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 }