8290464: Optimize ResourceArea zapping on ResourceMark release

src/hotspot/share/memory/resourceArea.hpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1997, 2020, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2022, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -117,16 +117,34 @@ class ResourceArea: public Arena {
              size_in_bytes(), state._size_in_bytes);
       set_size_in_bytes(state._size_in_bytes);
       state._chunk->next_chop();
+      assert(_hwm != state._hwm, "Sanity check: HWM moves when we have later chunks");
     } else {
       assert(size_in_bytes() == state._size_in_bytes, "Sanity check");
     }
-    _chunk = state._chunk;      // Roll back to saved chunk.
-    _hwm = state._hwm;
-    _max = state._max;
 
-    // Clear out this chunk (to detect allocation bugs)
-    if (ZapResourceArea) {
-      memset(state._hwm, badResourceValue, state._max - state._hwm);
+    if (_hwm != state._hwm) {
+      // HWM moved: resource area was used. Roll back!
+
+      char* replaced_hwm = _hwm;
+
+      _chunk = state._chunk;
+      _hwm = state._hwm;
+      _max = state._max;
+
+      // Clear out this chunk (to detect allocation bugs).
+      // If current chunk contains the replaced HWM, this means we are
+      // doing the rollback within the same chunk, and we only need to
+      // clear up to replaced HWM.
+      if (ZapResourceArea) {
+        char* limit = _chunk->contains(replaced_hwm) ? replaced_hwm : _max;
+        assert(limit >= _hwm, "Sanity check: non-negative memset size");
+        memset(_hwm, badResourceValue, limit - _hwm);
+      }
+    } else {
+      // No allocations. Nothing to rollback. Check it.
+      assert(_chunk == state._chunk, "Sanity check: idempotence");
+      assert(_hwm == state._hwm,     "Sanity check: idempotence");
+      assert(_max == state._max,     "Sanity check: idempotence");
     }
   }
 };