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   virtual void print() const;
 224   virtual void print_on(outputStream* st) const;
 225   virtual void print_short() const;
 226   virtual void print_short_on(outputStream* st) const;
 227 
 228 
 229   // Accessor for parallel sequential tasks.
 230   SequentialSubTasksDone* par_seq_tasks() { return &_par_seq_tasks; }
 231 
 232   // IF "this" is a ContiguousSpace, return it, else return NULL.
 233   virtual ContiguousSpace* toContiguousSpace() {
 234     return NULL;
 235   }
 236 
 237   // Debugging
 238   virtual void verify() const = 0;
 239 };
 240 
 241 // A MemRegionClosure (ResourceObj) whose "do_MemRegion" function applies an
 242 // OopClosure to (the addresses of) all the ref-containing fields that could
 243 // be modified by virtue of the given MemRegion being dirty. (Note that
 244 // because of the imprecise nature of the write barrier, this may iterate
 245 // over oops beyond the region.)
 246 // This base type for dirty card to oop closures handles memory regions
 247 // in non-contiguous spaces with no boundaries, and should be sub-classed
 248 // to support other space types. See ContiguousDCTOC for a sub-class
 249 // that works with ContiguousSpaces.
 250 
 251 class DirtyCardToOopClosure: public MemRegionClosureRO {
 252 protected:
 253   ExtendedOopClosure* _cl;
 254   Space* _sp;
 255   CardTableModRefBS::PrecisionStyle _precision;
 256   HeapWord* _boundary;          // If non-NULL, process only non-NULL oops
 257                                 // pointing below boundary.
 258   HeapWord* _min_done;          // ObjHeadPreciseArray precision requires
 259                                 // a downwards traversal; this is the
 260                                 // lowest location already done (or,
 261                                 // alternatively, the lowest address that
 262                                 // shouldn't be done again.  NULL means infinity.)
 263   NOT_PRODUCT(HeapWord* _last_bottom;)
 264   NOT_PRODUCT(HeapWord* _last_explicit_min_done;)
 265 
 266   // Get the actual top of the area on which the closure will
 267   // operate, given where the top is assumed to be (the end of the
 268   // memory region passed to do_MemRegion) and where the object
 269   // at the top is assumed to start. For example, an object may
 270   // start at the top but actually extend past the assumed top,
 271   // in which case the top becomes the end of the object.
 272   virtual HeapWord* get_actual_top(HeapWord* top, HeapWord* top_obj);
 273 
 274   // Walk the given memory region from bottom to (actual) top
 275   // looking for objects and applying the oop closure (_cl) to
 276   // them. The base implementation of this treats the area as
 277   // blocks, where a block may or may not be an object. Sub-
 278   // classes should override this to provide more accurate
 279   // or possibly more efficient walking.
 280   virtual void walk_mem_region(MemRegion mr, HeapWord* bottom, HeapWord* top);
 281 
 282 public:
 283   DirtyCardToOopClosure(Space* sp, ExtendedOopClosure* cl,
 284                         CardTableModRefBS::PrecisionStyle precision,
 285                         HeapWord* boundary) :
 286     _sp(sp), _cl(cl), _precision(precision), _boundary(boundary),
 287     _min_done(NULL) {
 288     NOT_PRODUCT(_last_bottom = NULL);
 289     NOT_PRODUCT(_last_explicit_min_done = NULL);
 290   }
 291 
 292   void do_MemRegion(MemRegion mr);
 293 
 294   void set_min_done(HeapWord* min_done) {
 295     _min_done = min_done;
 296     NOT_PRODUCT(_last_explicit_min_done = _min_done);
 297   }
 298 #ifndef PRODUCT
 299   void set_last_bottom(HeapWord* last_bottom) {
 300     _last_bottom = last_bottom;
 301   }
 302 #endif
 303 };
 304 
 305 // A structure to represent a point at which objects are being copied
 306 // during compaction.
 307 class CompactPoint : public StackObj {
 308 public:
 309   Generation* gen;
 310   CompactibleSpace* space;
 311   HeapWord* threshold;
 312 
 313   CompactPoint(Generation* g = NULL) :
 314     gen(g), space(NULL), threshold(0) {}
 315 };
 316 
 317 // A space that supports compaction operations.  This is usually, but not
 318 // necessarily, a space that is normally contiguous.  But, for example, a
 319 // free-list-based space whose normal collection is a mark-sweep without
 320 // compaction could still support compaction in full GC's.
