8351167: ZGC: Lazily initialize livemap

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

@@ -45,9 +45,16 @@ ZLiveMap::ZLiveMap(uint32_t size)
     _live_bytes(0),
     _segment_live_bits(0),
     _segment_claim_bits(0),
-    _bitmap(bitmap_size(size, NumSegments)),
+    _bitmap_size(bitmap_size(size, NumSegments)),
+    _bitmap(0),
     _segment_shift(log2i_exact(segment_size())) {}
 
+void ZLiveMap::allocate_bitmap() {
+  if (_bitmap.size() != _bitmap_size) {
+    _bitmap.initialize(_bitmap_size, false /* clear */);
+  }
+}
+
 void ZLiveMap::reset(ZGenerationId id) {
   ZGeneration* const generation = ZGeneration::generation(id);
   const uint32_t seqnum_initializing = (uint32_t)-1;
@@ -64,6 +71,10 @@ void ZLiveMap::reset(ZGenerationId id) {
       _live_bytes = 0;
       _live_objects = 0;
 
+      // We lazily initialize the bitmap the first time the page is
+      // marked, i.e. a bit is about to be set for the first time.
+      allocate_bitmap();
+
       // Clear segment claimed/live bits
       segment_live_bits().clear();
       segment_claim_bits().clear();
@@ -127,8 +138,10 @@ void ZLiveMap::reset_segment(BitMap::idx_t segment) {
 
 void ZLiveMap::resize(uint32_t size) {
   const size_t new_bitmap_size = bitmap_size(size, NumSegments);
-  if (_bitmap.size() != new_bitmap_size) {
+  _bitmap_size = new_bitmap_size;
+  _segment_shift = log2i_exact(segment_size());
+
+  if (_bitmap.size() != 0 && _bitmap.size() != new_bitmap_size) {
     _bitmap.reinitialize(new_bitmap_size, false /* clear */);
-    _segment_shift = log2i_exact(segment_size());
   }
 }

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

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, 2024, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2015, 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
@@ -42,6 +42,7 @@ class ZLiveMap {
   volatile size_t   _live_bytes;
   BitMap::bm_word_t _segment_live_bits;
   BitMap::bm_word_t _segment_claim_bits;
+  size_t            _bitmap_size;
   ZBitMap           _bitmap;
   int               _segment_shift;
 
@@ -65,6 +66,8 @@ class ZLiveMap {
 
   bool claim_segment(BitMap::idx_t segment);
 
+  void allocate_bitmap();
+
   void reset(ZGenerationId id);
   void reset_segment(BitMap::idx_t segment);
 

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

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, 2024, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2015, 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
@@ -88,7 +88,7 @@ inline BitMap::idx_t ZLiveMap::next_live_segment(BitMap::idx_t segment) const {
 }
 
 inline BitMap::idx_t ZLiveMap::segment_size() const {
-  return _bitmap.size() / NumSegments;
+  return _bitmap_size / NumSegments;
 }
 
 inline BitMap::idx_t ZLiveMap::index_to_segment(BitMap::idx_t index) const {

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

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, 2024, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2015, 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
@@ -76,7 +76,7 @@ inline uint32_t ZPage::object_max_count() const {
     return 1;
 
   default:
-    return (uint32_t)(size() >> object_alignment_shift());
+    return checked_cast<uint32_t>(size() >> object_alignment_shift());
   }
 }