8339161: ZGC: Remove unused remembered sets

Reviewed-by: aboldtch, stefank

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

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2015, 2024, 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
@@ -248,8 +248,7 @@ void ZHeap::free_page(ZPage* page) {
   _page_table.remove(page);
 
   if (page->is_old()) {
-    page->verify_remset_cleared_current();
+    page->remset_delete();
-    page->verify_remset_cleared_previous();
   }
 
   // Free page
@@ -261,12 +260,10 @@ size_t ZHeap::free_empty_pages(const ZArray<ZPage*>* pages) {
   // Remove page table entries
   ZArrayIterator<ZPage*> iter(pages);
   for (ZPage* page; iter.next(&page);) {
-    if (page->is_old()) {
-      // The remset of pages should be clean when installed into the page
-      // cache.
-      page->remset_clear();
-    }
     _page_table.remove(page);
+    if (page->is_old()) {
+      page->remset_delete();
+    }
     freed += page->size();
   }
 

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

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2015, 2024, 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
@@ -78,27 +78,16 @@ void ZPage::reset_seqnum() {
   Atomic::store(&_seqnum_other, ZGeneration::generation(_generation_id == ZGenerationId::young ? ZGenerationId::old : ZGenerationId::young)->seqnum());
 }
 
-void ZPage::remset_initialize() {
+void ZPage::remset_alloc() {
-  // Remsets should only be initialized once and only for old pages.
+  // Remsets should only be allocated/initialized once and only for old pages.
   assert(!_remembered_set.is_initialized(), "Should not be initialized");
   assert(is_old(), "Only old pages need a remset");
 
   _remembered_set.initialize(size());
 }
 
-void ZPage::remset_initialize_or_verify_cleared() {
+void ZPage::remset_delete() {
-  assert(is_old(), "Only old pages need a remset");
+  _remembered_set.delete_all();
-
-  if (_remembered_set.is_initialized()) {
-    verify_remset_cleared_current();
-    verify_remset_cleared_previous();
-  } else {
-    remset_initialize();
-  }
-}
-
-void ZPage::remset_clear() {
-  _remembered_set.clear_all();
 }
 
 void ZPage::reset(ZPageAge age) {
@@ -123,7 +112,6 @@ void ZPage::reset_top_for_allocation() {
 void ZPage::reset_type_and_size(ZPageType type) {
   _type = type;
   _livemap.resize(object_max_count());
-  _remembered_set.resize(size());
 }
 
 ZPage* ZPage::retype(ZPageType type) {
@@ -216,10 +204,6 @@ void ZPage::verify_remset_cleared_previous() const {
   }
 }
 
-void ZPage::clear_remset_current() {
-  _remembered_set.clear_current();
-}
-
 void ZPage::clear_remset_previous() {
   _remembered_set.clear_previous();
 }

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

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2015, 2024, 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
@@ -155,9 +155,8 @@ public:
   void clear_remset_range_non_par_current(uintptr_t l_offset, size_t size);
   void swap_remset_bitmaps();
 
-  void remset_initialize();
+  void remset_alloc();
-  void remset_initialize_or_verify_cleared();
+  void remset_delete();
-  void remset_clear();
 
   ZBitMap::ReverseIterator remset_reverse_iterator_previous();
   BitMap::Iterator remset_iterator_limited_current(uintptr_t l_offset, size_t size);
@@ -182,7 +181,6 @@ public:
   void verify_remset_cleared_current() const;
   void verify_remset_cleared_previous() const;
 
-  void clear_remset_current();
   void clear_remset_previous();
 
   void* remset_current();

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

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2015, 2024, 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
@@ -733,7 +733,7 @@ retry:
   page->reset_top_for_allocation();
   page->reset_livemap();
   if (age == ZPageAge::old) {
-    page->remset_initialize_or_verify_cleared();
+    page->remset_alloc();
   }
 
   // Update allocation statistics. Exclude gc relocations to avoid

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

@@ -848,7 +848,7 @@ private:
     to_page->reset(to_age);
     to_page->reset_top_for_allocation();
     if (promotion) {
-      to_page->remset_initialize();
+      to_page->remset_alloc();
     }
 
     // Verify that the inactive remset is clear when resetting the page for
@@ -943,35 +943,15 @@ public:
     return ZGeneration::old()->active_remset_is_current();
   }
 
