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  25 
  26 #ifndef SHARE_GC_SHENANDOAH_SHENANDOAHSCANREMEMBERED_HPP
  27 #define SHARE_GC_SHENANDOAH_SHENANDOAHSCANREMEMBERED_HPP
  28 
  29 // Terminology used within this source file:
  30 //
  31 // Card Entry:   This is the information that identifies whether a
  32 //               particular card-table entry is Clean or Dirty.  A clean
  33 //               card entry denotes that the associated memory does not
  34 //               hold references to young-gen memory.
  35 //
  36 // Card Region, aka
  37 // Card Memory:  This is the region of memory that is assocated with a
  38 //               particular card entry.
  39 //
  40 // Card Cluster: A card cluster represents 64 card entries.  A card
  41 //               cluster is the minimal amount of work performed at a
  42 //               time by a parallel thread.  Note that the work required
  43 //               to scan a card cluster is somewhat variable in that the
  44 //               required effort depends on how many cards are dirty, how
  45 //               many references are held within the objects that span a
  46 //               DIRTY card's memory, and on the size of the object
  47 //               that spans the end of a DIRTY card's memory (because
  48 //               that object will be scanned in its entirety). For these
  49 //               reasons, it is advisable for the multiple worker threads
  50 //               to be flexible in the number of clusters to be
  51 //               processed by each thread.
  52 //
  53 // A cluster represents a "natural" quantum of work to be performed by
  54 // a parallel GC thread's background remembered set scanning efforts.
  55 // The notion of cluster is similar to the notion of stripe in the
  56 // implementation of parallel GC card scanning.  However, a cluster is
  57 // typically smaller than a stripe, enabling finer grain division of
  58 // labor between multiple threads.
  59 //
  60 // For illustration, consider the following possible JVM configurations:
  61 //
  62 //   Scenario 1:
  63 //     RegionSize is 128 MB
  64 //     Span of a card entry is 512 B
  65 //     Each card table entry consumes 1 B
  66 //     Assume one long word of card table entries represents a cluster.
  67 //       This long word holds 8 card table entries, spanning a
  68 //       total of 4KB
  69 //     The number of clusters per region is 128 MB / 4 KB = 32K
  70 //
  71 //   Scenario 2:
  72 //     RegionSize is 128 MB
  73 //     Span of each card entry is 128 B
  74 //     Each card table entry consumes 1 bit
  75 //     Assume one int word of card tables represents a cluster.
  76 //       This int word holds 32 card table entries, spanning a
  77 //       total of 4KB
  78 //     The number of clusters per region is 128 MB / 4 KB = 32K
  79 //
  80 //   Scenario 3:
  81 //     RegionSize is 128 MB
  82 //     Span of each card entry is 512 B
  83 //     Each card table entry consumes 1 bit
  84 //     Assume one long word of card tables represents a cluster.
  85 //       This long word holds 64 card table entries, spanning a
  86 //       total of 32 KB
  87 //     The number of clusters per region is 128 MB / 32 KB = 4K
  88 //
  89 // At the start of a new young-gen concurrent mark pass, the gang of
  90 // Shenandoah worker threads collaborate in performing the following
  91 // actions:
  92 //
  93 //  Let old_regions = number of ShenandoahHeapRegion comprising
  94 //    old-gen memory
  95 //  Let region_size = ShenandoahHeapRegion::region_size_bytes()
  96 //    represent the number of bytes in each region
  97 //  Let clusters_per_region = region_size / 512
  98 //  Let rs represent the relevant RememberedSet implementation
  99 //    (an instance of ShenandoahDirectCardMarkRememberedSet or an instance
 100 //     of a to-be-implemented ShenandoahBufferWithSATBRememberedSet)
 101 //
 102 //  for each ShenandoahHeapRegion old_region in the whole heap
 103 //    determine the cluster number of the first cluster belonging
 104 //      to that region
 105 //    for each cluster contained within that region
 106 //      Assure that exactly one worker thread initializes each
 107 //      cluster of overreach memory by invoking:
 108 //
 109 //        rs->initialize_overreach(cluster_no, cluster_count)
 110 //
 111 //      in separate threads.  (Divide up the clusters so that
 112 //      different threads are responsible for initializing different
 113 //      clusters.  Initialization cost is essentially identical for
 114 //      each cluster.)
 115 //
 116 //  Next, we repeat the process for invocations of process_clusters.
 117 //  for each ShenandoahHeapRegion old_region in the whole heap
 118 //    determine the cluster number of the first cluster belonging
 119 //      to that region
 120 //    for each cluster contained within that region
 121 //      Assure that exactly one worker thread processes each
 122 //      cluster, each thread making a series of invocations of the
 123 //      following:
 124 //
 125 //        rs->process_clusters(worker_id, ReferenceProcessor *,
 126 //                             ShenandoahConcurrentMark *, cluster_no, cluster_count,
 127 //                             HeapWord *end_of_range, OopClosure *oops);
 128 //
 129 //  For efficiency, divide up the clusters so that different threads
 130 //  are responsible for processing different clusters.  Processing costs
 131 //  may vary greatly between clusters for the following reasons:
 132 //
 133 //        a) some clusters contain mostly dirty cards and other
 134 //           clusters contain mostly clean cards
 135 //        b) some clusters contain mostly primitive data and other
 136 //           clusters contain mostly reference data
 137 //        c) some clusters are spanned by very large objects that
 138 //           begin in some other cluster.  When a large object
 139 //           beginning in a preceding cluster spans large portions of
 140 //           this cluster, the processing of this cluster gets a
 141 //           "free ride" because the thread responsible for processing
 142 //           the cluster that holds the object's header does the
 143 //           processing.
 144 //        d) in the case that the end of this cluster is spanned by a
 145 //           very large object, the processing of this cluster will
 146 //           be responsible for examining the entire object,
 147 //           potentially requiring this thread to process large amounts
 148 //           of memory pertaining to other clusters.
 149 //
 150 // Though an initial division of labor between marking threads may
 151 // assign equal numbers of clusters to be scanned by each thread, it
 152 // should be expected that some threads will finish their assigned
 153 // work before others.  Therefore, some amount of the full remembered
 154 // set scanning effort should be held back and assigned incrementally
 155 // to the threads that end up with excess capacity.  Consider the
 156 // following strategy for dividing labor:
 157 //
 158 //        1. Assume there are 8 marking threads and 1024 remembered
 159 //           set clusters to be scanned.
