On Wed, Feb 3, 2021 at 11:12 AM Junio C Hamano <gitster@xxxxxxxxx> wrote:
>
> "Elijah Newren via GitGitGadget" <gitgitgadget@xxxxxxxxx> 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.
