1 /*
2 * Copyright (c) 2019, 2025, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 */
23
24 #include "gc/z/zThreadLocalData.hpp"
25 #include "gc/z/zObjArrayAllocator.hpp"
26 #include "gc/z/zUtils.inline.hpp"
27 #include "oops/arrayKlass.hpp"
28 #include "runtime/interfaceSupport.inline.hpp"
29 #include "utilities/debug.hpp"
30 #include "utilities/globalDefinitions.hpp"
31
32 ZObjArrayAllocator::ZObjArrayAllocator(Klass* klass, size_t word_size, int length, bool do_zero, Thread* thread)
33 : ObjArrayAllocator(klass, word_size, length, do_zero, thread) {}
34
35 void ZObjArrayAllocator::yield_for_safepoint() const {
36 ThreadBlockInVM tbivm(JavaThread::cast(_thread));
37 }
38
39 oop ZObjArrayAllocator::initialize(HeapWord* mem) const {
40 // ZGC specializes the initialization by performing segmented clearing
41 // to allow shorter time-to-safepoints.
42
43 if (!_do_zero) {
44 // No need for ZGC specialization
45 return ObjArrayAllocator::initialize(mem);
46 }
47
48 // A max segment size of 64K was chosen because microbenchmarking
49 // suggested that it offered a good trade-off between allocation
50 // time and time-to-safepoint
51 const size_t segment_max = ZUtils::bytes_to_words(64 * K);
52
53 if (_word_size <= segment_max || ArrayKlass::cast(_klass)->is_flatArray_klass()) {
54 // To small to use segmented clearing
55 return ObjArrayAllocator::initialize(mem);
56 }
57
58 // Segmented clearing
59
60 // The array is going to be exposed before it has been completely
61 // cleared, therefore we can't expose the header at the end of this
62 // function. Instead explicitly initialize it according to our needs.
63
64 // Signal to the ZIterator that this is an invisible root, by setting
65 // the mark word to "marked". Reset to prototype() after the clearing.
66 if (UseCompactObjectHeaders) {
67 oopDesc::release_set_mark(mem, _klass->prototype_header().set_marked());
68 } else {
69 if (EnableValhalla) {
70 arrayOopDesc::set_mark(mem, _klass->prototype_header().set_marked());
71 } else {
72 arrayOopDesc::set_mark(mem, markWord::prototype().set_marked());
73 }
74 arrayOopDesc::release_set_klass(mem, _klass);
75 }
76 assert(_length >= 0, "length should be non-negative");
77 arrayOopDesc::set_length(mem, _length);
78
79 // Keep the array alive across safepoints through an invisible
80 // root. Invisible roots are not visited by the heap iterator
81 // and the marking logic will not attempt to follow its elements.
82 // Relocation and remembered set code know how to dodge iterating
83 // over such objects.
84 ZThreadLocalData::set_invisible_root(_thread, (zaddress_unsafe*)&mem);
85
86 const BasicType element_type = ArrayKlass::cast(_klass)->element_type();
87 const size_t base_offset_in_bytes = (size_t)arrayOopDesc::base_offset_in_bytes(element_type);
88 const size_t process_start_offset_in_bytes = align_up(base_offset_in_bytes, (size_t)BytesPerWord);
89
90 if (process_start_offset_in_bytes != base_offset_in_bytes) {
91 // initialize_memory can only fill word aligned memory,
92 // fill the first 4 bytes here.
93 assert(process_start_offset_in_bytes - base_offset_in_bytes == 4, "Must be 4-byte aligned");
94 assert(!is_reference_type(element_type), "Only TypeArrays can be 4-byte aligned");
95 *reinterpret_cast<int*>(reinterpret_cast<char*>(mem) + base_offset_in_bytes) = 0;
96 }
97
98 // Note: initialize_memory may clear padding bytes at the end
99 const size_t process_start_offset = ZUtils::bytes_to_words(process_start_offset_in_bytes);
100 const size_t process_size = _word_size - process_start_offset;
101
102 uint32_t old_seqnum_before = ZGeneration::old()->seqnum();
103 uint32_t young_seqnum_before = ZGeneration::young()->seqnum();
104 uintptr_t color_before = ZPointerStoreGoodMask;
105 auto gc_safepoint_happened = [&]() {
106 return old_seqnum_before != ZGeneration::old()->seqnum() ||
107 young_seqnum_before != ZGeneration::young()->seqnum() ||
108 color_before != ZPointerStoreGoodMask;
109 };
110
111 bool seen_gc_safepoint = false;
112
113 auto initialize_memory = [&]() {
114 for (size_t processed = 0; processed < process_size; processed += segment_max) {
115 // Clear segment
116 uintptr_t* const start = (uintptr_t*)(mem + process_start_offset + processed);
117 const size_t remaining = process_size - processed;
118 const size_t segment = MIN2(remaining, segment_max);
119 // Usually, the young marking code has the responsibility to color
120 // raw nulls, before they end up in the old generation. However, the
121 // invisible roots are hidden from the marking code, and therefore
122 // we must color the nulls already here in the initialization. The
123 // color we choose must be store bad for any subsequent stores, regardless
124 // of how many GC flips later it will arrive. That's why we OR in 11
125 // (ZPointerRememberedMask) in the remembered bits, similar to how
126 // forgotten old oops also have 11, for the very same reason.
127 // However, we opportunistically try to color without the 11 remembered
128 // bits, hoping to not get interrupted in the middle of a GC safepoint.
129 // Most of the time, we manage to do that, and can the avoid having GC
130 // barriers trigger slow paths for this.
131 const uintptr_t colored_null = seen_gc_safepoint ? (ZPointerStoreGoodMask | ZPointerRememberedMask)
132 : ZPointerStoreGoodMask;
133 const uintptr_t fill_value = is_reference_type(element_type) ? colored_null : 0;
134 ZUtils::fill(start, segment, fill_value);
135
136 // Safepoint
137 yield_for_safepoint();
138
139 // Deal with safepoints
140 if (is_reference_type(element_type) && !seen_gc_safepoint && gc_safepoint_happened()) {
141 // The first time we observe a GC safepoint in the yield point,
142 // we have to restart processing with 11 remembered bits.
143 seen_gc_safepoint = true;
144 return false;
145 }
146 }
147 return true;
148 };
149
150 mem_zap_start_padding(mem);
151
152 if (!initialize_memory()) {
153 // Re-color with 11 remset bits if we got intercepted by a GC safepoint
154 const bool result = initialize_memory();
155 assert(result, "Array initialization should always succeed the second time");
156 }
157
158 mem_zap_end_padding(mem);
159
160 ZThreadLocalData::clear_invisible_root(_thread);
161
162 // Signal to the ZIterator that this is no longer an invisible root
163 if (UseCompactObjectHeaders || EnableValhalla) {
164 oopDesc::release_set_mark(mem, _klass->prototype_header());
165 } else {
166 oopDesc::release_set_mark(mem, markWord::prototype());
167 }
168
169 return cast_to_oop(mem);
170 }