1 /*
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   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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   6  * under the terms of the GNU General Public License version 2 only, as
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  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
  13  * accompanied this code).
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  24 
  25 #ifndef SHARE_VM_GC_SHARED_SPACE_HPP
  26 #define SHARE_VM_GC_SHARED_SPACE_HPP
  27 
  28 #include "gc/shared/blockOffsetTable.hpp"
  29 #include "gc/shared/cardTableModRefBS.hpp"
  30 #include "gc/shared/workgroup.hpp"
  31 #include "memory/allocation.hpp"
  32 #include "memory/iterator.hpp"
  33 #include "memory/memRegion.hpp"
  34 #include "oops/markOop.hpp"
  35 #include "runtime/mutexLocker.hpp"
  36 #include "utilities/macros.hpp"
  37 
  38 // A space is an abstraction for the "storage units" backing
  39 // up the generation abstraction. It includes specific
  40 // implementations for keeping track of free and used space,
  41 // for iterating over objects and free blocks, etc.
  42 
  43 // Forward decls.
  44 class Space;
  45 class BlockOffsetArray;
  46 class BlockOffsetArrayContigSpace;
  47 class Generation;
  48 class CompactibleSpace;
  49 class BlockOffsetTable;
  50 class CardTableRS;
  51 class DirtyCardToOopClosure;
  52 
  53 // A Space describes a heap area. Class Space is an abstract
  54 // base class.
  55 //
  56 // Space supports allocation, size computation and GC support is provided.
  57 //
  58 // Invariant: bottom() and end() are on page_size boundaries and
  59 // bottom() <= top() <= end()
  60 // top() is inclusive and end() is exclusive.
  61 
  62 class Space: public CHeapObj<mtGC> {
  63   friend class VMStructs;
  64  protected:
  65   HeapWord* _bottom;
  66   HeapWord* _end;
  67 
  68   // Used in support of save_marks()
  69   HeapWord* _saved_mark_word;
  70 
  71   // A sequential tasks done structure. This supports
  72   // parallel GC, where we have threads dynamically
  73   // claiming sub-tasks from a larger parallel task.
  74   SequentialSubTasksDone _par_seq_tasks;
  75 
  76   Space():
  77     _bottom(NULL), _end(NULL) { }
  78 
  79  public:
  80   // Accessors
  81   HeapWord* bottom() const         { return _bottom; }
  82   HeapWord* end() const            { return _end;    }
  83   virtual void set_bottom(HeapWord* value) { _bottom = value; }
  84   virtual void set_end(HeapWord* value)    { _end = value; }
  85 
  86   virtual HeapWord* saved_mark_word() const  { return _saved_mark_word; }
  87 
  88   void set_saved_mark_word(HeapWord* p) { _saved_mark_word = p; }
  89 
  90   // Returns true if this object has been allocated since a
  91   // generation's "save_marks" call.
  92   virtual bool obj_allocated_since_save_marks(const oop obj) const {
  93     return (HeapWord*)obj >= saved_mark_word();
  94   }
  95 
  96   virtual MemRegionClosure* preconsumptionDirtyCardClosure() const {
  97     return NULL;
  98   }
  99 
 100   // Returns a subregion of the space containing only the allocated objects in
 101   // the space.
 102   virtual MemRegion used_region() const = 0;
 103 
 104   // Returns a region that is guaranteed to contain (at least) all objects
 105   // allocated at the time of the last call to "save_marks".  If the space
 106   // initializes its DirtyCardToOopClosure's specifying the "contig" option
 107   // (that is, if the space is contiguous), then this region must contain only
 108   // such objects: the memregion will be from the bottom of the region to the
 109   // saved mark.  Otherwise, the "obj_allocated_since_save_marks" method of
 110   // the space must distinguish between objects in the region allocated before
 111   // and after the call to save marks.
 112   MemRegion used_region_at_save_marks() const {
 113     return MemRegion(bottom(), saved_mark_word());
 114   }
 115 
 116   // Initialization.
 117   // "initialize" should be called once on a space, before it is used for
 118   // any purpose.  The "mr" arguments gives the bounds of the space, and
 119   // the "clear_space" argument should be true unless the memory in "mr" is
 120   // known to be zeroed.
 121   virtual void initialize(MemRegion mr, bool clear_space, bool mangle_space);
 122 
 123   // The "clear" method must be called on a region that may have
 124   // had allocation performed in it, but is now to be considered empty.
