1 /*
   2  * Copyright (c) 2001, 2018, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  *
  23  */
  24 
  25 #include "precompiled.hpp"
  26 #include "gc/shared/cardTableRS.hpp"
  27 #include "gc/shared/genCollectedHeap.hpp"
  28 #include "gc/shared/generation.hpp"
  29 #include "gc/shared/space.inline.hpp"
  30 #include "memory/allocation.inline.hpp"
  31 #include "oops/access.inline.hpp"
  32 #include "oops/oop.inline.hpp"
  33 #include "runtime/atomic.hpp"
  34 #include "runtime/java.hpp"
  35 #include "runtime/os.hpp"
  36 #include "utilities/macros.hpp"
  37 
  38 class HasAccumulatedModifiedOopsClosure : public CLDClosure {
  39   bool _found;
  40  public:
  41   HasAccumulatedModifiedOopsClosure() : _found(false) {}
  42   void do_cld(ClassLoaderData* cld) {
  43     if (_found) {
  44       return;
  45     }
  46 
  47     if (cld->has_accumulated_modified_oops()) {
  48       _found = true;
  49     }
  50   }
  51   bool found() {
  52     return _found;
  53   }
  54 };
  55 
  56 bool CLDRemSet::mod_union_is_clear() {
  57   HasAccumulatedModifiedOopsClosure closure;
  58   ClassLoaderDataGraph::cld_do(&closure);
  59 
  60   return !closure.found();
  61 }
  62 
  63 
  64 class ClearCLDModUnionClosure : public CLDClosure {
  65  public:
  66   void do_cld(ClassLoaderData* cld) {
  67     if (cld->has_accumulated_modified_oops()) {
  68       cld->clear_accumulated_modified_oops();
  69     }
  70   }
  71 };
  72 
  73 void CLDRemSet::clear_mod_union() {
  74   ClearCLDModUnionClosure closure;
  75   ClassLoaderDataGraph::cld_do(&closure);
  76 }
  77 
  78 
  79 jbyte CardTableRS::find_unused_youngergenP_card_value() {
  80   for (jbyte v = youngergenP1_card;
  81        v < cur_youngergen_and_prev_nonclean_card;
  82        v++) {
  83     bool seen = false;
  84     for (int g = 0; g < _regions_to_iterate; g++) {
  85       if (_last_cur_val_in_gen[g] == v) {
  86         seen = true;
  87         break;
  88       }
  89     }
  90     if (!seen) {
  91       return v;
  92     }
  93   }
  94   ShouldNotReachHere();
  95   return 0;
  96 }
  97 
  98 void CardTableRS::prepare_for_younger_refs_iterate(bool parallel) {
  99   // Parallel or sequential, we must always set the prev to equal the
 100   // last one written.
 101   if (parallel) {
 102     // Find a parallel value to be used next.
 103     jbyte next_val = find_unused_youngergenP_card_value();
 104     set_cur_youngergen_card_val(next_val);
 105 
 106   } else {
 107     // In an sequential traversal we will always write youngergen, so that
 108     // the inline barrier is  correct.
 109     set_cur_youngergen_card_val(youngergen_card);
 110   }
 111 }
 112 
 113 void CardTableRS::younger_refs_iterate(Generation* g,
 114                                        OopsInGenClosure* blk,
 115                                        uint n_threads) {
 116   // The indexing in this array is slightly odd. We want to access
 117   // the old generation record here, which is at index 2.
 118   _last_cur_val_in_gen[2] = cur_youngergen_card_val();
 119   g->younger_refs_iterate(blk, n_threads);
 120 }
 121 
 122 inline bool ClearNoncleanCardWrapper::clear_card(jbyte* entry) {
 123   if (_is_par) {
 124     return clear_card_parallel(entry);
 125   } else {
 126     return clear_card_serial(entry);
 127   }
 128 }
 129 
 130 inline bool ClearNoncleanCardWrapper::clear_card_parallel(jbyte* entry) {
 131   while (true) {
 132     // In the parallel case, we may have to do this several times.