 321 //
 322 // The compaction operations are implemented by the
 323 // scan_and_{adjust_pointers,compact,forward} function templates.
 324 // The following are, non-virtual, auxiliary functions used by these function templates:
 325 // - scan_limit()
 326 // - scanned_block_is_obj()
 327 // - scanned_block_size()
 328 // - adjust_obj_size()
 329 // - obj_size()
 330 // These functions are to be used exclusively by the scan_and_* function templates,
 331 // and must be defined for all (non-abstract) subclasses of CompactibleSpace.
 332 //
 333 // NOTE: Any subclasses to CompactibleSpace wanting to change/define the behavior
 334 // in any of the auxiliary functions must also override the corresponding
 335 // prepare_for_compaction/adjust_pointers/compact functions using them.
 336 // If not, such changes will not be used or have no effect on the compaction operations.
 337 //
 338 // This translates to the following dependencies:
 339 // Overrides/definitions of
 340 //  - scan_limit
 341 //  - scanned_block_is_obj
 342 //  - scanned_block_size
 343 // require override/definition of prepare_for_compaction().
 344 // Similar dependencies exist between
 345 //  - adjust_obj_size  and adjust_pointers()
 346 //  - obj_size         and compact().
 347 //
 348 // Additionally, this also means that changes to block_size() or block_is_obj() that
 349 // should be effective during the compaction operations must provide a corresponding
 350 // definition of scanned_block_size/scanned_block_is_obj respectively.
 351 class CompactibleSpace: public Space {
 352   friend class VMStructs;
 353   friend class CompactibleFreeListSpace;
 354 private:
 355   HeapWord* _compaction_top;
 356   CompactibleSpace* _next_compaction_space;
 357 
 358   // Auxiliary functions for scan_and_{forward,adjust_pointers,compact} support.
 359   inline size_t adjust_obj_size(size_t size) const {
 360     return size;
 361   }
 362 
 363   inline size_t obj_size(const HeapWord* addr) const {
 364     return oop(addr)->size();
 365   }
 366 
 367 public:
 368   CompactibleSpace() :
 369    _compaction_top(NULL), _next_compaction_space(NULL) {}
 370 
 371   virtual void initialize(MemRegion mr, bool clear_space, bool mangle_space);
 372   virtual void clear(bool mangle_space);
 373 
 374   // Used temporarily during a compaction phase to hold the value
 375   // top should have when compaction is complete.
 376   HeapWord* compaction_top() const { return _compaction_top;    }
 377 
 378   void set_compaction_top(HeapWord* value) {
 379     assert(value == NULL || (value >= bottom() && value <= end()),
 380       "should point inside space");
 381     _compaction_top = value;
 382   }
 383 
 384   // Perform operations on the space needed after a compaction
 385   // has been performed.
 386   virtual void reset_after_compaction() = 0;
 387 
 388   // Returns the next space (in the current generation) to be compacted in
 389   // the global compaction order.  Also is used to select the next
 390   // space into which to compact.
 391 
 392   virtual CompactibleSpace* next_compaction_space() const {
 393     return _next_compaction_space;
 394   }
 395 
 396   void set_next_compaction_space(CompactibleSpace* csp) {
 397     _next_compaction_space = csp;
 398   }
 399 
 400   // MarkSweep support phase2
 401 
 402   // Start the process of compaction of the current space: compute
 403   // post-compaction addresses, and insert forwarding pointers.  The fields
 404   // "cp->gen" and "cp->compaction_space" are the generation and space into
 405   // which we are currently compacting.  This call updates "cp" as necessary,
 406   // and leaves the "compaction_top" of the final value of
 407   // "cp->compaction_space" up-to-date.  Offset tables may be updated in
 408   // this phase as if the final copy had occurred; if so, "cp->threshold"
 409   // indicates when the next such action should be taken.
 410   virtual void prepare_for_compaction(CompactPoint* cp) = 0;
 411   // MarkSweep support phase3
 412   virtual void adjust_pointers();
 413   // MarkSweep support phase4
 414   virtual void compact();
 415 
 416   // The maximum percentage of objects that can be dead in the compacted
 417   // live part of a compacted space ("deadwood" support.)
