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 
 54   // Clear leading 32 bits, if necessary.
 55   int base_offset = arrayOopDesc::base_offset_in_bytes(element_type);
 56   if (!is_aligned(base_offset, HeapWordSize)) {
 57     assert(is_aligned(base_offset, BytesPerInt), "array base must be 32 bit aligned");
 58     *reinterpret_cast<jint*>(reinterpret_cast<char*>(mem) + base_offset) = 0;
 59     base_offset += BytesPerInt;
 60   }
 61   assert(is_aligned(base_offset, HeapWordSize), "remaining array base must be 64 bit aligned");
 62 
 63   const size_t header = heap_word_size(base_offset);
 64   const size_t payload_size = _word_size - header;
 65   if (payload_size <= segment_max) {
 66     // To small to use segmented clearing
 67     return ObjArrayAllocator::initialize(mem);
 68   }
 69 
 70   // Segmented clearing
 71 
 72   // The array is going to be exposed before it has been completely
 73   // cleared, therefore we can't expose the header at the end of this
 74   // function. Instead explicitly initialize it according to our needs.
 75 
 76   // Signal to the ZIterator that this is an invisible root, by setting
 77   // the mark word to "marked". Reset to prototype() after the clearing.
 78   if (UseCompactObjectHeaders) {
 79     oopDesc::release_set_mark(mem, _klass->prototype_header().set_marked());
 80   } else {
 81     arrayOopDesc::set_mark(mem, markWord::prototype().set_marked());
 82     arrayOopDesc::release_set_klass(mem, _klass);
 83   }
 84   assert(_length >= 0, "length should be non-negative");
 85   arrayOopDesc::set_length(mem, _length);
 86 
 87   // Keep the array alive across safepoints through an invisible
 88   // root. Invisible roots are not visited by the heap iterator
 89   // and the marking logic will not attempt to follow its elements.
 90   // Relocation and remembered set code know how to dodge iterating
 91   // over such objects.
 92   ZThreadLocalData::set_invisible_root(_thread, (zaddress_unsafe*)&mem);
 93 
 94   uint32_t old_seqnum_before = ZGeneration::old()->seqnum();
 95   uint32_t young_seqnum_before = ZGeneration::young()->seqnum();
 96   uintptr_t color_before = ZPointerStoreGoodMask;
 97   auto gc_safepoint_happened = [&]() {
 98     return old_seqnum_before != ZGeneration::old()->seqnum() ||
 99            young_seqnum_before != ZGeneration::young()->seqnum() ||
100            color_before != ZPointerStoreGoodMask;
101   };
102 
103   bool seen_gc_safepoint = false;
104 
105   auto initialize_memory = [&]() {
106     for (size_t processed = 0; processed < payload_size; processed += segment_max) {
107       // Clear segment
108       uintptr_t* const start = (uintptr_t*)(mem + header + processed);
109       const size_t remaining = payload_size - processed;
110       const size_t segment = MIN2(remaining, segment_max);
111       // Usually, the young marking code has the responsibility to color
112       // raw nulls, before they end up in the old generation. However, the
113       // invisible roots are hidden from the marking code, and therefore
114       // we must color the nulls already here in the initialization. The
115       // color we choose must be store bad for any subsequent stores, regardless
116       // of how many GC flips later it will arrive. That's why we OR in 11
117       // (ZPointerRememberedMask) in the remembered bits, similar to how
118       // forgotten old oops also have 11, for the very same reason.
119       // However, we opportunistically try to color without the 11 remembered
120       // bits, hoping to not get interrupted in the middle of a GC safepoint.
121       // Most of the time, we manage to do that, and can the avoid having GC
122       // barriers trigger slow paths for this.
123       const uintptr_t colored_null = seen_gc_safepoint ? (ZPointerStoreGoodMask | ZPointerRememberedMask)
124                                                        : ZPointerStoreGoodMask;
125       const uintptr_t fill_value = is_reference_type(element_type) ? colored_null : 0;
126       ZUtils::fill(start, segment, fill_value);
127 
128       // Safepoint
129       yield_for_safepoint();
130 
131       // Deal with safepoints
132       if (!seen_gc_safepoint && gc_safepoint_happened()) {
133         // The first time we observe a GC safepoint in the yield point,
134         // we have to restart processing with 11 remembered bits.
135         seen_gc_safepoint = true;
136         return false;
137       }
138     }
139     return true;
140   };
141 
142   if (!initialize_memory()) {
143     // Re-color with 11 remset bits if we got intercepted by a GC safepoint
144     const bool result = initialize_memory();
145     assert(result, "Array initialization should always succeed the second time");
146   }
147 
148   mem_zap_end_padding(mem);
149 
150   ZThreadLocalData::clear_invisible_root(_thread);
151 
152   // Signal to the ZIterator that this is no longer an invisible root
153   if (UseCompactObjectHeaders) {
154     oopDesc::release_set_mark(mem, _klass->prototype_header());
155   } else {
156     oopDesc::release_set_mark(mem, markWord::prototype());
157   }
158 
159   return cast_to_oop(mem);
160 }
--- EOF ---