378 lines
16 KiB
C++
378 lines
16 KiB
C++
/*
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* Copyright (c) 1997, 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_OOPS_MARKWORD_HPP
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#define SHARE_OOPS_MARKWORD_HPP
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#include "metaprogramming/integralConstant.hpp"
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#include "metaprogramming/primitiveConversions.hpp"
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#include "oops/oopsHierarchy.hpp"
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#include "runtime/globals.hpp"
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// The markWord describes the header of an object.
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//
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// Bit-format of an object header (most significant first, big endian layout below):
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//
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// 32 bits:
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// --------
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// hash:25 ------------>| age:4 biased_lock:1 lock:2 (normal object)
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// JavaThread*:23 epoch:2 age:4 biased_lock:1 lock:2 (biased object)
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//
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// 64 bits:
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// --------
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// unused:25 hash:31 -->| unused_gap:1 age:4 biased_lock:1 lock:2 (normal object)
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// JavaThread*:54 epoch:2 unused_gap:1 age:4 biased_lock:1 lock:2 (biased object)
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//
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// - hash contains the identity hash value: largest value is
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// 31 bits, see os::random(). Also, 64-bit vm's require
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// a hash value no bigger than 32 bits because they will not
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// properly generate a mask larger than that: see library_call.cpp
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//
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// - the biased lock pattern is used to bias a lock toward a given
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// thread. When this pattern is set in the low three bits, the lock
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// is either biased toward a given thread or "anonymously" biased,
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// indicating that it is possible for it to be biased. When the
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// lock is biased toward a given thread, locking and unlocking can
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// be performed by that thread without using atomic operations.
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// When a lock's bias is revoked, it reverts back to the normal
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// locking scheme described below.
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//
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// Note that we are overloading the meaning of the "unlocked" state
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// of the header. Because we steal a bit from the age we can
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// guarantee that the bias pattern will never be seen for a truly
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// unlocked object.
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//
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// Note also that the biased state contains the age bits normally
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// contained in the object header. Large increases in scavenge
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// times were seen when these bits were absent and an arbitrary age
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// assigned to all biased objects, because they tended to consume a
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// significant fraction of the eden semispaces and were not
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// promoted promptly, causing an increase in the amount of copying
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// performed. The runtime system aligns all JavaThread* pointers to
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// a very large value (currently 128 bytes (32bVM) or 256 bytes (64bVM))
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// to make room for the age bits & the epoch bits (used in support of
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// biased locking).
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//
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// [JavaThread* | epoch | age | 1 | 01] lock is biased toward given thread
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// [0 | epoch | age | 1 | 01] lock is anonymously biased
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//
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// - the two lock bits are used to describe three states: locked/unlocked and monitor.
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//
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// [ptr | 00] locked ptr points to real header on stack
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// [header | 0 | 01] unlocked regular object header
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// [ptr | 10] monitor inflated lock (header is wapped out)
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// [ptr | 11] marked used to mark an object
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// [0 ............ 0| 00] inflating inflation in progress
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//
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// We assume that stack/thread pointers have the lowest two bits cleared.
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//
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// - INFLATING() is a distinguished markword value of all zeros that is
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// used when inflating an existing stack-lock into an ObjectMonitor.
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// See below for is_being_inflated() and INFLATING().
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class BasicLock;
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class ObjectMonitor;
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class JavaThread;
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class outputStream;
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class markWord {
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private:
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uintptr_t _value;
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public:
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explicit markWord(uintptr_t value) : _value(value) {}
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markWord() { /* uninitialized */}
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// It is critical for performance that this class be trivially
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// destructable, copyable, and assignable.
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static markWord from_pointer(void* ptr) {
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return markWord((uintptr_t)ptr);
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}
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void* to_pointer() const {
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return (void*)_value;
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}
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bool operator==(const markWord& other) const {
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return _value == other._value;
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}
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bool operator!=(const markWord& other) const {
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return !operator==(other);
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}
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// Conversion
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uintptr_t value() const { return _value; }
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// Constants
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static const int age_bits = 4;
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static const int lock_bits = 2;
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static const int biased_lock_bits = 1;
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static const int max_hash_bits = BitsPerWord - age_bits - lock_bits - biased_lock_bits;
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static const int hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits;
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static const int unused_gap_bits = LP64_ONLY(1) NOT_LP64(0);
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static const int epoch_bits = 2;
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// The biased locking code currently requires that the age bits be
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// contiguous to the lock bits.