 125   virtual void clear(bool mangle_space);
 126 
 127   // For detecting GC bugs.  Should only be called at GC boundaries, since
 128   // some unused space may be used as scratch space during GC's.
 129   // We also call this when expanding a space to satisfy an allocation
 130   // request. See bug #4668531
 131   virtual void mangle_unused_area() = 0;
 132   virtual void mangle_unused_area_complete() = 0;
 133 
 134   // Testers
 135   bool is_empty() const              { return used() == 0; }
 136   bool not_empty() const             { return used() > 0; }
 137 
 138   // Returns true iff the given the space contains the
 139   // given address as part of an allocated object. For
 140   // certain kinds of spaces, this might be a potentially
 141   // expensive operation. To prevent performance problems
 142   // on account of its inadvertent use in product jvm's,
 143   // we restrict its use to assertion checks only.
 144   bool is_in(const void* p) const {
 145     return used_region().contains(p);
 146   }
 147 
 148   // Returns true iff the given reserved memory of the space contains the
 149   // given address.
 150   bool is_in_reserved(const void* p) const { return _bottom <= p && p < _end; }
 151 
 152   // Returns true iff the given block is not allocated.
 153   virtual bool is_free_block(const HeapWord* p) const = 0;
 154 
 155   // Test whether p is double-aligned
 156   static bool is_aligned(void* p) {
 157     return ((intptr_t)p & (sizeof(double)-1)) == 0;
 158   }
 159 
 160   // Size computations.  Sizes are in bytes.
 161   size_t capacity()     const { return byte_size(bottom(), end()); }
 162   virtual size_t used() const = 0;
 163   virtual size_t free() const = 0;
 164 
 165   // Iterate over all the ref-containing fields of all objects in the
 166   // space, calling "cl.do_oop" on each.  Fields in objects allocated by
 167   // applications of the closure are not included in the iteration.
 168   virtual void oop_iterate(ExtendedOopClosure* cl);
 169 
 170   // Iterate over all objects in the space, calling "cl.do_object" on
 171   // each.  Objects allocated by applications of the closure are not
 172   // included in the iteration.
 173   virtual void object_iterate(ObjectClosure* blk) = 0;
 174   // Similar to object_iterate() except only iterates over
 175   // objects whose internal references point to objects in the space.
 176   virtual void safe_object_iterate(ObjectClosure* blk) = 0;
 177 
 178   // Create and return a new dirty card to oop closure. Can be
 179   // overridden to return the appropriate type of closure
 180   // depending on the type of space in which the closure will
 181   // operate. ResourceArea allocated.
 182   virtual DirtyCardToOopClosure* new_dcto_cl(ExtendedOopClosure* cl,
 183                                              CardTableModRefBS::PrecisionStyle precision,
 184                                              HeapWord* boundary,
 185                                              bool parallel);
 186 
 187   // If "p" is in the space, returns the address of the start of the
 188   // "block" that contains "p".  We say "block" instead of "object" since
 189   // some heaps may not pack objects densely; a chunk may either be an
 190   // object or a non-object.  If "p" is not in the space, return NULL.
 191   virtual HeapWord* block_start_const(const void* p) const = 0;
 192 
 193   // The non-const version may have benevolent side effects on the data
 194   // structure supporting these calls, possibly speeding up future calls.
 195   // The default implementation, however, is simply to call the const
 196   // version.
 197   virtual HeapWord* block_start(const void* p);
 198 
 199   // Requires "addr" to be the start of a chunk, and returns its size.
 200   // "addr + size" is required to be the start of a new chunk, or the end
 201   // of the active area of the heap.
 202   virtual size_t block_size(const HeapWord* addr) const = 0;
 203 
 204   // Requires "addr" to be the start of a block, and returns "TRUE" iff
 205   // the block is an object.
 206   virtual bool block_is_obj(const HeapWord* addr) const = 0;
 207 
 208   // Requires "addr" to be the start of a block, and returns "TRUE" iff
 209   // the block is an object and the object is alive.
 210   virtual bool obj_is_alive(const HeapWord* addr) const;
 211 
 212   // Allocation (return NULL if full).  Assumes the caller has established
 213   // mutually exclusive access to the space.