 418   virtual size_t allowed_dead_ratio() const { return 0; };
 419 
 420   // Some contiguous spaces may maintain some data structures that should
 421   // be updated whenever an allocation crosses a boundary.  This function
 422   // returns the first such boundary.
 423   // (The default implementation returns the end of the space, so the
 424   // boundary is never crossed.)
 425   virtual HeapWord* initialize_threshold() { return end(); }
 426 
 427   // "q" is an object of the given "size" that should be forwarded;
 428   // "cp" names the generation ("gen") and containing "this" (which must
 429   // also equal "cp->space").  "compact_top" is where in "this" the
 430   // next object should be forwarded to.  If there is room in "this" for
 431   // the object, insert an appropriate forwarding pointer in "q".
 432   // If not, go to the next compaction space (there must
 433   // be one, since compaction must succeed -- we go to the first space of
 434   // the previous generation if necessary, updating "cp"), reset compact_top
 435   // and then forward.  In either case, returns the new value of "compact_top".
 436   // If the forwarding crosses "cp->threshold", invokes the "cross_threshold"
 437   // function of the then-current compaction space, and updates "cp->threshold
 438   // accordingly".
 439   virtual HeapWord* forward(oop q, size_t size, CompactPoint* cp,
 440                     HeapWord* compact_top);
 441 
 442   // Return a size with adjustments as required of the space.
 443   virtual size_t adjust_object_size_v(size_t size) const { return size; }
 444 
 445 protected:
 446   // Used during compaction.
 447   HeapWord* _first_dead;
 448   HeapWord* _end_of_live;
 449 
 450   // Minimum size of a free block.
 451   virtual size_t minimum_free_block_size() const { return 0; }
 452 
 453   // This the function is invoked when an allocation of an object covering
 454   // "start" to "end occurs crosses the threshold; returns the next
 455   // threshold.  (The default implementation does nothing.)
 456   virtual HeapWord* cross_threshold(HeapWord* start, HeapWord* the_end) {
 457     return end();
 458   }
 459 
 460   // Requires "allowed_deadspace_words > 0", that "q" is the start of a
 461   // free block of the given "word_len", and that "q", were it an object,
 462   // would not move if forwarded.  If the size allows, fill the free
 463   // block with an object, to prevent excessive compaction.  Returns "true"
 464   // iff the free region was made deadspace, and modifies
 465   // "allowed_deadspace_words" to reflect the number of available deadspace
 466   // words remaining after this operation.
 467   bool insert_deadspace(size_t& allowed_deadspace_words, HeapWord* q,
 468                         size_t word_len);
 469 
 470   // Below are template functions for scan_and_* algorithms (avoiding virtual calls).
 471   // The space argument should be a subclass of CompactibleSpace, implementing
 472   // scan_limit(), scanned_block_is_obj(), and scanned_block_size(),
 473   // and possibly also overriding obj_size(), and adjust_obj_size().
 474   // These functions should avoid virtual calls whenever possible.
 475 
 476   // Frequently calls adjust_obj_size().
 477   template <class SpaceType>
 478   static inline void scan_and_adjust_pointers(SpaceType* space);
 479 
 480   // Frequently calls obj_size().
 481   template <class SpaceType>
 482   static inline void scan_and_compact(SpaceType* space);
 483 
 484   // Frequently calls scanned_block_is_obj() and scanned_block_size().
 485   // Requires the scan_limit() function.
 486   template <class SpaceType>
 487   static inline void scan_and_forward(SpaceType* space, CompactPoint* cp);
 488 };
 489 
 490 class GenSpaceMangler;
 491 
 492 // A space in which the free area is contiguous.  It therefore supports
 493 // faster allocation, and compaction.
 494 class ContiguousSpace: public CompactibleSpace {
 495   friend class VMStructs;
 496   // Allow scan_and_forward function to call (private) overrides for auxiliary functions on this class
 497   template <typename SpaceType>
 498   friend void CompactibleSpace::scan_and_forward(SpaceType* space, CompactPoint* cp);
 499 
 500  private:
 501   // Auxiliary functions for scan_and_forward support.
 502   // See comments for CompactibleSpace for more information.
