1 #ifndef SIMPLIFIED_CLIFFC_HASHTABLE_H
2 #define SIMPLIFIED_CLIFFC_HASHTABLE_H
14 This header file declares and defines a simplified version of Cliff Click's
15 NonblockingHashMap. It contains all the necessary structrues and main
16 functions. In simplified_cliffc_hashtable.cc file, it has the definition for
20 template<typename TypeK, typename TypeV>
21 class cliffc_hashtable;
24 Corresponding the the Object[] array in Cliff Click's Java implementation.
25 It keeps the first two slots for CHM (Hashtable control unit) and the hash
26 records (an array of hash used for fast negative key-equality check).
34 int real_size = sizeof(atomic<void*>) * 2 + 2;
35 _data = new atomic<void*>[real_size];
36 // The control block should be initialized in resize()
37 // Init the hash record array
38 int *hashes = new int[_size];
40 for (i = 0; i < _size; i++) {
43 _data[1].store(hashes, memory_order_relaxed);
44 // Init the data to Null slot
45 for (i = 2; i < real_size; i++) {
46 _data[i].store(NULL, memory_order_relaxed);
51 int *hashes = (int*) _data[1].load(memory_order_relaxed);
59 shared_ptr<void> _ptr;
61 slot(bool prime, shared_ptr<void> ptr) {
70 TypeK must have defined function "int hashCode()" which return the hash
71 code for the its object, and "int equals(TypeK anotherKey)" which is
72 used to judge equality.
73 TypeK and TypeV should define their own copy constructor.
74 To make the memory management safe and similar to Cliff Click's Java
75 implementation, we use shared_ptr instead of normal pointer in terms of the
76 pointers that point to TypeK and TypeV.
78 template<typename TypeK, typename TypeV>
79 class cliffc_hashtable {
81 # The synchronization we have for the hashtable gives us the property of
82 # serializability, so we should have a sequential hashtable when we check the
83 # correctness. The key thing is to identify all the commit point.
88 CLASS = cliffc_hashtable;
91 spec_hashtable<TypeK, TypeV*> map;
92 spec_hashtable<TypeK, Tag> id_map;
95 map = spec_hashtable<TypeK, TypeV*>();
96 id_map = spec_hashtable<TypeK, TypeV*>();
98 static bool equals_val(TypeV *ptr1, TypeV *ptr2) {
102 # Update the tag for the current key slot if the corresponding tag
103 # is NULL, otherwise just return that tag. It will update the next
104 # available tag too if it requires a new tag for that key slot.
105 static Tag getKeyTag(TypeK &key) {
106 if (id_map.get(key) == NULL) {
107 Tag cur_tag = tag.current();
108 id_map.put(key, cur_tag);
112 return id_map.get(key);
128 PutIfAbsent(COND_PutIfAbsentSucc),
130 RemoveIfMatch(COND_RemoveIfMatchSucc),
132 ReplaceIfMatch(COND_ReplaceIfMatchSucc)
135 Write_interface -> Read_interface
141 The control structure for the hashtable
145 friend class cliffc_hashtable;
147 atomic<kvs_data*> _newkvs;
149 // Size of active K,V pairs
152 // Count of used slots
155 // The next part of the table to copy
156 atomic_int _copy_idx;
158 // Work-done reporting
159 atomic_int _copy_done;
163 _size.store(size, memory_order_relaxed);
164 _slots.store(0, memory_order_relaxed);
166 _copy_idx.store(0, memory_order_relaxed);
167 _copy_done.store(0, memory_order_release);
174 // Heuristic to decide if the table is too full
175 bool table_full(int reprobe_cnt, int len) {
177 reprobe_cnt >= REPROBE_LIMIT &&
178 _slots.load(memory_order_relaxed) >= reprobe_limit(len);
181 kvs_data* resize(cliffc_hashtable *topmap, kvs_data *kvs) {
182 kvs_data *newkvs = _newkvs.load(memory_order_acquire);
186 // No copy in-progress, start one; Only double the table size
187 int oldlen = kvs->_size;