 133     jbyte entry_val = *entry;
 134     assert(entry_val != CardTableRS::clean_card_val(),
 135            "We shouldn't be looking at clean cards, and this should "
 136            "be the only place they get cleaned.");
 137     if (CardTableRS::card_is_dirty_wrt_gen_iter(entry_val)
 138         || _ct->is_prev_youngergen_card_val(entry_val)) {
 139       jbyte res =
 140         Atomic::cmpxchg(CardTableRS::clean_card_val(), entry, entry_val);
 141       if (res == entry_val) {
 142         break;
 143       } else {
 144         assert(res == CardTableRS::cur_youngergen_and_prev_nonclean_card,
 145                "The CAS above should only fail if another thread did "
 146                "a GC write barrier.");
 147       }
 148     } else if (entry_val ==
 149                CardTableRS::cur_youngergen_and_prev_nonclean_card) {
 150       // Parallelism shouldn't matter in this case.  Only the thread
 151       // assigned to scan the card should change this value.
 152       *entry = _ct->cur_youngergen_card_val();
 153       break;
 154     } else {
 155       assert(entry_val == _ct->cur_youngergen_card_val(),
 156              "Should be the only possibility.");
 157       // In this case, the card was clean before, and become
 158       // cur_youngergen only because of processing of a promoted object.
 159       // We don't have to look at the card.
 160       return false;
 161     }
 162   }
 163   return true;
 164 }
 165 
 166 
 167 inline bool ClearNoncleanCardWrapper::clear_card_serial(jbyte* entry) {
 168   jbyte entry_val = *entry;
 169   assert(entry_val != CardTableRS::clean_card_val(),
 170          "We shouldn't be looking at clean cards, and this should "
 171          "be the only place they get cleaned.");
 172   assert(entry_val != CardTableRS::cur_youngergen_and_prev_nonclean_card,
 173          "This should be possible in the sequential case.");
 174   *entry = CardTableRS::clean_card_val();
 175   return true;
 176 }
 177 
 178 ClearNoncleanCardWrapper::ClearNoncleanCardWrapper(
 179   DirtyCardToOopClosure* dirty_card_closure, CardTableRS* ct, bool is_par) :
 180     _dirty_card_closure(dirty_card_closure), _ct(ct), _is_par(is_par) {
 181 }
 182 
 183 bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) {
 184   return (((intptr_t)entry) & (BytesPerWord-1)) == 0;
 185 }
 186 
 187 // The regions are visited in *decreasing* address order.
 188 // This order aids with imprecise card marking, where a dirty
 189 // card may cause scanning, and summarization marking, of objects
 190 // that extend onto subsequent cards.
 191 void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) {
 192   assert(mr.word_size() > 0, "Error");
 193   assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned");
 194   // mr.end() may not necessarily be card aligned.
 195   jbyte* cur_entry = _ct->byte_for(mr.last());
 196   const jbyte* limit = _ct->byte_for(mr.start());
 197   HeapWord* end_of_non_clean = mr.end();
 198   HeapWord* start_of_non_clean = end_of_non_clean;
 199   while (cur_entry >= limit) {
 200     HeapWord* cur_hw = _ct->addr_for(cur_entry);
 201     if ((*cur_entry != CardTableRS::clean_card_val()) && clear_card(cur_entry)) {
 202       // Continue the dirty range by opening the
 203       // dirty window one card to the left.
 204       start_of_non_clean = cur_hw;
 205     } else {
 206       // We hit a "clean" card; process any non-empty
 207       // "dirty" range accumulated so far.
 208       if (start_of_non_clean < end_of_non_clean) {
 209         const MemRegion mrd(start_of_non_clean, end_of_non_clean);
 210         _dirty_card_closure->do_MemRegion(mrd);
 211       }
 212 
 213       // fast forward through potential continuous whole-word range of clean cards beginning at a word-boundary
 214       if (is_word_aligned(cur_entry)) {
 215         jbyte* cur_row = cur_entry - BytesPerWord;
 216         while (cur_row >= limit && *((intptr_t*)cur_row) ==  CardTableRS::clean_card_row_val()) {
 217           cur_row -= BytesPerWord;
 218         }
 219         cur_entry = cur_row + BytesPerWord;
 220         cur_hw = _ct->addr_for(cur_entry);
 221       }
 222 
 223       // Reset the dirty window, while continuing to look
 224       // for the next dirty card that will start a
 225       // new dirty window.