 503   inline HeapWord* scan_limit() const {
 504     return top();
 505   }
 506 
 507   inline bool scanned_block_is_obj(const HeapWord* addr) const {
 508     return true; // Always true, since scan_limit is top
 509   }
 510 
 511   inline size_t scanned_block_size(const HeapWord* addr) const {
 512     return oop(addr)->size();
 513   }
 514 
 515  protected:
 516   HeapWord* _top;
 517   HeapWord* _concurrent_iteration_safe_limit;
 518   // A helper for mangling the unused area of the space in debug builds.
 519   GenSpaceMangler* _mangler;
 520 
 521   GenSpaceMangler* mangler() { return _mangler; }
 522 
 523   // Allocation helpers (return NULL if full).
 524   inline HeapWord* allocate_impl(size_t word_size);
 525   inline HeapWord* par_allocate_impl(size_t word_size);
 526 
 527  public:
 528   ContiguousSpace();
 529   ~ContiguousSpace();
 530 
 531   virtual void initialize(MemRegion mr, bool clear_space, bool mangle_space);
 532   virtual void clear(bool mangle_space);
 533 
 534   // Accessors
 535   HeapWord* top() const            { return _top;    }
 536   void set_top(HeapWord* value)    { _top = value; }
 537 
 538   void set_saved_mark()            { _saved_mark_word = top();    }
 539   void reset_saved_mark()          { _saved_mark_word = bottom(); }
 540 
 541   bool saved_mark_at_top() const { return saved_mark_word() == top(); }
 542 
 543   // In debug mode mangle (write it with a particular bit
 544   // pattern) the unused part of a space.
 545 
 546   // Used to save the an address in a space for later use during mangling.
 547   void set_top_for_allocations(HeapWord* v) PRODUCT_RETURN;
 548   // Used to save the space's current top for later use during mangling.
 549   void set_top_for_allocations() PRODUCT_RETURN;
 550 
 551   // Mangle regions in the space from the current top up to the
 552   // previously mangled part of the space.
 553   void mangle_unused_area() PRODUCT_RETURN;
 554   // Mangle [top, end)
 555   void mangle_unused_area_complete() PRODUCT_RETURN;
 556 
 557   // Do some sparse checking on the area that should have been mangled.
 558   void check_mangled_unused_area(HeapWord* limit) PRODUCT_RETURN;
 559   // Check the complete area that should have been mangled.
 560   // This code may be NULL depending on the macro DEBUG_MANGLING.
 561   void check_mangled_unused_area_complete() PRODUCT_RETURN;
 562 
 563   // Size computations: sizes in bytes.
 564   size_t capacity() const        { return byte_size(bottom(), end()); }
 565   size_t used() const            { return byte_size(bottom(), top()); }
 566   size_t free() const            { return byte_size(top(),    end()); }
 567 
 568   virtual bool is_free_block(const HeapWord* p) const;
 569 
 570   // In a contiguous space we have a more obvious bound on what parts
 571   // contain objects.
 572   MemRegion used_region() const { return MemRegion(bottom(), top()); }
 573 
 574   // Allocation (return NULL if full)
 575   virtual HeapWord* allocate(size_t word_size);
 576   virtual HeapWord* par_allocate(size_t word_size);
 577   HeapWord* allocate_aligned(size_t word_size);
 578 
 579   // Iteration
 580   void oop_iterate(ExtendedOopClosure* cl);
 581   void object_iterate(ObjectClosure* blk);
 582   // For contiguous spaces this method will iterate safely over objects
 583   // in the space (i.e., between bottom and top) when at a safepoint.
 584   void safe_object_iterate(ObjectClosure* blk);
 585 
 586   // Iterate over as many initialized objects in the space as possible,
 587   // calling "cl.do_object_careful" on each. Return NULL if all objects
 588   // in the space (at the start of the iteration) were iterated over.
 589   // Return an address indicating the extent of the iteration in the
 590   // event that the iteration had to return because of finding an
 591   // uninitialized object in the space, or if the closure "cl"
 592   // signaled early termination.
