Re: [PATCH] cache-tree: remove use of strbuf_addf in update_one

["René Scharfe" <l.s.r@web.de>]

Am 10.08.2017 um 20:47 schrieb Kevin Willford:
> String formatting can be a performance issue when there are
> hundreds of thousands of trees.
> 
> Change to stop using the strbuf_addf and just add the strings
> or characters individually.
> 
> There are a limited number of modes so added a switch for the
> known ones and a default case if something comes through that
> are not a known one for git.
> 
> Signed-off-by: Kevin Willford <kewillf@microsoft.com>
> ---
>   cache-tree.c | 24 +++++++++++++++++++++++-
>   1 file changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/cache-tree.c b/cache-tree.c
> index 2440d1dc89..41744b3db7 100644
> --- a/cache-tree.c
> +++ b/cache-tree.c
> @@ -390,7 +390,29 @@ static int update_one(struct cache_tree *it,
>   			continue;
>   
>   		strbuf_grow(&buffer, entlen + 100);
> -		strbuf_addf(&buffer, "%o %.*s%c", mode, entlen, path + baselen, '\0');
> +
> +		switch (mode) {
> +		case 0100644:
> +			strbuf_add(&buffer, "100644 ", 7);
> +			break;
> +		case 0100664:
> +			strbuf_add(&buffer, "100664 ", 7);
> +			break;
> +		case 0100755:
> +			strbuf_add(&buffer, "100755 ", 7);
> +			break;
> +		case 0120000:
> +			strbuf_add(&buffer, "120000 ", 7);
> +			break;
> +		case 0160000:
> +			strbuf_add(&buffer, "160000 ", 7);
> +			break;

You can avoid specifying the string length by using strbuf_addstr.  The
compiler can determine that value; the resulting object code should be
the same with -O2.

> +		default:
> +			strbuf_addf(&buffer, "%o ", mode);
> +			break;
> +		}
> +		strbuf_add(&buffer, path + baselen, entlen);
> +		strbuf_addch(&buffer, '\0');

How much of the performance improvement is due to these two (especially
%.*s)?  Looking forward to the perf script to find out myself. :)

>   		strbuf_add(&buffer, sha1, 20);
>   
>   #if DEBUG
>