markword
注釋
該文件目錄在：
\openjdk-jdk8u\hotspot\src\share\vm\oops\markOop.hpp
#ifndef SHARE_VM_OOPS_MARKOOP_HPP
#define SHARE_VM_OOPS_MARKOOP_HPP
#include "oops/oop.hpp"
// The markOop describes the header of an object.
//
// Note that the mark is not a real oop but just a word.
// It is placed in the oop hierarchy for historical reasons.
//
// Bit-format of an object header (most significant first, big endian layout below):
//
// 32 bits:
// --------
// hash:25 ------------>| age:4 biased_lock:1 lock:2 (normal object)
// JavaThread*:23 epoch:2 age:4 biased_lock:1 lock:2 (biased object)
// size:32 ------------------------------------------>| (CMS free block)
// PromotedObject*:29 ---------->| promo_bits:3 ----->| (CMS promoted object)
這里咱們翻譯下
markOop描述對象頭。
注意的是，對象頭不是一個真正的oop，而只是一個word（大家可以回想下學計算機基礎課程的時候，一個word大概是32字節，64位機器上，則是64字節。）
它只是因為歷史原因而被放在oop的繼承結構中。
對象頭的格式（32字節）：
正常時：
25bit的hash---------------------------------- 4bit的 gc 年齡 -------偏向鎖標志1個bit------------lock標志，2個bit
偏向時：
23個bit，存放偏向的線程的指針；2bit，存放epoch；4bit，存放gc年齡；1bit，偏向鎖標志；2bit，鎖標志
cms free block：
跳過，不懂
CMS promoted object（cms提升后的對象，是指從新生代提升到老年代的對象？）
29bit，存放PromotedObject的指針；3bit，存放promo_bits
繼續下一段：
// 64 bits:
// --------
// unused:25 hash:31 -->| unused:1 age:4 biased_lock:1 lock:2 (normal object)
// JavaThread*:54 epoch:2 unused:1 age:4 biased_lock:1 lock:2 (biased object)
// PromotedObject*:61 --------------------->| promo_bits:3 ----->| (CMS promoted object)
// size:64 ----------------------------------------------------->| (CMS free block)
//
// unused:25 hash:31 -->| cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && normal object)
// JavaThread*:54 epoch:2 cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && biased object)
// narrowOop:32 unused:24 cms_free:1 unused:4 promo_bits:3 ----->| (COOPs && CMS promoted object)
// unused:21 size:35 -->| cms_free:1 unused:7 ------------------>| (COOPs && CMS free block)
翻譯：
64 bit：
正常對象：
25bit,未使用；31bit，hashcode；1bit，沒使用；4bit,gc年齡；1bit，偏向鎖標志；2bit,lock標志
偏向鎖狀態時：
54bit，當前偏向的線程的指針；2bit，epoch；1bit，沒使用；4bit，gc年齡；1bit，偏向鎖標志；2bit,lock標志
cms提升后的對象：
61bit，PromotedObject*；3bit，promo_bits
cms free obj：
不懂，跳過。
然后我們先說下，上面還有幾個沒翻譯，插播個名詞，COOPs，壓縮對象指針技術，對象指針壓縮在Java SE 6u23 默認開啟。在此之前，可以使用-XX:+UseCompressedOops來開啟。可以看看這個鏈接：
https://blog.csdn.net/superfjj/article/details/107455559
ok，我們繼續：
//
// unused:25 hash:31 -->| cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && normal object)
// JavaThread*:54 epoch:2 cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && biased object)
// narrowOop:32 unused:24 cms_free:1 unused:4 promo_bits:3 ----->| (COOPs && CMS promoted object)
// unused:21 size:35 -->| cms_free:1 unused:7 ------------------>| (COOPs && CMS free block)
翻譯：
64 bit：
正常對象（和前面沒開coops時比，沒變化）：
25bit,未使用；31bit，hashcode；1bit，沒使用；4bit,gc年齡；1bit，偏向鎖標志；2bit,lock標志
偏向鎖狀態時（和前面沒開coops時比，沒變化）：
54bit，當前偏向的線程的指針；2bit，epoch；1bit，沒使用；4bit，gc年齡；1bit，偏向鎖標志；2bit,lock標志
COOPs && CMS promoted object:
narrowOop:32 unused:24 cms_free:1 unused:4 promo_bits:3
COOPs && CMS free block:
unused:21 size:35 -->| cms_free:1 unused:7
// - 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
// and c1_CodePatterns_sparc.cpp.
