[DAG] Fix Memory ordering check in ReduceLoadOpStore.

When merging through a TokenFactor we need to check that the
load may be ordered such that no other aliasing memory operations may
happen. It is not sufficient to just check that the load is a member
of the chain token factor as it there may be a indirect chain. Require
the load's chain has only one use.

This fixes PR37826.

Reviewers: spatel, davide, efriedma, craig.topper, RKSimon

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D49388

llvm-svn: 337560

1 files changed, 18 insertions, 16 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 9a43aa69fec..60f7bcbb536 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -13067,22 +13067,6 @@ CheckForMaskedLoad(SDValue V, SDValue Ptr, SDValue Chain) {
   LoadSDNode *LD = cast<LoadSDNode>(V->getOperand(0));
   if (LD->getBasePtr() != Ptr) return Result;  // Not from same pointer.
 
-  // The store should be chained directly to the load or be an operand of a
-  // tokenfactor.
-  if (LD == Chain.getNode())
-    ; // ok.
-  else if (Chain->getOpcode() != ISD::TokenFactor)
-    return Result; // Fail.
-  else {
-    bool isOk = false;
-    for (const SDValue &ChainOp : Chain->op_values())
-      if (ChainOp.getNode() == LD) {
-        isOk = true;
-        break;
-      }
-    if (!isOk) return Result;
-  }
-
   // This only handles simple types.
   if (V.getValueType() != MVT::i16 &&
       V.getValueType() != MVT::i32 &&
@@ -13119,6 +13103,24 @@ CheckForMaskedLoad(SDValue V, SDValue Ptr, SDValue Chain) {
   // is aligned the same as the access width.
   if (NotMaskTZ && NotMaskTZ/8 % MaskedBytes) return Result;
 
+  // For narrowing to be valid, it must be the case that the load the
+  // immediately preceeding memory operation before the store.
+  if (LD == Chain.getNode())
+    ; // ok.
+  else if (Chain->getOpcode() == ISD::TokenFactor &&
+           SDValue(LD, 1).hasOneUse()) {
+    // LD has only 1 chain use so they are no indirect dependencies.
+    bool isOk = false;
+    for (const SDValue &ChainOp : Chain->op_values())
+      if (ChainOp.getNode() == LD) {
+        isOk = true;
+        break;
+      }
+    if (!isOk)
+      return Result;
+  } else
+    return Result; // Fail.
+
   Result.first = MaskedBytes;
   Result.second = NotMaskTZ/8;
   return Result;