188 int sz = _size.load(memory_order_relaxed);
191 // Just follow Cliff Click's heuristic to decide the new size
192 if (sz >= (oldlen >> 2)) { // If we are 25% full
193 newsz = oldlen << 1; // Double size
194 if (sz >= (oldlen >> 1))
195 newsz = oldlen << 2; // Double double size
198 // We do not record the record timestamp
199 if (newsz <= oldlen) newsz = oldlen << 1;
200 // Do not shrink ever
201 if (newsz < oldlen) newsz = oldlen;
203 // Last check cause the 'new' below is expensive
204 newkvs = _newkvs.load(memory_order_acquire);
205 if (newkvs != NULL) return newkvs;
207 newkvs = new kvs_data(newsz);
208 void *chm = (void*) new CHM(sz);
209 newkvs->_data[0].store(chm, memory_order_relaxed);
211 kvs_data *cur_newkvs;
212 // Another check after the slow allocation
213 if ((cur_newkvs = _newkvs.load(memory_order_acquire)) != NULL)
215 // CAS the _newkvs to the allocated table
216 kvs_data *desired = (kvs_data*) NULL;
217 kvs_data *expected = (kvs_data*) newkvs;
218 if (!_newkvs.compare_exchange_strong(desired, expected, memory_order_release,
219 memory_order_release)) {
220 // Should clean the allocated area
222 newkvs = _newkvs.load(memory_order_acquire);
227 void help_copy_impl(cliffc_hashtable *topmap, kvs_data *oldkvs,
229 assert (get_chm(oldkvs) == this);
230 kvs_data *newkvs = _newkvs.load(memory_order_acquire);
231 int oldlen = oldkvs->_size;
232 int min_copy_work = oldlen > 1024 ? 1024 : oldlen;
234 // Just follow Cliff Click's code here
235 int panic_start = -1;
237 while (_copy_done.load(memory_order_acquire) < oldlen) {
238 copyidx = _copy_idx.load(memory_order_acquire);
239 if (panic_start == -1) { // No painc
240 copyidx = _copy_idx.load(memory_order_acquire);
241 while (copyidx < (oldlen << 1) &&
242 !_copy_idx.compare_exchange_strong(copyidx, copyidx +
243 min_copy_work, memory_order_release, memory_order_release))
244 copyidx = _copy_idx.load(memory_order_relaxed);
245 if (!(copyidx < (oldlen << 1)))
246 panic_start = copyidx;
249 // Now copy the chunk of work we claimed
251 for (int i = 0; i < min_copy_work; i++)
252 if (copy_slot(topmap, (copyidx + i) & (oldlen - 1), oldkvs,
256 copy_check_and_promote(topmap, oldkvs, workdone);
258 copyidx += min_copy_work;
259 if (!copy_all && panic_start == -1)
260 return; // We are done with the work we claim
262 copy_check_and_promote(topmap, oldkvs, 0); // See if we can promote
265 kvs_data* copy_slot_and_check(cliffc_hashtable *topmap, kvs_data
266 *oldkvs, int idx, void *should_help) {
267 kvs_data *newkvs = _newkvs.load(memory_order_acquire);
268 // We're only here cause the caller saw a Prime
269 if (copy_slot(topmap, idx, oldkvs, _newkvs))
270 copy_check_and_promote(topmap, oldkvs, 1); // Record the slot copied
271 return (should_help == NULL) ? newkvs : topmap->help_copy(newkvs);
274 void copy_check_and_promote(cliffc_hashtable *topmap, kvs_data*
275 oldkvs, int workdone) {
276 int oldlen = oldkvs->_size;
277 int copyDone = _copy_done.load(memory_order_relaxed);
280 copyDone = _copy_done.load(memory_order_relaxed);
281 if (_copy_done.compare_exchange_weak(copyDone, copyDone +
282 workdone, memory_order_relaxed, memory_order_relaxed))
287 // Promote the new table to the current table
288 if (copyDone + workdone == oldlen &&
289 topmap->_kvs.load(memory_order_acquire) == oldkvs)
290 topmap->_kvs.compare_exchange_strong(oldkvs, _newkvs, memory_order_release,
291 memory_order_release);
294 bool copy_slot(cliffc_hashtable *topmap, int idx, kvs_data *oldkvs,
297 while ((key_slot = key(oldkvs, idx)) == NULL)
298 CAS_key(oldkvs, idx, NULL, TOMBSTONE);
