1 /*
  2  * Copyright (c) 2019, 2024, 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 "precompiled.hpp"
 25 #include "gc/z/zThreadLocalData.hpp"
 26 #include "gc/z/zObjArrayAllocator.hpp"
 27 #include "gc/z/zUtils.inline.hpp"
 28 #include "oops/arrayKlass.hpp"
 29 #include "runtime/interfaceSupport.inline.hpp"
 30 #include "utilities/debug.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   const BasicType element_type = ArrayKlass::cast(_klass)->element_type();
 53   const size_t header = arrayOopDesc::header_size(element_type);
 54   const size_t payload_size = _word_size - header;
 55 











 56   if (payload_size <= segment_max) {
 57     // To small to use segmented clearing
 58     return ObjArrayAllocator::initialize(mem);
 59   }
 60 
 61   // Segmented clearing
 62 
 63   // The array is going to be exposed before it has been completely
 64   // cleared, therefore we can't expose the header at the end of this
 65   // function. Instead explicitly initialize it according to our needs.
 66 
 67   // Signal to the ZIterator that this is an invisible root, by setting
 68   // the mark word to "marked". Reset to prototype() after the clearing.
 69   arrayOopDesc::set_mark(mem, markWord::prototype().set_marked());
 70   arrayOopDesc::release_set_klass(mem, _klass);




 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   uint32_t old_seqnum_before = ZGeneration::old()->seqnum();
 82   uint32_t young_seqnum_before = ZGeneration::young()->seqnum();
 83   uintptr_t color_before = ZPointerStoreGoodMask;
 84   auto gc_safepoint_happened = [&]() {
 85     return old_seqnum_before != ZGeneration::old()->seqnum() ||
 86            young_seqnum_before != ZGeneration::young()->seqnum() ||
 87            color_before != ZPointerStoreGoodMask;
 88   };
 89 
 90   bool seen_gc_safepoint = false;
 91 
 92   auto initialize_memory = [&]() {
 93     for (size_t processed = 0; processed < payload_size; processed += segment_max) {
 94       // Clear segment
 95       uintptr_t* const start = (uintptr_t*)(mem + header + processed);
 96       const size_t remaining = payload_size - processed;
 97       const size_t segment = MIN2(remaining, segment_max);
 98       // Usually, the young marking code has the responsibility to color
 99       // raw nulls, before they end up in the old generation. However, the
100       // invisible roots are hidden from the marking code, and therefore
101       // we must color the nulls already here in the initialization. The
102       // color we choose must be store bad for any subsequent stores, regardless
103       // of how many GC flips later it will arrive. That's why we OR in 11
104       // (ZPointerRememberedMask) in the remembered bits, similar to how
105       // forgotten old oops also have 11, for the very same reason.
106       // However, we opportunistically try to color without the 11 remembered
107       // bits, hoping to not get interrupted in the middle of a GC safepoint.
108       // Most of the time, we manage to do that, and can the avoid having GC
109       // barriers trigger slow paths for this.
110       const uintptr_t colored_null = seen_gc_safepoint ? (ZPointerStoreGoodMask | ZPointerRememberedMask)
111                                                        : ZPointerStoreGoodMask;
112       const uintptr_t fill_value = is_reference_type(element_type) ? colored_null : 0;
113       ZUtils::fill(start, segment, fill_value);
114 
115       // Safepoint
116       yield_for_safepoint();
117 
118       // Deal with safepoints
119       if (!seen_gc_safepoint && gc_safepoint_happened()) {
120         // The first time we observe a GC safepoint in the yield point,
121         // we have to restart processing with 11 remembered bits.
122         seen_gc_safepoint = true;
123         return false;
124       }
125     }
126     return true;
127   };
128 
129   if (!initialize_memory()) {
130     // Re-color with 11 remset bits if we got intercepted by a GC safepoint
131     const bool result = initialize_memory();
132     assert(result, "Array initialization should always succeed the second time");
133   }
134 
135   mem_zap_end_padding(mem);
136 
137   ZThreadLocalData::clear_invisible_root(_thread);
138 
139   // Signal to the ZIterator that this is no longer an invisible root
140   oopDesc::release_set_mark(mem, markWord::prototype());




141 
142   return cast_to_oop(mem);
143 }
--- EOF ---