 214   virtual HeapWord* allocate(size_t word_size) = 0;
 215 
 216   // Allocation (return NULL if full).  Enforces mutual exclusion internally.
 217   virtual HeapWord* par_allocate(size_t word_size) = 0;
 218 
 219   // Mark-sweep-compact support: all spaces can update pointers to objects
 220   // moving as a part of compaction.
 221   virtual void adjust_pointers() = 0;
 222 
 223   // PrintHeapAtGC support
 224   virtual void print() const;
 225   virtual void print_on(outputStream* st) const;
 226   virtual void print_short() const;
 227   virtual void print_short_on(outputStream* st) const;
 228 
 229 
 230   // Accessor for parallel sequential tasks.
 231   SequentialSubTasksDone* par_seq_tasks() { return &_par_seq_tasks; }
 232 
 233   // IF "this" is a ContiguousSpace, return it, else return NULL.
 234   virtual ContiguousSpace* toContiguousSpace() {
 235     return NULL;
 236   }
 237 
 238   // Debugging
 239   virtual void verify() const = 0;
 240 };
 241 
 242 // A MemRegionClosure (ResourceObj) whose "do_MemRegion" function applies an
 243 // OopClosure to (the addresses of) all the ref-containing fields that could
 244 // be modified by virtue of the given MemRegion being dirty. (Note that
 245 // because of the imprecise nature of the write barrier, this may iterate
 246 // over oops beyond the region.)
 247 // This base type for dirty card to oop closures handles memory regions
 248 // in non-contiguous spaces with no boundaries, and should be sub-classed
 249 // to support other space types. See ContiguousDCTOC for a sub-class
 250 // that works with ContiguousSpaces.
 251 
 252 class DirtyCardToOopClosure: public MemRegionClosureRO {
 253 protected:
 254   ExtendedOopClosure* _cl;
 255   Space* _sp;
 256   CardTableModRefBS::PrecisionStyle _precision;
 257   HeapWord* _boundary;          // If non-NULL, process only non-NULL oops
 258                                 // pointing below boundary.
 259   HeapWord* _min_done;          // ObjHeadPreciseArray precision requires
 260                                 // a downwards traversal; this is the
 261                                 // lowest location already done (or,
 262                                 // alternatively, the lowest address that
 263                                 // shouldn't be done again.  NULL means infinity.)
 264   NOT_PRODUCT(HeapWord* _last_bottom;)
 265   NOT_PRODUCT(HeapWord* _last_explicit_min_done;)
 266 
 267   // Get the actual top of the area on which the closure will
 268   // operate, given where the top is assumed to be (the end of the
 269   // memory region passed to do_MemRegion) and where the object
 270   // at the top is assumed to start. For example, an object may
 271   // start at the top but actually extend past the assumed top,
 272   // in which case the top becomes the end of the object.
 273   virtual HeapWord* get_actual_top(HeapWord* top, HeapWord* top_obj);
 274 
 275   // Walk the given memory region from bottom to (actual) top
 276   // looking for objects and applying the oop closure (_cl) to
 277   // them. The base implementation of this treats the area as
 278   // blocks, where a block may or may not be an object. Sub-
 279   // classes should override this to provide more accurate
 280   // or possibly more efficient walking.
 281   virtual void walk_mem_region(MemRegion mr, HeapWord* bottom, HeapWord* top);
 282 
 283 public:
 284   DirtyCardToOopClosure(Space* sp, ExtendedOopClosure* cl,
 285                         CardTableModRefBS::PrecisionStyle precision,
 286                         HeapWord* boundary) :
 287     _sp(sp), _cl(cl), _precision(precision), _boundary(boundary),
 288     _min_done(NULL) {
 289     NOT_PRODUCT(_last_bottom = NULL);
 290     NOT_PRODUCT(_last_explicit_min_done = NULL);
 291   }
 292 
 293   void do_MemRegion(MemRegion mr);
 294 
 295   void set_min_done(HeapWord* min_done) {
 296     _min_done = min_done;
 297     NOT_PRODUCT(_last_explicit_min_done = _min_done);
 298   }
 299 #ifndef PRODUCT
 300   void set_last_bottom(HeapWord* last_bottom) {
 301     _last_bottom = last_bottom;
 302   }
 303 #endif
 304 };
 305 
 306 // A structure to represent a point at which objects are being copied
 307 // during compaction.
