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/*
* Copyright (c) 2019, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*
*/
#include "precompiled.hpp"
#include "gc/g1/g1FreeIdSet.hpp"
#include "memory/allocation.inline.hpp"
#include "runtime/atomic.hpp"
#include "utilities/debug.hpp"
#include "utilities/globalDefinitions.hpp"
#include "utilities/macros.hpp"
#include "utilities/powerOfTwo.hpp"
G1FreeIdSet::G1FreeIdSet(uint start, uint size) :
_sem(size), // counting semaphore for available ids
_next(NULL), // array of "next" indices
_start(start), // first id value
_size(size), // number of available ids
_head_index_mask(0), // mask for extracting index from a _head value.
_head(0) // low part: index; high part: update counter
{
assert(size != 0, "precondition");
assert(start <= (UINT_MAX - size),
"start (%u) + size (%u) overflow: ", start, size);
// 2^shift must be greater than size. Equal is not permitted, because
// size is the "end of list" value, and can be the index part of _head.
uint shift = log2i(size) + 1;
assert(shift <= (BitsPerWord / 2), "excessive size %u", size);
_head_index_mask = (uintx(1) << shift) - 1;
assert(size <= _head_index_mask, "invariant");
_next = NEW_C_HEAP_ARRAY(uint, size, mtGC);
for (uint i = 0; i < size; ++i) {
_next[i] = i + 1;
}
}
G1FreeIdSet::~G1FreeIdSet() {
FREE_C_HEAP_ARRAY(uint, _next);
}
uint G1FreeIdSet::head_index(uintx head) const {
return head & _head_index_mask;
}
uintx G1FreeIdSet::make_head(uint index, uintx old_head) const {
// Include incremented old update counter to avoid ABA problem.
return index | ((old_head & ~_head_index_mask) + 1 + _head_index_mask);
}
const uint Claimed = UINT_MAX;
uint G1FreeIdSet::claim_par_id() {
_sem.wait();
// Semaphore gate permits passage by no more than the number of
// available ids, so there must be one that we can claim. But there
// may be multiple threads trying to claim ids at the same time.
uintx old_head = Atomic::load(&_head);
uint index;
while (true) {
index = head_index(old_head);
assert(index < _size, "invariant");
uintx new_head = make_head(_next[index], old_head);
new_head = Atomic::cmpxchg(&_head, old_head, new_head);
if (new_head == old_head) break;
old_head = new_head;
}
DEBUG_ONLY(_next[index] = Claimed;)
return _start + index;
}
void G1FreeIdSet::release_par_id(uint id) {
uint index = id - _start;
assert(index < _size, "invalid id %u", id);
assert(_next[index] == Claimed, "precondition");
uintx old_head = Atomic::load(&_head);
while (true) {
_next[index] = head_index(old_head);
uintx new_head = make_head(index, old_head);
new_head = Atomic::cmpxchg(&_head, old_head, new_head);
if (new_head == old_head) break;
old_head = new_head;
}
// Now that id has been released, permit another thread through the gate.
_sem.signal();
}
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