 160 //        2. Assign each thread to scan 64 clusters.  This leaves
 161 //           512 (1024 - (8*64)) clusters to still be scanned.
 162 //        3. As the 8 server threads complete previous cluster
 163 //           scanning assignments, issue each of the next 8 scanning
 164 //           assignments as units of 32 additional cluster each.
 165 //           In the case that there is high variance in effort
 166 //           associated with previous cluster scanning assignments,
 167 //           multiples of these next assignments may be serviced by
 168 //           the server threads that were previously assigned lighter
 169 //           workloads.
 170 //        4. Make subsequent scanning assignments as follows:
 171 //             a) 8 assignments of size 16 clusters
 172 //             b) 8 assignments of size 8 clusters
 173 //             c) 16 assignments of size 4 clusters
 174 //
 175 //    When there is no more remembered set processing work to be
 176 //    assigned to a newly idled worker thread, that thread can move
 177 //    on to work on other tasks associated with root scanning until such
 178 //    time as all clusters have been examined.
 179 //
 180 //  Once all clusters have been processed, the gang of GC worker
 181 //  threads collaborate to merge the overreach data.
 182 //
 183 //  for each ShenandoahHeapRegion old_region in the whole heap
 184 //    determine the cluster number of the first cluster belonging
 185 //      to that region
 186 //    for each cluster contained within that region
 187 //      Assure that exactly one worker thread initializes each
 188 //      cluster of overreach memory by invoking:
 189 //
 190 //        rs->merge_overreach(cluster_no, cluster_count)
 191 //
 192 //      in separate threads.  (Divide up the clusters so that
 193 //      different threads are responsible for merging different
 194 //      clusters.  Merging cost is essentially identical for
 195 //      each cluster.)
 196 //
 197 // Though remembered set scanning is designed to run concurrently with
 198 // mutator threads, the current implementation of remembered set
 199 // scanning runs in parallel during a GC safepoint.  Furthermore, the
 200 // current implementation of remembered set scanning never clears a
 201 // card once it has been marked.  Since the current implementation
 202 // never clears marked pages, the current implementation does not
 203 // invoke initialize_overreach() or merge_overreach().
 204 //
 205 // These limitations will be addressed in future enhancements to the
 206 // existing implementation.
 207 
 208 #include <stdint.h>
 209 #include "memory/iterator.hpp"
 210 #include "gc/shared/workerThread.hpp"
 211 #include "gc/shenandoah/shenandoahCardTable.hpp"
 212 #include "gc/shenandoah/shenandoahHeapRegion.hpp"
 213 #include "gc/shenandoah/shenandoahTaskqueue.hpp"
 214 
 215 class ShenandoahReferenceProcessor;
 216 class ShenandoahConcurrentMark;
 217 class ShenandoahHeap;
 218 class ShenandoahRegionIterator;
 219 class ShenandoahMarkingContext;
 220 
 221 class CardTable;
 222 
 223 class ShenandoahDirectCardMarkRememberedSet: public CHeapObj<mtGC> {
 224 
 225 private:
 226 
 227   // Use symbolic constants defined in cardTable.hpp
 228   //  CardTable::card_shift = 9;
 229   //  CardTable::card_size = 512;
 230   //  CardTable::card_size_in_words = 64;
 231 
 232   //  CardTable::clean_card_val()
 233   //  CardTable::dirty_card_val()
 234 
 235   ShenandoahHeap *_heap;
 236   ShenandoahCardTable *_card_table;
 237   size_t _card_shift;
 238   size_t _total_card_count;
 239   size_t _cluster_count;
 240   HeapWord *_whole_heap_base;   // Points to first HeapWord of data contained within heap memory
 241   HeapWord *_whole_heap_end;
 242   uint8_t *_byte_map;           // Points to first entry within the card table
 243   uint8_t *_byte_map_base;      // Points to byte_map minus the bias computed from address of heap memory
 244   uint8_t *_overreach_map;      // Points to first entry within the overreach card table
 245   uint8_t *_overreach_map_base; // Points to overreach_map minus the bias computed from address of heap memory
 246 
 247   uint64_t _wide_clean_value;
 248 
 249 public:
 250   // count is the number of cards represented by the card table.
 251   ShenandoahDirectCardMarkRememberedSet(ShenandoahCardTable *card_table, size_t total_card_count);
 252   ~ShenandoahDirectCardMarkRememberedSet();
 253 
 254   // Card index is zero-based relative to _byte_map.
 255   size_t total_cards();
 256   size_t card_index_for_addr(HeapWord *p);
 257   HeapWord *addr_for_card_index(size_t card_index);
 258   bool is_card_dirty(size_t card_index);
 259   bool is_write_card_dirty(size_t card_index);
 260   void mark_card_as_dirty(size_t card_index);
 261   void mark_range_as_dirty(size_t card_index, size_t num_cards);
 262   void mark_card_as_clean(size_t card_index);
 263   void mark_read_card_as_clean(size_t card_index);
 264   void mark_range_as_clean(size_t card_index, size_t num_cards);
 265   void mark_overreach_card_as_dirty(size_t card_index);
 266   bool is_card_dirty(HeapWord *p);
 267   void mark_card_as_dirty(HeapWord *p);
 268   void mark_range_as_dirty(HeapWord *p, size_t num_heap_words);
 269   void mark_card_as_clean(HeapWord *p);
 270   void mark_range_as_clean(HeapWord *p, size_t num_heap_words);
 271   void mark_overreach_card_as_dirty(void *p);
 272   size_t cluster_count();
 273 
 274   // Called by multiple GC threads at start of concurrent mark and evacuation phases.  Each parallel GC thread typically
 275   // initializes a different subranges of all overreach entries.
 276   void initialize_overreach(size_t first_cluster, size_t count);
 277 
 278   // Called by GC thread at end of concurrent mark or evacuation phase.  Each parallel GC thread typically merges different
 279   // subranges of all overreach entries.
 280   void merge_overreach(size_t first_cluster, size_t count);
 281 
 282   // Called by GC thread at start of concurrent mark to exchange roles of read and write remembered sets.
 283   // Not currently used because mutator write barrier does not honor changes to the location of card table.
