On Sat, Jun 08, 2019 at 07:42:42AM -0700, Jeff Hostetler via GitGitGadget wrote:
> From: Jeff Hostetler <jeffhost@xxxxxxxxxxxxx>
>
> Teach register_rename_src() to see if new file pair
> can simply be appended to the rename_src[] array before
> performing the binary search to find the proper insertion
> point.
>
> This is a performance optimization. This routine is called
> during run_diff_files in status and the caller is iterating
> over the sorted index, so we should expect to be able to
> append in the normal case. The existing insert logic is
> preserved so we don't have to assume that, but simply take
> advantage of it if possible.
Could you add a command and performance figures to the commit message
to show off the benefits of this patch?
>From the description it's clear that it's a performance optimization,
but it's unclear whether it has a noticeable, or at least measurable
effect, or it's "only" a micro-optimization.
I tried something more substantial than a simple 'git status':
# without this patch
$ time perf record -g ./git log --oneline -m --name-only v2.20.0 >/dev/null
[ ... ]
real 2m4.320s
user 2m0.913s
sys 0m2.284s
$ perf report -g none |grep -E '(diffcore_rename|register_rename_src)'
52.40% 0.79% git git [.] diffcore_rename
0.01% 0.01% git git [.] register_rename_src
but it looks like that while more than half of the considerable
runtime is spent detecting renames, the time spent in
register_rename_src(), the function optimized in this patch, is
negligible.
> Signed-off-by: Jeff Hostetler <jeffhost@xxxxxxxxxxxxx>
> Signed-off-by: Johannes Schindelin <johannes.schindelin@xxxxxx>
> ---
> diffcore-rename.c | 13 +++++++++++++
> 1 file changed, 13 insertions(+)
>
> diff --git a/diffcore-rename.c b/diffcore-rename.c
> index 07bd34b631..5bfc5f6c22 100644
> --- a/diffcore-rename.c
> +++ b/diffcore-rename.c
> @@ -82,6 +82,18 @@ static struct diff_rename_src *register_rename_src(struct diff_filepair *p)
>
> first = 0;
> last = rename_src_nr;
> +
> + if (last > 0) {
> + struct diff_rename_src *src = &(rename_src[last-1]);
> + int cmp = strcmp(one->path, src->p->one->path);
> + if (!cmp)
> + return src;
> + if (cmp > 0) {
> + first = last;
> + goto append_it;
> + }
> + }
> +
> while (last > first) {
> int next = (last + first) >> 1;
> struct diff_rename_src *src = &(rename_src[next]);
> @@ -95,6 +107,7 @@ static struct diff_rename_src *register_rename_src(struct diff_filepair *p)
> first = next+1;
> }
>
> +append_it:
> /* insert to make it at "first" */
> ALLOC_GROW(rename_src, rename_src_nr + 1, rename_src_alloc);
> rename_src_nr++;
> --
> gitgitgadget