300 // First CAS old to Prime
301 slot *oldval = val(oldkvs, idx, NULL);
302 while (!is_prime(oldval)) {
303 slot *box = (oldval == NULL || oldval == TOMBSTONE)
304 ? TOMBPRIME : new slot(true, oldval->_ptr);
305 if (CAS_val(oldkvs, idx, oldval, box)) {
306 if (box == TOMBPRIME)
307 return 1; // Copy done
308 // Otherwise we CAS'd the box
309 oldval = box; // Record updated oldval
312 oldval = val(oldkvs, idx, NULL); // Else re-try
315 if (oldval == TOMBPRIME) return false; // Copy already completed here
317 slot *old_unboxed = new slot(false, oldval->_ptr);
318 int copied_into_new = (putIfMatch(topmap, newkvs, key_slot, old_unboxed,
321 // Old value is exposed in the new table
322 while (!CAS_val(oldkvs, idx, oldval, TOMBPRIME))
323 oldval = val(oldkvs, idx, NULL);
325 return copied_into_new;
332 static const int Default_Init_Size = 8; // Intial table size
334 static slot* const MATCH_ANY;
335 static slot* const NO_MATCH_OLD;
337 static slot* const TOMBPRIME;
338 static slot* const TOMBSTONE;
340 static const int REPROBE_LIMIT = 10; // Forces a table-resize
342 atomic<kvs_data*> _kvs;
346 // Should initialize the CHM for the construction of the table
347 // For other CHM in kvs_data, they should be initialzed in resize()
348 // because the size is determined dynamically
349 kvs_data *kvs = new kvs_data(Default_Init_Size);
350 void *chm = (void*) new CHM(0);
351 kvs->_data[0].store(chm, memory_order_relaxed);
352 _kvs.store(kvs, memory_order_release);
355 cliffc_hashtable(int init_size) {
356 // Should initialize the CHM for the construction of the table
357 // For other CHM in kvs_data, they should be initialzed in resize()
358 // because the size is determined dynamically
359 kvs_data *kvs = new kvs_data(init_size);
360 void *chm = (void*) new CHM(0);
361 kvs->_data[0].store(chm, memory_order_relaxed);
362 _kvs.store(kvs, memory_order_release);
368 @Commit_point_set: Read_Val_Point1 | Read_Val_Point2 | Read_Val_Point3
369 @ID: __sequential.getKeyTag(key)
371 TypeV *_Old_Val = __sequential.map.get(key)
373 __sequential.equals_val(_Old_Val, __RET__)
376 shared_ptr<TypeV> get(TypeK& key) {
377 void *key_ptr = (void*) new TypeK(key);
378 slot *key_slot = new slot(false, shared_ptr<void>(key_ptr));
379 int fullhash = hash(key_slot);
380 slot *V = get_impl(this, _kvs, key_slot, fullhash);
381 if (V == NULL) return NULL;
382 assert (!is_prime(V));
383 return static_pointer_cast<TypeV>(V->_ptr);
389 @Commit_point_set: Write_Val_Point
390 @ID: __sequential.getKeyTag(key)
392 # Remember this old value at checking point
393 TypeV *_Old_Val = __sequential.map.get(key)
394 __sequential.map.put(key, &val);
396 __sequential.equals_val(__RET__, _Old_Val)
399 shared_ptr<TypeV> put(TypeK& key, TypeV& val) {
400 return putIfMatch(key, val, NO_MATCH_OLD);
405 @Interface: PutIfAbsent
407 Write_Val_Point | PutIfAbsent_Fail_Point
408 @Condition: __sequential.map.get(key) == NULL
410 COND_PutIfAbsentSucc :: __RET__ == NULL
411 @ID: __sequential.getKeyTag(key)
413 TypeV *_Old_Val = __sequential.map.get(key)
415 __sequential.map.put(key, &value);
417 __COND_SAT__ ? __RET__ == NULL : __sequential.equals_val(_Old_Val, __RET__)
420 shared_ptr<TypeV> putIfAbsent(TypeK& key, TypeV& value) {
421 return putIfMatch(key, val, TOMBSTONE);
426 @Interface: RemoveAny
427 @Commit_point_set: Write_Val_Point
428 @ID: __sequential.getKeyTag(key)
430 TypeV *_Old_Val = __sequential.map.get(key)
431 __sequential.map.put(key, NULL);
433 __sequential.equals_val(__RET__, _Old_Val)
436 shared_ptr<TypeV> remove(TypeK& key) {
437 return putIfMatch(key, TOMBSTONE, NO_MATCH_OLD);
