Mercurial > jdk8-mips64-public > hotspot / file revision

 /*

  * Copyright (c) 2001, 2010, Oracle and/or its affiliates. All rights reserved.

  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

  *

  * This code is free software; you can redistribute it and/or modify it

  * under the terms of the GNU General Public License version 2 only, as

  * published by the Free Software Foundation.

  *

  * This code is distributed in the hope that it will be useful, but WITHOUT

  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

  * version 2 for more details (a copy is included in the LICENSE file that

  * accompanied this code).

  *

  * You should have received a copy of the GNU General Public License version

  * 2 along with this work; if not, write to the Free Software Foundation,

  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

  *

  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

  * or visit www.oracle.com if you need additional information or have any

  * questions.

  *

  */

 // A "CollectedHeap" is an implementation of a java heap for HotSpot.  This

 // is an abstract class: there may be many different kinds of heaps.  This

 // class defines the functions that a heap must implement, and contains

 // infrastructure common to all heaps.

 class BarrierSet;

 class ThreadClosure;

 class AdaptiveSizePolicy;

 class Thread;

 class CollectorPolicy;

 //

 // CollectedHeap

 //   SharedHeap

 //     GenCollectedHeap

 //     G1CollectedHeap

 //   ParallelScavengeHeap

 //

 class CollectedHeap : public CHeapObj {

   friend class VMStructs;

   friend class IsGCActiveMark; // Block structured external access to _is_gc_active

   friend class constantPoolCacheKlass; // allocate() method inserts is_conc_safe

 #ifdef ASSERT

   static int       _fire_out_of_memory_count;

 #endif

   // Used for filler objects (static, but initialized in ctor).

   static size_t _filler_array_max_size;

   // Used in support of ReduceInitialCardMarks; only consulted if COMPILER2 is being used

   bool _defer_initial_card_mark;

  protected:

   MemRegion _reserved;

   BarrierSet* _barrier_set;

   bool _is_gc_active;

   int _n_par_threads;

   unsigned int _total_collections;          // ... started

   unsigned int _total_full_collections;     // ... started

   NOT_PRODUCT(volatile size_t _promotion_failure_alot_count;)

   NOT_PRODUCT(volatile size_t _promotion_failure_alot_gc_number;)

   // Reason for current garbage collection.  Should be set to

   // a value reflecting no collection between collections.

   GCCause::Cause _gc_cause;

   GCCause::Cause _gc_lastcause;

   PerfStringVariable* _perf_gc_cause;

   PerfStringVariable* _perf_gc_lastcause;

   // Constructor

   CollectedHeap();

   // Do common initializations that must follow instance construction,

   // for example, those needing virtual calls.

   // This code could perhaps be moved into initialize() but would

   // be slightly more awkward because we want the latter to be a

   // pure virtual.

   void pre_initialize();

   // Create a new tlab

   virtual HeapWord* allocate_new_tlab(size_t size);

   // Accumulate statistics on all tlabs.

   virtual void accumulate_statistics_all_tlabs();

   // Reinitialize tlabs before resuming mutators.

   virtual void resize_all_tlabs();

  protected:

   // Allocate from the current thread's TLAB, with broken-out slow path.

   inline static HeapWord* allocate_from_tlab(Thread* thread, size_t size);

   static HeapWord* allocate_from_tlab_slow(Thread* thread, size_t size);

   // Allocate an uninitialized block of the given size, or returns NULL if

   // this is impossible.

   inline static HeapWord* common_mem_allocate_noinit(size_t size, bool is_noref, TRAPS);

   // Like allocate_init, but the block returned by a successful allocation

   // is guaranteed initialized to zeros.

   inline static HeapWord* common_mem_allocate_init(size_t size, bool is_noref, TRAPS);

   // Same as common_mem version, except memory is allocated in the permanent area

   // If there is no permanent area, revert to common_mem_allocate_noinit

   inline static HeapWord* common_permanent_mem_allocate_noinit(size_t size, TRAPS);

   // Same as common_mem version, except memory is allocated in the permanent area

   // If there is no permanent area, revert to common_mem_allocate_init

   inline static HeapWord* common_permanent_mem_allocate_init(size_t size, TRAPS);

   // Helper functions for (VM) allocation.

