Re: [PATCH v2 07/10] strmap: enable faster clearing and reusing of strmaps

> From: Elijah Newren <newren@gmail.com>
> 
> When strmaps are used heavily, such as is done by my new merge-ort
> algorithm, and strmaps need to be cleared but then re-used (because of
> e.g. picking multiple commits to cherry-pick, or due to a recursive
> merge having several different merges while recursing), free-ing and
> reallocating map->table repeatedly can add up in time, especially since
> it will likely be reallocated to a much smaller size but the previous
> merge provides a good guide to the right size to use for the next merge.
> 
> Introduce strmap_partial_clear() to take advantage of this type of
> situation; it will act similar to strmap_clear() except that
> map->table's entries are zeroed instead of map->table being free'd.
> Making use of this function reduced the cost of reset_maps() by about
> 20% in mert-ort, and dropped the overall runtime of my rebase testcase
> by just under 2%.

> +/*
> + * Similar to strmap_clear() but leaves map->map->table allocated and
> + * pre-sized so that subsequent uses won't need as many rehashings.
> + */
> +void strmap_partial_clear(struct strmap *map, int free_values);

> +void strmap_partial_clear(struct strmap *map, int free_util)
> +{
> +	strmap_free_entries_(map, free_util);
> +	hashmap_partial_clear(&map->map);
> +}