442 @Interface: RemoveIfMatch
444 Write_Val_Point | RemoveIfMatch_Fail_Point
446 __sequential.equals_val(__sequential.map.get(key), &val)
448 COND_RemoveIfMatchSucc :: __RET__ == true
449 @ID: __sequential.getKeyTag(key)
452 __sequential.map.put(key, NULL);
454 __COND_SAT__ ? __RET__ : !__RET__
457 bool remove(TypeK& key, TypeV& val) {
458 slot *val_slot = val == NULL ? NULL : new slot(false, val);
459 return putIfMatch(key, TOMBSTONE, val) == val;
465 @Interface: ReplaceAny
468 @ID: __sequential.getKeyTag(key)
470 TypeV *_Old_Val = __sequential.map.get(key)
472 __sequential.equals_val(__RET__, _Old_Val)
475 shared_ptr<TypeV> replace(TypeK& key, TypeV& val) {
476 return putIfMatch(key, val, MATCH_ANY);
481 @Interface: ReplaceIfMatch
483 Write_Val_Point | ReplaceIfMatch_Fail_Point
485 __sequential.equals_val(__sequential.map.get(key), &oldval)
487 COND_ReplaceIfMatchSucc :: __RET__ == true
488 @ID: __sequential.getKeyTag(key)
491 __sequential.map.put(key, &newval);
493 __COND_SAT__ ? __RET__ : !__RET__
496 bool replace(TypeK& key, TypeV& oldval, TypeV& newval) {
497 return putIfMatch(key, newval, oldval) == oldval;
501 static CHM* get_chm(kvs_data* kvs) {
502 return (CHM*) kvs->_data[0].load(memory_order_relaxed);
505 static int* get_hashes(kvs_data *kvs) {
506 return (int *) kvs->_data[1].load(memory_order_relaxed);
509 // Preserve happens-before semantics on newly inserted keys
510 static inline slot* key(kvs_data *kvs, int idx) {
511 assert (idx >= 0 && idx < kvs->_size);
512 // Corresponding to the volatile read in get_impl() and putIfMatch in
513 // Cliff Click's Java implementation
514 return (slot*) kvs->_data[idx * 2 + 2].load(memory_order_acquire);
518 The atomic operation in val() function is a "potential" commit point,
519 which means in some case it is a real commit point while it is not for
520 some other cases. This so happens because the val() function is such a
521 fundamental function that many internal operation will call. Our
522 strategy is that we label any potential commit points and check if they
523 really are the commit points later.
525 // Preserve happens-before semantics on newly inserted values
526 static inline slot* val(kvs_data *kvs, int idx) {
527 assert (idx >= 0 && idx < kvs->_size);
528 // Corresponding to the volatile read in get_impl() and putIfMatch in
529 // Cliff Click's Java implementation
530 slot *res = (slot*) kvs->_data[idx * 2 + 3].load(memory_order_acquire);
533 # This is a complicated potential commit point since many many functions are
535 @Potential_commit_point_define: true
536 @Label: Read_Val_Point
544 static int hash(slot *key_slot) {
545 assert(key_slot != NULL && key_slot->_ptr != NULL);
546 shared_ptr<TypeK> key = static_pointer_cast<TypeK>(key_slot->_ptr);
547 int h = key->hashCode();
548 // Spread bits according to Cliff Click's code
549 h += (h << 15) ^ 0xffffcd7d;
553 h += (h << 2) + (h << 14);
554 return h ^ (h >> 16);
557 // Heuristic to decide if reprobed too many times.
558 // Be careful here: Running over the limit on a 'get' acts as a 'miss'; on a
559 // put it triggers a table resize. Several places MUST have exact agreement.
560 static int reprobe_limit(int len) {
561 return REPROBE_LIMIT + (len >> 2);
564 static inline bool is_prime(slot *val) {
565 return (val != NULL) && val->_prime;
568 // Check for key equality. Try direct pointer comparison first (fast
569 // negative teset) and then the full 'equals' call
570 static bool keyeq(slot *K, slot *key_slot, int *hashes, int hash,
572 // Caller should've checked this.
