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
31 ZObjArrayAllocator::ZObjArrayAllocator(Klass* klass, size_t word_size, int length, bool do_zero, Thread* thread)
32 : ObjArrayAllocator(klass, word_size, length, do_zero, thread) {}
33
34 void ZObjArrayAllocator::yield_for_safepoint() const {
35 ThreadBlockInVM tbivm(JavaThread::cast(_thread));
36 }
37
38 oop ZObjArrayAllocator::initialize(HeapWord* mem) const {
39 // ZGC specializes the initialization by performing segmented clearing
40 // to allow shorter time-to-safepoints.
41
42 if (!_do_zero) {
43 // No need for ZGC specialization
44 return ObjArrayAllocator::initialize(mem);
45 }
46
47 // A max segment size of 64K was chosen because microbenchmarking
48 // suggested that it offered a good trade-off between allocation
49 // time and time-to-safepoint
50 const size_t segment_max = ZUtils::bytes_to_words(64 * K);
51
52 if (_word_size <= segment_max) {
53 // To small to use segmented clearing
54 return ObjArrayAllocator::initialize(mem);
55 }
56
57 // Segmented clearing
58
59 // The array is going to be exposed before it has been completely
60 // cleared, therefore we can't expose the header at the end of this
61 // function. Instead explicitly initialize it according to our needs.
62
63 // Signal to the ZIterator that this is an invisible root, by setting
64 // the mark word to "marked". Reset to prototype() after the clearing.
65 if (UseCompactObjectHeaders) {
66 oopDesc::release_set_mark(mem, _klass->prototype_header().set_marked());
67 } else {
68 arrayOopDesc::set_mark(mem, markWord::prototype().set_marked());
69 arrayOopDesc::release_set_klass(mem, _klass);
70 }
71 assert(_length >= 0, "length should be non-negative");
72 arrayOopDesc::set_length(mem, _length);
73
74 // Keep the array alive across safepoints through an invisible
75 // root. Invisible roots are not visited by the heap iterator
76 // and the marking logic will not attempt to follow its elements.
77 // Relocation and remembered set code know how to dodge iterating
78 // over such objects.
79 ZThreadLocalData::set_invisible_root(_thread, (zaddress_unsafe*)&mem);
80
81 const BasicType element_type = ArrayKlass::cast(_klass)->element_type();
82 const size_t base_offset_in_bytes = (size_t)arrayOopDesc::base_offset_in_bytes(element_type);
83 const size_t process_start_offset_in_bytes = align_up(base_offset_in_bytes, (size_t)BytesPerWord);
84
85 if (process_start_offset_in_bytes != base_offset_in_bytes) {
86 // initialize_memory can only fill word aligned memory,
87 // fill the first 4 bytes here.
88 assert(process_start_offset_in_bytes - base_offset_in_bytes == 4, "Must be 4-byte aligned");
89 assert(!is_reference_type(element_type), "Only TypeArrays can be 4-byte aligned");
90 *reinterpret_cast<int*>(reinterpret_cast<char*>(mem) + base_offset_in_bytes) = 0;
91 }
92
93 // Note: initialize_memory may clear padding bytes at the end
94 const size_t process_start_offset = ZUtils::bytes_to_words(process_start_offset_in_bytes);
95 const size_t process_size = _word_size - process_start_offset;
96
97 uint32_t old_seqnum_before = ZGeneration::old()->seqnum();
98 uint32_t young_seqnum_before = ZGeneration::young()->seqnum();
99 uintptr_t color_before = ZPointerStoreGoodMask;
100 auto gc_safepoint_happened = [&]() {
101 return old_seqnum_before != ZGeneration::old()->seqnum() ||
102 young_seqnum_before != ZGeneration::young()->seqnum() ||
103 color_before != ZPointerStoreGoodMask;
104 };
105
106 bool seen_gc_safepoint = false;
107
108 auto initialize_memory = [&]() {
109 for (size_t processed = 0; processed < process_size; processed += segment_max) {
110 // Clear segment
111 uintptr_t* const start = (uintptr_t*)(mem + process_start_offset + processed);
112 const size_t remaining = process_size - processed;
113 const size_t segment = MIN2(remaining, segment_max);
114 // Usually, the young marking code has the responsibility to color
115 // raw nulls, before they end up in the old generation. However, the
116 // invisible roots are hidden from the marking code, and therefore
117 // we must color the nulls already here in the initialization. The
118 // color we choose must be store bad for any subsequent stores, regardless
119 // of how many GC flips later it will arrive. That's why we OR in 11
120 // (ZPointerRememberedMask) in the remembered bits, similar to how
121 // forgotten old oops also have 11, for the very same reason.
122 // However, we opportunistically try to color without the 11 remembered
123 // bits, hoping to not get interrupted in the middle of a GC safepoint.
124 // Most of the time, we manage to do that, and can the avoid having GC
125 // barriers trigger slow paths for this.
126 const uintptr_t colored_null = seen_gc_safepoint ? (ZPointerStoreGoodMask | ZPointerRememberedMask)
127 : ZPointerStoreGoodMask;
128 const uintptr_t fill_value = is_reference_type(element_type) ? colored_null : 0;
129 ZUtils::fill(start, segment, fill_value);
130
131 // Safepoint
132 yield_for_safepoint();
133
134 // Deal with safepoints
135 if (is_reference_type(element_type) && !seen_gc_safepoint && gc_safepoint_happened()) {
136 // The first time we observe a GC safepoint in the yield point,
137 // we have to restart processing with 11 remembered bits.
138 seen_gc_safepoint = true;
139 return false;
140 }
141 }
142 return true;
143 };
144
145 mem_zap_start_padding(mem);
146
147 if (!initialize_memory()) {
148 // Re-color with 11 remset bits if we got intercepted by a GC safepoint
149 const bool result = initialize_memory();
150 assert(result, "Array initialization should always succeed the second time");
151 }
152
153 mem_zap_end_padding(mem);
154
155 ZThreadLocalData::clear_invisible_root(_thread);
156
157 // Signal to the ZIterator that this is no longer an invisible root
158 if (UseCompactObjectHeaders) {
159 oopDesc::release_set_mark(mem, _klass->prototype_header());
160 } else {
161 oopDesc::release_set_mark(mem, markWord::prototype());
162 }
163
164 return cast_to_oop(mem);
165 }