   inline static void post_allocation_setup_common(KlassHandle klass,

                                                   HeapWord* obj, size_t size);

   inline static void post_allocation_setup_no_klass_install(KlassHandle klass,

                                                             HeapWord* objPtr,

                                                             size_t size);

   inline static void post_allocation_setup_obj(KlassHandle klass,

                                                HeapWord* obj, size_t size);

   inline static void post_allocation_setup_array(KlassHandle klass,

                                                  HeapWord* obj, size_t size,

                                                  int length);

   // Clears an allocated object.

   inline static void init_obj(HeapWord* obj, size_t size);

   // Filler object utilities.

   static inline size_t filler_array_hdr_size();

   static inline size_t filler_array_min_size();

   static inline size_t filler_array_max_size();

   DEBUG_ONLY(static void fill_args_check(HeapWord* start, size_t words);)

   DEBUG_ONLY(static void zap_filler_array(HeapWord* start, size_t words, bool zap = true);)

   // Fill with a single array; caller must ensure filler_array_min_size() <=

   // words <= filler_array_max_size().

   static inline void fill_with_array(HeapWord* start, size_t words, bool zap = true);

   // Fill with a single object (either an int array or a java.lang.Object).

   static inline void fill_with_object_impl(HeapWord* start, size_t words, bool zap = true);

   // Verification functions

   virtual void check_for_bad_heap_word_value(HeapWord* addr, size_t size)

     PRODUCT_RETURN;

   virtual void check_for_non_bad_heap_word_value(HeapWord* addr, size_t size)

     PRODUCT_RETURN;

   debug_only(static void check_for_valid_allocation_state();)

  public:

   enum Name {

     Abstract,

     SharedHeap,

     GenCollectedHeap,

     ParallelScavengeHeap,

     G1CollectedHeap

   };

   virtual CollectedHeap::Name kind() const { return CollectedHeap::Abstract; }

   /**

    * Returns JNI error code JNI_ENOMEM if memory could not be allocated,

    * and JNI_OK on success.

    */

   virtual jint initialize() = 0;

   // In many heaps, there will be a need to perform some initialization activities

   // after the Universe is fully formed, but before general heap allocation is allowed.

   // This is the correct place to place such initialization methods.

   virtual void post_initialize() = 0;

   MemRegion reserved_region() const { return _reserved; }

   address base() const { return (address)reserved_region().start(); }

   // Future cleanup here. The following functions should specify bytes or

   // heapwords as part of their signature.

   virtual size_t capacity() const = 0;

   virtual size_t used() const = 0;

   // Return "true" if the part of the heap that allocates Java

   // objects has reached the maximal committed limit that it can

   // reach, without a garbage collection.

   virtual bool is_maximal_no_gc() const = 0;

   virtual size_t permanent_capacity() const = 0;

   virtual size_t permanent_used() const = 0;

   // Support for java.lang.Runtime.maxMemory():  return the maximum amount of

   // memory that the vm could make available for storing 'normal' java objects.

   // This is based on the reserved address space, but should not include space

   // that the vm uses internally for bookkeeping or temporary storage (e.g.,

   // perm gen space or, in the case of the young gen, one of the survivor

   // spaces).

   virtual size_t max_capacity() const = 0;

   // Returns "TRUE" if "p" points into the reserved area of the heap.

   bool is_in_reserved(const void* p) const {

     return _reserved.contains(p);

   }

   bool is_in_reserved_or_null(const void* p) const {

     return p == NULL || is_in_reserved(p);

   }

   // Returns "TRUE" if "p" points to the head of an allocated object in the

   // heap. Since this method can be expensive in general, we restrict its

   // use to assertion checking only.

   virtual bool is_in(const void* p) const = 0;

   bool is_in_or_null(const void* p) const {

     return p == NULL || is_in(p);

   }

   // Let's define some terms: a "closed" subset of a heap is one that

   //

   // 1) contains all currently-allocated objects, and

   //

   // 2) is closed under reference: no object in the closed subset

   //    references one outside the closed subset.

   //

   // Membership in a heap's closed subset is useful for assertions.

   // Clearly, the entire heap is a closed subset, so the default

   // implementation is to use "is_in_reserved".  But this may not be too

   // liberal to perform useful checking.  Also, the "is_in" predicate

   // defines a closed subset, but may be too expensive, since "is_in"

   // verifies that its argument points to an object head.  The

   // "closed_subset" method allows a heap to define an intermediate

   // predicate, allowing more precise checking than "is_in_reserved" at

   // lower cost than "is_in."

