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  * 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).

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

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 #ifndef SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUP_HPP

 #define SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUP_HPP

 //

 // String Deduplication

 //

 // String deduplication aims to reduce the heap live-set by deduplicating identical

 // instances of String so that they share the same backing character array.

 //

 // The deduplication process is divided in two main parts, 1) finding the objects to

 // deduplicate, and 2) deduplicating those objects. The first part is done as part of

 // a normal GC cycle when objects are marked or evacuated. At this time a check is

 // applied on each object to check if it is a candidate for deduplication. If so, the

 // object is placed on the deduplication queue for later processing. The second part,

 // processing the objects on the deduplication queue, is a concurrent phase which

 // starts right after the stop-the-wold marking/evacuation phase. This phase is

 // executed by the deduplication thread, which pulls deduplication candidates of the

 // deduplication queue and tries to deduplicate them.

 //

 // A deduplication hashtable is used to keep track of all unique character arrays

 // used by String objects. When deduplicating, a lookup is made in this table to see

 // if there is already an identical character array somewhere on the heap. If so, the

 // String object is adjusted to point to that character array, releasing the reference

 // to the original array allowing it to eventually be garbage collected. If the lookup

 // fails the character array is instead inserted into the hashtable so that this array

 // can be shared at some point in the future.

 //

 // Candidate selection

 //

 // An object is considered a deduplication candidate if all of the following

 // statements are true:

 //

 // - The object is an instance of java.lang.String

 //

 // - The object is being evacuated from a young heap region

 //

 // - The object is being evacuated to a young/survivor heap region and the

 //   object's age is equal to the deduplication age threshold

 //

 //   or

 //

 //   The object is being evacuated to an old heap region and the object's age is

 //   less than the deduplication age threshold

 //

 // Once an string object has been promoted to an old region, or its age is higher

 // than the deduplication age threshold, is will never become a candidate again.

 // This approach avoids making the same object a candidate more than once.

 //

 // Interned strings are a bit special. They are explicitly deduplicated just before

 // being inserted into the StringTable (to avoid counteracting C2 optimizations done

 // on string literals), then they also become deduplication candidates if they reach

 // the deduplication age threshold or are evacuated to an old heap region. The second

 // attempt to deduplicate such strings will be in vain, but we have no fast way of

 // filtering them out. This has not shown to be a problem, as the number of interned

 // strings is usually dwarfed by the number of normal (non-interned) strings.

 //

 // For additional information on string deduplication, please see JEP 192,

 // http://openjdk.java.net/jeps/192

 //

 #include "memory/allocation.hpp"

 #include "oops/oop.hpp"

 class OopClosure;

 class BoolObjectClosure;

 class ThreadClosure;

 class outputStream;

 class G1StringDedupTable;

 //

 // Main interface for interacting with string deduplication.

 //

 class G1StringDedup : public AllStatic {

 private:

   // Single state for checking if both G1 and string deduplication is enabled.

   static bool _enabled;

   // Candidate selection policies, returns true if the given object is

   // candidate for string deduplication.

   static bool is_candidate_from_mark(oop obj);

   static bool is_candidate_from_evacuation(bool from_young, bool to_young, oop obj);

 public:

   // Returns true if both G1 and string deduplication is enabled.

   static bool is_enabled() {

     return _enabled;

   }

   static void initialize();

   // Immediately deduplicates the given String object, bypassing the

   // the deduplication queue.

   static void deduplicate(oop java_string);

   // Enqueues a deduplication candidate for later processing by the deduplication

   // thread. Before enqueuing, these functions apply the appropriate candidate

   // selection policy to filters out non-candidates.

   static void enqueue_from_mark(oop java_string);

   static void enqueue_from_evacuation(bool from_young, bool to_young,

                                       unsigned int queue, oop java_string);

   static void oops_do(OopClosure* keep_alive);

   static void unlink(BoolObjectClosure* is_alive);

   static void unlink_or_oops_do(BoolObjectClosure* is_alive, OopClosure* keep_alive,

                                 bool allow_resize_and_rehash = true);

   static void threads_do(ThreadClosure* tc);

   static void print_worker_threads_on(outputStream* st);

   static void verify();

 };

 //

 // This closure encapsulates the state and the closures needed when scanning

 // the deduplication queue and table during the unlink_or_oops_do() operation.

 // A single instance of this closure is created and then shared by all worker

 // threads participating in the scan. The _next_queue and _next_bucket fields

 // provide a simple mechanism for GC workers to claim exclusive access to a

 // queue or a table partition.

 //

 class G1StringDedupUnlinkOrOopsDoClosure : public StackObj {

 private:

   BoolObjectClosure*  _is_alive;

   OopClosure*         _keep_alive;

   G1StringDedupTable* _resized_table;

   G1StringDedupTable* _rehashed_table;

   size_t              _next_queue;

   size_t              _next_bucket;

 public:

   G1StringDedupUnlinkOrOopsDoClosure(BoolObjectClosure* is_alive,

                                      OopClosure* keep_alive,

                                      bool allow_resize_and_rehash);

   ~G1StringDedupUnlinkOrOopsDoClosure();

   bool is_resizing() {

     return _resized_table != NULL;

   }

   G1StringDedupTable* resized_table() {

     return _resized_table;

   }

   bool is_rehashing() {

     return _rehashed_table != NULL;

   }

   // Atomically claims the next available queue for exclusive access by

   // the current thread. Returns the queue number of the claimed queue.

   size_t claim_queue() {

     return (size_t)Atomic::add_ptr(1, &_next_queue) - 1;

   }

   // Atomically claims the next available table partition for exclusive

   // access by the current thread. Returns the table bucket number where

   // the claimed partition starts.

   size_t claim_table_partition(size_t partition_size) {

     return (size_t)Atomic::add_ptr(partition_size, &_next_bucket) - partition_size;

   }

   // Applies and returns the result from the is_alive closure, or

   // returns true if no such closure was provided.

   bool is_alive(oop o) {

     if (_is_alive != NULL) {

       return _is_alive->do_object_b(o);

     }

     return true;

   }

   // Applies the keep_alive closure, or does nothing if no such

   // closure was provided.

   void keep_alive(oop* p) {

     if (_keep_alive != NULL) {

       _keep_alive->do_oop(p);

     }

   }

 };

 #endif // SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUP_HPP