/* * Copyright (c) 1997, 2024, 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. * */ #ifndef SHARE_OOPS_MARKWORD_HPP #define SHARE_OOPS_MARKWORD_HPP #include "metaprogramming/primitiveConversions.hpp" #include "oops/compressedKlass.hpp" #include "oops/oopsHierarchy.hpp" #include "runtime/globals.hpp" #include // The markWord describes the header of an object. // // Bit-format of an object header (most significant first, big endian layout below): // // 32 bits: // -------- // hash:25 ------------>| age:4 self-fwd:1 lock:2 (normal object) // // 64 bits: // -------- // unused:22 hash:31 -->| unused_gap:4 age:4 self-fwd:1 lock:2 (normal object) // // 64 bits (with compact headers): // ------------------------------- // klass:22 hash:31 -->| unused_gap:4 age:4 self-fwd:1 lock:2 (normal object) // // - hash contains the identity hash value: largest value is // 31 bits, see os::random(). Also, 64-bit vm's require // a hash value no bigger than 32 bits because they will not // properly generate a mask larger than that: see library_call.cpp // // - the two lock bits are used to describe three states: locked/unlocked and monitor. // // [ptr | 00] locked ptr points to real header on stack (stack-locking in use) // [header | 00] locked locked regular object header (fast-locking in use) // [header | 01] unlocked regular object header // [ptr | 10] monitor inflated lock (header is swapped out, UseObjectMonitorTable == false) // [header | 10] monitor inflated lock (UseObjectMonitorTable == true) // [ptr | 11] marked used to mark an object // [0 ............ 0| 00] inflating inflation in progress (stack-locking in use) // // We assume that stack/thread pointers have the lowest two bits cleared. // // - INFLATING() is a distinguished markword value of all zeros that is // used when inflating an existing stack-lock into an ObjectMonitor. // See below for is_being_inflated() and INFLATING(). class BasicLock; class ObjectMonitor; class JavaThread; class outputStream; class markWord { private: uintptr_t _value; public: explicit markWord(uintptr_t value) : _value(value) {} markWord() = default; // Doesn't initialize _value. // It is critical for performance that this class be trivially // destructable, copyable, and assignable. ~markWord() = default; markWord(const markWord&) = default; markWord& operator=(const markWord&) = default; static markWord from_pointer(void* ptr) { return markWord((uintptr_t)ptr); } void* to_pointer() const { return (void*)_value; } bool operator==(const markWord& other) const { return _value == other._value; } bool operator!=(const markWord& other) const { return !operator==(other); } // Conversion uintptr_t value() const { return _value; } // Constants static const int age_bits = 4; static const int lock_bits = 2; static const int self_fwd_bits = 1; static const int max_hash_bits = BitsPerWord - age_bits - lock_bits - self_fwd_bits; static const int hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits; static const int unused_gap_bits = LP64_ONLY(4) NOT_LP64(0); // Reserved for Valhalla. static const int lock_shift = 0; static const int self_fwd_shift = lock_shift + lock_bits; static const int age_shift = self_fwd_shift + self_fwd_bits; static const int hash_shift = age_shift + age_bits + unused_gap_bits; static const uintptr_t lock_mask = right_n_bits(lock_bits); static const uintptr_t lock_mask_in_place = lock_mask << lock_shift; static const uintptr_t self_fwd_mask = right_n_bits(self_fwd_bits); static const uintptr_t self_fwd_mask_in_place = self_fwd_mask << self_fwd_shift; static const uintptr_t age_mask = right_n_bits(age_bits); static const uintptr_t age_mask_in_place = age_mask << age_shift; static const uintptr_t hash_mask = right_n_bits(hash_bits); static const uintptr_t hash_mask_in_place = hash_mask << hash_shift; #ifdef _LP64 // Used only with compact headers: // We store the (narrow) Klass* in the bits 43 to 64. // These are for bit-precise extraction of the narrow Klass* from the 64-bit Markword static constexpr int klass_offset_in_bytes = 4; static constexpr int klass_shift = hash_shift + hash_bits; static constexpr int klass_shift_at_offset = klass_shift - klass_offset_in_bytes * BitsPerByte; static constexpr int klass_bits = 22; static constexpr uintptr_t klass_mask = right_n_bits(klass_bits); static constexpr uintptr_t klass_mask_in_place = klass_mask << klass_shift; #endif static const uintptr_t locked_value = 0; static const uintptr_t unlocked_value = 1; static const uintptr_t monitor_value = 2; static const uintptr_t marked_value = 3; static const uintptr_t no_hash = 0 ; // no hash value assigned static const uintptr_t no_hash_in_place = (uintptr_t)no_hash << hash_shift; static const uintptr_t no_lock_in_place = unlocked_value; static const uint max_age = age_mask; // Creates a markWord with all bits set to zero. static markWord zero() { return markWord(uintptr_t(0)); } // lock accessors (note that these assume lock_shift == 0) bool is_locked() const { return (mask_bits(value(), lock_mask_in_place) != unlocked_value); } bool is_unlocked() const { return (mask_bits(value(), lock_mask_in_place) == unlocked_value); } bool is_marked() const { return (mask_bits(value(), lock_mask_in_place) == marked_value); } bool is_forwarded() const { // Returns true for normal forwarded (0b011) and self-forwarded (0b1xx). return mask_bits(value(), lock_mask_in_place | self_fwd_mask_in_place) >= static_cast(marked_value); } bool is_neutral() const { // Not locked, or marked - a "clean" neutral state return (mask_bits(value(), lock_mask_in_place) == unlocked_value); } // Special temporary state of the