Re: git diff too slow for a file

> Ever since the xdiff library had been introduced to git, all its callers
> have used the flag XDF_NEED_MINIMAL.  It makes sure that the smallest
> possible diff is produced, but that takes quite some time if there are
> lots of differences that can be expressed in multiple ways.
>
> This flag makes a difference for only 0.1% of the non-merge commits in
> the git repo of Linux, both in terms of diff size and execution time.
> The patches there are mostly nice and small.
>
> SungHyun Nam however reported a case in a different repo where a diff
> took more than 20 times longer to generate with XDF_NEED_MINIMAL than
> without.  Rebasing became really slow.
>
> This patch removes this flag from all callers.  The default of xdiff is
> saner because it has minimal to no impact in the normal case of small
> diffs and doesn't incur that much of a speed penalty for large ones.

> diff --git a/merge-file.c b/merge-file.c
> index c336c93..db4d0d5 100644
> --- a/merge-file.c
> +++ b/merge-file.c
> @@ -66,7 +66,7 @@ static int generate_common_file(mmfile_t *res, mmfile_t *f1, mmfile_t *f2)
>  	xdemitcb_t ecb;
>  
>  	memset(&xpp, 0, sizeof(xpp));
> -	xpp.flags = XDF_NEED_MINIMAL;
> +	xpp.flags = 0;
>  	memset(&xecfg, 0, sizeof(xecfg));
>  	xecfg.ctxlen = 3;
>  	xecfg.flags = XDL_EMIT_COMMON;