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We have two overloads of _M_find_before_node but they have quite different performance characteristics, which isn't necessarily obvious. The original version, _M_find_before_node(bucket, key, hash_code), looks only in the specified bucket, doing a linear search within that bucket for an element that compares equal to the key. This is the typical fast lookup for hash containers, assuming the load factor is low so that each bucket isn't too large. The newer _M_find_before_node(key) was added in r12-6272-ge3ef832a9e8d6a and could be naively assumed to calculate the hash code and bucket for key and then call the efficient _M_find_before_node(bkt, key, code) function. But in fact it does a linear search of the entire container. This is potentially very slow and should only be used for a suitably small container, as determined by the __small_size_threshold() function. We don't even have a comment pointing out this O(N) performance of the newer overload. Additionally, the newer overload is only ever used in exactly one place, which would suggest it could just be removed. However there are several places that do the linear search of the whole container with an explicit loop each time. This adds a new member function, _M_locate, and uses it to replace most uses of _M_find_node and the loops doing linear searches. This new member function does both forms of lookup, the linear search for small sizes and the _M_find_node(bkt, key, code) lookup within a single bucket. The new function returns a __location_type which is a struct that contains a pointer to the first node matching the key (if such a node is present), or the hash code and bucket index for the key. The hash code and bucket index allow the caller to know where a new node with that key should be inserted, for the cases where the lookup didn't find a matching node. The result struct actually contains a pointer to the node *before* the one that was located, as that is needed for it to be useful in erase and extract members. There is a member function that returns the found node, i.e. _M_before->_M_nxt downcast to __node_ptr, which should be used in most cases. This new function greatly simplifies the functions that currently have to do two kinds of lookup and explicitly check the current size against the small size threshold. Additionally, now that try_emplace is defined directly in _Hashtable (not in _Insert_base) we can use _M_locate in there too, to speed up some try_emplace calls. Previously it did not do the small-size linear search. It would be possible to add a function to get a __location_type from an iterator, and then rewrite some functions like _M_erase and _M_extract_node to take a __location_type parameter. While that might be conceptually nice, it wouldn't really make the code any simpler or more readable than it is now. That isn't done in this change. libstdc++-v3/ChangeLog: * include/bits/hashtable.h (__location_type): New struct. (_M_locate): New member function. (_M_find_before_node(const key_type&)): Remove. (_M_find_node): Move variable initialization into condition. (_M_find_node_tr): Likewise. (operator=(initializer_list<T>), try_emplace, _M_reinsert_node) (_M_merge_unique, find, erase(const key_type&)): Use _M_locate for lookup. |
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file: libstdc++-v3/README New users may wish to point their web browsers to the file index.html in the 'doc/html' subdirectory. It contains brief building instructions and notes on how to configure the library in interesting ways.