< prev index next > src/hotspot/share/oops/markWord.hpp
Print this page
// [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().
+ //
+ //
+ //
+ // Valhalla
+ //
+ // <CMH: merge this doc into the text above>
+ //
+ // Project Valhalla has mark word encoding requirements for the following oops:
+ //
+ // * inline types: have alternative bytecode behavior, e.g. can not be locked
+ // - "larval state": mutable state, but only during object init, observable
+ // by only by a single thread (generally do not mutate markWord)
+ //
+ // * flat arrays: load/decode of klass layout helper is expensive for aaload
+ //
+ // * "null free" arrays: load/decode of klass layout helper again for aaload
+ //
+ // EnableValhalla
+ //
+ // Formerly known as "biased lock bit", "unused_gap" is free to use: using this
+ // bit to indicate inline type, combined with "unlocked" lock bits, means we
+ // will not interfere with lock encodings (displaced, inflating, and monitor),
+ // since inline types can't be locked.
+ //
+ // Further state encoding
+ //
+ // 32 bit plaforms currently have no further room for encoding. No room for
+ // "denormalized layout helper bits", these fast mark word tests can only be made on
+ // 64 bit platforms. 32-bit platforms need to load the klass->_layout_helper. This
+ // said, the larval state bit is still required for operation, stealing from the hash
+ // code is simplest mechanism.
+ //
+ // Valhalla specific encodings
+ //
+ // Revised Bit-format of an object header (most significant first, big endian layout below):
+ //
+ // 32 bits:
+ // --------
+ // hash:24 ------------>| larval:1 age:4 inline_type:1 lock:2
+ //
+ // 64 bits:
+ // --------
+ // unused:1 | <-- hash:31 -->| unused:22 larval:1 age:4 flat_array:1 null_free_array:1 inline_type:1 lock:2
+ //
+ // The "fast" static type bits (flat_array, null_free_array, and inline_type)
+ // are placed lowest next to lock bits to more easily decode forwarding pointers.
+ // G1 for example, implicitly clears age bits ("G1FullGCCompactionPoint::forward()")
+ // using "oopDesc->forwardee()", so it necessary for "markWord::decode_pointer()"
+ // to return a non-nullptr for this case, but not confuse the static type bits for
+ // a pointer.
+ //
+ // Static types bits are recorded in the "klass->prototype_header()", displaced
+ // mark should simply use the prototype header as "slow path", rather chasing
+ // monitor or stack lock races.
+ //
+ // Lock patterns (note inline types can't be locked/monitor/inflating)...
+ //
+ // [ptr | 000] locked ptr points to real header on stack
+ // [header | ?01] unlocked regular object header
+ // [ptr | 010] monitor inflated lock (header is wapped out)
+ // [ptr | ?11] marked used to mark an object
+ // [0 ............ | 000] inflating inflation in progress
+ //
+ //
class BasicLock;
class ObjectMonitor;
class JavaThread;
class outputStream;
}
// Conversion
uintptr_t value() const { return _value; }
! // Constants
- static const int age_bits = 4;
static const int lock_bits = 2;
! static const int first_unused_gap_bits = 1;
! static const int max_hash_bits = BitsPerWord - age_bits - lock_bits - first_unused_gap_bits;
static const int hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits;
! static const int second_unused_gap_bits = LP64_ONLY(1) NOT_LP64(0);
static const int lock_shift = 0;
! static const int age_shift = lock_bits + first_unused_gap_bits;
! static const int hash_shift = age_shift + age_bits + second_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 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;
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 {
return (mask_bits(value(), lock_mask_in_place) == 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); }
}
// Conversion
uintptr_t value() const { return _value; }
! // Constants, in least significant bit order
static const int lock_bits = 2;
! static const int first_unused_gap_bits = 1; // When !EnableValhalla
! // EnableValhalla: static prototype header bits (fast path instead of klass layout_helper)
+ static const int inline_type_bits = 1;
+ static const int null_free_array_bits = LP64_ONLY(1) NOT_LP64(0);
+ static const int flat_array_bits = LP64_ONLY(1) NOT_LP64(0);
+ // instance state
+ static const int age_bits = 4;
+ static const int larval_bits = 1;
+ static const int max_hash_bits = BitsPerWord - age_bits - lock_bits - inline_type_bits - larval_bits - flat_array_bits - null_free_array_bits;
static const int hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits;
! static const int second_unused_gap_bits = LP64_ONLY(1) NOT_LP64(0); // !EnableValhalla: unused
static const int lock_shift = 0;
! static const int inline_type_shift = lock_bits;
! static const int null_free_array_shift = inline_type_shift + inline_type_bits;
+ static const int flat_array_shift = null_free_array_shift + null_free_array_bits;
+ static const int age_shift = flat_array_shift + flat_array_bits;
+ static const int unused_gap_shift = age_shift + age_bits; // !EnableValhalla: unused
+ static const int larval_shift = age_shift + age_bits;