   // One important case is a heap composed of disjoint contiguous spaces,

   // such as the Garbage-First collector.  Such heaps have a convenient

   // closed subset consisting of the allocated portions of those

   // contiguous spaces.

   // Return "TRUE" iff the given pointer points into the heap's defined

   // closed subset (which defaults to the entire heap).

   virtual bool is_in_closed_subset(const void* p) const {

     return is_in_reserved(p);

   }

   bool is_in_closed_subset_or_null(const void* p) const {

     return p == NULL || is_in_closed_subset(p);

   }

   // XXX is_permanent() and is_in_permanent() should be better named

   // to distinguish one from the other.

   // Returns "TRUE" if "p" is allocated as "permanent" data.

   // If the heap does not use "permanent" data, returns the same

   // value is_in_reserved() would return.

   // NOTE: this actually returns true if "p" is in reserved space

   // for the space not that it is actually allocated (i.e. in committed

   // space). If you need the more conservative answer use is_permanent().

   virtual bool is_in_permanent(const void *p) const = 0;

   bool is_in_permanent_or_null(const void *p) const {

     return p == NULL || is_in_permanent(p);

   }

   // Returns "TRUE" if "p" is in the committed area of  "permanent" data.

   // If the heap does not use "permanent" data, returns the same

   // value is_in() would return.

   virtual bool is_permanent(const void *p) const = 0;

   bool is_permanent_or_null(const void *p) const {

     return p == NULL || is_permanent(p);

   }

   // An object is scavengable if its location may move during a scavenge.

   // (A scavenge is a GC which is not a full GC.)

   // Currently, this just means it is not perm (and not null).

   // This could change if we rethink what's in perm-gen.

   bool is_scavengable(const void *p) const {

     return !is_in_permanent_or_null(p);

   }

   // Returns "TRUE" if "p" is a method oop in the

   // current heap, with high probability. This predicate

   // is not stable, in general.

   bool is_valid_method(oop p) const;

   void set_gc_cause(GCCause::Cause v) {

      if (UsePerfData) {

        _gc_lastcause = _gc_cause;

        _perf_gc_lastcause->set_value(GCCause::to_string(_gc_lastcause));

        _perf_gc_cause->set_value(GCCause::to_string(v));

      }

     _gc_cause = v;

   }

   GCCause::Cause gc_cause() { return _gc_cause; }

   // Number of threads currently working on GC tasks.

   int n_par_threads() { return _n_par_threads; }

   // May be overridden to set additional parallelism.

   virtual void set_par_threads(int t) { _n_par_threads = t; };

   // Preload classes into the shared portion of the heap, and then dump

   // that data to a file so that it can be loaded directly by another

   // VM (then terminate).

   virtual void preload_and_dump(TRAPS) { ShouldNotReachHere(); }

   // General obj/array allocation facilities.

   inline static oop obj_allocate(KlassHandle klass, int size, TRAPS);

   inline static oop array_allocate(KlassHandle klass, int size, int length, TRAPS);

   inline static oop large_typearray_allocate(KlassHandle klass, int size, int length, TRAPS);

   // Special obj/array allocation facilities.

   // Some heaps may want to manage "permanent" data uniquely. These default

   // to the general routines if the heap does not support such handling.

   inline static oop permanent_obj_allocate(KlassHandle klass, int size, TRAPS);

   // permanent_obj_allocate_no_klass_install() does not do the installation of

   // the klass pointer in the newly created object (as permanent_obj_allocate()

   // above does).  This allows for a delay in the installation of the klass

   // pointer that is needed during the create of klassKlass's.  The

   // method post_allocation_install_obj_klass() is used to install the

   // klass pointer.

   inline static oop permanent_obj_allocate_no_klass_install(KlassHandle klass,

                                                             int size,

                                                             TRAPS);

   inline static void post_allocation_install_obj_klass(KlassHandle klass,

                                                        oop obj,

                                                        int size);

   inline static oop permanent_array_allocate(KlassHandle klass, int size, int length, TRAPS);

   // Raw memory allocation facilities

   // The obj and array allocate methods are covers for these methods.

