1 /*
   2  * Copyright (c) 2001, 2018, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  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
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
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  23  */
  24 
  25 #ifndef SHARE_VM_GC_SHARED_COLLECTEDHEAP_HPP
  26 #define SHARE_VM_GC_SHARED_COLLECTEDHEAP_HPP
  27 
  28 #include "gc/shared/gcCause.hpp"
  29 #include "gc/shared/gcWhen.hpp"
  30 #include "memory/allocation.hpp"
  31 #include "runtime/handles.hpp"
  32 #include "runtime/perfData.hpp"
  33 #include "runtime/safepoint.hpp"
  34 #include "utilities/debug.hpp"
  35 #include "utilities/events.hpp"
  36 #include "utilities/formatBuffer.hpp"
  37 #include "utilities/growableArray.hpp"
  38 
  39 // A "CollectedHeap" is an implementation of a java heap for HotSpot.  This
  40 // is an abstract class: there may be many different kinds of heaps.  This
  41 // class defines the functions that a heap must implement, and contains
  42 // infrastructure common to all heaps.
  43 
  44 class AdaptiveSizePolicy;
  45 class BarrierSet;
  46 class CollectorPolicy;
  47 class GCHeapSummary;
  48 class GCTimer;
  49 class GCTracer;
  50 class GCMemoryManager;
  51 class MemoryPool;
  52 class MetaspaceSummary;
  53 class SoftRefPolicy;
  54 class Thread;
  55 class ThreadClosure;
  56 class VirtualSpaceSummary;
  57 class WorkGang;
  58 class nmethod;
  59 
  60 class GCMessage : public FormatBuffer<1024> {
  61  public:
  62   bool is_before;
  63 
  64  public:
  65   GCMessage() {}
  66 };
  67 
  68 class CollectedHeap;
  69 
  70 class GCHeapLog : public EventLogBase<GCMessage> {
  71  private:
  72   void log_heap(CollectedHeap* heap, bool before);
  73 
  74  public:
  75   GCHeapLog() : EventLogBase<GCMessage>("GC Heap History") {}
  76 
  77   void log_heap_before(CollectedHeap* heap) {
  78     log_heap(heap, true);
  79   }
  80   void log_heap_after(CollectedHeap* heap) {
  81     log_heap(heap, false);
  82   }
  83 };
  84 
  85 //
  86 // CollectedHeap
  87 //   GenCollectedHeap
  88 //     SerialHeap
  89 //     CMSHeap
  90 //   G1CollectedHeap
  91 //   ParallelScavengeHeap
  92 //
  93 class CollectedHeap : public CHeapObj<mtInternal> {
  94   friend class VMStructs;
  95   friend class JVMCIVMStructs;
  96   friend class IsGCActiveMark; // Block structured external access to _is_gc_active
  97 
  98  private:
  99 #ifdef ASSERT
 100   static int       _fire_out_of_memory_count;
 101 #endif
 102 
 103   GCHeapLog* _gc_heap_log;
 104 
 105   MemRegion _reserved;
 106 
 107  protected:
 108   bool _is_gc_active;
 109 
 110   // Used for filler objects (static, but initialized in ctor).
 111   static size_t _filler_array_max_size;
 112 
 113   unsigned int _total_collections;          // ... started
 114   unsigned int _total_full_collections;     // ... started
 115   NOT_PRODUCT(volatile size_t _promotion_failure_alot_count;)
 116   NOT_PRODUCT(volatile size_t _promotion_failure_alot_gc_number;)
 117 
 118   // Reason for current garbage collection.  Should be set to
 119   // a value reflecting no collection between collections.
 120   GCCause::Cause _gc_cause;
 121   GCCause::Cause _gc_lastcause;
 122   PerfStringVariable* _perf_gc_cause;
 123   PerfStringVariable* _perf_gc_lastcause;
 124 
 125   // Constructor
 126   CollectedHeap();
 127 
 128   // Create a new tlab. All TLAB allocations must go through this.