//
// - the biased lock pattern is used to bias a lock toward a given
// thread. When this pattern is set in the low three bits, the lock
// is either biased toward a given thread or "anonymously" biased,
// indicating that it is possible for it to be biased. When the
// lock is biased toward a given thread, locking and unlocking can
// be performed by that thread without using atomic operations.
// When a lock's bias is revoked, it reverts back to the normal
// locking scheme described below.
//
// Note that we are overloading the meaning of the "unlocked" state
// of the header. Because we steal a bit from the age we can
// guarantee that the bias pattern will never be seen for a truly
// unlocked object.
//
// Note also that the biased state contains the age bits normally
// contained in the object header. Large increases in scavenge
// times were seen when these bits were absent and an arbitrary age
// assigned to all biased objects, because they tended to consume a
// significant fraction of the eden semispaces and were not
// promoted promptly, causing an increase in the amount of copying
// performed.
// The runtime system aligns all JavaThread* pointers to
// a very large value (currently 128 bytes (32bVM) or 256 bytes (64bVM))
// to make room for the age bits & the epoch bits (used in support of
// biased locking), and for the CMS "freeness" bit in the 64bVM (+COOPs).
翻譯：
hash字段，包含了唯一的hash value：最大的值是31bit。另外，64bit的虛擬機時，hash value也不能超過32bit；因為不能恰當地生成一個大於它的掩碼。
偏向鎖標志，是用來把一個鎖，偏向一個指定的現場。當在最后三位，設置了該模式后，這個鎖，要么偏向一個指定的現場，要么被匿名偏向（表示可能被偏向）。當這個鎖，被偏向一個指定的線程時，該線程進行加鎖和解鎖時，無需原子操作（有點費解）
當該鎖的偏向標志被撤銷時，它會回到正常的鎖定的模式。
注意，我們這里重載了header中，未鎖定狀態的意義。因為我們從age中偷了一位，這樣我們就可以保證，對於一個真正沒被鎖定的對象，偏向標志不會被看到。
譯者補充：我們再把偏向標志的拿過來看一下：
// JavaThread*:54 epoch:2 cms_free:1 age:4 biased_lock:1 lock:2 (biased object)
從age偷了一位，難道之前age是5bit，最大gc年齡32嗎，現在4位，所以最大gc年齡為16.
不是很理解這句話。
注意的是，偏向狀態時，包含了age的bit位。當這幾個位缺失時，且給所有被偏向對象，賦值一個任意的gc 年齡時，我們會看到，清理垃圾的時間大幅上升，因為他們將消耗一部分的eden空間，且不能被迅速提升到老年代，導致了一定量的拷貝工作（譯者：從eden拷貝到s區）。
//
// [JavaThread* | epoch | age | 1 | 01] lock is biased toward given thread
// [0 | epoch | age | 1 | 01] lock is anonymously biased
//
// - the two lock bits are used to describe three states: locked/unlocked and monitor.
//
// [ptr | 00] locked ptr points to real header on stack
// [header | 0 | 01] unlocked regular object header
// [ptr | 10] monitor inflated lock (header is wapped out)
// [ptr | 11] marked used by markSweep to mark an object
// not valid at any other time
//
// We assume that stack/thread pointers have the lowest two bits cleared.