   // The permanent allocation method should default to mem_allocate if

   // permanent memory isn't supported.

   virtual HeapWord* mem_allocate(size_t size,

                                  bool is_noref,

                                  bool is_tlab,

                                  bool* gc_overhead_limit_was_exceeded) = 0;

   virtual HeapWord* permanent_mem_allocate(size_t size) = 0;

   // The boundary between a "large" and "small" array of primitives, in words.

   virtual size_t large_typearray_limit() = 0;

   // Utilities for turning raw memory into filler objects.

   //

   // min_fill_size() is the smallest region that can be filled.

   // fill_with_objects() can fill arbitrary-sized regions of the heap using

   // multiple objects.  fill_with_object() is for regions known to be smaller

   // than the largest array of integers; it uses a single object to fill the

   // region and has slightly less overhead.

   static size_t min_fill_size() {

     return size_t(align_object_size(oopDesc::header_size()));

   }

   static void fill_with_objects(HeapWord* start, size_t words, bool zap = true);

   static void fill_with_object(HeapWord* start, size_t words, bool zap = true);

   static void fill_with_object(MemRegion region, bool zap = true) {

     fill_with_object(region.start(), region.word_size(), zap);

   }

   static void fill_with_object(HeapWord* start, HeapWord* end, bool zap = true) {

     fill_with_object(start, pointer_delta(end, start), zap);

   }

   // Some heaps may offer a contiguous region for shared non-blocking

   // allocation, via inlined code (by exporting the address of the top and

   // end fields defining the extent of the contiguous allocation region.)

   // This function returns "true" iff the heap supports this kind of

   // allocation.  (Default is "no".)

   virtual bool supports_inline_contig_alloc() const {

     return false;

   }

   // These functions return the addresses of the fields that define the

   // boundaries of the contiguous allocation area.  (These fields should be

   // physically near to one another.)

   virtual HeapWord** top_addr() const {

     guarantee(false, "inline contiguous allocation not supported");

     return NULL;

   }

   virtual HeapWord** end_addr() const {

     guarantee(false, "inline contiguous allocation not supported");

     return NULL;

   }

   // Some heaps may be in an unparseable state at certain times between

   // collections. This may be necessary for efficient implementation of

   // certain allocation-related activities. Calling this function before

   // attempting to parse a heap ensures that the heap is in a parsable

   // state (provided other concurrent activity does not introduce

   // unparsability). It is normally expected, therefore, that this

   // method is invoked with the world stopped.

   // NOTE: if you override this method, make sure you call

   // super::ensure_parsability so that the non-generational

   // part of the work gets done. See implementation of

   // CollectedHeap::ensure_parsability and, for instance,

   // that of GenCollectedHeap::ensure_parsability().

   // The argument "retire_tlabs" controls whether existing TLABs

   // are merely filled or also retired, thus preventing further

   // allocation from them and necessitating allocation of new TLABs.

   virtual void ensure_parsability(bool retire_tlabs);

   // Return an estimate of the maximum allocation that could be performed

   // without triggering any collection or expansion activity.  In a

   // generational collector, for example, this is probably the largest

   // allocation that could be supported (without expansion) in the youngest

   // generation.  It is "unsafe" because no locks are taken; the result

   // should be treated as an approximation, not a guarantee, for use in

   // heuristic resizing decisions.

   virtual size_t unsafe_max_alloc() = 0;

   // Section on thread-local allocation buffers (TLABs)

   // If the heap supports thread-local allocation buffers, it should override

   // the following methods:

   // Returns "true" iff the heap supports thread-local allocation buffers.

   // The default is "no".

   virtual bool supports_tlab_allocation() const {

     return false;

   }

   // The amount of space available for thread-local allocation buffers.

   virtual size_t tlab_capacity(Thread *thr) const {

     guarantee(false, "thread-local allocation buffers not supported");

     return 0;

   }

   // An estimate of the maximum allocation that could be performed

   // for thread-local allocation buffers without triggering any

   // collection or expansion activity.

   virtual size_t unsafe_max_tlab_alloc(Thread *thr) const {

     guarantee(false, "thread-local allocation buffers not supported");

     return 0;

   }

   // Can a compiler initialize a new object without store barriers?

