Re: [PATCH v2 07/20] merge-ort: avoid repeating fill_tree_descriptor() on the same tree

[Elijah Newren <newren@gmail.com>]

On Wed, Nov 11, 2020 at 6:51 AM Derrick Stolee <stolee@gmail.com> wrote:
>
> On 11/2/2020 3:43 PM, Elijah Newren wrote:
> > @@ -99,6 +99,15 @@ static int collect_merge_info_callback(int n,
> >       unsigned mbase_null = !(mask & 1);
> >       unsigned side1_null = !(mask & 2);
> >       unsigned side2_null = !(mask & 4);
> > +     unsigned side1_matches_mbase = (!side1_null && !mbase_null &&
> > +                                     names[0].mode == names[1].mode &&
> > +                                     oideq(&names[0].oid, &names[1].oid));
> > +     unsigned side2_matches_mbase = (!side2_null && !mbase_null &&
> > +                                     names[0].mode == names[2].mode &&
> > +                                     oideq(&names[0].oid, &names[2].oid));
> > +     unsigned sides_match = (!side1_null && !side2_null &&
> > +                             names[1].mode == names[2].mode &&
> > +                             oideq(&names[1].oid, &names[2].oid));
>
> If the *_null values were in an array, instead, then all of these
> lines could be grouped as a macro:
>
>         unsigned null_oid[3] = {
>                 !(mask & 1),
>                 !(mask & 2),
>                 !(mask & 4)
>         };
>
>         #define trivial_merge(i,j) (!null_oid[i] && !null_oid[j] && \
>                                     names[i].mode == names[j].mode && \
>                                     oideq(&names[i].oid, &names[j].oid))
>
>         unsigned side1_matches_mbase = trivial_merge(0, 1);
>         unsigned side2_matches_mbase = trivial_merge(0, 2);
>         unsigned sides_match = trivial_merge(1, 2);

Hmm, I like it.  I think I'll rename trivial_merge() to
non_null_match() (trivial merge suggests it can immediately be
resolved which is not necessarily true if rename detection is on), but
otherwise I'll use this.

> I briefly considered making these last three an array, as well,
> except the loop below doesn't use 'i' in a symmetrical way:
>
> > +                     if (i == 1 && side1_matches_mbase)
> > +                             t[1] = t[0];
> > +                     else if (i == 2 && side2_matches_mbase)
> > +                             t[2] = t[0];
> > +                     else if (i == 2 && sides_match)
> > +                             t[2] = t[1];
>
> Since the 'i == 2' case has two possible options, it wouldn't be
> possible to just have 'side_matches[i]' here.
>
> > +                     else {
> > +                             const struct object_id *oid = NULL;
> > +                             if (dirmask & 1)
> > +                                     oid = &names[i].oid;
> > +                             buf[i] = fill_tree_descriptor(opt->repo,
> > +                                                           t + i, oid);
> > +                     }
>
> I do appreciate the reduced recursion here!

Technically, not my own optimization; I just copied from
unpack-trees.c:traverse_trees_recursive() -- though the code looks
slightly different because I didn't want to compare oids multiple
times (I use the side match variables earlier in the function as
well).