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[libc++] Optimize __hash_table::erase(iterator, iterator) #152471

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[libc++] Optimize __hash_table::erase(iterator, iterator) #152471

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54 changes: 49 additions & 5 deletions libcxx/include/__hash_table
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Original file line number Diff line number Diff line change
Expand Up @@ -1848,12 +1848,56 @@ __hash_table<_Tp, _Hash, _Equal, _Alloc>::erase(const_iterator __p) {
template <class _Tp, class _Hash, class _Equal, class _Alloc>
typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::iterator
__hash_table<_Tp, _Hash, _Equal, _Alloc>::erase(const_iterator __first, const_iterator __last) {
for (const_iterator __p = __first; __first != __last; __p = __first) {
++__first;
erase(__p);
if (__first == __last)
return iterator(__last.__node_);

// current node
__next_pointer __current = __first.__node_;
size_type __bucket_count = bucket_count();
size_t __chash = std::__constrain_hash(__current->__hash(), __bucket_count);
// find previous node
__next_pointer __before_first = __bucket_list_[__chash];
for (; __before_first->__next_ != __current; __before_first = __before_first->__next_)
;

__next_pointer __end = __last.__node_;

// If __before_first is in the same bucket, clear this bucket first without re-linking it
if (__before_first != __first_node_.__ptr() &&
std::__constrain_hash(__before_first->__hash(), __bucket_count) == __chash) {
while (__current != __end) {
if (auto __next_chash = std::__constrain_hash(__current->__hash(), __bucket_count); __next_chash != __chash) {
__chash = __next_chash;
break;
}
auto __next = __current->__next_;
__node_traits::deallocate(__node_alloc(), __current->__upcast(), 1);
__current = __next;
--__size_;
}
}
__next_pointer __np = __last.__node_;
return iterator(__np);

while (__current != __end) {
auto __next = __current->__next_;
__node_traits::deallocate(__node_alloc(), __current->__upcast(), 1);
__current = __next;
--__size_;

// When switching buckets, set the old bucket to be empty and update the next bucket to have __before_first as its
// before-first element
if (__next) {
if (auto __next_chash = std::__constrain_hash(__next->__hash(), __bucket_count); __next_chash != __chash) {
__bucket_list_[__chash] = nullptr;
__chash = __next_chash;
__bucket_list_[__chash] = __before_first;
}
}
}

// re-link __before_start with __last
__before_first->__next_ = __current;

return iterator(__last.__node_);
}

template <class _Tp, class _Hash, class _Equal, class _Alloc>
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