+ static const int hash_shift = LP64_ONLY(32) NOT_LP64(larval_shift + larval_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 inline_type_mask = right_n_bits(lock_bits + inline_type_bits);
+ static const uintptr_t inline_type_mask_in_place = inline_type_mask << lock_shift;
+ static const uintptr_t inline_type_bit_in_place = 1 << inline_type_shift;
+ static const uintptr_t null_free_array_mask = right_n_bits(null_free_array_bits);
+ static const uintptr_t null_free_array_mask_in_place = (null_free_array_mask << null_free_array_shift) | lock_mask_in_place;
+ static const uintptr_t null_free_array_bit_in_place = (1 << null_free_array_shift);
+ static const uintptr_t flat_array_mask = right_n_bits(flat_array_bits);
+ static const uintptr_t flat_array_mask_in_place = (flat_array_mask << flat_array_shift) | null_free_array_mask_in_place | lock_mask_in_place;
+ static const uintptr_t flat_array_bit_in_place = (1 << flat_array_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 larval_mask = right_n_bits(larval_bits);
+ static const uintptr_t larval_mask_in_place = (larval_mask << larval_shift) | inline_type_mask_in_place;
+ static const uintptr_t larval_bit_in_place = (1 << larval_shift);
+
static const uintptr_t hash_mask = right_n_bits(hash_bits);
static const uintptr_t hash_mask_in_place = hash_mask << hash_shift;
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 inline_type_pattern = inline_type_bit_in_place | unlocked_value;
+ static const uintptr_t null_free_array_pattern = null_free_array_bit_in_place | unlocked_value;
+ static const uintptr_t flat_array_pattern = flat_array_bit_in_place | null_free_array_pattern;
+ // Has static klass prototype, used for decode/encode pointer
+ static const uintptr_t static_prototype_mask = LP64_ONLY(right_n_bits(inline_type_bits + flat_array_bits + null_free_array_bits)) NOT_LP64(right_n_bits(inline_type_bits));
+ static const uintptr_t static_prototype_mask_in_place = static_prototype_mask << lock_bits;
+ static const uintptr_t static_prototype_value_max = (1 << age_shift) - 1;
+
+ static const uintptr_t larval_pattern = larval_bit_in_place | inline_type_pattern;
+
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)); }
+ bool is_inline_type() const {
+ return (mask_bits(value(), inline_type_mask_in_place) == inline_type_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(), lock_mask_in_place) == unlocked_value);
}
bool is_marked() const {
return (mask_bits(value(), lock_mask_in_place) == marked_value);
}
+
+ // is unlocked and not an inline type (which cannot be involved in locking, displacement or inflation)
+ // i.e. test both lock bits and the inline type bit together
+ bool is_neutral() const { // Not locked, or marked - a "clean" neutral state
+ return (mask_bits(value(), inline_type_mask_in_place) == unlocked_value);
+ }
+
bool is_forwarded() const {
return (mask_bits(value(), lock_mask_in_place) == marked_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); }
// 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 oopDesc* obj) 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.
// 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 oopDesc* obj) const {
! return (!is_unlocked() || !has_no_hash() || (EnableValhalla && is_larval_state()));
}
// 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.
bool has_no_hash() const {
return hash() == no_hash;
}
// 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 oop forwardee() const {
return cast_to_oop(decode_pointer());
}
};
bool has_no_hash() const {
return hash() == no_hash;
}
+ // private buffered value operations
+ markWord enter_larval_state() const {
+ return markWord(value() | larval_bit_in_place);
+ }
+ markWord exit_larval_state() const {
+ return markWord(value() & ~larval_bit_in_place);
+ }
+ bool is_larval_state() const {
+ return (mask_bits(value(), larval_mask_in_place) == larval_pattern);
+ }
+
+ #ifdef _LP64 // 64 bit encodings only
+ bool is_flat_array() const {
+ return (mask_bits(value(), flat_array_mask_in_place) == flat_array_pattern);
+ }
+
+ bool is_null_free_array() const {
+ return (mask_bits(value(), null_free_array_mask_in_place) == null_free_array_pattern);
+ }
+ #else
+ bool is_flat_array() const {
+ fatal("Should not ask this for mark word, ask oopDesc");
+ return false;
+ }
+
+ bool is_null_free_array() const {
+ fatal("Should not ask this for mark word, ask oopDesc");
+ return false;
+ }
+ #endif
// Prototype mark for initialization
static markWord prototype() {
return markWord( no_hash_in_place | no_lock_in_place );
}
+ static markWord inline_type_prototype() {
+ return markWord(inline_type_pattern);
+ }
+
+ #ifdef _LP64 // 64 bit encodings only
+ static markWord flat_array_prototype() {
+ return markWord(flat_array_pattern);
+ }
+
+ static markWord null_free_array_prototype() {
+ return markWord(null_free_array_pattern);
+ }
+ #endif
+
// 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 (EnableValhalla && _value < static_prototype_value_max) ? nullptr :
+ (void*) (clear_lock_bits().value());
+ }
inline oop forwardee() const {
return cast_to_oop(decode_pointer());
}
};
< prev index next >