 308 class CompactPoint : public StackObj {
 309 public:
 310   Generation* gen;
 311   CompactibleSpace* space;
 312   HeapWord* threshold;
 313 
 314   CompactPoint(Generation* g = NULL) :
 315     gen(g), space(NULL), threshold(0) {}
 316 };
 317 
 318 // A space that supports compaction operations.  This is usually, but not
 319 // necessarily, a space that is normally contiguous.  But, for example, a
 320 // free-list-based space whose normal collection is a mark-sweep without
 321 // compaction could still support compaction in full GC's.
 322 //
 323 // The compaction operations are implemented by the
 324 // scan_and_{adjust_pointers,compact,forward} function templates.
 325 // The following are, non-virtual, auxiliary functions used by these function templates:
 326 // - scan_limit()
 327 // - scanned_block_is_obj()
 328 // - scanned_block_size()
 329 // - adjust_obj_size()
 330 // - obj_size()
 331 // These functions are to be used exclusively by the scan_and_* function templates,
 332 // and must be defined for all (non-abstract) subclasses of CompactibleSpace.
 333 //
 334 // NOTE: Any subclasses to CompactibleSpace wanting to change/define the behavior
 335 // in any of the auxiliary functions must also override the corresponding
 336 // prepare_for_compaction/adjust_pointers/compact functions using them.
 337 // If not, such changes will not be used or have no effect on the compaction operations.
 338 //
 339 // This translates to the following dependencies:
 340 // Overrides/definitions of
 341 //  - scan_limit
 342 //  - scanned_block_is_obj
 343 //  - scanned_block_size
 344 // require override/definition of prepare_for_compaction().
 345 // Similar dependencies exist between
 346 //  - adjust_obj_size  and adjust_pointers()
 347 //  - obj_size         and compact().
 348 //
 349 // Additionally, this also means that changes to block_size() or block_is_obj() that
 350 // should be effective during the compaction operations must provide a corresponding
 351 // definition of scanned_block_size/scanned_block_is_obj respectively.
 352 class CompactibleSpace: public Space {
 353   friend class VMStructs;
 354   friend class CompactibleFreeListSpace;
 355 private:
 356   HeapWord* _compaction_top;
 357   CompactibleSpace* _next_compaction_space;
 358 
 359   // Auxiliary functions for scan_and_{forward,adjust_pointers,compact} support.
 360   inline size_t adjust_obj_size(size_t size) const {
 361     return size;
 362   }
 363 
 364   inline size_t obj_size(const HeapWord* addr) const {
 365     return oop(addr)->size();
 366   }
 367 
 368 public:
 369   CompactibleSpace() :
 370    _compaction_top(NULL), _next_compaction_space(NULL) {}
 371 
 372   virtual void initialize(MemRegion mr, bool clear_space, bool mangle_space);
 373   virtual void clear(bool mangle_space);
 374 
 375   // Used temporarily during a compaction phase to hold the value
 376   // top should have when compaction is complete.
 377   HeapWord* compaction_top() const { return _compaction_top;    }
 378 
 379   void set_compaction_top(HeapWord* value) {
 380     assert(value == NULL || (value >= bottom() && value <= end()),
 381       "should point inside space");
 382     _compaction_top = value;
 383   }
 384 
 385   // Perform operations on the space needed after a compaction
 386   // has been performed.
 387   virtual void reset_after_compaction() = 0;
 388 
 389   // Returns the next space (in the current generation) to be compacted in
 390   // the global compaction order.  Also is used to select the next
 391   // space into which to compact.
 392 
 393   virtual CompactibleSpace* next_compaction_space() const {
 394     return _next_compaction_space;
 395   }
 396 
 397   void set_next_compaction_space(CompactibleSpace* csp) {
 398     _next_compaction_space = csp;
 399   }
 400 
 401   // MarkSweep support phase2
 402 
 403   // Start the process of compaction of the current space: compute
 404   // post-compaction addresses, and insert forwarding pointers.  The fields
 405   // "cp->gen" and "cp->compaction_space" are the generation and space into
 406   // which we are currently compacting.  This call updates "cp" as necessary,
 407   // and leaves the "compaction_top" of the final value of
 408   // "cp->compaction_space" up-to-date.  Offset tables may be updated in
 409   // this phase as if the final copy had occurred; if so, "cp->threshold"
 410   // indicates when the next such action should be taken.