 284   void swap_remset() {  _card_table->swap_card_tables(); }
 285 
 286   void merge_write_table(HeapWord* start, size_t word_count) {
 287     size_t card_index = card_index_for_addr(start);
 288     size_t num_cards = word_count / CardTable::card_size_in_words();
 289     size_t iterations = num_cards / (sizeof (intptr_t) / sizeof (CardTable::CardValue));
 290     intptr_t* read_table_ptr = (intptr_t*) &(_card_table->read_byte_map())[card_index];
 291     intptr_t* write_table_ptr = (intptr_t*) &(_card_table->write_byte_map())[card_index];
 292     for (size_t i = 0; i < iterations; i++) {
 293       intptr_t card_value = *write_table_ptr;
 294       *read_table_ptr++ &= card_value;
 295       write_table_ptr++;
 296     }
 297   }
 298 
 299   HeapWord* whole_heap_base() { return _whole_heap_base; }
 300   HeapWord* whole_heap_end() { return _whole_heap_end; }
 301 
 302   // Instead of swap_remset, the current implementation of concurrent remembered set scanning does reset_remset
 303   // in parallel threads, each invocation processing one entire HeapRegion at a time.  Processing of a region
 304   // consists of copying the write table to the read table and cleaning the write table.
 305   void reset_remset(HeapWord* start, size_t word_count) {
 306     size_t card_index = card_index_for_addr(start);
 307     size_t num_cards = word_count / CardTable::card_size_in_words();
 308     size_t iterations = num_cards / (sizeof (intptr_t) / sizeof (CardTable::CardValue));
 309     intptr_t* read_table_ptr = (intptr_t*) &(_card_table->read_byte_map())[card_index];
 310     intptr_t* write_table_ptr = (intptr_t*) &(_card_table->write_byte_map())[card_index];
 311     for (size_t i = 0; i < iterations; i++) {
 312       *read_table_ptr++ = *write_table_ptr;
 313       *write_table_ptr++ = CardTable::clean_card_row_val();
 314     }
 315   }
 316 
 317   // Called by GC thread after scanning old remembered set in order to prepare for next GC pass
 318   void clear_old_remset() {  _card_table->clear_read_table(); }
 319 
 320 };
 321 
 322 // A ShenandoahCardCluster represents the minimal unit of work
 323 // performed by independent parallel GC threads during scanning of
 324 // remembered sets.
 325 //
 326 // The GC threads that perform card-table remembered set scanning may
 327 // overwrite card-table entries to mark them as clean in the case that
 328 // the associated memory no longer holds references to young-gen
 329 // memory.  Rather than access the card-table entries directly, all GC
 330 // thread access to card-table information is made by way of the
 331 // ShenandoahCardCluster data abstraction.  This abstraction
 332 // effectively manages access to multiple possible underlying
 333 // remembered set implementations, including a traditional card-table
 334 // approach and a SATB-based approach.
 335 //
 336 // The API services represent a compromise between efficiency and
 337 // convenience.
 338 //
 339 // In the initial implementation, we assume that scanning of card
 340 // table entries occurs only while the JVM is at a safe point.  Thus,
 341 // there is no synchronization required between GC threads that are
 342 // scanning card-table entries and marking certain entries that were
 343 // previously dirty as clean, and mutator threads which would possibly
 344 // be marking certain card-table entries as dirty.
 345 //
 346 // There is however a need to implement concurrency control and memory
 347 // coherency between multiple GC threads that scan the remembered set
 348 // in parallel.  The desire is to divide the complete scanning effort
 349 // into multiple clusters of work that can be independently processed
 350 // by individual threads without need for synchronizing efforts
 351 // between the work performed by each task.  The term "cluster" of
 352 // work is similar to the term "stripe" as used in the implementation
 353 // of Parallel GC.
 354 //
 355 // Complexity arises when an object to be scanned crosses the boundary
 356 // between adjacent cluster regions.  Here is the protocol that is
 357 // followed:
 358 //
 359 //  1. We implement a supplemental data structure known as the overreach
 360 //     card table.  The thread that is responsible for scanning each
 361 //     cluster of card-table entries is granted exclusive access to
 362 //     modify the associated card-table entries.  In the case that a
 363 //     thread scans a very large object that reaches into one or more
 364 //     following clusters, that thread has exclusive access to the
 365 //     overreach card table for all of the entries belonging to the
 366 //     following clusters that are spanned by this large object.
 367 //     After all clusters have been scanned, the scanning threads
 368 //     briefly synchronize to merge the contents of the overreach
 369 //     entries with the traditional card table entries using logical-
 370 //     and operations.
 371 //  2. Every object is scanned in its "entirety" by the thread that is
 372 //     responsible for the cluster that holds its starting address.
 373 //     Entirety is in quotes because there are various situations in
 374 //     which some portions of the object will not be scanned by this
 375 //     thread:
 376 //     a) If an object spans multiple card regions, all of which are
 377 //        contained within the same cluster, the scanning thread
 378 //        consults the existing card-table entries and does not scan
 379 //        portions of the object that are not currently dirty.
 380 //     b) For any cluster that is spanned in its entirety by a very
 381 //        large object, the GC thread that scans this object assumes
 382 //        full responsibility for maintenance of the associated
 383 //        card-table entries.
 384 //     c) If a cluster is partially spanned by an object originating
 385 //        in a preceding cluster, the portion of the object that
 386 //        partially spans the following cluster is scanned in its
 387 //        entirety (because the thread that is responsible for
 388 //        scanning the object cannot rely upon the card-table entries
 389 //        associated with the following cluster).  Whenever references
 390 //        to young-gen memory are found within the scanned data, the
 391 //        associated overreach card table entries are marked as dirty
 392 //        by the scanning thread.
 393 //  3. If a cluster is spanned in its entirety by an object that
 394 //     originates within a preceding cluster's memory, the thread
 395 //     assigned to examine this cluster does absolutely nothing.  The
 396 //     thread assigned to scan the cluster that holds the object's
 397 //     starting address takes full responsibility for scanning the
 398 //     entire object and updating the associated card-table entries.