 226       end_of_non_clean = cur_hw;
 227       start_of_non_clean = cur_hw;
 228     }
 229     // Note that "cur_entry" leads "start_of_non_clean" in
 230     // its leftward excursion after this point
 231     // in the loop and, when we hit the left end of "mr",
 232     // will point off of the left end of the card-table
 233     // for "mr".
 234     cur_entry--;
 235   }
 236   // If the first card of "mr" was dirty, we will have
 237   // been left with a dirty window, co-initial with "mr",
 238   // which we now process.
 239   if (start_of_non_clean < end_of_non_clean) {
 240     const MemRegion mrd(start_of_non_clean, end_of_non_clean);
 241     _dirty_card_closure->do_MemRegion(mrd);
 242   }
 243 }
 244 
 245 // clean (by dirty->clean before) ==> cur_younger_gen
 246 // dirty                          ==> cur_youngergen_and_prev_nonclean_card
 247 // precleaned                     ==> cur_youngergen_and_prev_nonclean_card
 248 // prev-younger-gen               ==> cur_youngergen_and_prev_nonclean_card
 249 // cur-younger-gen                ==> cur_younger_gen
 250 // cur_youngergen_and_prev_nonclean_card ==> no change.
 251 void CardTableRS::write_ref_field_gc_par(void* field, oop new_val) {
 252   volatile jbyte* entry = byte_for(field);
 253   do {
 254     jbyte entry_val = *entry;
 255     // We put this first because it's probably the most common case.
 256     if (entry_val == clean_card_val()) {
 257       // No threat of contention with cleaning threads.
 258       *entry = cur_youngergen_card_val();
 259       return;
 260     } else if (card_is_dirty_wrt_gen_iter(entry_val)
 261                || is_prev_youngergen_card_val(entry_val)) {
 262       // Mark it as both cur and prev youngergen; card cleaning thread will
 263       // eventually remove the previous stuff.
 264       jbyte new_val = cur_youngergen_and_prev_nonclean_card;
 265       jbyte res = Atomic::cmpxchg(new_val, entry, entry_val);
 266       // Did the CAS succeed?
 267       if (res == entry_val) return;
 268       // Otherwise, retry, to see the new value.
 269       continue;
 270     } else {
 271       assert(entry_val == cur_youngergen_and_prev_nonclean_card
 272              || entry_val == cur_youngergen_card_val(),
 273              "should be only possibilities.");
 274       return;
 275     }
 276   } while (true);
 277 }
 278 
 279 void CardTableRS::younger_refs_in_space_iterate(Space* sp,
 280                                                 OopsInGenClosure* cl,
 281                                                 uint n_threads) {


 282   const MemRegion urasm = sp->used_region_at_save_marks();



 283 #ifdef ASSERT
 284   // Convert the assertion check to a warning if we are running
 285   // CMS+ParNew until related bug is fixed.
 286   MemRegion ur    = sp->used_region();
 287   assert(ur.contains(urasm) || (UseConcMarkSweepGC),


 288          "Did you forget to call save_marks()? "
 289          "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
 290          "[" PTR_FORMAT ", " PTR_FORMAT ")",
 291          p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end()));
 292   // In the case of CMS+ParNew, issue a warning
 293   if (!ur.contains(urasm)) {
 294     assert(UseConcMarkSweepGC, "Tautology: see assert above");
 295     log_warning(gc)("CMS+ParNew: Did you forget to call save_marks()? "
 296                     "[" PTR_FORMAT ", " PTR_FORMAT ") is not contained in "
 297                     "[" PTR_FORMAT ", " PTR_FORMAT ")",
 298                     p2i(urasm.start()), p2i(urasm.end()), p2i(ur.start()), p2i(ur.end()));
 299     MemRegion ur2 = sp->used_region();
 300     MemRegion urasm2 = sp->used_region_at_save_marks();
 301     if (!ur.equals(ur2)) {
 302       log_warning(gc)("CMS+ParNew: Flickering used_region()!!");
 303     }
 304     if (!urasm.equals(urasm2)) {
 305       log_warning(gc)("CMS+ParNew: Flickering used_region_at_save_marks()!!");
 306     }
 307     ShouldNotReachHere();
 308   }
 309 #endif
 310   non_clean_card_iterate_possibly_parallel(sp, urasm, cl, this, n_threads);
 311 }

 312 
 313 void CardTableRS::clear_into_younger(Generation* old_gen) {
 314   assert(GenCollectedHeap::heap()->is_old_gen(old_gen),
 315          "Should only be called for the old generation");
 316   // The card tables for the youngest gen need never be cleared.
