8347337: ZGC: String dedups short-lived strings

Reviewed-by: kbarrett, aboldtch, eosterlund

Author: stefank

src/hotspot/share/gc/z/zForwarding.hpp

@@ -59,8 +59,8 @@ class ZForwarding {
   const size_t           _object_alignment_shift;
   const AttachedArray    _entries;
   ZPage* const           _page;
-  ZPageAge               _from_age;
-  ZPageAge               _to_age;
+  const ZPageAge         _from_age;
+  const ZPageAge         _to_age;
   volatile bool          _claimed;
   mutable ZConditionLock _ref_lock;
   volatile int32_t       _ref_count;

src/hotspot/share/gc/z/zMark.cpp

@@ -27,7 +27,6 @@
 #include "code/nmethod.hpp"
 #include "gc/shared/continuationGCSupport.inline.hpp"
 #include "gc/shared/gc_globals.hpp"
-#include "gc/shared/stringdedup/stringDedup.hpp"
 #include "gc/shared/suspendibleThreadSet.hpp"
 #include "gc/shared/workerThread.hpp"
 #include "gc/z/zAbort.inline.hpp"
@@ -389,25 +388,6 @@ void ZMark::follow_object(oop obj, bool finalizable) {
   }
 }
 
-static void try_deduplicate(ZMarkContext* context, oop obj) {
-  if (!StringDedup::is_enabled()) {
-    // Not enabled
-    return;
-  }
-
-  if (!java_lang_String::is_instance(obj)) {
-    // Not a String object
-    return;
-  }
-
-  if (java_lang_String::test_and_set_deduplication_requested(obj)) {
-    // Already requested deduplication
-    return;
-  }
-
-  // Request deduplication
-  context->string_dedup_requests()->add(obj);
-}
 
 void ZMark::mark_and_follow(ZMarkContext* context, ZMarkStackEntry entry) {
   // Decode flags
@@ -449,13 +429,7 @@ void ZMark::mark_and_follow(ZMarkContext* context, ZMarkStackEntry entry) {
     if (is_array(addr)) {
       follow_array_object(objArrayOop(to_oop(addr)), finalizable);
     } else {
-      const oop obj = to_oop(addr);
-      follow_object(obj, finalizable);
-
-      if (!finalizable) {
-        // Try deduplicate
-        try_deduplicate(context, obj);
-      }
+      follow_object(to_oop(addr), finalizable);
     }
   }
 }

src/hotspot/share/gc/z/zMarkContext.hpp

@@ -24,7 +24,6 @@
 #ifndef SHARE_GC_Z_ZMARKCONTEXT_HPP
 #define SHARE_GC_Z_ZMARKCONTEXT_HPP
 
-#include "gc/shared/stringdedup/stringDedup.hpp"
 #include "gc/z/zMarkCache.hpp"
 #include "memory/allocation.hpp"
 
@@ -37,7 +36,6 @@ class ZMarkContext : public StackObj {
   ZMarkStripe*                  _stripe;
   ZMarkThreadLocalStacks* const _stacks;
   size_t                        _nstripes;
-  StringDedup::Requests         _string_dedup_requests;
 
 public:
   ZMarkContext(size_t nstripes,
@@ -48,7 +46,6 @@ class ZMarkContext : public StackObj {
   ZMarkStripe* stripe();
   void set_stripe(ZMarkStripe* stripe);
   ZMarkThreadLocalStacks* stacks();
-  StringDedup::Requests* string_dedup_requests();
 
   size_t nstripes();
   void set_nstripes(size_t nstripes);

src/hotspot/share/gc/z/zMarkContext.inline.hpp

@@ -32,8 +32,7 @@ inline ZMarkContext::ZMarkContext(size_t nstripes,
   : _cache(nstripes),
     _stripe(stripe),
     _stacks(stacks),
-    _nstripes(nstripes),
-    _string_dedup_requests() {}
+    _nstripes(nstripes) {}
 
 inline ZMarkCache* ZMarkContext::cache() {
   return &_cache;
@@ -51,10 +50,6 @@ inline ZMarkThreadLocalStacks* ZMarkContext::stacks() {
   return _stacks;
 }
 
