junction/ConcurrentMap_Grampa.h

   1 /*------------------------------------------------------------------------
   2   Junction: Concurrent data structures in C++
   3   Copyright (c) 2016 Jeff Preshing
   4
   5   Distributed under the Simplified BSD License.
   6   Original location: https://github.com/preshing/junction
   7
   8   This software is distributed WITHOUT ANY WARRANTY; without even the
   9   implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
  10   See the LICENSE file for more information.
  11 ------------------------------------------------------------------------*/
  12
  13 #ifndef JUNCTION_CONCURRENTMAP_GRAMPA_H
  14 #define JUNCTION_CONCURRENTMAP_GRAMPA_H
  15
  16 #include <junction/Core.h>
  17 #include <junction/details/Grampa.h>
  18 #include <junction/QSBR.h>
  19 #include <turf/Heap.h>
  20 #include <turf/Trace.h>
  21
  22 namespace junction {
  23
  24 TURF_TRACE_DECLARE(ConcurrentMap_Grampa, 27)
  25
  26 template <typename K, typename V, class KT = DefaultKeyTraits<K>, class VT = DefaultValueTraits<V>>
  27 class ConcurrentMap_Grampa {
  28 public:
  29     typedef K Key;
  30     typedef V Value;
  31     typedef KT KeyTraits;
  32     typedef VT ValueTraits;
  33     typedef typename turf::util::BestFit<Key>::Unsigned Hash;
  34     typedef details::Grampa<ConcurrentMap_Grampa> Details;
  35
  36 private:
  37     turf::Atomic<uptr> m_root;
  38
  39     bool locateTable(typename Details::Table*& table, ureg& sizeMask, Hash hash) {
  40         ureg root = m_root.load(turf::Consume);
  41         if (root & 1) {
  42             typename Details::FlatTree* flatTree = (typename Details::FlatTree*) (root & ~ureg(1));
  43             for (;;) {
  44                 ureg leafIdx = (hash >> flatTree->safeShift);
  45                 table = flatTree->getTables()[leafIdx].load(turf::Relaxed);
  46                 if (ureg(table) != Details::RedirectFlatTree) {
  47                     sizeMask = (Details::LeafSize - 1);
  48                     return true;
  49                 }
  50                 TURF_TRACE(ConcurrentMap_Grampa, 0, "[locateTable] flattree lookup redirected", uptr(flatTree), uptr(leafIdx));
  51                 typename Details::FlatTreeMigration* migration = Details::getExistingFlatTreeMigration(flatTree);
  52                 migration->run();
  53                 migration->m_completed.wait();
  54                 flatTree = migration->m_destination;
  55             }
  56         } else {
  57             if (!root)
  58                 return false;
  59             table = (typename Details::Table*) root;
  60             sizeMask = table->sizeMask;
  61             return true;
  62         }
  63     }
  64
  65     void createInitialTable(ureg initialSize) {
  66         if (!m_root.load(turf::Relaxed)) {
  67             // This could perform DCLI, but let's avoid needing a mutex instead.
  68             typename Details::Table* table = Details::Table::create(initialSize, 0, sizeof(Hash) * 8);
  69             if (m_root.compareExchange(uptr(NULL), uptr(table), turf::Release)) {
  70                 TURF_TRACE(ConcurrentMap_Grampa, 1, "[createInitialTable] race to create initial table", uptr(this), 0);
  71                 table->destroy();
  72             }
  73         }
  74     }
  75
  76 public:
  77     ConcurrentMap_Grampa(ureg initialSize = 0) : m_root(uptr(NULL)) {
  78         // FIXME: Support initialSize argument
  79         TURF_UNUSED(initialSize);
  80     }
  81
  82     ~ConcurrentMap_Grampa() {
  83         ureg root = m_root.loadNonatomic();
  84         if (root & 1) {
  85             typename Details::FlatTree* flatTree = (typename Details::FlatTree*) (root & ~ureg(1));
  86             ureg size = (Hash(-1) >> flatTree->safeShift) + 1;
  87             typename Details::Table* lastTableGCed = NULL;
  88             for (ureg i = 0; i < size; i++) {
  89                 typename Details::Table* t = flatTree->getTables()[i].loadNonatomic();
  90                 TURF_ASSERT(ureg(t) != Details::RedirectFlatTree);
  91                 if (t != lastTableGCed) {
  92                     t->destroy();
  93                     lastTableGCed = t;
  94                 }
  95             }
  96             flatTree->destroy();
  97         } else if (root) {
  98             typename Details::Table* t = (typename Details::Table*) root;
  99             t->destroy();
 100         }
 101     }
 102
 103     // publishTableMigration() is called by exactly one thread from Details::TableMigration::run()
 104     // after all the threads participating in the migration have completed their work.