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static const int lock_shift = 0;
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static const int biased_lock_shift = lock_bits;
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static const int age_shift = lock_bits + biased_lock_bits;
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static const int unused_gap_shift = age_shift + age_bits;
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static const int hash_shift = unused_gap_shift + unused_gap_bits;
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static const int epoch_shift = hash_shift;
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static const uintptr_t lock_mask = right_n_bits(lock_bits);
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static const uintptr_t lock_mask_in_place = lock_mask << lock_shift;
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static const uintptr_t biased_lock_mask = right_n_bits(lock_bits + biased_lock_bits);
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static const uintptr_t biased_lock_mask_in_place= biased_lock_mask << lock_shift;
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static const uintptr_t biased_lock_bit_in_place = 1 << biased_lock_shift;
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static const uintptr_t age_mask = right_n_bits(age_bits);
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static const uintptr_t age_mask_in_place = age_mask << age_shift;
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static const uintptr_t epoch_mask = right_n_bits(epoch_bits);
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static const uintptr_t epoch_mask_in_place = epoch_mask << epoch_shift;
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static const uintptr_t hash_mask = right_n_bits(hash_bits);
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static const uintptr_t hash_mask_in_place = hash_mask << hash_shift;
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// Alignment of JavaThread pointers encoded in object header required by biased locking
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static const size_t biased_lock_alignment = 2 << (epoch_shift + epoch_bits);
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static const uintptr_t locked_value = 0;
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static const uintptr_t unlocked_value = 1;
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static const uintptr_t monitor_value = 2;
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static const uintptr_t marked_value = 3;
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static const uintptr_t biased_lock_pattern = 5;
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static const uintptr_t no_hash = 0 ; // no hash value assigned
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static const uintptr_t no_hash_in_place = (address_word)no_hash << hash_shift;
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static const uintptr_t no_lock_in_place = unlocked_value;
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static const uint max_age = age_mask;
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static const int max_bias_epoch = epoch_mask;
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// Creates a markWord with all bits set to zero.
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static markWord zero() { return markWord(uintptr_t(0)); }
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// Biased Locking accessors.
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// These must be checked by all code which calls into the
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// ObjectSynchronizer and other code. The biasing is not understood
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// by the lower-level CAS-based locking code, although the runtime
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// fixes up biased locks to be compatible with it when a bias is
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// revoked.
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bool has_bias_pattern() const {
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return (mask_bits(value(), biased_lock_mask_in_place) == biased_lock_pattern);
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}
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JavaThread* biased_locker() const {
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assert(has_bias_pattern(), "should not call this otherwise");
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return (JavaThread*) mask_bits(value(), ~(biased_lock_mask_in_place | age_mask_in_place | epoch_mask_in_place));
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}
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// Indicates that the mark has the bias bit set but that it has not
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// yet been biased toward a particular thread
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bool is_biased_anonymously() const {
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return (has_bias_pattern() && (biased_locker() == NULL));
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}
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// Indicates epoch in which this bias was acquired. If the epoch
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// changes due to too many bias revocations occurring, the biases
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// from the previous epochs are all considered invalid.
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int bias_epoch() const {
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assert(has_bias_pattern(), "should not call this otherwise");
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return (mask_bits(value(), epoch_mask_in_place) >> epoch_shift);
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}
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markWord set_bias_epoch(int epoch) {
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assert(has_bias_pattern(), "should not call this otherwise");
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assert((epoch & (~epoch_mask)) == 0, "epoch overflow");
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return markWord(mask_bits(value(), ~epoch_mask_in_place) | (epoch << epoch_shift));
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}
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markWord incr_bias_epoch() {
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return set_bias_epoch((1 + bias_epoch()) & epoch_mask);
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}
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// Prototype mark for initialization
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static markWord biased_locking_prototype() {
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return markWord( biased_lock_pattern );
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}
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// lock accessors (note that these assume lock_shift == 0)
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bool is_locked() const {
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return (mask_bits(value(), lock_mask_in_place) != unlocked_value);
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}
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bool is_unlocked() const {
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return (mask_bits(value(), biased_lock_mask_in_place) == unlocked_value);
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}
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bool is_marked() const {
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return (mask_bits(value(), lock_mask_in_place) == marked_value);
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}
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bool is_neutral() const { return (mask_bits(value(), biased_lock_mask_in_place) == unlocked_value); }
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// Special temporary state of the markWord while being inflated.
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// Code that looks at mark outside a lock need to take this into account.
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bool is_being_inflated() const { return (value() == 0); }
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// Distinguished markword value - used when inflating over
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// an existing stack-lock. 0 indicates the markword is "BUSY".
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// Lockword mutators that use a LD...CAS idiom should always
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// check for and avoid overwriting a 0 value installed by some
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// other thread. (They should spin or block instead. The 0 value
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// is transient and *should* be short-lived).
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static markWord INFLATING() { return zero(); } // inflate-in-progress
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// Should this header be preserved during GC?
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template <typename KlassProxy>
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inline bool must_be_preserved(KlassProxy klass) const;
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// Should this header (including its age bits) be preserved in the
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// case of a promotion failure during scavenge?
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// Note that we special case this situation. We want to avoid
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// calling BiasedLocking::preserve_marks()/restore_marks() (which
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// decrease the number of mark words that need to be preserved
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// during GC) during each scavenge. During scavenges in which there
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// is no promotion failure, we actually don't need to call the above
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// routines at all, since we don't mutate and re-initialize the
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// marks of promoted objects using init_mark(). However, during
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// scavenges which result in promotion failure, we do re-initialize
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// the mark words of objects, meaning that we should have called
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// these mark word preservation routines. Currently there's no good
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// place in which to call them in any of the scavengers (although
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// guarded by appropriate locks we could make one), but the
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// observation is that promotion failures are quite rare and
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// reducing the number of mark words preserved during them isn't a
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// high priority.