 317   // There's a bit of subtlety in the clear() and invalidate()
 318   // methods that we exploit here and in invalidate_or_clear()
 319   // below to avoid missing cards at the fringes. If clear() or
 320   // invalidate() are changed in the future, this code should
 321   // be revisited. 20040107.ysr
 322   clear(old_gen->prev_used_region());
 323 }
 324 
 325 void CardTableRS::invalidate_or_clear(Generation* old_gen) {
 326   assert(GenCollectedHeap::heap()->is_old_gen(old_gen),
 327          "Should only be called for the old generation");
 328   // Invalidate the cards for the currently occupied part of
 329   // the old generation and clear the cards for the
 330   // unoccupied part of the generation (if any, making use
 331   // of that generation's prev_used_region to determine that
 332   // region). No need to do anything for the youngest
 333   // generation. Also see note#20040107.ysr above.
 334   MemRegion used_mr = old_gen->used_region();
 335   MemRegion to_be_cleared_mr = old_gen->prev_used_region().minus(used_mr);
 336   if (!to_be_cleared_mr.is_empty()) {
 337     clear(to_be_cleared_mr);
 338   }
 339   invalidate(used_mr);
 340 }
 341 
 342 
 343 class VerifyCleanCardClosure: public OopClosure {
 344 private:
 345   HeapWord* _boundary;
 346   HeapWord* _begin;
 347   HeapWord* _end;
 348 protected:
 349   template <class T> void do_oop_work(T* p) {
 350     HeapWord* jp = (HeapWord*)p;
 351     assert(jp >= _begin && jp < _end,
 352            "Error: jp " PTR_FORMAT " should be within "
 353            "[_begin, _end) = [" PTR_FORMAT "," PTR_FORMAT ")",
 354            p2i(jp), p2i(_begin), p2i(_end));
 355     oop obj = RawAccess<>::oop_load(p);
 356     guarantee(obj == NULL || (HeapWord*)obj >= _boundary,
 357               "pointer " PTR_FORMAT " at " PTR_FORMAT " on "
 358               "clean card crosses boundary" PTR_FORMAT,
 359               p2i(obj), p2i(jp), p2i(_boundary));
 360   }
 361 
 362 public:
 363   VerifyCleanCardClosure(HeapWord* b, HeapWord* begin, HeapWord* end) :
 364     _boundary(b), _begin(begin), _end(end) {
 365     assert(b <= begin,
 366            "Error: boundary " PTR_FORMAT " should be at or below begin " PTR_FORMAT,
 367            p2i(b), p2i(begin));
 368     assert(begin <= end,
 369            "Error: begin " PTR_FORMAT " should be strictly below end " PTR_FORMAT,
 370            p2i(begin), p2i(end));
 371   }
 372 
 373   virtual void do_oop(oop* p)       { VerifyCleanCardClosure::do_oop_work(p); }
 374   virtual void do_oop(narrowOop* p) { VerifyCleanCardClosure::do_oop_work(p); }
 375 };
 376 
 377 class VerifyCTSpaceClosure: public SpaceClosure {
 378 private:
 379   CardTableRS* _ct;
 380   HeapWord* _boundary;
 381 public:
 382   VerifyCTSpaceClosure(CardTableRS* ct, HeapWord* boundary) :
 383     _ct(ct), _boundary(boundary) {}
 384   virtual void do_space(Space* s) { _ct->verify_space(s, _boundary); }
 385 };
 386 
 387 class VerifyCTGenClosure: public GenCollectedHeap::GenClosure {
 388   CardTableRS* _ct;
 389 public:
 390   VerifyCTGenClosure(CardTableRS* ct) : _ct(ct) {}
 391   void do_generation(Generation* gen) {
 392     // Skip the youngest generation.