 105     // There are no racing writes to the same range of hashes.
 106     void publishTableMigration(typename Details::TableMigration* migration) {
 107         TURF_TRACE(ConcurrentMap_Grampa, 2, "[publishTableMigration] called", uptr(migration), 0);
 108         if (migration->m_safeShift == 0) {
 109             // This TableMigration replaces the entire map with a single table.
 110             TURF_ASSERT(migration->m_baseHash == 0);
 111             TURF_ASSERT(migration->m_numDestinations == 1);
 112             ureg oldRoot = m_root.loadNonatomic();  // There are no racing writes to m_root.
 113             // Store the single table in m_root directly.
 114             typename Details::Table* newTable = migration->getDestinations()[0];
 115             m_root.store(uptr(newTable), turf::Release);   // Make table contents visible
 116             newTable->isPublished.signal();
 117             if ((oldRoot & 1) == 0) {
 118                 TURF_TRACE(ConcurrentMap_Grampa, 3, "[publishTableMigration] replacing single root with single root", uptr(migration), 0);
 119                 // If oldRoot is a table, it must be the original source of the migration.
 120                 TURF_ASSERT((typename Details::Table*) oldRoot == migration->getSources()[0].table);
 121                 // Don't GC it here. The caller will GC it since it's a source of the TableMigration.
 122             } else {
 123                 TURF_TRACE(ConcurrentMap_Grampa, 4, "[publishTableMigration] replacing flattree with single root", uptr(migration), 0);
 124                 // The entire previous flattree is being replaced.
 125                 Details::garbageCollectFlatTree((typename Details::FlatTree*) (oldRoot & ~ureg(1)));
 126             }
 127             // Caller will GC the TableMigration.
 128         } else {
 129             // We are either publishing a subtree of one or more tables, or replacing the entire map with multiple tables.
 130             // In either case, there will be a flattree after this function returns.
 131             TURF_ASSERT(migration->m_safeShift < sizeof(Hash) * 8);     // If m_numDestinations > 1, some index bits must remain after shifting
 132             ureg oldRoot = m_root.load(turf::Consume);
 133             if ((oldRoot & 1) == 0) {
 134                 // There's no flattree yet. This means the TableMigration is publishing the full range of hashes.
 135                 TURF_ASSERT(migration->m_baseHash == 0);
 136                 TURF_ASSERT((Hash(-1) >> migration->m_safeShift) == (migration->m_numDestinations - 1));
 137                 // The oldRoot should be the original source of the migration.
 138                 TURF_ASSERT((typename Details::Table*) oldRoot == migration->getSources()[0].table);
 139                 // Furthermore, it is guaranteed that there are no racing writes to m_root.
 140                 // Create a new flattree and store it to m_root.
 141                 TURF_TRACE(ConcurrentMap_Grampa, 5, "[publishTableMigration] replacing single root with flattree", uptr(migration), 0);
 142                 typename Details::FlatTree* flatTree = Details::FlatTree::create(migration->m_safeShift);
 143                 typename Details::Table* prevTable = NULL;
 144                 for (ureg i = 0; i < migration->m_numDestinations; i++) {
 145                     typename Details::Table* newTable = migration->getDestinations()[i];
 146                     flatTree->getTables()[i].storeNonatomic(newTable);
 147                     if (newTable != prevTable) {
 148                         newTable->isPublished.signal();
 149                         prevTable = newTable;
 150                     }
 151                 }
 152                 m_root.store(uptr(flatTree) | 1, turf::Release);  // Ensure visibility of flatTree->tables
 153                 // Caller will GC the TableMigration.
 154                 // Caller will also GC the old oldRoot since it's a source of the TableMigration.
 155             } else {
 156                 // There is an existing flattree, and we are publishing one or more tables to it.
 157                 // Attempt to publish the subtree in a loop.
 158                 // The loop is necessary because we might get redirected in the middle of publishing.