 399 //  4. If a cluster is spanned partially by an object that originates
 400 //     within a preceding cluster's memory, the thread assigned to
 401 //     examine this cluster marks the card-table entry as clean for
 402 //     each card table that is fully spanned by this overreaching
 403 //     object.  If a card-table entry's memory is partially spanned
 404 //     by the overreaching object, the thread sets the card-table
 405 //     entry to clean if it was previously dirty and if the portion
 406 //     of the card-table entry's memory that is not spanned by the
 407 //     overreaching object does not hold pointers to young-gen
 408 //     memory.
 409 //  5. While examining a particular card belonging to a particular
 410 //     cluster, if an object reaches beyond the end of its card
 411 //     memory, the thread "scans" all portions of the object that
 412 //     correspond to DIRTY card entries within the current cluster and
 413 //     all portions of the object that reach into following clustesr.
 414 //     After this object is scanned, continue scanning with the memory
 415 //     that follows this object if this memory pertains to the same
 416 //     cluster.  Otherwise, consider this cluster's memory to have
 417 //     been fully examined.
 418 //
 419 // Discussion:
 420 //  Though this design results from careful consideration of multiple
 421 //  design objectives, it is subject to various criticisms.  Some
 422 //  discussion of the design choices is provided here:
 423 //
 424 //  1. Note that remembered sets are a heuristic technique to avoid
 425 //     the need to scan all of old-gen memory with each young-gen
 426 //     collection.  If we sometimes scan a bit more memory than is
 427 //     absolutely necessary, that should be considered a reasonable
 428 //     compromise.  This compromise is already present in the sizing
 429 //     of card table memory areas.  Note that a single dirty pointer
 430 //     within a 512-byte card region forces the "unnecessary" scanning
 431 //     of 63 = ((512 - 8 = 504) / 8) pointers.
 432 //  2. One undesirable aspect of this design is that we sometimes have
 433 //     to scan large amounts of memory belonging to very large
 434 //     objects, even for parts of the very large object that do not
 435 //     correspond to dirty card table entries.  Note that this design
 436 //     limits the amount of non-dirty scanning that might have to
 437 //     be performed for these very large objects.  In particular, only
 438 //     the last part of the very large object that extends into but
 439 //     does not completely span a particular cluster is unnecessarily
 440 //     scanned.  Thus, for each very large object, the maximum
 441 //     over-scan is the size of memory spanned by a single cluster.
 442 //  3. The representation of pointer location descriptive information
 443 //     within Klass representations is not designed for efficient
 444 //     "random access".  An alternative approach to this design would
 445 //     be to scan very large objects multiple times, once for each
 446 //     cluster that is spanned by the object's range.  This reduces
 447 //     unnecessary overscan, but it introduces different sorts of
 448 //     overhead effort:
 449 //       i) For each spanned cluster, we have to look up the start of
 450 //          the crossing object.
 451 //      ii) Each time we scan the very large object, we have to
 452 //          sequentially walk through its pointer location
 453 //          descriptors, skipping over all of the pointers that
 454 //          precede the start of the range of addresses that we
 455 //          consider relevant.
 456 
 457 
 458 // Because old-gen heap memory is not necessarily contiguous, and
 459 // because cards are not necessarily maintained for young-gen memory,
 460 // consecutive card numbers do not necessarily correspond to consecutive
 461 // address ranges.  For the traditional direct-card-marking
 462 // implementation of this interface, consecutive card numbers are
 463 // likely to correspond to contiguous regions of memory, but this
 464 // should not be assumed.  Instead, rely only upon the following:
 465 //
 466 //  1. All card numbers for cards pertaining to the same
 467 //     ShenandoahHeapRegion are consecutively numbered.
 468 //  2. In the case that neighboring ShenandoahHeapRegions both
 469 //     represent old-gen memory, the card regions that span the
 470 //     boundary between these neighboring heap regions will be
 471 //     consecutively numbered.
 472 //  3. (A corollary) In the case that an old-gen object spans the
 473 //     boundary between two heap regions, the card regions that
 474 //     correspond to the span of this object will be consecutively
 475 //     numbered.
 476 
 477 
 478 // ShenandoahCardCluster abstracts access to the remembered set
 479 // and also keeps track of crossing map information to allow efficient
 480 // resolution of object start addresses.
 481 //
 482 // ShenandoahCardCluster supports all of the services of
 483 // RememberedSet, plus it supports register_object() and lookup_object().
 484 //
 485 // There are two situations under which we need to know the location
 486 // at which the object spanning the start of a particular card-table
 487 // memory region begins:
 488 //
 489 // 1. When we begin to scan dirty card memory that is not the
 490 //    first card region within a cluster, and the object that
 491 //    crosses into this card memory was not previously scanned,
 492 //    we need to find where that object starts so we can scan it.
 493 //    (Asides: if the objects starts within a previous cluster, it
 494 //     has already been scanned.  If the object starts within this
 495 //     cluster and it spans at least one card region that is dirty
 496 //     and precedes this card region within the cluster, then it has
 497 //     already been scanned.)
 498 // 2. When we are otherwise done scanning a complete cluster, if the
 499 //    last object within the cluster reaches into the following
 500 //    cluster, we need to scan this object.  Thus, we need to find
 501 //    its starting location.
 502 //
 503 // The RememberedSet template parameter is intended to represent either
 504 //     ShenandoahDirectCardMarkRememberedSet, or a to-be-implemented
 505 //     ShenandoahBufferWithSATBRememberedSet.
 506 template<typename RememberedSet>
 507 class ShenandoahCardCluster: public CHeapObj<mtGC> {
 508 
 509 private:
 510   RememberedSet *_rs;
 511 
 512 public:
 513   static const size_t CardsPerCluster = 64;
 514 
 515 private:
 516   typedef struct cross_map { uint8_t first; uint8_t last; } xmap;
 517   typedef union crossing_info { uint16_t short_word; xmap offsets; } crossing_info;
 518 
 519   // ObjectStartsInCardRegion bit is set within a crossing_info.offsets.start iff at least one object starts within
 520   // a particular card region.  We pack this bit into start byte under assumption that start byte is accessed less
 521   // frequently that last byte.  This is true when number of clean cards is greater than number of dirty cards.
 522   static const uint16_t ObjectStartsInCardRegion = 0x80;
 523   static const uint16_t FirstStartBits           = 0x3f;
 524 
 525   crossing_info *object_starts;
 526 
 527 public:
 528   // If we're setting first_start, assume the card has an object.