 593   HeapWord* object_iterate_careful(ObjectClosureCareful* cl);
 594   HeapWord* concurrent_iteration_safe_limit() {
 595     assert(_concurrent_iteration_safe_limit <= top(),
 596            "_concurrent_iteration_safe_limit update missed");
 597     return _concurrent_iteration_safe_limit;
 598   }
 599   // changes the safe limit, all objects from bottom() to the new
 600   // limit should be properly initialized
 601   void set_concurrent_iteration_safe_limit(HeapWord* new_limit) {
 602     assert(new_limit <= top(), "uninitialized objects in the safe range");
 603     _concurrent_iteration_safe_limit = new_limit;
 604   }
 605 
 606 
 607 #if INCLUDE_ALL_GCS
 608   // In support of parallel oop_iterate.
 609   #define ContigSpace_PAR_OOP_ITERATE_DECL(OopClosureType, nv_suffix)  \
 610     void par_oop_iterate(MemRegion mr, OopClosureType* blk);
 611 
 612     ALL_PAR_OOP_ITERATE_CLOSURES(ContigSpace_PAR_OOP_ITERATE_DECL)
 613   #undef ContigSpace_PAR_OOP_ITERATE_DECL
 614 #endif // INCLUDE_ALL_GCS
 615 
 616   // Compaction support
 617   virtual void reset_after_compaction() {
 618     assert(compaction_top() >= bottom() && compaction_top() <= end(), "should point inside space");
 619     set_top(compaction_top());
 620     // set new iteration safe limit
 621     set_concurrent_iteration_safe_limit(compaction_top());
 622   }
 623 
 624   // Override.
 625   DirtyCardToOopClosure* new_dcto_cl(ExtendedOopClosure* cl,
 626                                      CardTableModRefBS::PrecisionStyle precision,
 627                                      HeapWord* boundary,
 628                                      bool parallel);
 629 
 630   // Apply "blk->do_oop" to the addresses of all reference fields in objects
 631   // starting with the _saved_mark_word, which was noted during a generation's
 632   // save_marks and is required to denote the head of an object.
 633   // Fields in objects allocated by applications of the closure
 634   // *are* included in the iteration.
 635   // Updates _saved_mark_word to point to just after the last object
 636   // iterated over.
 637 #define ContigSpace_OOP_SINCE_SAVE_MARKS_DECL(OopClosureType, nv_suffix)  \
 638   void oop_since_save_marks_iterate##nv_suffix(OopClosureType* blk);
 639 
 640   ALL_SINCE_SAVE_MARKS_CLOSURES(ContigSpace_OOP_SINCE_SAVE_MARKS_DECL)
 641 #undef ContigSpace_OOP_SINCE_SAVE_MARKS_DECL
 642 
 643   // Same as object_iterate, but starting from "mark", which is required
 644   // to denote the start of an object.  Objects allocated by
 645   // applications of the closure *are* included in the iteration.
 646   virtual void object_iterate_from(HeapWord* mark, ObjectClosure* blk);
 647 
 648   // Very inefficient implementation.
 649   virtual HeapWord* block_start_const(const void* p) const;
 650   size_t block_size(const HeapWord* p) const;
 651   // If a block is in the allocated area, it is an object.
 652   bool block_is_obj(const HeapWord* p) const { return p < top(); }
 653 
 654   // Addresses for inlined allocation
 655   HeapWord** top_addr() { return &_top; }
 656   HeapWord** end_addr() { return &_end; }
 657 
 658   // Overrides for more efficient compaction support.
 659   void prepare_for_compaction(CompactPoint* cp);
 660 
 661   virtual void print_on(outputStream* st) const;
 662 
 663   // Checked dynamic downcasts.
 664   virtual ContiguousSpace* toContiguousSpace() {
 665     return this;
 666   }
 667 
 668   // Debugging
 669   virtual void verify() const;
 670 
 671   // Used to increase collection frequency.  "factor" of 0 means entire
 672   // space.
 673   void allocate_temporary_filler(int factor);
 674 };
 675 
 676 
 677 // A dirty card to oop closure that does filtering.
 678 // It knows how to filter out objects that are outside of the _boundary.
 679 class Filtering_DCTOC : public DirtyCardToOopClosure {
 680 protected:
 681   // Override.
 682   void walk_mem_region(MemRegion mr,
 683                        HeapWord* bottom, HeapWord* top);
 684 
 685   // Walk the given memory region, from bottom to top, applying
 686   // the given oop closure to (possibly) all objects found. The
 687   // given oop closure may or may not be the same as the oop
 688   // closure with which this closure was created, as it may
 689   // be a filtering closure which makes use of the _boundary.