574 shared_ptr<TypeK> key_ptr = static_pointer_cast<TypeK>(key_slot->_ptr);
577 ((hashes[hash] == 0 || hashes[hash] == fullhash) &&
579 key_ptr->equals(K->_ptr));
582 static bool valeq(slot *val_slot1, slot *val_slot2) {
583 assert (val_slot1 != NULL);
584 shared_ptr<TypeK> ptr1 = static_pointer_cast<TypeV>(val_slot1->_ptr);
585 if (val_slot2 == NULL || ptr1 == NULL) return false;
586 return ptr1->equals(val_slot2->_ptr);
589 // Together with key() preserve the happens-before relationship on newly
591 static inline bool CAS_key(kvs_data *kvs, int idx, void *expected, void *desired) {
592 return kvs->_data[2 * idx + 2].compare_exchange_strong(expected,
593 desired, memory_order_release, memory_order_release);
597 Same as the val() function, we only label the CAS operation as the
598 potential commit point.
600 // Together with val() preserve the happens-before relationship on newly
602 static inline bool CAS_val(kvs_data *kvs, int idx, void *expected, void
604 bool res = kvs->_data[2 * idx + 3].compare_exchange_strong(expected,
605 desired, memory_order_release, memory_order_release);
607 # If it is a successful put instead of a copy or any other internal
608 # operantions, expected != NULL
610 @Potential_commit_point_define: __ATOMIC_RET__ == true
611 @Label: Write_Val_Point
617 slot* get_impl(cliffc_hashtable *topmap, kvs_data *kvs, slot* key_slot, int
619 int len = kvs->_size;
620 CHM *chm = get_chm(kvs);
621 int *hashes = get_hashes(kvs);
623 int idx = fullhash & (len - 1);
626 slot *K = key(kvs, idx);
627 slot *V = val(kvs, idx);
630 @Commit_point_define: V == NULL
631 @Potential_commit_point_label: Read_Val_Point
632 @Label: Get_Success_Point_1
636 if (V == NULL) return NULL; // A miss
638 if (keyeq(K, key_slot, hashes, idx, fullhash)) {
639 // Key hit! Check if table-resize in progress
643 @Commit_point_define: true
644 @Potential_commit_point_label: Read_Val_Point
645 @Label: Get_Success_Point_2
648 return (V == TOMBSTONE) ? NULL : V; // Return this value
650 // Otherwise, finish the copy & retry in the new table
651 return get_impl(topmap, chm->copy_slot_and_check(topmap, kvs,
652 idx, key_slot), key_slot, fullhash);
655 if (++reprobe_cnt >= REPROBE_LIMIT ||
656 key_slot == TOMBSTONE) {
657 // Retry in new table
658 // Atomic read (acquire) can be here
659 kvs_data *newkvs = chm->_newkvs.load(memory_order_acquire);
662 @Commit_point_define_check: newkvs == NULL
663 @Label: Get_Success_Point_3
666 return newkvs == NULL ? NULL : get_impl(topmap,
667 topmap->help_copy(newkvs), key_slot, fullhash);
670 idx = (idx + 1) & (len - 1); // Reprobe by 1
674 // A wrapper of the essential function putIfMatch()
675 shared_ptr<TypeV> putIfMatch(TypeK& key, TypeV& value, slot *old_val) {
676 // TODO: Should throw an exception rather return NULL
677 if (old_val == NULL) {
680 void *key_ptr = (void*) new TypeK(key);
681 slot *key_slot = new slot(false, shared_ptr<void>(key_ptr));
683 void *val_ptr = (void*) new TypeV(value);
684 slot *value_slot = new slot(false, shared_ptr<void>(val_ptr));
685 slot *res = putIfMatch(this, _kvs, key_slot, value_slot, old_val);
686 // Only when copy_slot() call putIfMatch() will it return NULL
687 assert (res != NULL);
688 assert (!is_prime(res));
689 return res == TOMBSTONE ? NULL : static_pointer_cast<TypeV>(res->_ptr);
