1 /*
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 10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 12  * version 2 for more details (a copy is included in the LICENSE file that
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 24 
 25 #ifndef SHARE_OOPS_MARKWORD_HPP
 26 #define SHARE_OOPS_MARKWORD_HPP
 27 
 28 #include "metaprogramming/integralConstant.hpp"
 29 #include "metaprogramming/primitiveConversions.hpp"
 30 #include "oops/oopsHierarchy.hpp"
 31 #include "runtime/globals.hpp"
 32 
 33 // The markWord describes the header of an object.
 34 //
 35 // Bit-format of an object header (most significant first, big endian layout below):
 36 //
 37 //  32 bits:
 38 //  --------
 39 //             hash:25 ------------>| age:4  unused_gap:1  lock:2 (normal object)
 40 //
 41 //  64 bits:
 42 //  --------
 43 //  unused:25 hash:31 -->| unused_gap:1  age:4  unused_gap:1  lock:2 (normal object)
 44 //
 45 //  - hash contains the identity hash value: largest value is
 46 //    31 bits, see os::random().  Also, 64-bit vm's require
 47 //    a hash value no bigger than 32 bits because they will not
 48 //    properly generate a mask larger than that: see library_call.cpp
 49 //
 50 //  - the two lock bits are used to describe three states: locked/unlocked and monitor.
 51 //
 52 //    [ptr             | 00]  locked             ptr points to real header on stack
 53 //    [header          | 01]  unlocked           regular object header
 54 //    [ptr             | 10]  monitor            inflated lock (header is wapped out)
 55 //    [ptr             | 11]  marked             used to mark an object
 56 //    [0 ............ 0| 00]  inflating          inflation in progress
 57 //
 58 //    We assume that stack/thread pointers have the lowest two bits cleared.
 59 //
 60 //
 61 //  - INFLATING() is a distinguished markword value of all zeros that is
 62 //    used when inflating an existing stack-lock into an ObjectMonitor.
 63 //    See below for is_being_inflated() and INFLATING().
 64 //
 65 //
 66 //
 67 //  Valhalla
 68 //
 69 //  <CMH: merge this doc into the text above>
 70 //
 71 //  Project Valhalla has mark word encoding requirements for the following oops:
 72 //
 73 //  * inline types: have alternative bytecode behavior, e.g. can not be locked
 74 //    - "larval state": mutable state, but only during object init, observable
 75 //      by only by a single thread (generally do not mutate markWord)
 76 //
 77 //  * flat arrays: load/decode of klass layout helper is expensive for aaload
 78 //
 79 //  * "null free" arrays: load/decode of klass layout helper again for aaload
 80 //
 81 //  EnableValhalla
 82 //
 83 //  Formerly known as "biased lock bit", "unused_gap" is free to use: using this
 84 //  bit to indicate inline type, combined with "unlocked" lock bits, means we
 85 //  will not interfere with lock encodings (displaced, inflating, and monitor),
 86 //  since inline types can't be locked.
 87 //
 88 //  Further state encoding
 89 //
 90 //  32 bit plaforms currently have no further room for encoding. No room for
 91 //  "denormalized layout helper bits", these fast mark word tests can only be made on
 92 //  64 bit platforms. 32-bit platforms need to load the klass->_layout_helper. This
 93 //  said, the larval state bit is still required for operation, stealing from the hash
 94 //  code is simplest mechanism.
 95 //
 96 //  Valhalla specific encodings
 97 //
 98 //  Revised Bit-format of an object header (most significant first, big endian layout below):
 99 //
100 //  32 bits:
101 //  --------
102 //  hash:24 ------------>| larval:1 age:4 inline_type:1 lock:2
103 //
104 //  64 bits:
105 //  --------
106 //  unused:1 | <-- hash:31 -->| unused:22 larval:1 age:4 flat_array:1 null_free_array:1 inline_type:1 lock:2
107 //
108 //  The "fast" static type bits (flat_array, null_free_array, and inline_type)
109 //  are placed lowest next to lock bits to more easily decode forwarding pointers.
110 //  G1 for example, implicitly clears age bits ("G1FullGCCompactionPoint::forward()")
111 //  using "oopDesc->forwardee()", so it necessary for "markWord::decode_pointer()"
112 //  to return a non-NULL for this case, but not confuse the static type bits for
113 //  a pointer.