 529   inline void set_first_start(size_t card_index, uint8_t value) {
 530     object_starts[card_index].offsets.first = ObjectStartsInCardRegion | value;
 531   }
 532 
 533   inline void set_last_start(size_t card_index, uint8_t value) {
 534     object_starts[card_index].offsets.last = value;
 535   }
 536 
 537   inline void set_has_object_bit(size_t card_index) {
 538     object_starts[card_index].offsets.first |= ObjectStartsInCardRegion;
 539   }
 540 
 541   inline void clear_has_object_bit(size_t card_index) {
 542     object_starts[card_index].offsets.first &= ~ObjectStartsInCardRegion;
 543   }
 544 
 545   // Returns true iff an object is known to start within the card memory associated with card card_index.
 546   inline bool has_object(size_t card_index) {
 547     return (object_starts[card_index].offsets.first & ObjectStartsInCardRegion) != 0;
 548   }
 549 
 550   inline void clear_objects_in_range(HeapWord *addr, size_t num_words) {
 551     size_t card_index = _rs->card_index_for_addr(addr);
 552     size_t last_card_index = _rs->card_index_for_addr(addr + num_words - 1);
 553     while (card_index <= last_card_index)
 554       object_starts[card_index++].short_word = 0;
 555   }
 556 
 557   ShenandoahCardCluster(RememberedSet *rs) {
 558     _rs = rs;
 559     // TODO: We don't really need object_starts entries for every card entry.  We only need these for
 560     // the card entries that correspond to old-gen memory.  But for now, let's be quick and dirty.
 561     object_starts = (crossing_info *) malloc(rs->total_cards() * sizeof(crossing_info));
 562     if (object_starts == nullptr)
 563       fatal("Insufficient memory for initializing heap");
 564     for (size_t i = 0; i < rs->total_cards(); i++)
 565       object_starts[i].short_word = 0;
 566   }
 567 
 568   ~ShenandoahCardCluster() {
 569     if (object_starts != nullptr)
 570       free(object_starts);
 571     object_starts = nullptr;
 572   }
 573 
 574   // There is one entry within the object_starts array for each card entry.
 575   //
 576   // In the most recent implementation of ShenandoahScanRemembered::process_clusters(),
 577   // there is no need for the get_crossing_object_start() method function, so there is no
 578   // need to maintain the following information.  The comment is left in place for now in
 579   // case we find it necessary to add support for this service at a later time.
 580   //
 581   // Bits 0x7fff: If no object starts within this card region, the
 582   //              remaining bits of the object_starts array represent
 583   //              the absolute word offset within the enclosing
 584   //              cluster's memory of the starting address for the
 585   //              object that spans the start of this card region's
 586   //              memory.  If the spanning object begins in memory
 587   //              that precedes this card region's cluster, the value
 588   //              stored in these bits is the special value 0x7fff.
 589   //              (Note that the maximum value required to represent a
 590   //              spanning object from within the current cluster is
 591   //              ((63 * 64) - 8), which equals 0x0fbf.
 592   //
 593   // In the absence of the need to support get_crossing_object_start(),
 594   // here is discussion of performance:
 595   //
 596   //  Suppose multiple garbage objects are coalesced during GC sweep
 597   //  into a single larger "free segment".  As each two objects are
 598   //  coalesced together, the start information pertaining to the second
 599   //  object must be removed from the objects_starts array.  If the
 600   //  second object had been been the first object within card memory,
 601   //  the new first object is the object that follows that object if
 602   //  that starts within the same card memory, or NoObject if the
 603   //  following object starts within the following cluster.  If the
 604   //  second object had been the last object in the card memory,
 605   //  replace this entry with the newly coalesced object if it starts
 606   //  within the same card memory, or with NoObject if it starts in a
 607   //  preceding card's memory.
 608   //
 609   //  Suppose a large free segment is divided into a smaller free
 610   //  segment and a new object.  The second part of the newly divided
 611   //  memory must be registered as a new object, overwriting at most
 612   //  one first_start and one last_start entry.  Note that one of the
 613   //  newly divided two objects might be a new GCLAB.
 614   //
 615   //  Suppose postprocessing of a GCLAB finds that the original GCLAB
 616   //  has been divided into N objects.  Each of the N newly allocated
 617   //  objects will be registered, overwriting at most one first_start
 618   //  and one last_start entries.
 619   //
 620   //  No object registration operations are linear in the length of
 621   //  the registered objects.
 622   //
 623   // Consider further the following observations regarding object
 624   // registration costs:
 625   //
 626   //   1. The cost is paid once for each old-gen object (Except when
 627   //      an object is demoted and repromoted, in which case we would
 628   //      pay the cost again).
 629   //   2. The cost can be deferred so that there is no urgency during
 630   //      mutator copy-on-first-access promotion.  Background GC
 631   //      threads will update the object_starts array by post-
 632   //      processing the contents of retired PLAB buffers.
 633   //   3. The bet is that these costs are paid relatively rarely
 634   //      because:
 635   //      a) Most objects die young and objects that die in young-gen
 636   //         memory never need to be registered with the object_starts
 637   //         array.
 638   //      b) Most objects that are promoted into old-gen memory live
 639   //         there without further relocation for a relatively long
 640   //         time, so we get a lot of benefit from each investment
 641   //         in registering an object.
 642 
 643 public:
 644 
 645   // The starting locations of objects contained within old-gen memory
 646   // are registered as part of the remembered set implementation.  This
 647   // information is required when scanning dirty card regions that are
 648   // spanned by objects beginning within preceding card regions.  It
 649   // is necessary to find the first and last objects that begin within
 650   // this card region.  Starting addresses of objects are required to
 651   // find the object headers, and object headers provide information
 652   // about which fields within the object hold addresses.
 653   //
 654   // The old-gen memory allocator invokes register_object() for any
 655   // object that is allocated within old-gen memory.  This identifies
 656   // the starting addresses of objects that span boundaries between
 657   // card regions.
 658   //
 659   // It is not necessary to invoke register_object at the very instant
 660   // an object is allocated.  It is only necessary to invoke it
 661   // prior to the next start of a garbage collection concurrent mark
 662   // or concurrent update-references phase.  An "ideal" time to register
 663   // objects is during post-processing of a GCLAB after the GCLAB is
 664   // retired due to depletion of its memory.