 159                 TURF_TRACE(ConcurrentMap_Grampa, 6, "[publishTableMigration] publishing subtree to existing flattree", uptr(migration), 0);
 160                 typename Details::FlatTree* flatTree = (typename Details::FlatTree*) (oldRoot & ~ureg(1));
 161                 ureg subTreeEntriesPublished = 0;
 162                 typename Details::Table* tableToReplace = migration->getSources()[0].table;
 163                 // Wait here so that we only replace tables that are fully published.
 164                 // Otherwise, there will be a race between a subtree and its own children.
 165                 // (If all ManualResetEvent objects supported isPublished(), we could add a TURF_TRACE counter for this.
 166                 // In previous tests, such a counter does in fact get hit.)
 167                 tableToReplace->isPublished.wait();
 168                 typename Details::Table* prevTable = NULL;
 169                 for (;;) {
 170                 publishLoop:
 171                     if (migration->m_safeShift < flatTree->safeShift) {
 172                         // We'll need to migrate to larger flattree before publishing our new subtree.
 173                         // First, try to create a FlatTreeMigration with the necessary properties.
 174                         // This will fail if an existing FlatTreeMigration has already been created using the same source.
 175                         // In that case, we'll help complete the existing FlatTreeMigration, then we'll retry the loop.
 176                         TURF_TRACE(ConcurrentMap_Grampa, 7, "[publishTableMigration] existing flattree too small", uptr(migration), 0);
 177                         typename Details::FlatTreeMigration* flatTreeMigration = Details::createFlatTreeMigration(*this, flatTree, migration->m_safeShift);
 178                         tableToReplace->jobCoordinator.runOne(flatTreeMigration);
 179                         flatTreeMigration->m_completed.wait();      // flatTreeMigration->m_destination becomes entirely visible
 180                         flatTree = flatTreeMigration->m_destination;
 181                         // The FlatTreeMigration has already been GC'ed by the last worker.
 182                         // Retry the loop.
 183                     } else {
 184                         ureg repeat = ureg(1) << (migration->m_safeShift - flatTree->safeShift);
 185                         ureg dstStartIndex = migration->m_baseHash >> flatTree->safeShift;
 186                         // The subtree we're about to publish fits inside the flattree.
 187                         TURF_ASSERT(dstStartIndex + migration->m_numDestinations * repeat - 1 <= Hash(-1) >> flatTree->safeShift);
 188                         // If a previous attempt to publish got redirected, resume publishing into the new flattree,
 189                         // starting with the first subtree entry that has not yet been fully published, as given by subTreeEntriesPublished.
 190                         // (Note: We could, in fact, restart the publish operation starting at entry 0. That would be valid too.
 191                         // We are the only thread that can modify this particular range of the flattree at this time.)
 192                         turf::Atomic<typename Details::Table*>* dstLeaf = flatTree->getTables() + dstStartIndex + (subTreeEntriesPublished * repeat);
 193                         typename Details::Table** subFlatTree = migration->getDestinations();
 194                         while (subTreeEntriesPublished < migration->m_numDestinations) {
 195                             typename Details::Table* srcTable = subFlatTree[subTreeEntriesPublished];
 196                             for (ureg r = repeat; r > 0; r--) {
 197                                 typename Details::Table* probeTable = tableToReplace;
 198                                 while (!dstLeaf->compareExchangeStrong(probeTable, srcTable, turf::Relaxed)) {
 199                                     if (ureg(probeTable) == Details::RedirectFlatTree) {
 200                                         // We've been redirected.
 201                                         // Help with the FlatTreeMigration, then try again.
 202                                         TURF_TRACE(ConcurrentMap_Grampa, 8, "[publishTableMigration] redirected", uptr(migration), uptr(dstLeaf));
 203                                         typename Details::FlatTreeMigration* flatTreeMigration = Details::getExistingFlatTreeMigration(flatTree);
 204                                         tableToReplace->jobCoordinator.runOne(flatTreeMigration);
 205                                         flatTreeMigration->m_completed.wait();    // flatTreeMigration->m_destination becomes entirely visible
 206                                         flatTree = flatTreeMigration->m_destination;
 207                                         goto publishLoop;
 208                                     }
 209                                     // The only other possibility is that we were previously redirected, and the subtree entry got partially published.
 210                                     TURF_TRACE(ConcurrentMap_Grampa, 9, "[publishTableMigration] recovering from partial publish", uptr(migration), 0);
 211                                     TURF_ASSERT(probeTable == srcTable);
 212                                 }
 213                                 // The caller will GC the table) being replaced them since it's a source of the TableMigration.