114 //
115 //  Static types bits are recorded in the "klass->prototype_header()", displaced
116 //  mark should simply use the prototype header as "slow path", rather chasing
117 //  monitor or stack lock races.
118 //
119 //  Lock patterns (note inline types can't be locked/monitor/inflating)...
120 //
121 //  [ptr            | 000]  locked             ptr points to real header on stack
122 //  [header         | ?01]  unlocked           regular object header
123 //  [ptr            | 010]  monitor            inflated lock (header is wapped out)
124 //  [ptr            | ?11]  marked             used to mark an object
125 //  [0 ............ | 000]  inflating          inflation in progress
126 //
127 //
128 
129 class BasicLock;
130 class ObjectMonitor;
131 class JavaThread;
132 class outputStream;
133 
134 class markWord {
135  private:
136   uintptr_t _value;
137 
138  public:
139   explicit markWord(uintptr_t value) : _value(value) {}
140 
141   markWord() = default;         // Doesn't initialize _value.
142 
143   // It is critical for performance that this class be trivially
144   // destructable, copyable, and assignable.
145   ~markWord() = default;
146   markWord(const markWord&) = default;
147   markWord& operator=(const markWord&) = default;
148 
149   static markWord from_pointer(void* ptr) {
150     return markWord((uintptr_t)ptr);
151   }
152   void* to_pointer() const {
153     return (void*)_value;
154   }
155 
156   bool operator==(const markWord& other) const {
157     return _value == other._value;
158   }
159   bool operator!=(const markWord& other) const {
160     return !operator==(other);
161   }
162 
163   // Conversion
164   uintptr_t value() const { return _value; }
165 
166   // Constants, in least significant bit order
167   static const int lock_bits                      = 2;
168   static const int first_unused_gap_bits          = 1; // When !EnableValhalla
169   // EnableValhalla: static prototype header bits (fast path instead of klass layout_helper)
170   static const int inline_type_bits               = 1;
171   static const int null_free_array_bits           = LP64_ONLY(1) NOT_LP64(0);
172   static const int flat_array_bits                = LP64_ONLY(1) NOT_LP64(0);
173   // instance state
174   static const int age_bits                       = 4;
175   static const int larval_bits                    = 1;
176   static const int max_hash_bits                  = BitsPerWord - age_bits - lock_bits - inline_type_bits - larval_bits - flat_array_bits - null_free_array_bits;
177   static const int hash_bits                      = max_hash_bits > 31 ? 31 : max_hash_bits;
178   static const int second_unused_gap_bits         = LP64_ONLY(1) NOT_LP64(0); // !EnableValhalla: unused
179 
180   static const int lock_shift                     = 0;
181   static const int inline_type_shift              = lock_bits;
182   static const int null_free_array_shift          = inline_type_shift + inline_type_bits;
183   static const int flat_array_shift               = null_free_array_shift + null_free_array_bits;
184   static const int age_shift                      = flat_array_shift + flat_array_bits;
185   static const int unused_gap_shift               = age_shift + age_bits; // !EnableValhalla: unused
186   static const int larval_shift                   = age_shift + age_bits;
187   static const int hash_shift                     = LP64_ONLY(32) NOT_LP64(larval_shift + larval_bits);
188 
189   static const uintptr_t lock_mask                = right_n_bits(lock_bits);
190   static const uintptr_t lock_mask_in_place       = lock_mask << lock_shift;
191   static const uintptr_t inline_type_mask         = right_n_bits(lock_bits + inline_type_bits);
192   static const uintptr_t inline_type_mask_in_place = inline_type_mask << lock_shift;
193   static const uintptr_t inline_type_bit_in_place = 1 << inline_type_shift;
194   static const uintptr_t null_free_array_mask     = right_n_bits(null_free_array_bits);
195   static const uintptr_t null_free_array_mask_in_place = (null_free_array_mask << null_free_array_shift) | lock_mask_in_place;
196   static const uintptr_t null_free_array_bit_in_place  = (1 << null_free_array_shift);
197   static const uintptr_t flat_array_mask          = right_n_bits(flat_array_bits);
198   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;
199   static const uintptr_t flat_array_bit_in_place  = (1 << flat_array_shift);
200 
201   static const uintptr_t age_mask                 = right_n_bits(age_bits);