翻譯：
lock偏向某個線程時：
[JavaThread* | epoch | age | 1 | 01]
lock is anonymously biased（可能被偏向時）
[0 | epoch | age | 1 | 01]
后面的兩位，被用來描述三種狀態：鎖定、未鎖定、monitor。
鎖定時：
[ptr | 00]
ptr指向一個棧上的header
未鎖定時：
[header | 0 | 01]
header就是常規的對象頭
monitor時：
[ptr | 10]
ptr指向膨脹后的lock， header被包裝起來了
marked：
[ptr | 11] marked
markSweep，即標記清理時使用，標記一個對象無效
正文
以下為全文，下面的1處，我先講解下，這個定義了一個field：
uintptr_t value()
我也不熟c++，查了一下，https://blog.csdn.net/cs_zhanyb/article/details/16973379
在64位的機器上，intptr_t和uintptr_t分別是long int、unsigned long int的別名；在32位的機器上，intptr_t和uintptr_t分別是int、unsigned int的別名。
也就是說，在64位機器上，這個類型代表了unsigned long int，那既然是long，肯定是64位了，也就是說，此時它就是個無符號的long類型；
32位上，則代表了unsigned int，此時，它就是個無符號的int（32位）。
有同學問我，鎖膨脹時，指向objectmonitor的指針在哪里，ok，就是在這個里面。
大家繼續看下面：
class BasicLock;
class ObjectMonitor;
class JavaThread;
class markOopDesc: public oopDesc {
private:
// Conversion
// 1
uintptr_t value() const { return (uintptr_t) this; }
public:
// Constants
enum { age_bits = 4,
lock_bits = 2,
biased_lock_bits = 1,
max_hash_bits = BitsPerWord - age_bits - lock_bits - biased_lock_bits,
hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits,
cms_bits = LP64_ONLY(1) NOT_LP64(0),
epoch_bits = 2
};
// The biased locking code currently requires that the age bits be
// contiguous to the lock bits.
enum { lock_shift = 0,
biased_lock_shift = lock_bits,
age_shift = lock_bits + biased_lock_bits,
cms_shift = age_shift + age_bits,
hash_shift = cms_shift + cms_bits,
epoch_shift = hash_shift
};
enum { lock_mask = right_n_bits(lock_bits),
lock_mask_in_place = lock_mask << lock_shift,
biased_lock_mask = right_n_bits(lock_bits + biased_lock_bits),
biased_lock_mask_in_place= biased_lock_mask << lock_shift,
biased_lock_bit_in_place = 1 << biased_lock_shift,
age_mask = right_n_bits(age_bits),
age_mask_in_place = age_mask << age_shift,
epoch_mask = right_n_bits(epoch_bits),
epoch_mask_in_place = epoch_mask << epoch_shift,
cms_mask = right_n_bits(cms_bits),
cms_mask_in_place = cms_mask << cms_shift
};
// Alignment of JavaThread pointers encoded in object header required by biased locking
enum { biased_lock_alignment = 2 << (epoch_shift + epoch_bits)
};
enum { locked_value = 0,
unlocked_value = 1,
monitor_value = 2,
marked_value = 3,
biased_lock_pattern = 5
};
enum { no_hash = 0 }; // no hash value assigned
enum { no_hash_in_place = (address_word)no_hash << hash_shift,
no_lock_in_place = unlocked_value
};
enum { max_age = age_mask };
enum { max_bias_epoch = epoch_mask };
// Prototype mark for initialization
static markOop biased_locking_prototype() {
return markOop( biased_lock_pattern );
}
// 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(), biased_lock_mask_in_place) == unlocked_value);
}
bool is_marked() const {
return (mask_bits(value(), lock_mask_in_place) == marked_value);
}
bool is_neutral() const { return (mask_bits(value(), biased_lock_mask_in_place) == unlocked_value); }
// Special temporary state of the markOop 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 stacklock. 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).
static markOop INFLATING() { return (markOop) 0; } // inflate-in-progress
// Should this header be preserved during GC?
inline bool must_be_preserved(oop obj_containing_mark) const;
inline bool must_be_preserved_with_bias(oop obj_containing_mark) const;
// Should this header (including its age bits) be preserved in the
// case of a promotion failure during scavenge?