 393     if (GenCollectedHeap::heap()->is_young_gen(gen)) {
 394       return;
 395     }
 396     // Normally, we're interested in pointers to younger generations.
 397     VerifyCTSpaceClosure blk(_ct, gen->reserved().start());
 398     gen->space_iterate(&blk, true);
 399   }
 400 };
 401 
 402 void CardTableRS::verify_space(Space* s, HeapWord* gen_boundary) {
 403   // We don't need to do young-gen spaces.
 404   if (s->end() <= gen_boundary) return;
 405   MemRegion used = s->used_region();
 406 
 407   jbyte* cur_entry = byte_for(used.start());
 408   jbyte* limit = byte_after(used.last());
 409   while (cur_entry < limit) {
 410     if (*cur_entry == clean_card_val()) {
 411       jbyte* first_dirty = cur_entry+1;
 412       while (first_dirty < limit &&
 413              *first_dirty == clean_card_val()) {
 414         first_dirty++;
 415       }
 416       // If the first object is a regular object, and it has a
 417       // young-to-old field, that would mark the previous card.
 418       HeapWord* boundary = addr_for(cur_entry);
 419       HeapWord* end = (first_dirty >= limit) ? used.end() : addr_for(first_dirty);
 420       HeapWord* boundary_block = s->block_start(boundary);
 421       HeapWord* begin = boundary;             // Until proven otherwise.
 422       HeapWord* start_block = boundary_block; // Until proven otherwise.
 423       if (boundary_block < boundary) {
 424         if (s->block_is_obj(boundary_block) && s->obj_is_alive(boundary_block)) {
 425           oop boundary_obj = oop(boundary_block);
 426           if (!boundary_obj->is_objArray() &&
 427               !boundary_obj->is_typeArray()) {
 428             guarantee(cur_entry > byte_for(used.start()),
 429                       "else boundary would be boundary_block");
 430             if (*byte_for(boundary_block) != clean_card_val()) {
 431               begin = boundary_block + s->block_size(boundary_block);
 432               start_block = begin;
 433             }
 434           }
 435         }
 436       }
 437       // Now traverse objects until end.
 438       if (begin < end) {
 439         MemRegion mr(begin, end);
 440         VerifyCleanCardClosure verify_blk(gen_boundary, begin, end);
 441         for (HeapWord* cur = start_block; cur < end; cur += s->block_size(cur)) {
 442           if (s->block_is_obj(cur) && s->obj_is_alive(cur)) {
 443             oop(cur)->oop_iterate_no_header(&verify_blk, mr);
 444           }
 445         }
 446       }
 447       cur_entry = first_dirty;
 448     } else {
 449       // We'd normally expect that cur_youngergen_and_prev_nonclean_card
 450       // is a transient value, that cannot be in the card table
 451       // except during GC, and thus assert that:
 452       // guarantee(*cur_entry != cur_youngergen_and_prev_nonclean_card,
 453       //        "Illegal CT value");
 454       // That however, need not hold, as will become clear in the
 455       // following...
 456 
 457       // We'd normally expect that if we are in the parallel case,
 458       // we can't have left a prev value (which would be different
 459       // from the current value) in the card table, and so we'd like to
 460       // assert that:
 461       // guarantee(cur_youngergen_card_val() == youngergen_card
 462       //           || !is_prev_youngergen_card_val(*cur_entry),
 463       //           "Illegal CT value");
 464       // That, however, may not hold occasionally, because of
 465       // CMS or MSC in the old gen. To wit, consider the
 466       // following two simple illustrative scenarios:
 467       // (a) CMS: Consider the case where a large object L
 468       //     spanning several cards is allocated in the old
 469       //     gen, and has a young gen reference stored in it, dirtying
 470       //     some interior cards. A young collection scans the card,
 471       //     finds a young ref and installs a youngergenP_n value.
 472       //     L then goes dead. Now a CMS collection starts,
 473       //     finds L dead and sweeps it up. Assume that L is
 474       //     abutting _unallocated_blk, so _unallocated_blk is
 475       //     adjusted down to (below) L. Assume further that
 476       //     no young collection intervenes during this CMS cycle.
 477       //     The next young gen cycle will not get to look at this
 478       //     youngergenP_n card since it lies in the unoccupied
 479       //     part of the space.