 129   // To allow more flexible TLAB allocations min_size specifies
 130   // the minimum size needed, while requested_size is the requested
 131   // size based on ergonomics. The actually allocated size will be
 132   // returned in actual_size.
 133   virtual HeapWord* allocate_new_tlab(size_t min_size,
 134                                       size_t requested_size,
 135                                       size_t* actual_size);
 136 
 137   // Accumulate statistics on all tlabs.
 138   virtual void accumulate_statistics_all_tlabs();
 139 
 140   // Reinitialize tlabs before resuming mutators.
 141   virtual void resize_all_tlabs();
 142 
 143   // Allocate from the current thread's TLAB, with broken-out slow path.
 144   inline static HeapWord* allocate_from_tlab(Klass* klass, Thread* thread, size_t size);
 145   static HeapWord* allocate_from_tlab_slow(Klass* klass, Thread* thread, size_t size);
 146 
 147   // Allocate an uninitialized block of the given size, or returns NULL if
 148   // this is impossible.
 149   inline static HeapWord* common_mem_allocate_noinit(Klass* klass, size_t size, TRAPS);
 150 
 151   // Like allocate_init, but the block returned by a successful allocation
 152   // is guaranteed initialized to zeros.
 153   inline static HeapWord* common_mem_allocate_init(Klass* klass, size_t size, TRAPS);
 154 
 155   // Helper functions for (VM) allocation.
 156   inline static void post_allocation_setup_common(Klass* klass, HeapWord* obj);
 157   inline static void post_allocation_setup_no_klass_install(Klass* klass,
 158                                                             HeapWord* objPtr);
 159 
 160   inline static void post_allocation_setup_obj(Klass* klass, HeapWord* obj, int size);
 161 
 162   inline static void post_allocation_setup_array(Klass* klass,
 163                                                  HeapWord* obj, int length);
 164 
 165   inline static void post_allocation_setup_class(Klass* klass, HeapWord* obj, int size);
 166 
 167   // Clears an allocated object.
 168   inline static void init_obj(HeapWord* obj, size_t size);
 169 
 170   // Filler object utilities.
 171   static inline size_t filler_array_hdr_size();
 172   static inline size_t filler_array_min_size();
 173 
 174   DEBUG_ONLY(static void fill_args_check(HeapWord* start, size_t words);)
 175   DEBUG_ONLY(static void zap_filler_array(HeapWord* start, size_t words, bool zap = true);)
 176 
 177   // Fill with a single array; caller must ensure filler_array_min_size() <=
 178   // words <= filler_array_max_size().
 179   static inline void fill_with_array(HeapWord* start, size_t words, bool zap = true);
 180 
 181   // Fill with a single object (either an int array or a java.lang.Object).
 182   static inline void fill_with_object_impl(HeapWord* start, size_t words, bool zap = true);
 183 
 184   virtual void trace_heap(GCWhen::Type when, const GCTracer* tracer);
 185 
 186   // Internal allocation methods.
 187   inline static HeapWord* common_allocate_memory(Klass* klass, int size,
 188                                                  void (*post_setup)(Klass*, HeapWord*, int),
 189                                                  int size_for_post, bool init_memory,
 190                                                  TRAPS);
 191 
 192   // Internal allocation method for common obj/class/array allocations.
 193   inline static HeapWord* allocate_memory(Klass* klass, int size,
 194                                           void (*post_setup)(Klass*, HeapWord*, int),
 195                                           int size_for_post, bool init_memory,
 196                                           TRAPS);
 197 
 198   // Verification functions
 199   virtual void check_for_bad_heap_word_value(HeapWord* addr, size_t size)
 200     PRODUCT_RETURN;
 201   virtual void check_for_non_bad_heap_word_value(HeapWord* addr, size_t size)
 202     PRODUCT_RETURN;
 203   debug_only(static void check_for_valid_allocation_state();)
 204 
 205  public:
 206   enum Name {
 207     None,
 208     Serial,
 209     Parallel,
 210     CMS,
 211     G1
 212   };
 213 
 214   static inline size_t filler_array_max_size() {
 215     return _filler_array_max_size;
 216   }
 217 
 218   virtual Name kind() const = 0;
 219 
 220   virtual const char* name() const = 0;
 221 
 222   /**
 223    * Returns JNI error code JNI_ENOMEM if memory could not be allocated,
 224    * and JNI_OK on success.