 214                                 dstLeaf++;
 215                             }
 216                             if (prevTable != srcTable) {
 217                                 srcTable->isPublished.signal();
 218                                 prevTable = srcTable;
 219                             }
 220                             subTreeEntriesPublished++;
 221                         }
 222                         // We've successfully published the migrated sub-flattree.
 223                         // Caller will GC the TableMigration.
 224                         break;
 225                     }
 226                 }
 227             }
 228         }
 229     }
 230
 231     void publishFlatTreeMigration(typename Details::FlatTreeMigration* migration) {
 232         // There are no racing writes.
 233         // Old root must be the migration source (a flattree).
 234         TURF_ASSERT(m_root.loadNonatomic() == (ureg(migration->m_source) | 1));
 235         // Publish the new flattree, making entire table contents visible.
 236         m_root.store(uptr(migration->m_destination) | 1, turf::Release);
 237         // Don't GC the old flattree. The FlatTreeMigration will do that, since it's a source.
 238     }
 239
 240     // A Mutator represents a known cell in the hash table.
 241     // It's meant for manipulations within a temporary function scope.
 242     // Obviously you must not call QSBR::Update while holding a Mutator.
 243     // Any operation that modifies the table (exchangeValue, eraseValue)
 244     // may be forced to follow a redirected cell, which changes the Mutator itself.
 245     // Note that even if the Mutator was constructed from an existing cell,
 246     // exchangeValue() can still trigger a resize if the existing cell was previously marked deleted,
 247     // or if another thread deletes the key between the two steps.
 248     class Mutator {
 249     private:
 250         friend class ConcurrentMap_Grampa;
 251
 252         ConcurrentMap_Grampa& m_map;
 253         typename Details::Table* m_table;
 254         ureg m_sizeMask;
 255         typename Details::Cell* m_cell;
 256         Value m_value;
 257
 258         // Constructor: Find existing cell
 259         Mutator(ConcurrentMap_Grampa& map, Key key, bool) : m_map(map), m_value(Value(ValueTraits::NullValue)) {
 260             TURF_TRACE(ConcurrentMap_Grampa, 10, "[Mutator] find constructor called", uptr(map.m_root.load(turf::Relaxed)), uptr(key));
 261             Hash hash = KeyTraits::hash(key);
 262             for (;;) {
 263                 if (!m_map.locateTable(m_table, m_sizeMask, hash))
 264                     return;
 265                 m_cell = Details::find(hash, m_table, m_sizeMask);
 266                 if (!m_cell)
 267                     return;
 268                 m_value = m_cell->value.load(turf::Consume);
 269                 if (m_value != Value(ValueTraits::Redirect))
 270                     return;                        // Found an existing value
 271                 // We've encountered a Redirect value. Help finish the migration.
 272                 TURF_TRACE(ConcurrentMap_Grampa, 11, "[Mutator] find was redirected", uptr(m_table), 0);
 273                 m_table->jobCoordinator.participate();
 274                 // Try again using the latest root.
 275             }
 276         }
 277
 278         // Constructor: Insert cell
 279         Mutator(ConcurrentMap_Grampa& map, Key key) : m_map(map), m_value(Value(ValueTraits::NullValue)) {
 280             TURF_TRACE(ConcurrentMap_Grampa, 12, "[Mutator] insert constructor called", uptr(map.m_root.load(turf::Relaxed)), uptr(key));
 281             Hash hash = KeyTraits::hash(key);
 282             for (;;) {
 283                 if (!m_map.locateTable(m_table, m_sizeMask, hash)) {
 284                     m_map.createInitialTable(Details::MinTableSize);
 285                 } else {
 286                     ureg overflowIdx;
 287                     switch (Details::insert(hash, m_table, m_sizeMask, m_cell, overflowIdx)) {   // Modifies m_cell
 288                         case Details::InsertResult_InsertedNew: {
 289                             // We've inserted a new cell. Don't load m_cell->value.
 290                             return;
 291                         }
 292                         case Details::InsertResult_AlreadyFound: {
 293                             // The hash was already found in the table.
 294                             m_value = m_cell->value.load(turf::Consume);
 295                             if (m_value == Value(ValueTraits::Redirect)) {
 296                                 // We've encountered a Redirect value.
