Re: [PATCH v2 3/4] unpack-trees: reduce malloc in cache-tree walk

> This is a micro optimization that probably only shines on repos with
> deep directory structure. Instead of allocating and freeing a new
> cache_entry in every iteration, we reuse the last one and only update
> the parts that are new each iteration.
> 
> Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
> ---
>   unpack-trees.c | 29 ++++++++++++++++++++---------
>   1 file changed, 20 insertions(+), 9 deletions(-)
> 
> diff --git a/unpack-trees.c b/unpack-trees.c
> index 39566b28fb..c33ebaf001 100644
> --- a/unpack-trees.c
> +++ b/unpack-trees.c
> @@ -694,6 +694,8 @@ static int traverse_by_cache_tree(int pos, int nr_entries, int nr_names,
>   {
>   	struct cache_entry *src[MAX_UNPACK_TREES + 1] = { NULL, };
>   	struct unpack_trees_options *o = info->data;
> +	struct cache_entry *tree_ce = NULL;
> +	int ce_len = 0;
>   	int i, d;
>   
>   	if (!o->merge)
> @@ -708,30 +710,39 @@ static int traverse_by_cache_tree(int pos, int nr_entries, int nr_names,
>   	 * in the first place.
>   	 */
>   	for (i = 0; i < nr_entries; i++) {
> -		struct cache_entry *tree_ce;
> -		int len, rc;
> +		int new_ce_len, len, rc;
>   
>   		src[0] = o->src_index->cache[pos + i];
>   
>   		len = ce_namelen(src[0]);
> -		tree_ce = xcalloc(1, cache_entry_size(len));
> +		new_ce_len = cache_entry_size(len);
> +
> +		if (new_ce_len > ce_len) {
> +			new_ce_len <<= 1;
> +			tree_ce = xrealloc(tree_ce, new_ce_len);
> +			memset(tree_ce, 0, new_ce_len);
> +			ce_len = new_ce_len;
> +
> +			tree_ce->ce_flags = create_ce_flags(0);
> +
> +			for (d = 1; d <= nr_names; d++)
> +				src[d] = tree_ce;
> +		}

>   
>   		tree_ce->ce_mode = src[0]->ce_mode;
> -		tree_ce->ce_flags = create_ce_flags(0);
>   		tree_ce->ce_namelen = len;
>   		oidcpy(&tree_ce->oid, &src[0]->oid);
>   		memcpy(tree_ce->name, src[0]->name, len + 1);
>   
> -		for (d = 1; d <= nr_names; d++)
> -			src[d] = tree_ce;
> -
>   		rc = call_unpack_fn((const struct cache_entry * const *)src, o);
> -		free(tree_ce);
> -		if (rc < 0)
> +		if (rc < 0) {
> +			free(tree_ce);
>   			return rc;
> +		}
>   
>   		mark_ce_used(src[0], o);
>   	}
> +	free(tree_ce);
>   	if (o->debug_unpack)
>   		printf("Unpacked %d entries from %s to %s using cache-tree\n",
>   		       nr_entries,
>