-  void clear_remset_before_reuse(ZPage* page, bool in_place) {
+  void clear_remset_before_in_place_reuse(ZPage* page) {
     if (_forwarding->from_age() != ZPageAge::old) {
       // No remset bits
       return;
     }
 
-    if (in_place) {
+    // Clear 'previous' remset bits. For in-place relocated pages, the previous
-      // Clear 'previous' remset bits. For in-place relocated pages, the previous
+    // remset bits are always used, even when active_remset_is_current().
-      // remset bits are always used, even when active_remset_is_current().
+    page->clear_remset_previous();
-      page->clear_remset_previous();
-
-      return;
-    }
-
-    // Normal relocate
-
-    // Clear active remset bits
-    if (active_remset_is_current()) {
-      page->clear_remset_current();
-    } else {
-      page->clear_remset_previous();
-    }
-
-    // Verify that inactive remset bits are all cleared
-    if (active_remset_is_current()) {
-      page->verify_remset_cleared_previous();
-    } else {
-      page->verify_remset_cleared_current();
-    }
   }
 
   void finish_in_place_relocation() {
@@ -1017,7 +997,7 @@ public:
       ZPage* const page = _forwarding->detach_page();
 
       // Ensure that previous remset bits are cleared
-      clear_remset_before_reuse(page, true /* in_place */);
+      clear_remset_before_in_place_reuse(page);
 
       page->log_msg(" (relocate page done in-place)");
 
@@ -1029,11 +1009,6 @@ public:
       // Wait for all other threads to call release_page
       ZPage* const page = _forwarding->detach_page();
 
-      // Ensure that all remset bits are cleared
-      // Note: cleared after detach_page, when we know that
-      // the young generation isn't scanning the remset.
-      clear_remset_before_reuse(page, false /* in_place */);
-
       page->log_msg(" (relocate page done normal)");
 
       // Free page
@@ -1292,7 +1267,7 @@ public:
       new_page->reset(to_age);
       new_page->reset_livemap();
       if (promotion) {
-        new_page->remset_initialize();
+        new_page->remset_alloc();
       }
 
       if (promotion) {

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

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2021, 2024, 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
@@ -55,18 +55,10 @@ void ZRememberedSet::initialize(size_t page_size) {
   _bitmap[1].initialize(size_in_bits, true /* clear */);
 }
 
-void ZRememberedSet::resize(size_t page_size) {
+void ZRememberedSet::delete_all() {
-  // The bitmaps only need to be resized if remset has been
+  assert(is_initialized(), "precondition");
-  // initialized, and hence the bitmaps have been initialized.
+  _bitmap[0].resize(0);
-  if (is_initialized()) {
+  _bitmap[1].resize(0);
-    const BitMap::idx_t size_in_bits = to_bit_size(page_size);
-
-    // The bitmaps need to be cleared when free, but since this function is
-    // only used for shrinking the clear argument is correct but not crucial.
-    assert(size_in_bits <= _bitmap[0].size(), "Only used for shrinking");
-    _bitmap[0].resize(size_in_bits, true /* clear */);
-    _bitmap[1].resize(size_in_bits, true /* clear */);
-  }
 }
 
 bool ZRememberedSet::is_cleared_current() const {
@@ -77,15 +69,6 @@ bool ZRememberedSet::is_cleared_previous() const {
   return previous()->is_empty();
 }
 
-void ZRememberedSet::clear_all() {
-  _bitmap[0].clear_large();
-  _bitmap[1].clear_large();
-}
-
-void ZRememberedSet::clear_current() {
-  current()->clear_large();
-}
-
 void ZRememberedSet::clear_previous() {
   previous()->clear_large();
 }

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

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2021, 2024, 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
@@ -114,8 +114,7 @@ public:
 
   bool is_initialized() const;
   void initialize(size_t page_size);
-
+  void delete_all();
-  void resize(size_t page_size);
 
   bool at_current(uintptr_t offset) const;
   bool at_previous(uintptr_t offset) const;
@@ -133,8 +132,6 @@ public:
   bool is_cleared_current() const;
   bool is_cleared_previous() const;
 
-  void clear_all();
-  void clear_current();
   void clear_previous();
   void swap_remset_bitmaps();