 297                                 TURF_TRACE(ConcurrentMap_Grampa, 13, "[Mutator] insert was redirected", uptr(m_table), uptr(m_value));
 298                                 break;   // Help finish the migration.
 299                             }
 300                             return;     // Found an existing value
 301                         }
 302                         case Details::InsertResult_Overflow: {
 303                             Details::beginTableMigration(m_map, m_table, overflowIdx);
 304                             break;
 305                         }
 306                     }
 307                     // A migration has been started (either by us, or another thread). Participate until it's complete.
 308                     m_table->jobCoordinator.participate();
 309                 }
 310                 // Try again using the latest root.
 311             }
 312         }
 313
 314     public:
 315         Value getValue() const {
 316             // Return previously loaded value. Don't load it again.
 317             return m_value;
 318         }
 319
 320         Value exchangeValue(Value desired) {
 321             TURF_ASSERT(desired != Value(ValueTraits::NullValue));
 322             TURF_ASSERT(m_cell);    // Cell must have been found or inserted
 323             TURF_TRACE(ConcurrentMap_Grampa, 14, "[Mutator::exchangeValue] called", uptr(m_table), uptr(m_value));
 324             for (;;) {
 325                 Value oldValue = m_value;
 326                 if (m_cell->value.compareExchangeStrong(m_value, desired, turf::ConsumeRelease)) {
 327                     // Exchange was successful. Return previous value.
 328                     TURF_TRACE(ConcurrentMap_Grampa, 15, "[Mutator::exchangeValue] exchanged Value", uptr(m_value), uptr(desired));
 329                     Value result = m_value;
 330                     m_value = desired;  // Leave the mutator in a valid state
 331                     return result;
 332                 }
 333                 // The CAS failed and m_value has been updated with the latest value.
 334                 if (m_value != Value(ValueTraits::Redirect)) {
 335                     TURF_TRACE(ConcurrentMap_Grampa, 16, "[Mutator::exchangeValue] detected race to write value", uptr(m_table), uptr(m_value));
 336                     if (oldValue == Value(ValueTraits::NullValue) && m_value != Value(ValueTraits::NullValue)) {
 337                         TURF_TRACE(ConcurrentMap_Grampa, 17, "[Mutator::exchangeValue] racing write inserted new value", uptr(m_table), uptr(m_value));
 338                     }
 339                     // There was a racing write (or erase) to this cell.
 340                     // Pretend we exchanged with ourselves, and just let the racing write win.
 341                     return desired;
 342                 }
 343                 // We've encountered a Redirect value. Help finish the migration.
 344                 TURF_TRACE(ConcurrentMap_Grampa, 18, "[Mutator::exchangeValue] was redirected", uptr(m_table), uptr(m_value));
 345                 Hash hash = m_cell->hash.load(turf::Relaxed);
 346                 for (;;) {
 347                     // Help complete the migration.
 348                     m_table->jobCoordinator.participate();
 349                     // Try again in the latest table.
 350                     // FIXME: locateTable() could return false if the map is concurrently cleared (m_root set to 0).
 351                     // This is not concern yet since clear() is not implemented.
 352                     bool exists = m_map.locateTable(m_table, m_sizeMask, hash);
 353                     TURF_ASSERT(exists);
 354                     TURF_UNUSED(exists);
 355                     m_value = Value(ValueTraits::NullValue);
 356                     ureg overflowIdx;
 357                     switch (Details::insert(hash, m_table, m_sizeMask, m_cell, overflowIdx)) {   // Modifies m_cell
 358                     case Details::InsertResult_AlreadyFound:
 359                         m_value = m_cell->value.load(turf::Consume);
 360                         if (m_value == Value(ValueTraits::Redirect)) {
 361                             TURF_TRACE(ConcurrentMap_Grampa, 19, "[Mutator::exchangeValue] was re-redirected", uptr(m_table), uptr(m_value));
 362                             break;
 363                         }
 364                         goto breakOuter;
 365                     case Details::InsertResult_InsertedNew:
 366                         goto breakOuter;
 367                     case Details::InsertResult_Overflow:
 368                         TURF_TRACE(ConcurrentMap_Grampa, 20, "[Mutator::exchangeValue] overflow after redirect", uptr(m_table), overflowIdx);
 369                         Details::beginTableMigration(m_map, m_table, overflowIdx);
 370                         break;
 371                     }
 372                     // We were redirected... again
 373                 }
 374             breakOuter:
 375                 ;
 376                 // Try again in the new table.
