Re: [PATCH v1 1/2] string-list: use ALLOC_GROW macro when reallocing string_list

[Jeff King <peff@peff.net>]

On Wed, Apr 05, 2017 at 07:55:59PM +0000, git@jeffhostetler.com wrote:

> From: Jeff Hostetler <jeffhost@microsoft.com>
> 
> Use ALLOC_GROW() macro when reallocing a string_list array
> rather than simply increasing it by 32.  This is a performance
> optimization.
> 
> During status on a very large repo and there are many changes,
> a significant percentage of the total run time was spent
> reallocing the wt_status.changes array.
> 
> This change decreased the time in wt_status_collect_changes_worktree()
> from 125 seconds to 45 seconds on my very large repository.

Oof. Looks like the original was quadratic. I'm surprised this didn't
bite us more often. I guess we don't usually use string-lists for big
lists.

Aside from the redundant size-check that Stefan pointed out, the patch
looks obviously correct. I grepped for "alloc +=" and "alloc =.*+' to
see if there were any other cases, but didn't find any. Obviously that
is dependent on calling the variable "alloc", but that is normal for us
(and it does turn up a number of cases that do allocate correctly).

-Peff