 225    */
 226   virtual jint initialize() = 0;
 227 
 228   // In many heaps, there will be a need to perform some initialization activities
 229   // after the Universe is fully formed, but before general heap allocation is allowed.
 230   // This is the correct place to place such initialization methods.
 231   virtual void post_initialize();
 232 
 233   // Stop any onging concurrent work and prepare for exit.
 234   virtual void stop() {}
 235 
 236   // Stop and resume concurrent GC threads interfering with safepoint operations
 237   virtual void safepoint_synchronize_begin() {}
 238   virtual void safepoint_synchronize_end() {}
 239 
 240   void initialize_reserved_region(HeapWord *start, HeapWord *end);
 241   MemRegion reserved_region() const { return _reserved; }
 242   address base() const { return (address)reserved_region().start(); }
 243 
 244   virtual size_t capacity() const = 0;
 245   virtual size_t used() const = 0;
 246 
 247   // Return "true" if the part of the heap that allocates Java
 248   // objects has reached the maximal committed limit that it can
 249   // reach, without a garbage collection.
 250   virtual bool is_maximal_no_gc() const = 0;
 251 
 252   // Support for java.lang.Runtime.maxMemory():  return the maximum amount of
 253   // memory that the vm could make available for storing 'normal' java objects.
 254   // This is based on the reserved address space, but should not include space
 255   // that the vm uses internally for bookkeeping or temporary storage
 256   // (e.g., in the case of the young gen, one of the survivor
 257   // spaces).
 258   virtual size_t max_capacity() const = 0;
 259 
 260   // Returns "TRUE" if "p" points into the reserved area of the heap.
 261   bool is_in_reserved(const void* p) const {
 262     return _reserved.contains(p);
 263   }
 264 
 265   bool is_in_reserved_or_null(const void* p) const {
 266     return p == NULL || is_in_reserved(p);
 267   }
 268 
 269   // Returns "TRUE" iff "p" points into the committed areas of the heap.
 270   // This method can be expensive so avoid using it in performance critical
 271   // code.
 272   virtual bool is_in(const void* p) const = 0;
 273 
 274   DEBUG_ONLY(bool is_in_or_null(const void* p) const { return p == NULL || is_in(p); })
 275 
 276   // Let's define some terms: a "closed" subset of a heap is one that
 277   //
 278   // 1) contains all currently-allocated objects, and
 279   //
 280   // 2) is closed under reference: no object in the closed subset
 281   //    references one outside the closed subset.
 282   //
 283   // Membership in a heap's closed subset is useful for assertions.
 284   // Clearly, the entire heap is a closed subset, so the default
 285   // implementation is to use "is_in_reserved".  But this may not be too
 286   // liberal to perform useful checking.  Also, the "is_in" predicate
 287   // defines a closed subset, but may be too expensive, since "is_in"
 288   // verifies that its argument points to an object head.  The
 289   // "closed_subset" method allows a heap to define an intermediate
 290   // predicate, allowing more precise checking than "is_in_reserved" at
 291   // lower cost than "is_in."
 292 
 293   // One important case is a heap composed of disjoint contiguous spaces,
 294   // such as the Garbage-First collector.  Such heaps have a convenient
 295   // closed subset consisting of the allocated portions of those
 296   // contiguous spaces.
 297 
 298   // Return "TRUE" iff the given pointer points into the heap's defined
 299   // closed subset (which defaults to the entire heap).