 690   // We offer two signatures, so the FilteringClosure static type is
 691   // apparent.
 692   virtual void walk_mem_region_with_cl(MemRegion mr,
 693                                        HeapWord* bottom, HeapWord* top,
 694                                        ExtendedOopClosure* cl) = 0;
 695   virtual void walk_mem_region_with_cl(MemRegion mr,
 696                                        HeapWord* bottom, HeapWord* top,
 697                                        FilteringClosure* cl) = 0;
 698 
 699 public:
 700   Filtering_DCTOC(Space* sp, ExtendedOopClosure* cl,
 701                   CardTableModRefBS::PrecisionStyle precision,
 702                   HeapWord* boundary) :
 703     DirtyCardToOopClosure(sp, cl, precision, boundary) {}
 704 };
 705 
 706 // A dirty card to oop closure for contiguous spaces
 707 // (ContiguousSpace and sub-classes).
 708 // It is a FilteringClosure, as defined above, and it knows:
 709 //
 710 // 1. That the actual top of any area in a memory region
 711 //    contained by the space is bounded by the end of the contiguous
 712 //    region of the space.
 713 // 2. That the space is really made up of objects and not just
 714 //    blocks.
 715 
 716 class ContiguousSpaceDCTOC : public Filtering_DCTOC {
 717 protected:
 718   // Overrides.
 719   HeapWord* get_actual_top(HeapWord* top, HeapWord* top_obj);
 720 
 721   virtual void walk_mem_region_with_cl(MemRegion mr,
 722                                        HeapWord* bottom, HeapWord* top,
 723                                        ExtendedOopClosure* cl);
 724   virtual void walk_mem_region_with_cl(MemRegion mr,
 725                                        HeapWord* bottom, HeapWord* top,
 726                                        FilteringClosure* cl);
 727 
 728 public:
 729   ContiguousSpaceDCTOC(ContiguousSpace* sp, ExtendedOopClosure* cl,
 730                        CardTableModRefBS::PrecisionStyle precision,
 731                        HeapWord* boundary) :
 732     Filtering_DCTOC(sp, cl, precision, boundary)
 733   {}
 734 };
 735 
 736 // A ContigSpace that Supports an efficient "block_start" operation via
 737 // a BlockOffsetArray (whose BlockOffsetSharedArray may be shared with
 738 // other spaces.)  This is the abstract base class for old generation
 739 // (tenured) spaces.
 740 
 741 class OffsetTableContigSpace: public ContiguousSpace {
 742   friend class VMStructs;
 743  protected:
 744   BlockOffsetArrayContigSpace _offsets;
 745   Mutex _par_alloc_lock;
 746 
 747  public:
 748   // Constructor
 749   OffsetTableContigSpace(BlockOffsetSharedArray* sharedOffsetArray,
 750                          MemRegion mr);
 751 
 752   void set_bottom(HeapWord* value);
 753   void set_end(HeapWord* value);
 754 
 755   void clear(bool mangle_space);
 756 
 757   inline HeapWord* block_start_const(const void* p) const;
 758 
 759   // Add offset table update.
 760   virtual inline HeapWord* allocate(size_t word_size);
 761   inline HeapWord* par_allocate(size_t word_size);
 762 
 763   // MarkSweep support phase3
 764   virtual HeapWord* initialize_threshold();
 765   virtual HeapWord* cross_threshold(HeapWord* start, HeapWord* end);
 766 
 767   virtual void print_on(outputStream* st) const;
 768 
 769   // Debugging
 770   void verify() const;
 771 };
 772 
 773 
 774 // Class TenuredSpace is used by TenuredGeneration
 775 
 776 class TenuredSpace: public OffsetTableContigSpace {
 777   friend class VMStructs;
 778  protected:
 779   // Mark sweep support
 780   size_t allowed_dead_ratio() const;
 781  public:
 782   // Constructor
 783   TenuredSpace(BlockOffsetSharedArray* sharedOffsetArray,
 784                MemRegion mr) :
 785     OffsetTableContigSpace(sharedOffsetArray, mr) {}
 786 };
 787 #endif // SHARE_VM_GC_SHARED_SPACE_HPP