 411   virtual void prepare_for_compaction(CompactPoint* cp) = 0;
 412   // MarkSweep support phase3
 413   virtual void adjust_pointers();
 414   // MarkSweep support phase4
 415   virtual void compact();
 416 
 417   // The maximum percentage of objects that can be dead in the compacted
 418   // live part of a compacted space ("deadwood" support.)
 419   virtual size_t allowed_dead_ratio() const { return 0; };
 420 
 421   // Some contiguous spaces may maintain some data structures that should
 422   // be updated whenever an allocation crosses a boundary.  This function
 423   // returns the first such boundary.
 424   // (The default implementation returns the end of the space, so the
 425   // boundary is never crossed.)
 426   virtual HeapWord* initialize_threshold() { return end(); }
 427 
 428   // "q" is an object of the given "size" that should be forwarded;
 429   // "cp" names the generation ("gen") and containing "this" (which must
 430   // also equal "cp->space").  "compact_top" is where in "this" the
 431   // next object should be forwarded to.  If there is room in "this" for
 432   // the object, insert an appropriate forwarding pointer in "q".
 433   // If not, go to the next compaction space (there must
 434   // be one, since compaction must succeed -- we go to the first space of
 435   // the previous generation if necessary, updating "cp"), reset compact_top
 436   // and then forward.  In either case, returns the new value of "compact_top".
 437   // If the forwarding crosses "cp->threshold", invokes the "cross_threshold"
 438   // function of the then-current compaction space, and updates "cp->threshold
 439   // accordingly".
 440   virtual HeapWord* forward(oop q, size_t size, CompactPoint* cp,
 441                     HeapWord* compact_top);
 442 
 443   // Return a size with adjustments as required of the space.
 444   virtual size_t adjust_object_size_v(size_t size) const { return size; }
 445 
 446 protected:
 447   // Used during compaction.
 448   HeapWord* _first_dead;
 449   HeapWord* _end_of_live;
 450 
 451   // Minimum size of a free block.
 452   virtual size_t minimum_free_block_size() const { return 0; }
 453 
 454   // This the function is invoked when an allocation of an object covering
 455   // "start" to "end occurs crosses the threshold; returns the next
 456   // threshold.  (The default implementation does nothing.)
 457   virtual HeapWord* cross_threshold(HeapWord* start, HeapWord* the_end) {
 458     return end();
 459   }
 460 
 461   // Requires "allowed_deadspace_words > 0", that "q" is the start of a
 462   // free block of the given "word_len", and that "q", were it an object,
 463   // would not move if forwarded.  If the size allows, fill the free
 464   // block with an object, to prevent excessive compaction.  Returns "true"
 465   // iff the free region was made deadspace, and modifies
 466   // "allowed_deadspace_words" to reflect the number of available deadspace
 467   // words remaining after this operation.
 468   bool insert_deadspace(size_t& allowed_deadspace_words, HeapWord* q,
 469                         size_t word_len);
 470 
 471   // Below are template functions for scan_and_* algorithms (avoiding virtual calls).
 472   // The space argument should be a subclass of CompactibleSpace, implementing
 473   // scan_limit(), scanned_block_is_obj(), and scanned_block_size(),
 474   // and possibly also overriding obj_size(), and adjust_obj_size().
 475   // These functions should avoid virtual calls whenever possible.
 476 
 477   // Frequently calls adjust_obj_size().
 478   template <class SpaceType>
 479   static inline void scan_and_adjust_pointers(SpaceType* space);
 480 
 481   // Frequently calls obj_size().
 482   template <class SpaceType>
 483   static inline void scan_and_compact(SpaceType* space);
 484 
 485   // Frequently calls scanned_block_is_obj() and scanned_block_size().
 486   // Requires the scan_limit() function.
 487   template <class SpaceType>
 488   static inline void scan_and_forward(SpaceType* space, CompactPoint* cp);
 489 };
 490 
 491 class GenSpaceMangler;
 492 
 493 // A space in which the free area is contiguous.  It therefore supports
 494 // faster allocation, and compaction.
 495 class ContiguousSpace: public CompactibleSpace {
 496   friend class VMStructs;
 497   // Allow scan_and_forward function to call (private) overrides for auxiliary functions on this class
 498   template <typename SpaceType>
 499   friend void CompactibleSpace::scan_and_forward(SpaceType* space, CompactPoint* cp);
 500 
 501  private:
 502   // Auxiliary functions for scan_and_forward support.