202   static const uintptr_t age_mask_in_place        = age_mask << age_shift;
203 
204   static const uintptr_t larval_mask              = right_n_bits(larval_bits);
205   static const uintptr_t larval_mask_in_place     = (larval_mask << larval_shift) | inline_type_mask_in_place;
206   static const uintptr_t larval_bit_in_place      = (1 << larval_shift);
207 
208   static const uintptr_t hash_mask                = right_n_bits(hash_bits);
209   static const uintptr_t hash_mask_in_place       = hash_mask << hash_shift;
210 
211   static const uintptr_t locked_value             = 0;
212   static const uintptr_t unlocked_value           = 1;
213   static const uintptr_t monitor_value            = 2;
214   static const uintptr_t marked_value             = 3;
215 
216   static const uintptr_t inline_type_pattern      = inline_type_bit_in_place | unlocked_value;
217   static const uintptr_t null_free_array_pattern  = null_free_array_bit_in_place | unlocked_value;
218   static const uintptr_t flat_array_pattern       = flat_array_bit_in_place | null_free_array_pattern;
219   // Has static klass prototype, used for decode/encode pointer
220   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));
221   static const uintptr_t static_prototype_mask_in_place = static_prototype_mask << lock_bits;
222   static const uintptr_t static_prototype_value_max = (1 << age_shift) - 1;
223 
224   static const uintptr_t larval_pattern           = larval_bit_in_place | inline_type_pattern;
225 
226   static const uintptr_t no_hash                  = 0 ;  // no hash value assigned
227   static const uintptr_t no_hash_in_place         = (address_word)no_hash << hash_shift;
228   static const uintptr_t no_lock_in_place         = unlocked_value;
229 
230   static const uint max_age                       = age_mask;
231 
232   // Creates a markWord with all bits set to zero.
233   static markWord zero() { return markWord(uintptr_t(0)); }
234 
235   bool is_inline_type() const {
236     return (mask_bits(value(), inline_type_mask_in_place) == inline_type_pattern);
237   }
238 
239   // lock accessors (note that these assume lock_shift == 0)
240   bool is_locked()   const {
241     return (mask_bits(value(), lock_mask_in_place) != unlocked_value);
242   }
243   bool is_unlocked() const {
244     return (mask_bits(value(), lock_mask_in_place) == unlocked_value);
245   }
246   bool is_marked()   const {
247     return (mask_bits(value(), lock_mask_in_place) == marked_value);
248   }
249 
250   // is unlocked and not an inline type (which cannot be involved in locking, displacement or inflation)
251   // i.e. test both lock bits and the inline type bit together
252   bool is_neutral()  const {
253     return (mask_bits(value(), inline_type_mask_in_place) == unlocked_value);
254   }
255 
256   // Special temporary state of the markWord while being inflated.
257   // Code that looks at mark outside a lock need to take this into account.
258   bool is_being_inflated() const { return (value() == 0); }
259 
260   // Distinguished markword value - used when inflating over
261   // an existing stack-lock.  0 indicates the markword is "BUSY".
262   // Lockword mutators that use a LD...CAS idiom should always
263   // check for and avoid overwriting a 0 value installed by some
264   // other thread.  (They should spin or block instead.  The 0 value
265   // is transient and *should* be short-lived).
266   static markWord INFLATING() { return zero(); }    // inflate-in-progress
267 
268   // Should this header be preserved during GC?
269   bool must_be_preserved(const oopDesc* obj) const {
270     return (!is_unlocked() || !has_no_hash() || (EnableValhalla && is_larval_state()));
271   }
272 
273   // WARNING: The following routines are used EXCLUSIVELY by
274   // synchronization functions. They are not really gc safe.
275   // They must get updated if markWord layout get changed.
276   markWord set_unlocked() const {
277     return markWord(value() | unlocked_value);
278   }
279   bool has_locker() const {
280     return ((value() & lock_mask_in_place) == locked_value);
281   }
282   BasicLock* locker() const {
283     assert(has_locker(), "check");
284     return (BasicLock*) value();
285   }
286   bool has_monitor() const {
287     return ((value() & monitor_value) != 0);
288   }
289   ObjectMonitor* monitor() const {
290     assert(has_monitor(), "check");
291     // Use xor instead of &~ to provide one extra tag-bit check.
