--- old/src/share/vm/opto/memnode.hpp	2021-01-25 19:30:47.603960071 +0000
+++ new/src/share/vm/opto/memnode.hpp	2021-01-25 19:30:47.485958830 +0000
@@ -586,6 +586,8 @@
 
   // have all possible loads of the value stored been optimized away?
   bool value_never_loaded(PhaseTransform *phase) const;
+
+  MemBarNode* trailing_membar() const;
 };
 
 //------------------------------StoreBNode-------------------------------------
@@ -791,6 +793,7 @@
   virtual const class TypePtr *adr_type() const { return _adr_type; }  // returns bottom_type of address
 
   bool result_not_used() const;
+  MemBarNode* trailing_membar() const;
 };
 
 class LoadStoreConditionalNode : public LoadStoreNode {
@@ -1044,6 +1047,20 @@
   // Memory type this node is serializing.  Usually either rawptr or bottom.
   const TypePtr* _adr_type;
 
+  // How is this membar related to a nearby memory access?
+  enum {
+    Standalone,
+    TrailingLoad,
+    TrailingStore,
+    LeadingStore,
+    TrailingLoadStore,
+    LeadingLoadStore
+  } _kind;
+
+#ifdef ASSERT
+  uint _pair_idx;
+#endif
+
 public:
   enum {
     Precedent = TypeFunc::Parms  // optional edge to force precedence
@@ -1061,6 +1078,24 @@
   static MemBarNode* make(Compile* C, int opcode,
                           int alias_idx = Compile::AliasIdxBot,
                           Node* precedent = NULL);
+
+  MemBarNode* trailing_membar() const;
+  MemBarNode* leading_membar() const;
+
+  void set_trailing_load() { _kind = TrailingLoad; }
+  bool trailing_load() const { return _kind == TrailingLoad; }
+  bool trailing_store() const { return _kind == TrailingStore; }
+  bool leading_store() const { return _kind == LeadingStore; }
+  bool trailing_load_store() const { return _kind == TrailingLoadStore; }
+  bool leading_load_store() const { return _kind == LeadingLoadStore; }
+  bool trailing() const { return _kind == TrailingLoad || _kind == TrailingStore || _kind == TrailingLoadStore; }
+  bool leading() const { return _kind == LeadingStore || _kind == LeadingLoadStore; }
+  bool standalone() const { return _kind == Standalone; }
+
+  static void set_store_pair(MemBarNode* leading, MemBarNode* trailing);
+  static void set_load_store_pair(MemBarNode* leading, MemBarNode* trailing);
+
+  void remove(PhaseIterGVN *igvn);
 };
 
 // "Acquire" - no following ref can move before (but earlier refs can