   // This permission only extends from the creation of a new object

   // via a TLAB up to the first subsequent safepoint. If such permission

   // is granted for this heap type, the compiler promises to call

   // defer_store_barrier() below on any slow path allocation of

   // a new object for which such initializing store barriers will

   // have been elided.

   virtual bool can_elide_tlab_store_barriers() const = 0;

   // If a compiler is eliding store barriers for TLAB-allocated objects,

   // there is probably a corresponding slow path which can produce

   // an object allocated anywhere.  The compiler's runtime support

   // promises to call this function on such a slow-path-allocated

   // object before performing initializations that have elided

   // store barriers. Returns new_obj, or maybe a safer copy thereof.

   virtual oop new_store_pre_barrier(JavaThread* thread, oop new_obj);

   // Answers whether an initializing store to a new object currently

   // allocated at the given address doesn't need a store

   // barrier. Returns "true" if it doesn't need an initializing

   // store barrier; answers "false" if it does.

   virtual bool can_elide_initializing_store_barrier(oop new_obj) = 0;

   // If a compiler is eliding store barriers for TLAB-allocated objects,

   // we will be informed of a slow-path allocation by a call

   // to new_store_pre_barrier() above. Such a call precedes the

   // initialization of the object itself, and no post-store-barriers will

   // be issued. Some heap types require that the barrier strictly follows

   // the initializing stores. (This is currently implemented by deferring the

   // barrier until the next slow-path allocation or gc-related safepoint.)

   // This interface answers whether a particular heap type needs the card

   // mark to be thus strictly sequenced after the stores.

   virtual bool card_mark_must_follow_store() const = 0;

   // If the CollectedHeap was asked to defer a store barrier above,

   // this informs it to flush such a deferred store barrier to the

   // remembered set.

   virtual void flush_deferred_store_barrier(JavaThread* thread);

   // Can a compiler elide a store barrier when it writes

   // a permanent oop into the heap?  Applies when the compiler

   // is storing x to the heap, where x->is_perm() is true.

   virtual bool can_elide_permanent_oop_store_barriers() const = 0;

   // Does this heap support heap inspection (+PrintClassHistogram?)

   virtual bool supports_heap_inspection() const = 0;

   // Perform a collection of the heap; intended for use in implementing

   // "System.gc".  This probably implies as full a collection as the

   // "CollectedHeap" supports.

   virtual void collect(GCCause::Cause cause) = 0;

   // This interface assumes that it's being called by the

   // vm thread. It collects the heap assuming that the

   // heap lock is already held and that we are executing in

   // the context of the vm thread.

   virtual void collect_as_vm_thread(GCCause::Cause cause) = 0;

   // Returns the barrier set for this heap

   BarrierSet* barrier_set() { return _barrier_set; }

   // Returns "true" iff there is a stop-world GC in progress.  (I assume

   // that it should answer "false" for the concurrent part of a concurrent

   // collector -- dld).

   bool is_gc_active() const { return _is_gc_active; }

   // Total number of GC collections (started)

   unsigned int total_collections() const { return _total_collections; }

   unsigned int total_full_collections() const { return _total_full_collections;}

   // Increment total number of GC collections (started)

   // Should be protected but used by PSMarkSweep - cleanup for 1.4.2

   void increment_total_collections(bool full = false) {

     _total_collections++;

     if (full) {

       increment_total_full_collections();

     }

   }

   void increment_total_full_collections() { _total_full_collections++; }

   // Return the AdaptiveSizePolicy for the heap.

   virtual AdaptiveSizePolicy* size_policy() = 0;

   // Return the CollectorPolicy for the heap

   virtual CollectorPolicy* collector_policy() const = 0;

   // Iterate over all the ref-containing fields of all objects, calling

   // "cl.do_oop" on each. This includes objects in permanent memory.

   virtual void oop_iterate(OopClosure* cl) = 0;

   // Iterate over all objects, calling "cl.do_object" on each.

   // This includes objects in permanent memory.

   virtual void object_iterate(ObjectClosure* cl) = 0;

   // Similar to object_iterate() except iterates only

   // over live objects.

   virtual void safe_object_iterate(ObjectClosure* cl) = 0;

   // Behaves the same as oop_iterate, except only traverses

   // interior pointers contained in permanent memory. If there

   // is no permanent memory, does nothing.

   virtual void permanent_oop_iterate(OopClosure* cl) = 0;

   // Behaves the same as object_iterate, except only traverses

   // object contained in permanent memory. If there is no

   // permanent memory, does nothing.

   virtual void permanent_object_iterate(ObjectClosure* cl) = 0;

   // NOTE! There is no requirement that a collector implement these

   // functions.