 300   virtual bool is_in_closed_subset(const void* p) const {
 301     return is_in_reserved(p);
 302   }
 303 
 304   bool is_in_closed_subset_or_null(const void* p) const {
 305     return p == NULL || is_in_closed_subset(p);
 306   }
 307 
 308   void set_gc_cause(GCCause::Cause v) {
 309      if (UsePerfData) {
 310        _gc_lastcause = _gc_cause;
 311        _perf_gc_lastcause->set_value(GCCause::to_string(_gc_lastcause));
 312        _perf_gc_cause->set_value(GCCause::to_string(v));
 313      }
 314     _gc_cause = v;
 315   }
 316   GCCause::Cause gc_cause() { return _gc_cause; }
 317 
 318   // General obj/array allocation facilities.
 319   inline static oop obj_allocate(Klass* klass, int size, TRAPS);
 320   inline static oop array_allocate(Klass* klass, int size, int length, TRAPS);
 321   inline static oop array_allocate_nozero(Klass* klass, int size, int length, TRAPS);
 322   inline static oop class_allocate(Klass* klass, int size, TRAPS);
 323 
 324   // Raw memory allocation facilities
 325   // The obj and array allocate methods are covers for these methods.
 326   // mem_allocate() should never be
 327   // called to allocate TLABs, only individual objects.
 328   virtual HeapWord* mem_allocate(size_t size,
 329                                  bool* gc_overhead_limit_was_exceeded) = 0;
 330 
 331   // Utilities for turning raw memory into filler objects.
 332   //
 333   // min_fill_size() is the smallest region that can be filled.
 334   // fill_with_objects() can fill arbitrary-sized regions of the heap using
 335   // multiple objects.  fill_with_object() is for regions known to be smaller
 336   // than the largest array of integers; it uses a single object to fill the
 337   // region and has slightly less overhead.
 338   static size_t min_fill_size() {
 339     return size_t(align_object_size(oopDesc::header_size()));
 340   }
 341 
 342   static void fill_with_objects(HeapWord* start, size_t words, bool zap = true);
 343 
 344   static void fill_with_object(HeapWord* start, size_t words, bool zap = true);
 345   static void fill_with_object(MemRegion region, bool zap = true) {
 346     fill_with_object(region.start(), region.word_size(), zap);
 347   }
 348   static void fill_with_object(HeapWord* start, HeapWord* end, bool zap = true) {
 349     fill_with_object(start, pointer_delta(end, start), zap);
 350   }
 351 
 352   // Return the address "addr" aligned by "alignment_in_bytes" if such
 353   // an address is below "end".  Return NULL otherwise.
 354   inline static HeapWord* align_allocation_or_fail(HeapWord* addr,
 355                                                    HeapWord* end,
 356                                                    unsigned short alignment_in_bytes);
 357 
 358   // Some heaps may offer a contiguous region for shared non-blocking
 359   // allocation, via inlined code (by exporting the address of the top and
 360   // end fields defining the extent of the contiguous allocation region.)
 361 
 362   // This function returns "true" iff the heap supports this kind of
 363   // allocation.  (Default is "no".)
 364   virtual bool supports_inline_contig_alloc() const {
 365     return false;
 366   }
 367   // These functions return the addresses of the fields that define the
 368   // boundaries of the contiguous allocation area.  (These fields should be
 369   // physically near to one another.)
 370   virtual HeapWord* volatile* top_addr() const {
 371     guarantee(false, "inline contiguous allocation not supported");
 372     return NULL;
 373   }
 374   virtual HeapWord** end_addr() const {
 375     guarantee(false, "inline contiguous allocation not supported");
 376     return NULL;
 377   }
 378 
 379   // Some heaps may be in an unparseable state at certain times between
 380   // collections. This may be necessary for efficient implementation of
 381   // certain allocation-related activities. Calling this function before
 382   // attempting to parse a heap ensures that the heap is in a parsable
 383   // state (provided other concurrent activity does not introduce
 384   // unparsability). It is normally expected, therefore, that this
 385   // method is invoked with the world stopped.
 386   // NOTE: if you override this method, make sure you call
 387   // super::ensure_parsability so that the non-generational
 388   // part of the work gets done. See implementation of
 389   // CollectedHeap::ensure_parsability and, for instance,
 390   // that of GenCollectedHeap::ensure_parsability().