 503   // See comments for CompactibleSpace for more information.
 504   inline HeapWord* scan_limit() const {
 505     return top();
 506   }
 507 
 508   inline bool scanned_block_is_obj(const HeapWord* addr) const {
 509     return true; // Always true, since scan_limit is top
 510   }
 511 
 512   inline size_t scanned_block_size(const HeapWord* addr) const {
 513     return oop(addr)->size();
 514   }
 515 
 516  protected:
 517   HeapWord* _top;
 518   HeapWord* _concurrent_iteration_safe_limit;
 519   // A helper for mangling the unused area of the space in debug builds.
 520   GenSpaceMangler* _mangler;
 521 
 522   GenSpaceMangler* mangler() { return _mangler; }
 523 
 524   // Allocation helpers (return NULL if full).
 525   inline HeapWord* allocate_impl(size_t word_size);
 526   inline HeapWord* par_allocate_impl(size_t word_size);
 527 
 528  public:
 529   ContiguousSpace();
 530   ~ContiguousSpace();
 531 
 532   virtual void initialize(MemRegion mr, bool clear_space, bool mangle_space);
 533   virtual void clear(bool mangle_space);
 534 
 535   // Accessors
 536   HeapWord* top() const            { return _top;    }
 537   void set_top(HeapWord* value)    { _top = value; }
 538 
 539   void set_saved_mark()            { _saved_mark_word = top();    }
 540   void reset_saved_mark()          { _saved_mark_word = bottom(); }
 541 
 542   bool saved_mark_at_top() const { return saved_mark_word() == top(); }
 543 
 544   // In debug mode mangle (write it with a particular bit
 545   // pattern) the unused part of a space.
 546 
 547   // Used to save the an address in a space for later use during mangling.
 548   void set_top_for_allocations(HeapWord* v) PRODUCT_RETURN;
 549   // Used to save the space's current top for later use during mangling.
 550   void set_top_for_allocations() PRODUCT_RETURN;
 551 
 552   // Mangle regions in the space from the current top up to the
 553   // previously mangled part of the space.
 554   void mangle_unused_area() PRODUCT_RETURN;
 555   // Mangle [top, end)
 556   void mangle_unused_area_complete() PRODUCT_RETURN;
 557 
 558   // Do some sparse checking on the area that should have been mangled.
 559   void check_mangled_unused_area(HeapWord* limit) PRODUCT_RETURN;
 560   // Check the complete area that should have been mangled.
 561   // This code may be NULL depending on the macro DEBUG_MANGLING.
 562   void check_mangled_unused_area_complete() PRODUCT_RETURN;
 563 
 564   // Size computations: sizes in bytes.
 565   size_t capacity() const        { return byte_size(bottom(), end()); }
 566   size_t used() const            { return byte_size(bottom(), top()); }
 567   size_t free() const            { return byte_size(top(),    end()); }
 568 
 569   virtual bool is_free_block(const HeapWord* p) const;
 570 
 571   // In a contiguous space we have a more obvious bound on what parts
 572   // contain objects.
 573   MemRegion used_region() const { return MemRegion(bottom(), top()); }
 574 
 575   // Allocation (return NULL if full)
 576   virtual HeapWord* allocate(size_t word_size);
 577   virtual HeapWord* par_allocate(size_t word_size);
 578   HeapWord* allocate_aligned(size_t word_size);
 579 
 580   // Iteration
 581   void oop_iterate(ExtendedOopClosure* cl);
 582   void object_iterate(ObjectClosure* blk);
 583   // For contiguous spaces this method will iterate safely over objects
 584   // in the space (i.e., between bottom and top) when at a safepoint.
 585   void safe_object_iterate(ObjectClosure* blk);
 586 
 587   // Iterate over as many initialized objects in the space as possible,
 588   // calling "cl.do_object_careful" on each. Return NULL if all objects
 589   // in the space (at the start of the iteration) were iterated over.
 590   // Return an address indicating the extent of the iteration in the
 591   // event that the iteration had to return because of finding an
 592   // uninitialized object in the space, or if the closure "cl"
 593   // signaled early termination.