 377             }
 378         }
 379
 380         void setValue(Value desired) {
 381             exchangeValue(desired);
 382         }
 383
 384         Value eraseValue() {
 385             TURF_ASSERT(m_cell);    // Cell must have been found or inserted
 386             TURF_TRACE(ConcurrentMap_Grampa, 21, "[Mutator::eraseValue] called", uptr(m_table), uptr(m_value));
 387             for (;;) {
 388                 if (m_value == Value(ValueTraits::NullValue))
 389                     return m_value;
 390                 TURF_ASSERT(m_cell);    // m_value is non-NullValue, therefore cell must have been found or inserted.
 391                 if (m_cell->value.compareExchangeStrong(m_value, Value(ValueTraits::NullValue), turf::Consume)) {
 392                     // Exchange was successful and a non-NullValue value was erased and returned by reference in m_value.
 393                     TURF_ASSERT(m_value != Value(ValueTraits::NullValue));   // Implied by the test at the start of the loop.
 394                     Value result = m_value;
 395                     m_value = Value(ValueTraits::NullValue);   // Leave the mutator in a valid state
 396                     return result;
 397                 }
 398                 // The CAS failed and m_value has been updated with the latest value.
 399                 TURF_TRACE(ConcurrentMap_Grampa, 22, "[Mutator::eraseValue] detected race to write value", uptr(m_table), uptr(m_value));
 400                 if (m_value != Value(ValueTraits::Redirect)) {
 401                     // There was a racing write (or erase) to this cell.
 402                     // Pretend we erased nothing, and just let the racing write win.
 403                     return Value(ValueTraits::NullValue);
 404                 }
 405                 // We've been redirected to a new table.
 406                 TURF_TRACE(ConcurrentMap_Grampa, 23, "[Mutator::eraseValue] was redirected", uptr(m_table), uptr(m_cell));
 407                 Hash hash = m_cell->hash.load(turf::Relaxed);           // Re-fetch hash
 408                 for (;;) {
 409                     // Help complete the migration.
 410                     m_table->jobCoordinator.participate();
 411                     // Try again in the latest table.
 412                     if (!m_map.locateTable(m_table, m_sizeMask, hash))
 413                         m_cell = NULL;
 414                     else
 415                         m_cell = Details::find(hash, m_table, m_sizeMask);
 416                     if (!m_cell) {
 417                         m_value = Value(ValueTraits::NullValue);
 418                         return m_value;
 419                     }
 420                     m_value = m_cell->value.load(turf::Relaxed);
 421                     if (m_value != Value(ValueTraits::Redirect))
 422                         break;
 423                     TURF_TRACE(ConcurrentMap_Grampa, 24, "[Mutator::eraseValue] was re-redirected", uptr(m_table), uptr(m_cell));
 424                 }
 425             }
 426         }
 427     };
 428
 429     Mutator insert(Key key) {
 430         return Mutator(*this, key);
 431     }
 432
 433     Mutator find(Key key) {
 434         return Mutator(*this, key, false);
 435     }
 436
 437     // Lookup without creating a temporary Mutator.
 438     Value get(Key key) {
 439         Hash hash = KeyTraits::hash(key);
 440         TURF_TRACE(ConcurrentMap_Grampa, 25, "[get] called", uptr(this), uptr(hash));
 441         for (;;) {
 442             typename Details::Table* table;
 443             ureg sizeMask;
 444             if (!locateTable(table, sizeMask, hash))
 445                 return Value(ValueTraits::NullValue);
 446             typename Details::Cell* cell = Details::find(hash, table, sizeMask);
 447             if (!cell)
 448                 return Value(ValueTraits::NullValue);
 449             Value value = cell->value.load(turf::Consume);
 450             if (value != Value(ValueTraits::Redirect))
 451                 return value;                        // Found an existing value
 452             // We've been redirected to a new table. Help with the migration.
 453             TURF_TRACE(ConcurrentMap_Grampa, 26, "[get] was redirected", uptr(table), 0);
 454             table->jobCoordinator.participate();
 455             // Try again in the new table.