 391   // The argument "retire_tlabs" controls whether existing TLABs
 392   // are merely filled or also retired, thus preventing further
 393   // allocation from them and necessitating allocation of new TLABs.
 394   virtual void ensure_parsability(bool retire_tlabs);
 395 
 396   // Section on thread-local allocation buffers (TLABs)
 397   // If the heap supports thread-local allocation buffers, it should override
 398   // the following methods:
 399   // Returns "true" iff the heap supports thread-local allocation buffers.
 400   // The default is "no".
 401   virtual bool supports_tlab_allocation() const = 0;
 402 
 403   // The amount of space available for thread-local allocation buffers.
 404   virtual size_t tlab_capacity(Thread *thr) const = 0;
 405 
 406   // The amount of used space for thread-local allocation buffers for the given thread.
 407   virtual size_t tlab_used(Thread *thr) const = 0;
 408 
 409   virtual size_t max_tlab_size() const;
 410 
 411   // An estimate of the maximum allocation that could be performed
 412   // for thread-local allocation buffers without triggering any
 413   // collection or expansion activity.
 414   virtual size_t unsafe_max_tlab_alloc(Thread *thr) const {
 415     guarantee(false, "thread-local allocation buffers not supported");
 416     return 0;
 417   }
 418 
 419   // Perform a collection of the heap; intended for use in implementing
 420   // "System.gc".  This probably implies as full a collection as the
 421   // "CollectedHeap" supports.
 422   virtual void collect(GCCause::Cause cause) = 0;
 423 
 424   // Perform a full collection
 425   virtual void do_full_collection(bool clear_all_soft_refs) = 0;
 426 
 427   // This interface assumes that it's being called by the
 428   // vm thread. It collects the heap assuming that the
 429   // heap lock is already held and that we are executing in
 430   // the context of the vm thread.
 431   virtual void collect_as_vm_thread(GCCause::Cause cause);
 432 
 433   virtual MetaWord* satisfy_failed_metadata_allocation(ClassLoaderData* loader_data,
 434                                                        size_t size,
 435                                                        Metaspace::MetadataType mdtype);
 436 
 437   // Returns "true" iff there is a stop-world GC in progress.  (I assume
 438   // that it should answer "false" for the concurrent part of a concurrent
 439   // collector -- dld).
 440   bool is_gc_active() const { return _is_gc_active; }
 441 
 442   // Total number of GC collections (started)
 443   unsigned int total_collections() const { return _total_collections; }
 444   unsigned int total_full_collections() const { return _total_full_collections;}
 445 
 446   // Increment total number of GC collections (started)
 447   // Should be protected but used by PSMarkSweep - cleanup for 1.4.2
 448   void increment_total_collections(bool full = false) {
 449     _total_collections++;
 450     if (full) {
 451       increment_total_full_collections();
 452     }
 453   }
 454 
 455   void increment_total_full_collections() { _total_full_collections++; }
 456 
 457   // Return the CollectorPolicy for the heap
 458   virtual CollectorPolicy* collector_policy() const = 0;
 459 
 460   // Return the SoftRefPolicy for the heap;
 461   virtual SoftRefPolicy* soft_ref_policy() = 0;
 462 
 463   virtual GrowableArray<GCMemoryManager*> memory_managers() = 0;
 464   virtual GrowableArray<MemoryPool*> memory_pools() = 0;
 465 
 466   // Iterate over all objects, calling "cl.do_object" on each.
 467   virtual void object_iterate(ObjectClosure* cl) = 0;
 468 
 469   // Similar to object_iterate() except iterates only
 470   // over live objects.
 471   virtual void safe_object_iterate(ObjectClosure* cl) = 0;
 472 
 473   // NOTE! There is no requirement that a collector implement these
 474   // functions.