 480       //     Some young collections later the blocks on this
 481       //     card can be re-allocated either due to direct allocation
 482       //     or due to absorbing promotions. At this time, the
 483       //     before-gc verification will fail the above assert.
 484       // (b) MSC: In this case, an object L with a young reference
 485       //     is on a card that (therefore) holds a youngergen_n value.
 486       //     Suppose also that L lies towards the end of the used
 487       //     the used space before GC. An MSC collection
 488       //     occurs that compacts to such an extent that this
 489       //     card is no longer in the occupied part of the space.
 490       //     Since current code in MSC does not always clear cards
 491       //     in the unused part of old gen, this stale youngergen_n
 492       //     value is left behind and can later be covered by
 493       //     an object when promotion or direct allocation
 494       //     re-allocates that part of the heap.
 495       //
 496       // Fortunately, the presence of such stale card values is
 497       // "only" a minor annoyance in that subsequent young collections
 498       // might needlessly scan such cards, but would still never corrupt
 499       // the heap as a result. However, it's likely not to be a significant
 500       // performance inhibitor in practice. For instance,
 501       // some recent measurements with unoccupied cards eagerly cleared
 502       // out to maintain this invariant, showed next to no
 503       // change in young collection times; of course one can construct
 504       // degenerate examples where the cost can be significant.)
 505       // Note, in particular, that if the "stale" card is modified
 506       // after re-allocation, it would be dirty, not "stale". Thus,
 507       // we can never have a younger ref in such a card and it is
 508       // safe not to scan that card in any collection. [As we see
 509       // below, we do some unnecessary scanning
 510       // in some cases in the current parallel scanning algorithm.]
 511       //
 512       // The main point below is that the parallel card scanning code
 513       // deals correctly with these stale card values. There are two main
 514       // cases to consider where we have a stale "young gen" value and a
 515       // "derivative" case to consider, where we have a stale
 516       // "cur_younger_gen_and_prev_non_clean" value, as will become
 517       // apparent in the case analysis below.
 518       // o Case 1. If the stale value corresponds to a younger_gen_n
 519       //   value other than the cur_younger_gen value then the code
 520       //   treats this as being tantamount to a prev_younger_gen
 521       //   card. This means that the card may be unnecessarily scanned.
 522       //   There are two sub-cases to consider:
 523       //   o Case 1a. Let us say that the card is in the occupied part
 524       //     of the generation at the time the collection begins. In
 525       //     that case the card will be either cleared when it is scanned
 526       //     for young pointers, or will be set to cur_younger_gen as a
 527       //     result of promotion. (We have elided the normal case where
 528       //     the scanning thread and the promoting thread interleave
 529       //     possibly resulting in a transient
 530       //     cur_younger_gen_and_prev_non_clean value before settling
 531       //     to cur_younger_gen. [End Case 1a.]
 532       //   o Case 1b. Consider now the case when the card is in the unoccupied
 533       //     part of the space which becomes occupied because of promotions
 534       //     into it during the current young GC. In this case the card
 535       //     will never be scanned for young references. The current
 536       //     code will set the card value to either
 537       //     cur_younger_gen_and_prev_non_clean or leave
 538       //     it with its stale value -- because the promotions didn't
 539       //     result in any younger refs on that card. Of these two
 540       //     cases, the latter will be covered in Case 1a during
 541       //     a subsequent scan. To deal with the former case, we need
 542       //     to further consider how we deal with a stale value of
 543       //     cur_younger_gen_and_prev_non_clean in our case analysis
 544       //     below. This we do in Case 3 below. [End Case 1b]
 545       //   [End Case 1]
 546       // o Case 2. If the stale value corresponds to cur_younger_gen being
 547       //   a value not necessarily written by a current promotion, the
 548       //   card will not be scanned by the younger refs scanning code.
 549       //   (This is OK since as we argued above such cards cannot contain
 550       //   any younger refs.) The result is that this value will be
 551       //   treated as a prev_younger_gen value in a subsequent collection,
 552       //   which is addressed in Case 1 above. [End Case 2]
 553       // o Case 3. We here consider the "derivative" case from Case 1b. above
 554       //   because of which we may find a stale
 555       //   cur_younger_gen_and_prev_non_clean card value in the table.