   //

   // A CollectedHeap is divided into a dense sequence of "blocks"; that is,

   // each address in the (reserved) heap is a member of exactly

   // one block.  The defining characteristic of a block is that it is

   // possible to find its size, and thus to progress forward to the next

   // block.  (Blocks may be of different sizes.)  Thus, blocks may

   // represent Java objects, or they might be free blocks in a

   // free-list-based heap (or subheap), as long as the two kinds are

   // distinguishable and the size of each is determinable.

   // Returns the address of the start of the "block" that contains the

   // address "addr".  We say "blocks" instead of "object" since some heaps

   // may not pack objects densely; a chunk may either be an object or a

   // non-object.

   virtual HeapWord* block_start(const void* addr) const = 0;

   // Requires "addr" to be the start of a chunk, and returns its size.

   // "addr + size" is required to be the start of a new chunk, or the end

   // of the active area of the heap.

   virtual size_t block_size(const HeapWord* addr) const = 0;

   // Requires "addr" to be the start of a block, and returns "TRUE" iff

   // the block is an object.

   virtual bool block_is_obj(const HeapWord* addr) const = 0;

   // Returns the longest time (in ms) that has elapsed since the last

   // time that any part of the heap was examined by a garbage collection.

   virtual jlong millis_since_last_gc() = 0;

   // Perform any cleanup actions necessary before allowing a verification.

   virtual void prepare_for_verify() = 0;

   // Generate any dumps preceding or following a full gc

   void pre_full_gc_dump();

   void post_full_gc_dump();

   virtual void print() const = 0;

   virtual void print_on(outputStream* st) const = 0;

   // Print all GC threads (other than the VM thread)

   // used by this heap.

   virtual void print_gc_threads_on(outputStream* st) const = 0;

   void print_gc_threads() { print_gc_threads_on(tty); }

   // Iterator for all GC threads (other than VM thread)

   virtual void gc_threads_do(ThreadClosure* tc) const = 0;

   // Print any relevant tracing info that flags imply.

   // Default implementation does nothing.

   virtual void print_tracing_info() const = 0;

   // Heap verification

   virtual void verify(bool allow_dirty, bool silent, bool option) = 0;

   // Non product verification and debugging.

 #ifndef PRODUCT

   // Support for PromotionFailureALot.  Return true if it's time to cause a

   // promotion failure.  The no-argument version uses

   // this->_promotion_failure_alot_count as the counter.

   inline bool promotion_should_fail(volatile size_t* count);

   inline bool promotion_should_fail();

   // Reset the PromotionFailureALot counters.  Should be called at the end of a

   // GC in which promotion failure ocurred.

   inline void reset_promotion_should_fail(volatile size_t* count);

   inline void reset_promotion_should_fail();

 #endif  // #ifndef PRODUCT

 #ifdef ASSERT

   static int fired_fake_oom() {

     return (CIFireOOMAt > 1 && _fire_out_of_memory_count >= CIFireOOMAt);

   }

 #endif

  public:

   // This is a convenience method that is used in cases where

   // the actual number of GC worker threads is not pertinent but

   // only whether there more than 0.  Use of this method helps

   // reduce the occurrence of ParallelGCThreads to uses where the

   // actual number may be germane.

   static bool use_parallel_gc_threads() { return ParallelGCThreads > 0; }

 };

 // Class to set and reset the GC cause for a CollectedHeap.

 class GCCauseSetter : StackObj {

   CollectedHeap* _heap;

   GCCause::Cause _previous_cause;

  public:

   GCCauseSetter(CollectedHeap* heap, GCCause::Cause cause) {

     assert(SafepointSynchronize::is_at_safepoint(),

            "This method manipulates heap state without locking");

     _heap = heap;

     _previous_cause = _heap->gc_cause();

     _heap->set_gc_cause(cause);

   }

   ~GCCauseSetter() {

     assert(SafepointSynchronize::is_at_safepoint(),

           "This method manipulates heap state without locking");

     _heap->set_gc_cause(_previous_cause);

   }

 };