 475   //
 476   // A CollectedHeap is divided into a dense sequence of "blocks"; that is,
 477   // each address in the (reserved) heap is a member of exactly
 478   // one block.  The defining characteristic of a block is that it is
 479   // possible to find its size, and thus to progress forward to the next
 480   // block.  (Blocks may be of different sizes.)  Thus, blocks may
 481   // represent Java objects, or they might be free blocks in a
 482   // free-list-based heap (or subheap), as long as the two kinds are
 483   // distinguishable and the size of each is determinable.
 484 
 485   // Returns the address of the start of the "block" that contains the
 486   // address "addr".  We say "blocks" instead of "object" since some heaps
 487   // may not pack objects densely; a chunk may either be an object or a
 488   // non-object.
 489   virtual HeapWord* block_start(const void* addr) const = 0;
 490 
 491   // Requires "addr" to be the start of a chunk, and returns its size.
 492   // "addr + size" is required to be the start of a new chunk, or the end
 493   // of the active area of the heap.
 494   virtual size_t block_size(const HeapWord* addr) const = 0;
 495 
 496   // Requires "addr" to be the start of a block, and returns "TRUE" iff
 497   // the block is an object.
 498   virtual bool block_is_obj(const HeapWord* addr) const = 0;
 499 
 500   // Returns the longest time (in ms) that has elapsed since the last
 501   // time that any part of the heap was examined by a garbage collection.
 502   virtual jlong millis_since_last_gc() = 0;
 503 
 504   // Perform any cleanup actions necessary before allowing a verification.
 505   virtual void prepare_for_verify() = 0;
 506 
 507   // Generate any dumps preceding or following a full gc
 508  private:
 509   void full_gc_dump(GCTimer* timer, bool before);
 510 
 511   virtual void initialize_serviceability() = 0;
 512 
 513  public:
 514   void pre_full_gc_dump(GCTimer* timer);
 515   void post_full_gc_dump(GCTimer* timer);
 516 
 517   virtual VirtualSpaceSummary create_heap_space_summary();
 518   GCHeapSummary create_heap_summary();
 519 
 520   MetaspaceSummary create_metaspace_summary();
 521 
 522   // Print heap information on the given outputStream.
 523   virtual void print_on(outputStream* st) const = 0;
 524   // The default behavior is to call print_on() on tty.
 525   virtual void print() const {
 526     print_on(tty);
 527   }
 528   // Print more detailed heap information on the given
 529   // outputStream. The default behavior is to call print_on(). It is
 530   // up to each subclass to override it and add any additional output
 531   // it needs.
 532   virtual void print_extended_on(outputStream* st) const {
 533     print_on(st);
 534   }
 535 
 536   virtual void print_on_error(outputStream* st) const;
 537 
 538   // Print all GC threads (other than the VM thread)
 539   // used by this heap.
 540   virtual void print_gc_threads_on(outputStream* st) const = 0;
 541   // The default behavior is to call print_gc_threads_on() on tty.
 542   void print_gc_threads() {
 543     print_gc_threads_on(tty);
 544   }
 545   // Iterator for all GC threads (other than VM thread)
 546   virtual void gc_threads_do(ThreadClosure* tc) const = 0;
 547 
 548   // Print any relevant tracing info that flags imply.
 549   // Default implementation does nothing.
 550   virtual void print_tracing_info() const = 0;
 551 
 552   void print_heap_before_gc();
 553   void print_heap_after_gc();
 554 
 555   // An object is scavengable if its location may move during a scavenge.
 556   // (A scavenge is a GC which is not a full GC.)
 557   virtual bool is_scavengable(oop obj) = 0;
 558   // Registering and unregistering an nmethod (compiled code) with the heap.
 559   // Override with specific mechanism for each specialized heap type.
 560   virtual void register_nmethod(nmethod* nm) {}
 561   virtual void unregister_nmethod(nmethod* nm) {}
 562   virtual void verify_nmethod(nmethod* nmethod) {}
 563 
 564   void trace_heap_before_gc(const GCTracer* gc_tracer);
 565   void trace_heap_after_gc(const GCTracer* gc_tracer);
 566 
 567   // Heap verification
 568   virtual void verify(VerifyOption option) = 0;
 569 
 570   // Return true if concurrent phase control (via
 571   // request_concurrent_phase_control) is supported by this collector.