 556       //   Once again, as in Case 1, we consider two subcases, depending
 557       //   on whether the card lies in the occupied or unoccupied part
 558       //   of the space at the start of the young collection.
 559       //   o Case 3a. Let us say the card is in the occupied part of
 560       //     the old gen at the start of the young collection. In that
 561       //     case, the card will be scanned by the younger refs scanning
 562       //     code which will set it to cur_younger_gen. In a subsequent
 563       //     scan, the card will be considered again and get its final
 564       //     correct value. [End Case 3a]
 565       //   o Case 3b. Now consider the case where the card is in the
 566       //     unoccupied part of the old gen, and is occupied as a result
 567       //     of promotions during thus young gc. In that case,
 568       //     the card will not be scanned for younger refs. The presence
 569       //     of newly promoted objects on the card will then result in
 570       //     its keeping the value cur_younger_gen_and_prev_non_clean
 571       //     value, which we have dealt with in Case 3 here. [End Case 3b]
 572       //   [End Case 3]
 573       //
 574       // (Please refer to the code in the helper class
 575       // ClearNonCleanCardWrapper and in CardTable for details.)
 576       //
 577       // The informal arguments above can be tightened into a formal
 578       // correctness proof and it behooves us to write up such a proof,
 579       // or to use model checking to prove that there are no lingering
 580       // concerns.
 581       //
 582       // Clearly because of Case 3b one cannot bound the time for
 583       // which a card will retain what we have called a "stale" value.
 584       // However, one can obtain a Loose upper bound on the redundant
 585       // work as a result of such stale values. Note first that any
 586       // time a stale card lies in the occupied part of the space at
 587       // the start of the collection, it is scanned by younger refs
 588       // code and we can define a rank function on card values that
 589       // declines when this is so. Note also that when a card does not
 590       // lie in the occupied part of the space at the beginning of a
 591       // young collection, its rank can either decline or stay unchanged.
 592       // In this case, no extra work is done in terms of redundant
 593       // younger refs scanning of that card.
 594       // Then, the case analysis above reveals that, in the worst case,
 595       // any such stale card will be scanned unnecessarily at most twice.
 596       //
 597       // It is nonetheless advisable to try and get rid of some of this
 598       // redundant work in a subsequent (low priority) re-design of
 599       // the card-scanning code, if only to simplify the underlying
 600       // state machine analysis/proof. ysr 1/28/2002. XXX
 601       cur_entry++;
 602     }
 603   }
 604 }
 605 
 606 void CardTableRS::verify() {
 607   // At present, we only know how to verify the card table RS for
 608   // generational heaps.
 609   VerifyCTGenClosure blk(this);
 610   GenCollectedHeap::heap()->generation_iterate(&blk, false);
 611   CardTable::verify();
 612 }
 613 
 614 CardTableRS::CardTableRS(MemRegion whole_heap) :
 615   CardTable(whole_heap, /* scanned concurrently */ UseConcMarkSweepGC && CMSPrecleaningEnabled),
 616   _cur_youngergen_card_val(youngergenP1_card),
 617   // LNC functionality
 618   _lowest_non_clean(NULL),
 619   _lowest_non_clean_chunk_size(NULL),
 620   _lowest_non_clean_base_chunk_index(NULL),
 621   _last_LNC_resizing_collection(NULL)
 622 {
 623   // max_gens is really GenCollectedHeap::heap()->gen_policy()->number_of_generations()
 624   // (which is always 2, young & old), but GenCollectedHeap has not been initialized yet.