 594   HeapWord* object_iterate_careful(ObjectClosureCareful* cl);
 595   HeapWord* concurrent_iteration_safe_limit() {
 596     assert(_concurrent_iteration_safe_limit <= top(),
 597            "_concurrent_iteration_safe_limit update missed");
 598     return _concurrent_iteration_safe_limit;
 599   }
 600   // changes the safe limit, all objects from bottom() to the new
 601   // limit should be properly initialized
 602   void set_concurrent_iteration_safe_limit(HeapWord* new_limit) {
 603     assert(new_limit <= top(), "uninitialized objects in the safe range");
 604     _concurrent_iteration_safe_limit = new_limit;
 605   }
 606 
 607 
 608 #if INCLUDE_ALL_GCS
 609   // In support of parallel oop_iterate.
 610   #define ContigSpace_PAR_OOP_ITERATE_DECL(OopClosureType, nv_suffix)  \
 611     void par_oop_iterate(MemRegion mr, OopClosureType* blk);
 612 
 613     ALL_PAR_OOP_ITERATE_CLOSURES(ContigSpace_PAR_OOP_ITERATE_DECL)
 614   #undef ContigSpace_PAR_OOP_ITERATE_DECL
 615 #endif // INCLUDE_ALL_GCS
 616 
 617   // Compaction support
 618   virtual void reset_after_compaction() {
 619     assert(compaction_top() >= bottom() && compaction_top() <= end(), "should point inside space");
 620     set_top(compaction_top());
 621     // set new iteration safe limit
 622     set_concurrent_iteration_safe_limit(compaction_top());
 623   }
 624 
 625   // Override.
 626   DirtyCardToOopClosure* new_dcto_cl(ExtendedOopClosure* cl,
 627                                      CardTableModRefBS::PrecisionStyle precision,
 628                                      HeapWord* boundary,
 629                                      bool parallel);
 630 
 631   // Apply "blk->do_oop" to the addresses of all reference fields in objects
 632   // starting with the _saved_mark_word, which was noted during a generation's
 633   // save_marks and is required to denote the head of an object.
 634   // Fields in objects allocated by applications of the closure
 635   // *are* included in the iteration.
 636   // Updates _saved_mark_word to point to just after the last object
 637   // iterated over.
 638 #define ContigSpace_OOP_SINCE_SAVE_MARKS_DECL(OopClosureType, nv_suffix)  \
 639   void oop_since_save_marks_iterate##nv_suffix(OopClosureType* blk);
 640 
 641   ALL_SINCE_SAVE_MARKS_CLOSURES(ContigSpace_OOP_SINCE_SAVE_MARKS_DECL)
 642 #undef ContigSpace_OOP_SINCE_SAVE_MARKS_DECL
 643 
 644   // Same as object_iterate, but starting from "mark", which is required
 645   // to denote the start of an object.  Objects allocated by
 646   // applications of the closure *are* included in the iteration.
 647   virtual void object_iterate_from(HeapWord* mark, ObjectClosure* blk);
 648 
 649   // Very inefficient implementation.
 650   virtual HeapWord* block_start_const(const void* p) const;
 651   size_t block_size(const HeapWord* p) const;
 652   // If a block is in the allocated area, it is an object.
 653   bool block_is_obj(const HeapWord* p) const { return p < top(); }
 654 
 655   // Addresses for inlined allocation
 656   HeapWord** top_addr() { return &_top; }
 657   HeapWord** end_addr() { return &_end; }
 658 
 659   // Overrides for more efficient compaction support.
 660   void prepare_for_compaction(CompactPoint* cp);
 661 
 662   // PrintHeapAtGC support.
 663   virtual void print_on(outputStream* st) const;
 664 
 665   // Checked dynamic downcasts.
 666   virtual ContiguousSpace* toContiguousSpace() {
 667     return this;
 668   }
 669 
 670   // Debugging
 671   virtual void verify() const;
 672 
 673   // Used to increase collection frequency.  "factor" of 0 means entire
 674   // space.
 675   void allocate_temporary_filler(int factor);
 676 };
 677 
 678 
 679 // A dirty card to oop closure that does filtering.
 680 // It knows how to filter out objects that are outside of the _boundary.
 681 class Filtering_DCTOC : public DirtyCardToOopClosure {
 682 protected:
 683   // Override.
