[DAGCombine] Generalize Dead Store to overlapping stores.

Summary:
Remove stores that are immediately overwritten by larger
stores.

Reviewers: courbet, rnk

Reviewed By: rnk

Subscribers: javed.absar, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58467

llvm-svn: 354518

1 files changed, 17 insertions, 14 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 993ab4ae551..e46cda729f3 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -15414,25 +15414,28 @@ SDValue DAGCombiner::visitSTORE(SDNode *N) {
 
   if (StoreSDNode *ST1 = dyn_cast<StoreSDNode>(Chain)) {
     if (ST->isUnindexed() && !ST->isVolatile() && ST1->isUnindexed() &&
-        !ST1->isVolatile() && ST1->getBasePtr() == Ptr &&
-        ST->getMemoryVT() == ST1->getMemoryVT()) {
-      // If this is a store followed by a store with the same value to the same
-      // location, then the store is dead/noop.
-      if (ST1->getValue() == Value) {
-        // The store is dead, remove it.
+        !ST1->isVolatile()) {
+      if (ST1->getBasePtr() == Ptr && ST1->getValue() == Value &&
+          ST->getMemoryVT() == ST1->getMemoryVT()) {
+        // If this is a store followed by a store with the same value to the
+        // same location, then the store is dead/noop.
         return Chain;
       }
 
-      // If this is a store who's preceeding store to the same location
-      // and no one other node is chained to that store we can effectively
-      // drop the store. Do not remove stores to undef as they may be used as
-      // data sinks.
       if (OptLevel != CodeGenOpt::None && ST1->hasOneUse() &&
           !ST1->getBasePtr().isUndef()) {
-        // ST1 is fully overwritten and can be elided. Combine with it's chain
-        // value.
-        CombineTo(ST1, ST1->getChain());
-        return SDValue();
+        const BaseIndexOffset STBase = BaseIndexOffset::match(ST, DAG);
+        const BaseIndexOffset ChainBase = BaseIndexOffset::match(ST1, DAG);
+        unsigned STByteSize = ST->getMemoryVT().getSizeInBits() / 8;
+        unsigned ChainByteSize = ST1->getMemoryVT().getSizeInBits() / 8;
+        // If this is a store who's preceeding store to a subset of the current
+        // location and no one other node is chained to that store we can
+        // effectively drop the store. Do not remove stores to undef as they may
+        // be used as data sinks.
+        if (STBase.contains(STByteSize, ChainBase, ChainByteSize, DAG)) {
+          CombineTo(ST1, ST1->getChain());
+          return SDValue();
+        }
       }
     }
   }