG.getZExtOrTrunc(Idx, dl, TLI.getPointerTy());
   StackPtr = DAG.getNode(ISD::ADD, dl, Idx.getValueType(), Idx, StackPtr);
 
+  SDValue NewLoad;
+
   if (Op.getValueType().isVector())
-    return DAG.getLoad(Op.getValueType(), dl, Ch, StackPtr,MachinePointerInfo(),
-                       false, false, false, 0);
-  return DAG.getExtLoad(ISD::EXTLOAD, dl, Op.getValueType(), Ch, StackPtr,
-                        MachinePointerInfo(),
-                        Vec.getValueType().getVectorElementType(),
-                        false, false, false, 0);
+    NewLoad = DAG.getLoad(Op.getValueType(), dl, Ch, StackPtr,
+                          MachinePointerInfo(), false, false, false, 0);
+  else
+    NewLoad = DAG.getExtLoad(
+        ISD::EXTLOAD, dl, Op.getValueType(), Ch, StackPtr, MachinePointerInfo(),
+        Vec.getValueType().getVectorElementType(), false, false, false, 0);
+
+  // Replace the chain going out of the store, by the one out of the load.
+  DAG.ReplaceAllUsesOfValueWith(Ch, SDValue(NewLoad.getNode(), 1));
+
+  // We introduced a cycle though, so update the loads operands, making sure
+  // to use the original store's chain as an incoming chain.
+  SmallVector<SDValue, 6> NewLoadOperands(NewLoad->op_begin(),
+                                          NewLoad->op_end());
+  NewLoadOperands[0] = Ch;
+  NewLoad =
+      SDValue(DAG.UpdateNodeOperands(NewLoad.getNode(), NewLoadOperands), 0);
+  return NewLoad;
 }
 
 SDValue SelectionDAGLegalize::ExpandInsertToVectorThroughStack(SDValue Op) {