 625   uint max_gens = 2;
 626   _last_cur_val_in_gen = NEW_C_HEAP_ARRAY3(jbyte, max_gens + 1,
 627                          mtGC, CURRENT_PC, AllocFailStrategy::RETURN_NULL);
 628   if (_last_cur_val_in_gen == NULL) {
 629     vm_exit_during_initialization("Could not create last_cur_val_in_gen array.");
 630   }
 631   for (uint i = 0; i < max_gens + 1; i++) {
 632     _last_cur_val_in_gen[i] = clean_card_val();
 633   }
 634 }
 635 
 636 CardTableRS::~CardTableRS() {
 637   if (_last_cur_val_in_gen) {
 638     FREE_C_HEAP_ARRAY(jbyte, _last_cur_val_in_gen);
 639     _last_cur_val_in_gen = NULL;
 640   }
 641   if (_lowest_non_clean) {
 642     FREE_C_HEAP_ARRAY(CardArr, _lowest_non_clean);
 643     _lowest_non_clean = NULL;
 644   }
 645   if (_lowest_non_clean_chunk_size) {
 646     FREE_C_HEAP_ARRAY(size_t, _lowest_non_clean_chunk_size);
 647     _lowest_non_clean_chunk_size = NULL;
 648   }
 649   if (_lowest_non_clean_base_chunk_index) {
 650     FREE_C_HEAP_ARRAY(uintptr_t, _lowest_non_clean_base_chunk_index);
 651     _lowest_non_clean_base_chunk_index = NULL;
 652   }
 653   if (_last_LNC_resizing_collection) {
 654     FREE_C_HEAP_ARRAY(int, _last_LNC_resizing_collection);
 655     _last_LNC_resizing_collection = NULL;
 656   }
 657 }
 658 
 659 void CardTableRS::initialize() {
 660   CardTable::initialize();
 661   _lowest_non_clean =
 662     NEW_C_HEAP_ARRAY(CardArr, _max_covered_regions, mtGC);
 663   _lowest_non_clean_chunk_size =
 664     NEW_C_HEAP_ARRAY(size_t, _max_covered_regions, mtGC);
 665   _lowest_non_clean_base_chunk_index =
 666     NEW_C_HEAP_ARRAY(uintptr_t, _max_covered_regions, mtGC);
 667   _last_LNC_resizing_collection =
 668     NEW_C_HEAP_ARRAY(int, _max_covered_regions, mtGC);
 669   if (_lowest_non_clean == NULL
 670       || _lowest_non_clean_chunk_size == NULL
 671       || _lowest_non_clean_base_chunk_index == NULL
 672       || _last_LNC_resizing_collection == NULL)
 673     vm_exit_during_initialization("couldn't allocate an LNC array.");
 674   for (int i = 0; i < _max_covered_regions; i++) {
 675     _lowest_non_clean[i] = NULL;
 676     _lowest_non_clean_chunk_size[i] = 0;
 677     _last_LNC_resizing_collection[i] = -1;
 678   }
 679 }
 680 
 681 bool CardTableRS::card_will_be_scanned(jbyte cv) {
 682   return card_is_dirty_wrt_gen_iter(cv) || is_prev_nonclean_card_val(cv);
 683 }
 684 
 685 bool CardTableRS::card_may_have_been_dirty(jbyte cv) {
 686   return
 687     cv != clean_card &&
 688     (card_is_dirty_wrt_gen_iter(cv) ||
 689      CardTableRS::youngergen_may_have_been_dirty(cv));
 690 }
 691 
 692 void CardTableRS::non_clean_card_iterate_possibly_parallel(
 693   Space* sp,
 694   MemRegion mr,
 695   OopsInGenClosure* cl,
 696   CardTableRS* ct,
 697   uint n_threads)
 698 {
 699   if (!mr.is_empty()) {
 700     if (n_threads > 0) {
 701 #if INCLUDE_ALL_GCS
 702       non_clean_card_iterate_parallel_work(sp, mr, cl, ct, n_threads);
 703 #else  // INCLUDE_ALL_GCS
 704       fatal("Parallel gc not supported here.");
 705 #endif // INCLUDE_ALL_GCS
 706     } else {
 707       // clear_cl finds contiguous dirty ranges of cards to process and clear.
 708 
 709       // This is the single-threaded version used by DefNew.
 710       const bool parallel = false;
 711 
 712       DirtyCardToOopClosure* dcto_cl = sp->new_dcto_cl(cl, precision(), cl->gen_boundary(), parallel);
 713       ClearNoncleanCardWrapper clear_cl(dcto_cl, ct, parallel);
 714 
 715       clear_cl.do_MemRegion(mr);
 716     }
 717   }






 718 }
 719 
 720 bool CardTableRS::is_in_young(oop obj) const {
 721   return GenCollectedHeap::heap()->is_in_young(obj);
 722 }
--- EOF ---