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132 lines
5.2 KiB
C++
132 lines
5.2 KiB
C++
/*
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* Copyright (c) 2014, 2020, 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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#ifndef SHARE_GC_SHARED_STRINGDEDUP_STRINGDEDUP_HPP
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#define SHARE_GC_SHARED_STRINGDEDUP_STRINGDEDUP_HPP
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//
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// String Deduplication
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//
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// String deduplication aims to reduce the heap live-set by deduplicating identical
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// instances of String so that they share the same backing character array.
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//
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// The deduplication process is divided in two main parts, 1) finding the objects to
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// deduplicate, and 2) deduplicating those objects. The first part is done as part of
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// a normal GC cycle when objects are marked or evacuated. At this time a check is
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// applied on each object to check if it is a candidate for deduplication. If so, the
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// object is placed on the deduplication queue for later processing. The second part,
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// processing the objects on the deduplication queue, is a concurrent phase which
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// starts right after the stop-the-wold marking/evacuation phase. This phase is
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// executed by the deduplication thread, which pulls deduplication candidates of the
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// deduplication queue and tries to deduplicate them.
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//
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// A deduplication hashtable is used to keep track of all unique character arrays
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// used by String objects. When deduplicating, a lookup is made in this table to see
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// if there is already an identical character array somewhere on the heap. If so, the
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// String object is adjusted to point to that character array, releasing the reference
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// to the original array allowing it to eventually be garbage collected. If the lookup
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// fails the character array is instead inserted into the hashtable so that this array
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// can be shared at some point in the future.
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//
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// Candidate selection criteria is GC specific.
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//
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// Interned strings are a bit special. They are explicitly deduplicated just before
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// being inserted into the StringTable (to avoid counteracting C2 optimizations done
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// on string literals), then they also become deduplication candidates if they reach
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// the deduplication age threshold or are evacuated to an old heap region. The second
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// attempt to deduplicate such strings will be in vain, but we have no fast way of
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// filtering them out. This has not shown to be a problem, as the number of interned
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// strings is usually dwarfed by the number of normal (non-interned) strings.
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//
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// For additional information on string deduplication, please see JEP 192,
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// http://openjdk.java.net/jeps/192
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//
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#include "gc/shared/stringdedup/stringDedupQueue.hpp"
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#include "gc/shared/stringdedup/stringDedupStat.hpp"
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#include "gc/shared/stringdedup/stringDedupTable.hpp"
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#include "memory/allocation.hpp"
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#include "runtime/thread.hpp"
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class ThreadClosure;
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//
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// Main interface for interacting with string deduplication.
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//
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class StringDedup : public AllStatic {
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private:
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// Single state for checking if string deduplication is enabled.
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static bool _enabled;
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public:
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// Returns true if string deduplication is enabled.
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static bool is_enabled() {
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return _enabled;
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}
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// Stop the deduplication thread.
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static void stop();
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// Immediately deduplicates the given String object, bypassing the
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// the deduplication queue.
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static void deduplicate(oop java_string);
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static void parallel_unlink(StringDedupUnlinkOrOopsDoClosure* unlink, uint worker_id);
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static void threads_do(ThreadClosure* tc);
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static void verify();
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// GC support
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static void gc_prologue(bool resize_and_rehash_table);
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static void gc_epilogue();
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protected:
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// Initialize string deduplication.
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// Q: String Dedup Queue implementation
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// S: String Dedup Stat implementation
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template <typename Q, typename S>
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static void initialize_impl();
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};
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//
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// This closure encapsulates the closures needed when scanning
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// the deduplication queue and table during the unlink_or_oops_do() operation.
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//
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class StringDedupUnlinkOrOopsDoClosure : public StackObj {
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AlwaysTrueClosure _always_true;
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DoNothingClosure _do_nothing;
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BoolObjectClosure* _is_alive;
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OopClosure* _keep_alive;
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public:
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StringDedupUnlinkOrOopsDoClosure(BoolObjectClosure* is_alive,
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OopClosure* keep_alive);
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bool is_alive(oop o) { return _is_alive->do_object_b(o); }
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void keep_alive(oop* p) { _keep_alive->do_oop(p); }
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};
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#endif // SHARE_GC_SHARED_STRINGDEDUP_STRINGDEDUP_HPP
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