 456         }
 457     }
 458
 459     Value insert(Key key, Value desired) {
 460         Mutator iter(*this, key);
 461         return iter.exchangeValue(desired);
 462     }
 463
 464     Value exchange(Key key, Value desired) {
 465         Mutator iter(*this, key);
 466         return iter.exchangeValue(desired);
 467     }
 468
 469     Value erase(Key key) {
 470         Mutator iter(*this, key, false);
 471         return iter.eraseValue();
 472     }
 473
 474     // The easiest way to implement an Iterator is to prevent all Redirects.
 475     // The currrent Iterator does that by forbidding concurrent inserts.
 476     // To make it work with concurrent inserts, we'd need a way to block TableMigrations as the Iterator visits each table.
 477     // FlatTreeMigrations, too.
 478     class Iterator {
 479     private:
 480         typename Details::FlatTree* m_flatTree;
 481         ureg m_flatTreeIdx;
 482         typename Details::Table* m_table;
 483         ureg m_idx;
 484         Key m_hash;
 485         Value m_value;
 486
 487     public:
 488         Iterator(ConcurrentMap_Grampa& map) {
 489             // Since we've forbidden concurrent inserts (for now), nonatomic would suffice here, but let's plan ahead:
 490             ureg root = map.m_root.load(turf::Consume);
 491             if (root & 1) {
 492                 m_flatTree = (typename Details::FlatTree*) (root & ~ureg(1));
 493                 TURF_ASSERT(m_flatTree->getSize() > 0);
 494                 m_flatTreeIdx = 0;
 495                 m_table = m_flatTree->getTables()[0].load(turf::Consume);
 496                 TURF_ASSERT(m_table);
 497                 m_idx = -1;
 498             } else {
 499                 m_flatTree = NULL;
 500                 m_flatTreeIdx = 0;
 501                 m_table = (typename Details::Table*) root;
 502                 m_idx = -1;
 503             }
 504             if (m_table) {
 505                 next();
 506             } else {
 507                 m_hash = KeyTraits::NullHash;
 508                 m_value = Value(ValueTraits::NullValue);
 509             }
 510         }
 511
 512         void next() {
 513             TURF_ASSERT(m_table);
 514             TURF_ASSERT(isValid() || m_idx == -1);  // Either the Iterator is already valid, or we've just started iterating.
 515             for (;;) {
 516             searchInTable:
 517                 m_idx++;
 518                 if (m_idx <= m_table->sizeMask) {
 519                     // Index still inside range of table.
 520                     typename Details::CellGroup* group = m_table->getCellGroups() + (m_idx >> 2);
 521                     typename Details::Cell *cell = group->cells + (m_idx & 3);
 522                     m_hash = cell->hash.load(turf::Relaxed);
 523                     if (m_hash != KeyTraits::NullHash) {
 524                         // Cell has been reserved.
 525                         m_value = cell->value.load(turf::Relaxed);
 526                         TURF_ASSERT(m_value != Value(ValueTraits::Redirect));
 527                         if (m_value != Value(ValueTraits::NullValue))
 528                             return;     // Yield this cell.
 529                     }
 530                 } else {
 531                     // We've advanced past the end of this table.
 532                     if (m_flatTree) {
 533                         // Scan for the next unique table in the flattree.
 534                         while (++m_flatTreeIdx < m_flatTree->getSize()) {
 535                             typename Details::Table* nextTable = m_flatTree->getTables()[m_flatTreeIdx].load(turf::Consume);
 536                             if (nextTable != m_table) {
 537                                 // Found the next table.
 538                                 m_table = nextTable;
 539                                 m_idx = -1;
 540                                 goto searchInTable;       // Continue iterating in this table.
 541                             }
 542                         }
 543                     }
 544                     // That's the end of the entire map.
 545                     m_hash = KeyTraits::NullHash;
 546                     m_value = Value(ValueTraits::NullValue);
 547                     return;
 548                 }
 549             }
 550         }
 551
 552         bool isValid() const {
 553             return m_value != Value(ValueTraits::NullValue);
 554         }
 555
 556         Key getKey() const {
 557             TURF_ASSERT(isValid());
 558             // Since we've forbidden concurrent inserts (for now), nonatomic would suffice here, but let's plan ahead:
 559             return KeyTraits::dehash(m_hash);
 560         }
 561
 562         Value getValue() const {
 563             TURF_ASSERT(isValid());
 564             return m_value;
 565         }
 566     };
 567 };
 568
 569 } // namespace junction
 570
 571 #endif // JUNCTION_CONCURRENTMAP_GRAMPA_H