 572   // The default implementation returns false.
 573   virtual bool supports_concurrent_phase_control() const;
 574 
 575   // Return a NULL terminated array of concurrent phase names provided
 576   // by this collector.  Supports Whitebox testing.  These are the
 577   // names recognized by request_concurrent_phase(). The default
 578   // implementation returns an array of one NULL element.
 579   virtual const char* const* concurrent_phases() const;
 580 
 581   // Request the collector enter the indicated concurrent phase, and
 582   // wait until it does so.  Supports WhiteBox testing.  Only one
 583   // request may be active at a time.  Phases are designated by name;
 584   // the set of names and their meaning is GC-specific.  Once the
 585   // requested phase has been reached, the collector will attempt to
 586   // avoid transitioning to a new phase until a new request is made.
 587   // [Note: A collector might not be able to remain in a given phase.
 588   // For example, a full collection might cancel an in-progress
 589   // concurrent collection.]
 590   //
 591   // Returns true when the phase is reached.  Returns false for an
 592   // unknown phase.  The default implementation returns false.
 593   virtual bool request_concurrent_phase(const char* phase);
 594 
 595   // Provides a thread pool to SafepointSynchronize to use
 596   // for parallel safepoint cleanup.
 597   // GCs that use a GC worker thread pool may want to share
 598   // it for use during safepoint cleanup. This is only possible
 599   // if the GC can pause and resume concurrent work (e.g. G1
 600   // concurrent marking) for an intermittent non-GC safepoint.
 601   // If this method returns NULL, SafepointSynchronize will
 602   // perform cleanup tasks serially in the VMThread.
 603   virtual WorkGang* get_safepoint_workers() { return NULL; }
 604 
 605   // Support for object pinning. This is used by JNI Get*Critical()
 606   // and Release*Critical() family of functions. If supported, the GC
 607   // must guarantee that pinned objects never move.
 608   virtual bool supports_object_pinning() const;
 609   virtual oop pin_object(JavaThread* thread, oop obj);
 610   virtual void unpin_object(JavaThread* thread, oop obj);
 611 
 612   // Deduplicate the string, iff the GC supports string deduplication.
 613   virtual void deduplicate_string(oop str);
 614 
 615   virtual bool is_oop(oop object) const;
 616 
 617   // Non product verification and debugging.
 618 #ifndef PRODUCT
 619   // Support for PromotionFailureALot.  Return true if it's time to cause a
 620   // promotion failure.  The no-argument version uses
 621   // this->_promotion_failure_alot_count as the counter.
 622   bool promotion_should_fail(volatile size_t* count);
 623   bool promotion_should_fail();
 624 
 625   // Reset the PromotionFailureALot counters.  Should be called at the end of a
 626   // GC in which promotion failure occurred.
 627   void reset_promotion_should_fail(volatile size_t* count);
 628   void reset_promotion_should_fail();
 629 #endif  // #ifndef PRODUCT
 630 
 631 #ifdef ASSERT
 632   static int fired_fake_oom() {
 633     return (CIFireOOMAt > 1 && _fire_out_of_memory_count >= CIFireOOMAt);
 634   }
 635 #endif
 636 };
 637 
 638 // Class to set and reset the GC cause for a CollectedHeap.
 639 
 640 class GCCauseSetter : StackObj {
 641   CollectedHeap* _heap;
 642   GCCause::Cause _previous_cause;
 643  public:
 644   GCCauseSetter(CollectedHeap* heap, GCCause::Cause cause) {
 645     _heap = heap;
 646     _previous_cause = _heap->gc_cause();
 647     _heap->set_gc_cause(cause);
 648   }
 649 
 650   ~GCCauseSetter() {
 651     _heap->set_gc_cause(_previous_cause);
 652   }
 653 };
 654 
 655 #endif // SHARE_VM_GC_SHARED_COLLECTEDHEAP_HPP