693 Put, Remove, PutIfAbsent, etc will call this function. Return the old
694 value. If the returned value is equals to the expVal (or expVal is
695 NO_MATCH_OLD), then this function puts the val_slot to the table 'kvs'.
696 Only copy_slot will pass a NULL expVal, and putIfMatch only returns a
697 NULL if passed a NULL expVal.
699 static slot* putIfMatch(cliffc_hashtable *topmap, kvs_data *kvs, slot
700 *key_slot, slot *val_slot, slot *expVal) {
701 assert (val_slot != NULL);
702 assert (!is_prime(val_slot));
703 assert (!is_prime(expVal));
705 int fullhash = hash(key_slot);
706 int len = kvs->_size;
707 CHM *chm = get_chm(kvs);
708 int *hashes = get_hashes(kvs);
709 int idx = fullhash & (len - 1);
717 while (true) { // Spin till we get a key slot
719 V = val(kvs, idx, NULL);
720 if (K == NULL) { // Get a free slot
721 if (val_slot == TOMBSTONE) return val_slot;
722 // Claim the null key-slot
723 if (CAS_key(kvs, idx, NULL, key_slot)) {
724 chm->_slots.fetch_add(1, memory_order_relaxed); // Inc key-slots-used count
725 hashes[idx] = fullhash; // Memorize full hash
728 K = key(kvs, idx); // CAS failed, get updated value
732 // Key slot not null, there exists a Key here
733 if (keyeq(K, key_slot, hashes, idx, fullhash))
736 // Notice that the logic here should be consistent with that of get.
737 // The first predicate means too many reprobes means nothing in the
739 if (++reprobe_cnt >= reprobe_limit(len) ||
740 K == TOMBSTONE) { // Found a Tombstone key, no more keys
741 newkvs = chm->resize(topmap, kvs);
742 // Help along an existing copy
743 if (expVal != NULL) topmap->help_copy(newkvs);
744 return putIfMatch(topmap, newkvs, key_slot, val_slot, expVal);
747 idx = (idx + 1) & (len - 1); // Reprobe
748 } // End of spinning till we get a Key slot
750 if (val_slot == V) return V; // Fast cutout for no-change
752 // Here it tries to resize cause it doesn't want other threads to stop
753 // its progress (eagerly try to resize soon)
754 newkvs = chm->_newkvs.load(memory_order_acquire);
755 if (newkvs == NULL &&
756 ((V == NULL && chm->table_full(reprobe_cnt, len)) || is_prime(V)))
757 newkvs = chm->resize(topmap, kvs); // Force the copy to start
759 // Finish the copy and then put it in the new table
761 return putIfMatch(topmap, chm->copy_slot_and_check(topmap, kvs, idx,
762 expVal), key_slot, val_slot, expVal);
764 // Decided to update the existing table
766 assert (!is_prime(V));
768 if (expVal != NO_MATCH_OLD &&
770 (expVal != MATCH_ANY || V == TOMBSTONE || V == NULL) &&
771 !(V == NULL && expVal == TOMBSTONE) &&
772 (expVal == NULL || !valeq(expVal, V))) {
775 @Commit_point_define: expVal == TOMBSTONE
776 @Potential_commit_point_label: Read_Val_Point
777 @Label: PutIfAbsent_Fail_Point
778 # This is a check for the PutIfAbsent() when the value
784 @Commit_point_define: expVal != NULL && val_slot == TOMBSTONE
785 @Potential_commit_point_label: Read_Val_Point
786 @Label: RemoveIfMatch_Fail_Point
791 @Commit_point_define: !valeq(expVal, V)
792 @Potential_commit_point_label: Read_Val_Point
793 @Label: ReplaceIfMatch_Fail_Point
796 return V; // Do not update!
799 if (CAS_val(kvs, idx, V, val_slot)) {
802 # The only point where a successful put happens
803 @Commit_point_define: true
804 @Potential_commit_point_label: Write_Val_Point
805 @Label: Write_Success_Point
808 if (expVal != NULL) { // Not called by a table-copy
809 // CAS succeeded, should adjust size
810 // Both normal put's and table-copy calls putIfMatch, but
811 // table-copy does not increase the number of live K/V pairs
812 if ((V == NULL || V == TOMBSTONE) &&
813 val_slot != TOMBSTONE)
814 chm->_size.fetch_add(1, memory_order_relaxed);
815 if (!(V == NULL || V == TOMBSTONE) &&
816 val_slot == TOMBSTONE)
817 chm->_size.fetch_add(-1, memory_order_relaxed);
819 return (V == NULL && expVal != NULL) ? TOMBSTONE : V;
822 V = val(kvs, idx, NULL);
824 return putIfMatch(topmap, chm->copy_slot_and_check(topmap, kvs,
825 idx, expVal), key_slot, val_slot, expVal);
829 // Help along an existing table-resize. This is a fast cut-out wrapper.
830 kvs_data* help_copy(kvs_data *helper) {
831 kvs_data *topkvs = _kvs.load(memory_order_acquire);
832 CHM *topchm = get_chm(topkvs);
833 // No cpy in progress
834 if (topchm->_newkvs.load(memory_order_acquire) == NULL) return helper;
835 topchm->help_copy_impl(this, topkvs, false);