Re: [PATCH 2/2] diffcore-rename: filter rename_src list when possible

[Elijah Newren <newren@gmail.com>]

On Wed, Feb 3, 2021 at 11:12 AM Junio C Hamano <gitster@pobox.com> wrote:
>
> "Elijah Newren via GitGitGadget" <gitgitgadget@gmail.com> writes:
>
> > +static int remove_unneeded_paths_from_src(int num_src,
> > +                                       int detecting_copies)
> > +{
> > +     int i, new_num_src;
> > +
> > +     /*
> > +      * Note on reasons why we cull unneeded sources but not destinations:
> > +      *   1) Pairings are stored in rename_dst (not rename_src), which we
> > +      *      need to keep around.  So, we just can't cull rename_dst even
> > +      *      if we wanted to.  But doing so wouldn't help because...
> > +      *
> > +      *   2) There is a matrix pairwise comparison that follows the
> > +      *      "Performing inexact rename detection" progress message.
> > +      *      Iterating over the destinations is done in the outer loop,
> > +      *      hence we only iterate over each of those once and we can
> > +      *      easily skip the outer loop early if the destination isn't
> > +      *      relevant.  That's only one check per destination path to
> > +      *      skip.
> > +      *
> > +      *      By contrast, the sources are iterated in the inner loop; if
> > +      *      we check whether a source can be skipped, then we'll be
> > +      *      checking it N separate times, once for each destination.
> > +      *      We don't want to have to iterate over known-not-needed
> > +      *      sources N times each, so avoid that by removing the sources
> > +      *      from rename_src here.
> > +      */
> > +     if (detecting_copies)
> > +             return num_src; /* nothing to remove */
> > +     if (break_idx)
> > +             return num_src; /* culling incompatbile with break detection */
> > +
> > +     for (i = 0, new_num_src = 0; i < num_src; i++) {
> > +             /*
> > +              * renames are stored in rename_dst, so if a rename has
> > +              * already been detected using this source, we can just
> > +              * remove the source knowing rename_dst has its info.
> > +              */
> > +             if (rename_src[i].p->one->rename_used)
> > +                     continue;
> > +
> > +             if (new_num_src < i)
> > +                     memcpy(&rename_src[new_num_src], &rename_src[i],
> > +                            sizeof(struct diff_rename_src));
> > +             new_num_src++;
> > +     }
> > +
> > +     return new_num_src;
> > +}
>
> Essentially we are compacting rename_src[] array from num_src
> elements down to new_num_src elements; we are losing pointers, but I
> presume these are all borrowed pointers that we do not own and we
> are not responsible for freeing?  If we were to free them, the
> compaction would leave duplicates after the new tail (new_num_src)
> and we'd end up having to worry about double-freeing, so hopefully
> all we need to do is just to free the entire array of pointers, and
> not the pointees.
>
> Having to do this just once and being able to reduce the number of
> entries we need to iterate over does sound like a good simple
> optimization.

Correct, they are all just borrowed pointers and we only need to free
the array, not the pointers within the array.

> >  void diffcore_rename(struct diff_options *options)
> >  {
> >       int detect_rename = options->detect_rename;
> > @@ -463,10 +512,11 @@ void diffcore_rename(struct diff_options *options)
> >       struct diff_score *mx;
> >       int i, j, rename_count, skip_unmodified = 0;
> >       int num_destinations, dst_cnt;
> > -     int num_sources;
> > +     int num_sources, want_copies;
> >       struct progress *progress = NULL;
> >
> >       trace2_region_enter("diff", "setup", options->repo);
> > +     want_copies = (detect_rename == DIFF_DETECT_COPY);
> >       if (!minimum_score)
> >               minimum_score = DEFAULT_RENAME_SCORE;
> >
> > @@ -529,13 +579,10 @@ void diffcore_rename(struct diff_options *options)
> >               goto cleanup;
> >
> >       /*
> > -      * Calculate how many renames are left (but all the source
> > -      * files still remain as options for rename/copies!)
> > +      * Calculate how many renames are left
> >        */
> >       num_destinations = (rename_dst_nr - rename_count);
> > -     num_sources = rename_src_nr;
> > -     if (detect_rename != DIFF_DETECT_COPY)
> > -             num_sources -= rename_count;
> > +     num_sources = remove_unneeded_paths_from_src(rename_src_nr, want_copies);
>
> OK, this is in a sense an extended version of the previous step.
>
> I am not sure if rename_src_nr can be left out of sync with reality
> like this patch does, though.  The reference to that variable in
> register_rename_src() and find_exact_renames() are OK as we are not
> going to call them after we futz with the rename_src[] array, but
> the reference in prefetch(), which does not actually happen early
> but only when we start running estimate_similarity(), which is after
> we compacted the rename_src[] array, would be affected, no?

Yes, good catch.  This is the same issue Stolee caught.  I did set
rename_src_nr to the new num_sources in my "ort" branch, but when
breaking up the changes to upstream them, that line of code somehow
got separated into a later patch (that I haven't submitted yet) and I
just didn't notice it when reviewing this early series.  It belongs in
this patch as both of you pointed out.