markWord while being inflated. // Code that looks at mark outside a lock need to take this into account. bool is_being_inflated() const { return (value() == 0); } // Distinguished markword value - used when inflating over // an existing stack-lock. 0 indicates the markword is "BUSY". // Lockword mutators that use a LD...CAS idiom should always // check for and avoid overwriting a 0 value installed by some // other thread. (They should spin or block instead. The 0 value // is transient and *should* be short-lived). // Fast-locking does not use INFLATING. static markWord INFLATING() { return zero(); } // inflate-in-progress // Should this header be preserved during GC? bool must_be_preserved() const { return (!is_unlocked() || !has_no_hash()); } // WARNING: The following routines are used EXCLUSIVELY by // synchronization functions. They are not really gc safe. // They must get updated if markWord layout get changed. markWord set_unlocked() const { return markWord(value() | unlocked_value); } bool has_locker() const { assert(LockingMode == LM_LEGACY, "should only be called with legacy stack locking"); return (value() & lock_mask_in_place) == locked_value; } BasicLock* locker() const { assert(has_locker(), "check"); return (BasicLock*) value(); } bool is_fast_locked() const { assert(LockingMode == LM_LIGHTWEIGHT, "should only be called with new lightweight locking"); return (value() & lock_mask_in_place) == locked_value; } markWord set_fast_locked() const { // Clear the lock_mask_in_place bits to set locked_value: return markWord(value() & ~lock_mask_in_place); } bool has_monitor() const { return ((value() & lock_mask_in_place) == monitor_value); } ObjectMonitor* monitor() const { assert(has_monitor(), "check"); assert(!UseObjectMonitorTable, "Lightweight locking with OM table does not use markWord for monitors"); // Use xor instead of &~ to provide one extra tag-bit check. return (ObjectMonitor*) (value() ^ monitor_value); } bool has_displaced_mark_helper() const { intptr_t lockbits = value() & lock_mask_in_place; if (LockingMode == LM_LIGHTWEIGHT) { return !UseObjectMonitorTable && lockbits == monitor_value; } // monitor (0b10) | stack-locked (0b00)? return (lockbits & unlocked_value) == 0; } markWord displaced_mark_helper() const; void set_displaced_mark_helper(markWord m) const; markWord copy_set_hash(intptr_t hash) const { uintptr_t tmp = value() & (~hash_mask_in_place); tmp |= ((hash & hash_mask) << hash_shift); return markWord(tmp); } // it is only used to be stored into BasicLock as the // indicator that the lock is using heavyweight monitor static markWord unused_mark() { return markWord(marked_value); } // the following two functions create the markWord to be // stored into object header, it encodes monitor info static markWord encode(BasicLock* lock) { return from_pointer(lock); } static markWord encode(ObjectMonitor* monitor) { assert(!UseObjectMonitorTable, "Lightweight locking with OM table does not use markWord for monitors"); uintptr_t tmp = (uintptr_t) monitor; return markWord(tmp | monitor_value); } markWord set_has_monitor() const { return markWord((value() & ~lock_mask_in_place) | monitor_value); } // used to encode pointers during GC markWord clear_lock_bits() const { return markWord(value() & ~lock_mask_in_place); } // age operations markWord set_marked() { return markWord((value() & ~lock_mask_in_place) | marked_value); } markWord set_unmarked() { return markWord((value() & ~lock_mask_in_place) | unlocked_value); } uint age() const { return (uint) mask_bits(value() >> age_shift, age_mask); } markWord set_age(uint v) const { assert((v & ~age_mask) == 0, "shouldn't overflow age field"); return markWord((value() & ~age_mask_in_place) | ((v & age_mask) << age_shift)); } markWord incr_age() const { return age() == max_age ? markWord(_value) : set_age(age() + 1); } // hash operations intptr_t hash() const { return mask_bits(value() >> hash_shift, hash_mask); } bool has_no_hash() const { return hash() == no_hash; } inline Klass* klass() const; inline Klass* klass_or_null() const; inline Klass* klass_without_asserts() const; inline narrowKlass narrow_klass() const; inline markWord set_narrow_klass(narrowKlass narrow_klass) const; // Prototype mark for initialization static markWord prototype() { return markWord( no_hash_in_place | no_lock_in_place ); } // Debugging void print_on(outputStream* st, bool print_monitor_info = true) const; // Prepare address of oop for placement into mark inline static markWord encode_pointer_as_mark(void* p) { return from_pointer(p).set_marked(); } // Recover address of oop from encoded form used in mark inline void* decode_pointer() const { return (void*)clear_lock_bits().value(); } inline bool is_self_forwarded() const { NOT_LP64(assert(LockingMode != LM_LEGACY, "incorrect with LM_LEGACY on 32 bit");) return mask_bits(value(), self_fwd_mask_in_place) != 0; } inline markWord set_self_forwarded() const { NOT_LP64(assert(LockingMode != LM_LEGACY, "incorrect with LM_LEGACY on 32 bit");) return markWord(value() | self_fwd_mask_in_place); } inline markWord unset_self_forwarded() const { NOT_LP64(assert(LockingMode != LM_LEGACY, "incorrect with LM_LEGACY on 32 bit");) return markWord(value() & ~self_fwd_mask_in_place); } inline oop forwardee() const { return cast_to_oop(decode_pointer()); } }; // Support atomic operations. template<> struct PrimitiveConversions::Translate : public std::true_type { typedef markWord Value; typedef uintptr_t Decayed; static Decayed decay(const Value& x) { return x.value(); } static Value recover(Decayed x) { return Value(x); } }; #endif // SHARE_OOPS_MARKWORD_HPP