 684   void walk_mem_region(MemRegion mr,
 685                        HeapWord* bottom, HeapWord* top);
 686 
 687   // Walk the given memory region, from bottom to top, applying
 688   // the given oop closure to (possibly) all objects found. The
 689   // given oop closure may or may not be the same as the oop
 690   // closure with which this closure was created, as it may
 691   // be a filtering closure which makes use of the _boundary.
 692   // We offer two signatures, so the FilteringClosure static type is
 693   // apparent.
 694   virtual void walk_mem_region_with_cl(MemRegion mr,
 695                                        HeapWord* bottom, HeapWord* top,
 696                                        ExtendedOopClosure* cl) = 0;
 697   virtual void walk_mem_region_with_cl(MemRegion mr,
 698                                        HeapWord* bottom, HeapWord* top,
 699                                        FilteringClosure* cl) = 0;
 700 
 701 public:
 702   Filtering_DCTOC(Space* sp, ExtendedOopClosure* cl,
 703                   CardTableModRefBS::PrecisionStyle precision,
 704                   HeapWord* boundary) :
 705     DirtyCardToOopClosure(sp, cl, precision, boundary) {}
 706 };
 707 
 708 // A dirty card to oop closure for contiguous spaces
 709 // (ContiguousSpace and sub-classes).
 710 // It is a FilteringClosure, as defined above, and it knows:
 711 //
 712 // 1. That the actual top of any area in a memory region
 713 //    contained by the space is bounded by the end of the contiguous
 714 //    region of the space.
 715 // 2. That the space is really made up of objects and not just
 716 //    blocks.
 717 
 718 class ContiguousSpaceDCTOC : public Filtering_DCTOC {
 719 protected:
 720   // Overrides.
 721   HeapWord* get_actual_top(HeapWord* top, HeapWord* top_obj);
 722 
 723   virtual void walk_mem_region_with_cl(MemRegion mr,
 724                                        HeapWord* bottom, HeapWord* top,
 725                                        ExtendedOopClosure* cl);
 726   virtual void walk_mem_region_with_cl(MemRegion mr,
 727                                        HeapWord* bottom, HeapWord* top,
 728                                        FilteringClosure* cl);
 729 
 730 public:
 731   ContiguousSpaceDCTOC(ContiguousSpace* sp, ExtendedOopClosure* cl,
 732                        CardTableModRefBS::PrecisionStyle precision,
 733                        HeapWord* boundary) :
 734     Filtering_DCTOC(sp, cl, precision, boundary)
 735   {}
 736 };
 737 
 738 // A ContigSpace that Supports an efficient "block_start" operation via
 739 // a BlockOffsetArray (whose BlockOffsetSharedArray may be shared with
 740 // other spaces.)  This is the abstract base class for old generation
 741 // (tenured) spaces.
 742 
 743 class OffsetTableContigSpace: public ContiguousSpace {
 744   friend class VMStructs;
 745  protected:
 746   BlockOffsetArrayContigSpace _offsets;
 747   Mutex _par_alloc_lock;
 748 
 749  public:
 750   // Constructor
 751   OffsetTableContigSpace(BlockOffsetSharedArray* sharedOffsetArray,
 752                          MemRegion mr);
 753 
 754   void set_bottom(HeapWord* value);
 755   void set_end(HeapWord* value);
 756 
 757   void clear(bool mangle_space);
 758 
 759   inline HeapWord* block_start_const(const void* p) const;
 760 
 761   // Add offset table update.
 762   virtual inline HeapWord* allocate(size_t word_size);
 763   inline HeapWord* par_allocate(size_t word_size);
 764 
 765   // MarkSweep support phase3
 766   virtual HeapWord* initialize_threshold();
 767   virtual HeapWord* cross_threshold(HeapWord* start, HeapWord* end);
 768 
 769   virtual void print_on(outputStream* st) const;
 770 
 771   // Debugging
 772   void verify() const;
 773 };
 774 
 775 
 776 // Class TenuredSpace is used by TenuredGeneration
 777 
 778 class TenuredSpace: public OffsetTableContigSpace {
 779   friend class VMStructs;
 780  protected:
 781   // Mark sweep support
 782   size_t allowed_dead_ratio() const;
 783  public:
 784   // Constructor
 785   TenuredSpace(BlockOffsetSharedArray* sharedOffsetArray,
 786                MemRegion mr) :
 787     OffsetTableContigSpace(sharedOffsetArray, mr) {}
 788 };
 789 #endif // SHARE_VM_GC_SHARED_SPACE_HPP