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/*
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* Copyright (c) 2019, Oracle and/or its affiliates. All rights reserved.
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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* questions.
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*
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*/
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#include "precompiled.hpp"
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#include "gc/g1/g1FreeIdSet.hpp"
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#include "memory/allocation.inline.hpp"
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#include "runtime/atomic.hpp"
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#include "utilities/debug.hpp"
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#include "utilities/globalDefinitions.hpp"
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#include "utilities/macros.hpp"
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G1FreeIdSet::G1FreeIdSet(uint start, uint size) :
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_sem(size), // counting semaphore for available ids
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_next(NULL), // array of "next" indices
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_start(start), // first id value
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_size(size), // number of available ids
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_head_index_mask(0), // mask for extracting index from a _head value.
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_head(0) // low part: index; high part: update counter
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{
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assert(size != 0, "precondition");
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assert(start <= (UINT_MAX - size),
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"start (%u) + size (%u) overflow: ", start, size);
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// 2^shift must be greater than size. Equal is not permitted, because
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// size is the "end of list" value, and can be the index part of _head.
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uint shift = log2_intptr((uintptr_t)size) + 1;
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assert(shift <= (BitsPerWord / 2), "excessive size %u", size);
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_head_index_mask = (uintx(1) << shift) - 1;
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assert(size <= _head_index_mask, "invariant");
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_next = NEW_C_HEAP_ARRAY(uint, size, mtGC);
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for (uint i = 0; i < size; ++i) {
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_next[i] = i + 1;
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}
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}
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G1FreeIdSet::~G1FreeIdSet() {
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FREE_C_HEAP_ARRAY(uint, _next);
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}
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uint G1FreeIdSet::head_index(uintx head) const {
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return head & _head_index_mask;
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}
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uintx G1FreeIdSet::make_head(uint index, uintx old_head) const {
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// Include incremented old update counter to avoid ABA problem.
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return index | ((old_head & ~_head_index_mask) + 1 + _head_index_mask);
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}
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const uint Claimed = UINT_MAX;
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uint G1FreeIdSet::claim_par_id() {
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_sem.wait();
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// Semaphore gate permits passage by no more than the number of
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// available ids, so there must be one that we can claim. But there
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// may be multiple threads trying to claim ids at the same time.
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uintx old_head = Atomic::load(&_head);
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uint index;
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while (true) {
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index = head_index(old_head);
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assert(index < _size, "invariant");
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uintx new_head = make_head(_next[index], old_head);
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new_head = Atomic::cmpxchg(new_head, &_head, old_head);
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if (new_head == old_head) break;
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old_head = new_head;
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}
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DEBUG_ONLY(_next[index] = Claimed;)
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return _start + index;
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}
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void G1FreeIdSet::release_par_id(uint id) {
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uint index = id - _start;
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assert(index < _size, "invalid id %u", id);
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assert(_next[index] == Claimed, "precondition");
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uintx old_head = Atomic::load(&_head);
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while (true) {
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_next[index] = head_index(old_head);
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uintx new_head = make_head(index, old_head);
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new_head = Atomic::cmpxchg(new_head, &_head, old_head);
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if (new_head == old_head) break;
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old_head = new_head;
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}
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// Now that id has been released, permit another thread through the gate.
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_sem.signal();
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}
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