// Note that we special case this situation. We want to avoid
// calling BiasedLocking::preserve_marks()/restore_marks() (which
// decrease the number of mark words that need to be preserved
// during GC) during each scavenge. During scavenges in which there
// is no promotion failure, we actually don't need to call the above
// routines at all, since we don't mutate and re-initialize the
// marks of promoted objects using init_mark(). However, during
// scavenges which result in promotion failure, we do re-initialize
// the mark words of objects, meaning that we should have called
// these mark word preservation routines. Currently there's no good
// place in which to call them in any of the scavengers (although
// guarded by appropriate locks we could make one), but the
// observation is that promotion failures are quite rare and
// reducing the number of mark words preserved during them isn't a
// high priority.
inline bool must_be_preserved_for_promotion_failure(oop obj_containing_mark) const;
inline bool must_be_preserved_with_bias_for_promotion_failure(oop obj_containing_mark) const;
// Should this header be preserved during a scavenge where CMS is
// the old generation?
// (This is basically the same body as must_be_preserved_for_promotion_failure(),
// but takes the Klass* as argument instead)
inline bool must_be_preserved_for_cms_scavenge(Klass* klass_of_obj_containing_mark) const;
inline bool must_be_preserved_with_bias_for_cms_scavenge(Klass* klass_of_obj_containing_mark) const;
// WARNING: The following routines are used EXCLUSIVELY by
// synchronization functions. They are not really gc safe.
// They must get updated if markOop layout get changed.
markOop set_unlocked() const {
return markOop(value() | unlocked_value);
}
bool has_locker() const {
return ((value() & lock_mask_in_place) == locked_value);
}
BasicLock* locker() const {
assert(has_locker(), "check");
return (BasicLock*) value();
}
bool has_displaced_mark_helper() const {
return ((value() & unlocked_value) == 0);
}
markOop displaced_mark_helper() const {
assert(has_displaced_mark_helper(), "check");
intptr_t ptr = (value() & ~monitor_value);
return *(markOop*)ptr;
}
void set_displaced_mark_helper(markOop m) const {
assert(has_displaced_mark_helper(), "check");
intptr_t ptr = (value() & ~monitor_value);
*(markOop*)ptr = m;
}
markOop copy_set_hash(intptr_t hash) const {
intptr_t tmp = value() & (~hash_mask_in_place);
tmp |= ((hash & hash_mask) << hash_shift);
return (markOop)tmp;
}
// it is only used to be stored into BasicLock as the
// indicator that the lock is using heavyweight monitor
static markOop unused_mark() {
return (markOop) marked_value;
}
// the following two functions create the markOop to be
// stored into object header, it encodes monitor info
static markOop encode(BasicLock* lock) {
return (markOop) lock;
}
static markOop encode(ObjectMonitor* monitor) {
intptr_t tmp = (intptr_t) monitor;
return (markOop) (tmp | monitor_value);
}
static markOop encode(JavaThread* thread, uint age, int bias_epoch) {
intptr_t tmp = (intptr_t) thread;
assert(UseBiasedLocking && ((tmp & (epoch_mask_in_place | age_mask_in_place | biased_lock_mask_in_place)) == 0), "misaligned JavaThread pointer");
assert(age <= max_age, "age too large");
assert(bias_epoch <= max_bias_epoch, "bias epoch too large");
return (markOop) (tmp | (bias_epoch << epoch_shift) | (age << age_shift) | biased_lock_pattern);
}
// used to encode pointers during GC
markOop clear_lock_bits() { return markOop(value() & ~lock_mask_in_place); }
// age operations
markOop set_marked() { return markOop((value() & ~lock_mask_in_place) | marked_value); }
markOop set_unmarked() { return markOop((value() & ~lock_mask_in_place) | unlocked_value); }
uint age() const { return mask_bits(value() >> age_shift, age_mask); }
markOop set_age(uint v) const {
assert((v & ~age_mask) == 0, "shouldn't overflow age field");
return markOop((value() & ~age_mask_in_place) | (((uintptr_t)v & age_mask) << age_shift));
}
markOop incr_age() const { return age() == max_age ? markOop(this) : 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;
}
// Prototype mark for initialization
static markOop prototype() {
return markOop( no_hash_in_place | no_lock_in_place );
}
// Helper function for restoration of unmarked mark oops during GC
static inline markOop prototype_for_object(oop obj);
// Debugging
void print_on(outputStream* st) const;
// Prepare address of oop for placement into mark
inline static markOop encode_pointer_as_mark(void* p) { return markOop(p)->set_marked(); }
// Recover address of oop from encoded form used in mark
inline void* decode_pointer() { if (UseBiasedLocking && has_bias_pattern()) return NULL; return clear_lock_bits(); }
// These markOops indicate cms free chunk blocks and not objects.