 665   //
 666   // register_object() does not perform synchronization.  In the case
 667   // that multiple threads are registering objects whose starting
 668   // addresses are within the same cluster, races between these
 669   // threads may result in corruption of the object-start data
 670   // structures.  Parallel GC threads should avoid registering objects
 671   // residing within the same cluster by adhering to the following
 672   // coordination protocols:
 673   //
 674   //  1. Align thread-local GCLAB buffers with some TBD multiple of
 675   //     card clusters.  The card cluster size is 32 KB.  If the
 676   //     desired GCLAB size is 128 KB, align the buffer on a multiple
 677   //     of 4 card clusters.
 678   //  2. Post-process the contents of GCLAB buffers to register the
 679   //     objects allocated therein.  Allow one GC thread at a
 680   //     time to do the post-processing of each GCLAB.
 681   //  3. Since only one GC thread at a time is registering objects
 682   //     belonging to a particular allocation buffer, no locking
 683   //     is performed when registering these objects.
 684   //  4. Any remnant of unallocated memory within an expended GC
 685   //     allocation buffer is not returned to the old-gen allocation
 686   //     pool until after the GC allocation buffer has been post
 687   //     processed.  Before any remnant memory is returned to the
 688   //     old-gen allocation pool, the GC thread that scanned this GC
 689   //     allocation buffer performs a write-commit memory barrier.
 690   //  5. Background GC threads that perform tenuring of young-gen
 691   //     objects without a GCLAB use a CAS lock before registering
 692   //     each tenured object.  The CAS lock assures both mutual
 693   //     exclusion and memory coherency/visibility.  Note that an
 694   //     object tenured by a background GC thread will not overlap
 695   //     with any of the clusters that are receiving tenured objects
 696   //     by way of GCLAB buffers.  Multiple independent GC threads may
 697   //     attempt to tenure objects into a shared cluster.  This is why
 698   //     sychronization may be necessary.  Consider the following
 699   //     scenarios:
 700   //
 701   //     a) If two objects are tenured into the same card region, each
 702   //        registration may attempt to modify the first-start or
 703   //        last-start information associated with that card region.
 704   //        Furthermore, because the representations of first-start
 705   //        and last-start information within the object_starts array
 706   //        entry uses different bits of a shared uint_16 to represent
 707   //        each, it is necessary to lock the entire card entry
 708   //        before modifying either the first-start or last-start
 709   //        information within the entry.
 710   //     b) Suppose GC thread X promotes a tenured object into
 711   //        card region A and this tenured object spans into
 712   //        neighboring card region B.  Suppose GC thread Y (not equal
 713   //        to X) promotes a tenured object into cluster B.  GC thread X
 714   //        will update the object_starts information for card A.  No
 715   //        synchronization is required.
 716   //     c) In summary, when background GC threads register objects
 717   //        newly tenured into old-gen memory, they must acquire a
 718   //        mutual exclusion lock on the card that holds the starting
 719   //        address of the newly tenured object.  This can be achieved
 720   //        by using a CAS instruction to assure that the previous
 721   //        values of first-offset and last-offset have not been
 722   //        changed since the same thread inquired as to their most
 723   //        current values.
 724   //
 725   //     One way to minimize the need for synchronization between
 726   //     background tenuring GC threads is for each tenuring GC thread
 727   //     to promote young-gen objects into distinct dedicated cluster
 728   //     ranges.
 729   //  6. The object_starts information is only required during the
 730   //     starting of concurrent marking and concurrent evacuation
 731   //     phases of GC.  Before we start either of these GC phases, the
 732   //     JVM enters a safe point and all GC threads perform
 733   //     commit-write barriers to assure that access to the
 734   //     object_starts information is coherent.
 735 
 736 
 737   // Notes on synchronization of register_object():
 738   //
 739   //  1. For efficiency, there is no locking in the implementation of register_object()
 740   //  2. Thus, it is required that users of this service assure that concurrent/parallel invocations of
 741   //     register_object() do pertain to the same card's memory range.  See discussion below to undestand
 742   //     the risks.
 743   //  3. When allocating from a TLAB or GCLAB, the mutual exclusion can be guaranteed by assuring that each
 744   //     LAB's start and end are aligned on card memory boundaries.
 745   //  4. Use the same lock that guarantees exclusivity when performing free-list allocation within heap regions.
 746   //
 747   // Register the newly allocated object while we're holding the global lock since there's no synchronization
 748   // built in to the implementation of register_object().  There are potential races when multiple independent
 749   // threads are allocating objects, some of which might span the same card region.  For example, consider
 750   // a card table's memory region within which three objects are being allocated by three different threads:
 751   //
 752   // objects being "concurrently" allocated:
 753   //    [-----a------][-----b-----][--------------c------------------]
 754   //            [---- card table memory range --------------]
 755   //
 756   // Before any objects are allocated, this card's memory range holds no objects.  Note that:
 757   //   allocation of object a wants to set the has-object, first-start, and last-start attributes of the preceding card region.
 758   //   allocation of object b wants to set the has-object, first-start, and last-start attributes of this card region.
 759   //   allocation of object c also wants to set the has-object, first-start, and last-start attributes of this card region.
 760   //
 761   // The thread allocating b and the thread allocating c can "race" in various ways, resulting in confusion, such as last-start
 762   // representing object b while first-start represents object c.  This is why we need to require all register_object()
 763   // invocations associated with objects that are allocated from "free lists" to provide their own mutual exclusion locking
 764   // mechanism.
 765 
 766   // Reset the has_object() information to false for all cards in the range between from and to.
 767   void reset_object_range(HeapWord *from, HeapWord *to);
 768 
 769   // register_object() requires that the caller hold the heap lock
 770   // before calling it.
 771   void register_object(HeapWord* address);
 772 
 773   // register_object_wo_lock() does not require that the caller hold
 774   // the heap lock before calling it, under the assumption that the
 775   // caller has assure no other thread will endeavor to concurrently
 776   // register objects that start within the same card's memory region
 777   // as address.