-inline StringDedup::Requests* ZMarkContext::string_dedup_requests() {
-  return &_string_dedup_requests;
-}
-
 inline size_t ZMarkContext::nstripes() {
   return _nstripes;
 }

src/hotspot/share/gc/z/zRelocate.cpp

@@ -40,6 +40,7 @@
 #include "gc/z/zRootsIterator.hpp"
 #include "gc/z/zStackWatermark.hpp"
 #include "gc/z/zStat.hpp"
+#include "gc/z/zStringDedup.inline.hpp"
 #include "gc/z/zTask.hpp"
 #include "gc/z/zUncoloredRoot.inline.hpp"
 #include "gc/z/zVerify.hpp"
@@ -559,12 +560,14 @@ class ZRelocateMediumAllocator {
 template <typename Allocator>
 class ZRelocateWork : public StackObj {
 private:
-  Allocator* const   _allocator;
-  ZForwarding*       _forwarding;
-  ZPage*             _target[ZAllocator::_relocation_allocators];
-  ZGeneration* const _generation;
-  size_t             _other_promoted;
-  size_t             _other_compacted;
+  Allocator* const    _allocator;
+  ZForwarding*        _forwarding;
+  ZPage*              _target[ZAllocator::_relocation_allocators];
+  ZGeneration* const  _generation;
+  size_t              _other_promoted;
+  size_t              _other_compacted;
+  ZStringDedupContext _string_dedup_context;
+
 
   ZPage* target(ZPageAge age) {
     return _target[static_cast<uint>(age) - 1];
@@ -795,6 +798,13 @@ class ZRelocateWork : public StackObj {
     update_remset_promoted(to_addr);
   }
 
+  void maybe_string_dedup(zaddress to_addr) {
+    if (_forwarding->is_promotion()) {
+      // Only deduplicate promoted objects, and let short-lived strings simply die instead.
+      _string_dedup_context.request(to_oop(to_addr));
+    }
+  }
+
   bool try_relocate_object(zaddress from_addr) {
     const zaddress to_addr = try_relocate_object_inner(from_addr);
 
@@ -804,6 +814,8 @@ class ZRelocateWork : public StackObj {
 
     update_remset_for_fields(from_addr, to_addr);
 
+    maybe_string_dedup(to_addr);
+
     return true;
   }
 
@@ -1176,10 +1188,15 @@ class ZRelocateAddRemsetForFlipPromoted : public ZRestartableTask {
 
   virtual void work() {
     SuspendibleThreadSetJoiner sts_joiner;
+    ZStringDedupContext        string_dedup_context;
 
     for (ZPage* page; _iter.next(&page);) {
       page->object_iterate([&](oop obj) {
+        // Remap oops and add remset if needed
         ZIterator::basic_oop_iterate_safe(obj, remap_and_maybe_add_remset);
+
+        // String dedup
+        string_dedup_context.request(obj);
       });
 
       SuspendibleThreadSet::yield();

src/hotspot/share/gc/z/zStringDedup.hpp

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2025, 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
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+#ifndef SHARE_GC_Z_ZSTRINGDEDUP_HPP
+#define SHARE_GC_Z_ZSTRINGDEDUP_HPP
+
+#include "gc/shared/stringdedup/stringDedup.hpp"
+#include "gc/z/zAddress.hpp"
+#include "oops/oopsHierarchy.hpp"
+
+class ZStringDedupContext {
+private:
+  StringDedup::Requests _requests;
+
+public:
+  void request(oop obj);
+};
+
+#endif // SHARE_GC_Z_ZSTRINGDEDUP_HPP

src/hotspot/share/gc/z/zStringDedup.inline.hpp

@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) 2025, 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
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+#ifndef SHARE_GC_Z_ZSTRINGDEDUP_INLINE_HPP
+#define SHARE_GC_Z_ZSTRINGDEDUP_INLINE_HPP
+
+#include "gc/z/zStringDedup.hpp"
+
+#include "classfile/javaClasses.inline.hpp"
+#include "gc/shared/gc_globals.hpp"
+
+inline void ZStringDedupContext::request(oop obj) {
+  if (!StringDedup::is_enabled()) {
+    // Not enabled
+    return;
+  }
+
+  if (!java_lang_String::is_instance(obj)) {
+    // Not a String object
+    return;
+  }
+
+  if (java_lang_String::test_and_set_deduplication_requested(obj)) {
+    // Already requested deduplication
+    return;
+  }
+
+  // Request deduplication
+  _requests.add(obj);
+}
+
+#endif // SHARE_GC_Z_ZSTRINGDEDUP_INLINE_HPP