292     return (ObjectMonitor*) (value() ^ monitor_value);
293   }
294   bool has_displaced_mark_helper() const {
295     return ((value() & unlocked_value) == 0);
296   }
297   markWord displaced_mark_helper() const;
298   void set_displaced_mark_helper(markWord m) const;
299   markWord copy_set_hash(intptr_t hash) const {
300     uintptr_t tmp = value() & (~hash_mask_in_place);
301     tmp |= ((hash & hash_mask) << hash_shift);
302     return markWord(tmp);
303   }
304   // it is only used to be stored into BasicLock as the
305   // indicator that the lock is using heavyweight monitor
306   static markWord unused_mark() {
307     return markWord(marked_value);
308   }
309   // the following two functions create the markWord to be
310   // stored into object header, it encodes monitor info
311   static markWord encode(BasicLock* lock) {
312     return from_pointer(lock);
313   }
314   static markWord encode(ObjectMonitor* monitor) {
315     uintptr_t tmp = (uintptr_t) monitor;
316     return markWord(tmp | monitor_value);
317   }
318 
319   // used to encode pointers during GC
320   markWord clear_lock_bits() { return markWord(value() & ~lock_mask_in_place); }
321 
322   // age operations
323   markWord set_marked()   { return markWord((value() & ~lock_mask_in_place) | marked_value); }
324   markWord set_unmarked() { return markWord((value() & ~lock_mask_in_place) | unlocked_value); }
325 
326   uint     age()           const { return mask_bits(value() >> age_shift, age_mask); }
327   markWord set_age(uint v) const {
328     assert((v & ~age_mask) == 0, "shouldn't overflow age field");
329     return markWord((value() & ~age_mask_in_place) | ((v & age_mask) << age_shift));
330   }
331   markWord incr_age()      const { return age() == max_age ? markWord(_value) : set_age(age() + 1); }
332 
333   // hash operations
334   intptr_t hash() const {
335     return mask_bits(value() >> hash_shift, hash_mask);
336   }
337 
338   bool has_no_hash() const {
339     return hash() == no_hash;
340   }
341 
342   // private buffered value operations
343   markWord enter_larval_state() const {
344     return markWord(value() | larval_bit_in_place);
345   }
346   markWord exit_larval_state() const {
347     return markWord(value() & ~larval_bit_in_place);
348   }
349   bool is_larval_state() const {
350     return (mask_bits(value(), larval_mask_in_place) == larval_pattern);
351   }
352 
353 #ifdef _LP64 // 64 bit encodings only
354   bool is_flat_array() const {
355     return (mask_bits(value(), flat_array_mask_in_place) == flat_array_pattern);
356   }
357 
358   bool is_null_free_array() const {
359     return (mask_bits(value(), null_free_array_mask_in_place) == null_free_array_pattern);
360   }
361 #else
362   bool is_flat_array() const {
363     fatal("Should not ask this for mark word, ask oopDesc");
364     return false;
365   }
366 
367   bool is_null_free_array() const {
368     fatal("Should not ask this for mark word, ask oopDesc");
369     return false;
370   }
371 #endif
372   // Prototype mark for initialization
373   static markWord prototype() {
374     return markWord( no_hash_in_place | no_lock_in_place );
375   }
376 
377   static markWord inline_type_prototype() {
378     return markWord(inline_type_pattern);
379   }
380 
381 #ifdef _LP64 // 64 bit encodings only
382   static markWord flat_array_prototype() {
383     return markWord(flat_array_pattern);
384   }
385 
386   static markWord null_free_array_prototype() {
387     return markWord(null_free_array_pattern);
388   }
389 #endif
390 
391   // Debugging
392   void print_on(outputStream* st, bool print_monitor_info = true) const;
393 
394   // Prepare address of oop for placement into mark
395   inline static markWord encode_pointer_as_mark(void* p) { return from_pointer(p).set_marked(); }
396 
397   // Recover address of oop from encoded form used in mark
398   inline void* decode_pointer() {
399     return (EnableValhalla && _value < static_prototype_value_max) ? NULL :
400       (void*) (clear_lock_bits().value());
401   }
402 };
403 
404 // Support atomic operations.
405 template<>
406 struct PrimitiveConversions::Translate<markWord> : public TrueType {
407   typedef markWord Value;
408   typedef uintptr_t Decayed;
409 
410   static Decayed decay(const Value& x) { return x.value(); }
411   static Value recover(Decayed x) { return Value(x); }
412 };
413 
414 #endif // SHARE_OOPS_MARKWORD_HPP