 778   void register_object_wo_lock(HeapWord* address);
 779 
 780   // During the reference updates phase of GC, we walk through each old-gen memory region that was
 781   // not part of the collection set and we invalidate all unmarked objects.  As part of this effort,
 782   // we coalesce neighboring dead objects in order to make future remembered set scanning more
 783   // efficient (since future remembered set scanning of any card region containing consecutive
 784   // dead objects can skip over all of them at once by reading only a single dead object header
 785   // instead of having to read the header of each of the coalesced dead objects.
 786   //
 787   // At some future time, we may implement a further optimization: satisfy future allocation requests
 788   // by carving new objects out of the range of memory that represents the coalesced dead objects.
 789   //
 790   // Suppose we want to combine several dead objects into a single coalesced object.  How does this
 791   // impact our representation of crossing map information?
 792   //  1. If the newly coalesced range is contained entirely within a card range, that card's last
 793   //     start entry either remains the same or it is changed to the start of the coalesced region.
 794   //  2. For the card that holds the start of the coalesced object, it will not impact the first start
 795   //     but it may impact the last start.
 796   //  3. For following cards spanned entirely by the newly coalesced object, it will change has_object
 797   //     to false (and make first-start and last-start "undefined").
 798   //  4. For a following card that is spanned patially by the newly coalesced object, it may change
 799   //     first-start value, but it will not change the last-start value.
 800   //
 801   // The range of addresses represented by the arguments to coalesce_objects() must represent a range
 802   // of memory that was previously occupied exactly by one or more previously registered objects.  For
 803   // convenience, it is legal to invoke coalesce_objects() with arguments that span a single previously
 804   // registered object.
 805   //
 806   // The role of coalesce_objects is to change the crossing map information associated with all of the coalesced
 807   // objects.
 808   void coalesce_objects(HeapWord* address, size_t length_in_words);
 809 
 810   // The typical use case is going to look something like this:
 811   //   for each heapregion that comprises old-gen memory
 812   //     for each card number that corresponds to this heap region
 813   //       scan the objects contained therein if the card is dirty
 814   // To avoid excessive lookups in a sparse array, the API queries
 815   // the card number pertaining to a particular address and then uses the
 816   // card noumber for subsequent information lookups and stores.
 817 
 818   // If has_object(card_index), this returns the word offset within this card
 819   // memory at which the first object begins.  If !has_object(card_index), the
 820   // result is a don't care value.
 821   size_t get_first_start(size_t card_index);
 822 
 823   // If has_object(card_index), this returns the word offset within this card
 824   // memory at which the last object begins.  If !has_object(card_index), the
 825   // result is a don't care value.
 826   size_t get_last_start(size_t card_index);
 827 
 828 };
 829 
 830 // ShenandoahScanRemembered is a concrete class representing the
 831 // ability to scan the old-gen remembered set for references to
 832 // objects residing in young-gen memory.
 833 //
 834 // Scanning normally begins with an invocation of numRegions and ends
 835 // after all clusters of all regions have been scanned.
 836 //
 837 // Throughout the scanning effort, the number of regions does not
 838 // change.
 839 //
 840 // Even though the regions that comprise old-gen memory are not
 841 // necessarily contiguous, the abstraction represented by this class
 842 // identifies each of the old-gen regions with an integer value
 843 // in the range from 0 to (numRegions() - 1) inclusive.
 844 //
 845 
 846 template<typename RememberedSet>
 847 class ShenandoahScanRemembered: public CHeapObj<mtGC> {
 848 
 849 private:
 850 
 851   RememberedSet* _rs;
 852   ShenandoahCardCluster<RememberedSet>* _scc;
 853 
 854 public:
 855   // How to instantiate this object?
 856   //   ShenandoahDirectCardMarkRememberedSet *rs =
 857   //       new ShenandoahDirectCardMarkRememberedSet();
 858   //   scr = new
 859   //     ShenandoahScanRememberd<ShenandoahDirectCardMarkRememberedSet>(rs);
 860   //
 861   // or, after the planned implementation of
 862   // ShenandoahBufferWithSATBRememberedSet has been completed:
 863   //
 864   //   ShenandoahBufferWithSATBRememberedSet *rs =
 865   //       new ShenandoahBufferWithSATBRememberedSet();
 866   //   scr = new
 867   //     ShenandoahScanRememberd<ShenandoahBufferWithSATBRememberedSet>(rs);
 868 
 869 
 870   ShenandoahScanRemembered(RememberedSet *rs) {
 871     _rs = rs;
 872     _scc = new ShenandoahCardCluster<RememberedSet>(rs);
 873   }
 874 
 875   ~ShenandoahScanRemembered() {
 876     delete _scc;
 877   }
 878 
 879   // TODO:  We really don't want to share all of these APIs with arbitrary consumers of the ShenandoahScanRemembered abstraction.
 880   // But in the spirit of quick and dirty for the time being, I'm going to go ahead and publish everything for right now.  Some
 881   // of existing code already depends on having access to these services (because existing code has not been written to honor
 882   // full abstraction of remembered set scanning.  In the not too distant future, we want to try to make most, if not all, of
 883   // these services private.  Two problems with publicizing:
 884   //  1. Allowing arbitrary users to reach beneath the hood allows the users to make assumptions about underlying implementation.
 885   //     This will make it more difficult to change underlying implementation at a future time, such as when we eventually experiment
 886   //     with SATB-based implementation of remembered set representation.
 887   //  2. If we carefully control sharing of certain of these services, we can reduce the overhead of synchronization by assuring
 888   //     that all users follow protocols that avoid contention that might require synchronization.  When we publish these APIs, we
 889   //     lose control over who and how the data is accessed.  As a result, we are required to insert more defensive measures into
 890   //     the implementation, including synchronization locks.
 891 
 892 
 893   // Card index is zero-based relative to first spanned card region.