// In 64 bit, the markOop is set to distinguish them from oops.
// These are defined in 32 bit mode for vmStructs.
const static uintptr_t cms_free_chunk_pattern = 0x1;
// Constants for the size field.
enum { size_shift = cms_shift + cms_bits,
size_bits = 35 // need for compressed oops 32G
};
// These values are too big for Win64
const static uintptr_t size_mask = LP64_ONLY(right_n_bits(size_bits))
NOT_LP64(0);
const static uintptr_t size_mask_in_place =
(address_word)size_mask << size_shift;
#ifdef _LP64
static markOop cms_free_prototype() {
return markOop(((intptr_t)prototype() & ~cms_mask_in_place) |
((cms_free_chunk_pattern & cms_mask) << cms_shift));
}
uintptr_t cms_encoding() const {
return mask_bits(value() >> cms_shift, cms_mask);
}
bool is_cms_free_chunk() const {
return is_neutral() &&
(cms_encoding() & cms_free_chunk_pattern) == cms_free_chunk_pattern;
}
size_t get_size() const { return (size_t)(value() >> size_shift); }
static markOop set_size_and_free(size_t size) {
assert((size & ~size_mask) == 0, "shouldn't overflow size field");
return markOop(((intptr_t)cms_free_prototype() & ~size_mask_in_place) |
(((intptr_t)size & size_mask) << size_shift));
}
#endif // _LP64
};
#endif // SHARE_VM_OOPS_MARKOOP_HPP
然后我們講解下部分方法：
// Biased Locking accessors.
// These must be checked by all code which calls into the
// ObjectSynchronizer and other code. The biasing is not understood
// by the lower-level CAS-based locking code, although the runtime
// fixes up biased locks to be compatible with it when a bias is
// revoked.
// 是否設置了偏向標志
bool has_bias_pattern() const {
return (mask_bits(value(), biased_lock_mask_in_place) == biased_lock_pattern);
}
// 獲取偏向的線程的指針
JavaThread* biased_locker() const {
assert(has_bias_pattern(), "should not call this otherwise");
return (JavaThread*) ((intptr_t) (mask_bits(value(), ~(biased_lock_mask_in_place | age_mask_in_place | epoch_mask_in_place))));
}
// Indicates that the mark has the bias bit set but that it has not
// yet been biased toward a particular thread
// 是否可以偏向；但當前還沒有偏向任何線程
bool is_biased_anonymously() const {
return (has_bias_pattern() && (biased_locker() == NULL));
}
下面可以看到epoch的簡單意思：
如果因為太多次發生偏向鎖撤銷，那么epoch會發生變化。
// Indicates epoch in which this bias was acquired. If the epoch
// changes due to too many bias revocations occurring, the biases
// from the previous epochs are all considered invalid.
int bias_epoch() const {
assert(has_bias_pattern(), "should not call this otherwise");
return (mask_bits(value(), epoch_mask_in_place) >> epoch_shift);
}
markOop set_bias_epoch(int epoch) {
assert(has_bias_pattern(), "should not call this otherwise");
assert((epoch & (~epoch_mask)) == 0, "epoch overflow");
return markOop(mask_bits(value(), ~epoch_mask_in_place) | (epoch << epoch_shift));
}
markOop incr_bias_epoch() {
return set_bias_epoch((1 + bias_epoch()) & epoch_mask);
}
是否升級為monitor及返回對應的monitor的指針：
bool has_monitor() const {
return ((value() & monitor_value) != 0);
}
ObjectMonitor* monitor() const {
assert(has_monitor(), "check");
// Use xor instead of &~ to provide one extra tag-bit check.
return (ObjectMonitor*) (value() ^ monitor_value);
}