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template <typename KlassProxy>
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inline bool must_be_preserved_for_promotion_failure(KlassProxy klass) const;
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// WARNING: The following routines are used EXCLUSIVELY by
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// synchronization functions. They are not really gc safe.
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// They must get updated if markWord layout get changed.
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markWord set_unlocked() const {
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return markWord(value() | unlocked_value);
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}
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bool has_locker() const {
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return ((value() & lock_mask_in_place) == locked_value);
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}
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BasicLock* locker() const {
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assert(has_locker(), "check");
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return (BasicLock*) value();
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}
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bool has_monitor() const {
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return ((value() & monitor_value) != 0);
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}
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ObjectMonitor* monitor() const {
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assert(has_monitor(), "check");
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// Use xor instead of &~ to provide one extra tag-bit check.
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return (ObjectMonitor*) (value() ^ monitor_value);
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}
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bool has_displaced_mark_helper() const {
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return ((value() & unlocked_value) == 0);
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}
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markWord displaced_mark_helper() const {
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assert(has_displaced_mark_helper(), "check");
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uintptr_t ptr = (value() & ~monitor_value);
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return *(markWord*)ptr;
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}
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void set_displaced_mark_helper(markWord m) const {
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assert(has_displaced_mark_helper(), "check");
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uintptr_t ptr = (value() & ~monitor_value);
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((markWord*)ptr)->_value = m._value;
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}
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markWord copy_set_hash(intptr_t hash) const {
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uintptr_t tmp = value() & (~hash_mask_in_place);
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tmp |= ((hash & hash_mask) << hash_shift);
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return markWord(tmp);
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}
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// it is only used to be stored into BasicLock as the
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// indicator that the lock is using heavyweight monitor
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static markWord unused_mark() {
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return markWord(marked_value);
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}
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// the following two functions create the markWord to be
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// stored into object header, it encodes monitor info
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static markWord encode(BasicLock* lock) {
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return from_pointer(lock);
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}
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static markWord encode(ObjectMonitor* monitor) {
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uintptr_t tmp = (uintptr_t) monitor;
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return markWord(tmp | monitor_value);
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}
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static markWord encode(JavaThread* thread, uint age, int bias_epoch) {
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uintptr_t tmp = (uintptr_t) thread;
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assert(UseBiasedLocking && ((tmp & (epoch_mask_in_place | age_mask_in_place | biased_lock_mask_in_place)) == 0), "misaligned JavaThread pointer");
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assert(age <= max_age, "age too large");
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assert(bias_epoch <= max_bias_epoch, "bias epoch too large");
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return markWord(tmp | (bias_epoch << epoch_shift) | (age << age_shift) | biased_lock_pattern);
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}
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// used to encode pointers during GC
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markWord clear_lock_bits() { return markWord(value() & ~lock_mask_in_place); }
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// age operations
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markWord set_marked() { return markWord((value() & ~lock_mask_in_place) | marked_value); }
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markWord set_unmarked() { return markWord((value() & ~lock_mask_in_place) | unlocked_value); }
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uint age() const { return mask_bits(value() >> age_shift, age_mask); }
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markWord set_age(uint v) const {
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assert((v & ~age_mask) == 0, "shouldn't overflow age field");
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return markWord((value() & ~age_mask_in_place) | ((v & age_mask) << age_shift));
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}
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markWord incr_age() const { return age() == max_age ? markWord(_value) : set_age(age() + 1); }
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// hash operations
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intptr_t hash() const {
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return mask_bits(value() >> hash_shift, hash_mask);
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}
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bool has_no_hash() const {
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return hash() == no_hash;
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}
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// Prototype mark for initialization
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static markWord prototype() {
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return markWord( no_hash_in_place | no_lock_in_place );
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}
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// Helper function for restoration of unmarked mark oops during GC
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static inline markWord prototype_for_klass(const Klass* klass);
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// Debugging
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void print_on(outputStream* st) const;
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// Prepare address of oop for placement into mark
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inline static markWord encode_pointer_as_mark(void* p) { return from_pointer(p).set_marked(); }
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// Recover address of oop from encoded form used in mark
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inline void* decode_pointer() { if (UseBiasedLocking && has_bias_pattern()) return NULL; return (void*)clear_lock_bits().value(); }
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};
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// Support atomic operations.
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template<>
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struct PrimitiveConversions::Translate<markWord> : public TrueType {
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typedef markWord Value;
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typedef uintptr_t Decayed;
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static Decayed decay(const Value& x) { return x.value(); }
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static Value recover(Decayed x) { return Value(x); }
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};
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#endif // SHARE_OOPS_MARKWORD_HPP
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