 894   size_t total_cards();
 895   size_t card_index_for_addr(HeapWord *p);
 896   HeapWord *addr_for_card_index(size_t card_index);
 897   bool is_card_dirty(size_t card_index);
 898   bool is_write_card_dirty(size_t card_index) { return _rs->is_write_card_dirty(card_index); }
 899   void mark_card_as_dirty(size_t card_index);
 900   void mark_range_as_dirty(size_t card_index, size_t num_cards);
 901   void mark_card_as_clean(size_t card_index);
 902   void mark_read_card_as_clean(size_t card_index) { _rs->mark_read_card_clean(card_index); }
 903   void mark_range_as_clean(size_t card_index, size_t num_cards);
 904   void mark_overreach_card_as_dirty(size_t card_index);
 905   bool is_card_dirty(HeapWord *p);
 906   void mark_card_as_dirty(HeapWord *p);
 907   void mark_range_as_dirty(HeapWord *p, size_t num_heap_words);
 908   void mark_card_as_clean(HeapWord *p);
 909   void mark_range_as_clean(HeapWord *p, size_t num_heap_words);
 910   void mark_overreach_card_as_dirty(void *p);
 911   size_t cluster_count();
 912   void initialize_overreach(size_t first_cluster, size_t count);
 913   void merge_overreach(size_t first_cluster, size_t count);
 914 
 915   // Called by GC thread at start of concurrent mark to exchange roles of read and write remembered sets.
 916   void swap_remset() { _rs->swap_remset(); }
 917 
 918   void reset_remset(HeapWord* start, size_t word_count) { _rs->reset_remset(start, word_count); }
 919 
 920   void merge_write_table(HeapWord* start, size_t word_count) { _rs->merge_write_table(start, word_count); }
 921 
 922   // Called by GC thread after scanning old remembered set in order to prepare for next GC pass
 923   void clear_old_remset() { _rs->clear_old_remset(); }
 924 
 925   size_t cluster_for_addr(HeapWord *addr);
 926 
 927   void reset_object_range(HeapWord *from, HeapWord *to);
 928   void register_object(HeapWord *addr);
 929   void register_object_wo_lock(HeapWord *addr);
 930   void coalesce_objects(HeapWord *addr, size_t length_in_words);
 931 
 932   // Return true iff this object is "properly" registered.
 933   bool verify_registration(HeapWord* address, ShenandoahMarkingContext* ctx);
 934 
 935   // clear the cards to clean, and clear the object_starts info to no objects
 936   void mark_range_as_empty(HeapWord *addr, size_t length_in_words);
 937 
 938   // process_clusters() scans a portion of the remembered set during a JVM
 939   // safepoint as part of the root scanning activities that serve to
 940   // initiate concurrent scanning and concurrent evacuation.  Multiple
 941   // threads may scan different portions of the remembered set by
 942   // making parallel invocations of process_clusters() with each
 943   // invocation scanning different clusters of the remembered set.
 944   //
 945   // An invocation of process_clusters() examines all of the
 946   // intergenerational references spanned by count clusters starting
 947   // with first_cluster.  The oops argument is assumed to represent a
 948   // thread-local OopClosure into which addresses of intergenerational
 949   // pointer values will be accumulated for the purposes of root scanning.
 950   //
 951   // A side effect of executing process_clusters() is to update the card
 952   // table entries, marking dirty cards as clean if they no longer
 953   // hold references to young-gen memory.  (THIS IS NOT YET IMPLEMENTED.)
 954   //
 955   // The implementation of process_clusters() is designed to efficiently
 956   // minimize work in the large majority of cases for which the
 957   // associated cluster has very few dirty card-table entries.
 958   //
 959   // At initialization of concurrent marking, invoke process_clusters with
 960   // ClosureType equal to ShenandoahInitMarkRootsClosure.
 961   //
 962   // At initialization of concurrent evacuation, invoke process_clusters with
 963   // ClosureType equal to ShenandoahEvacuateUpdateRootsClosure.
 964 
 965   // This is big enough it probably shouldn't be in-lined.  On the other hand, there are only a few places this
 966   // code is called from, so it might as well be in-lined.  The "real" reason I'm inlining at the moment is because
 967   // the template expansions were making it difficult for the link/loader to resolve references to the template-
 968   // parameterized implementations of this service.
 969   template <typename ClosureType>
 970   inline void process_clusters(size_t first_cluster, size_t count, HeapWord *end_of_range, ClosureType *oops);
 971 
 972   template <typename ClosureType>
 973   inline void process_clusters(size_t first_cluster, size_t count, HeapWord *end_of_range, ClosureType *oops, bool use_write_table);
 974 
 975   template <typename ClosureType>
 976   inline void process_region(ShenandoahHeapRegion* region, ClosureType *cl);
 977 
 978   template <typename ClosureType>
 979   inline void process_region(ShenandoahHeapRegion* region, ClosureType *cl, bool use_write_table);
 980 
 981   // To Do:
 982   //  Create subclasses of ShenandoahInitMarkRootsClosure and
 983   //  ShenandoahEvacuateUpdateRootsClosure and any other closures
 984   //  that need to participate in remembered set scanning.  Within the
 985   //  subclasses, add a (probably templated) instance variable that
 986   //  refers to the associated ShenandoahCardCluster object.  Use this
 987   //  ShenandoahCardCluster instance to "enhance" the do_oops
 988   //  processing so that we can:
 989   //
 990   //   1. Avoid processing references that correspond to clean card
 991   //      regions, and
 992   //   2. Set card status to CLEAN when the associated card region no
 993   //      longer holds inter-generatioanal references.
 994   //
 995   //  To enable efficient implementation of these behaviors, we
 996   //  probably also want to add a few fields into the
 997   //  ShenandoahCardCluster object that allow us to precompute and
 998   //  remember the addresses at which card status is going to change
 999   //  from dirty to clean and clean to dirty.  The do_oops
1000   //  implementations will want to update this value each time they
1001   //  cross one of these boundaries.
1002   void roots_do(OopIterateClosure* cl);
1003 };
1004 
1005 typedef ShenandoahScanRemembered<ShenandoahDirectCardMarkRememberedSet> RememberedScanner;
1006 
1007 class ShenandoahScanRememberedTask : public WorkerTask {
1008  private:
1009   ShenandoahObjToScanQueueSet* _queue_set;
1010   ShenandoahObjToScanQueueSet* _old_queue_set;
1011   ShenandoahReferenceProcessor* _rp;
1012   ShenandoahRegionIterator* _regions;
1013  public:
1014   ShenandoahScanRememberedTask(ShenandoahObjToScanQueueSet* queue_set,
1015                                ShenandoahObjToScanQueueSet* old_queue_set,
1016                                ShenandoahReferenceProcessor* rp,
1017                                ShenandoahRegionIterator* regions);
1018 
1019   void work(uint worker_id);
1020 };
1021 #endif // SHARE_GC_SHENANDOAH_SHENANDOAHSCANREMEMBERED_HPP