On Mon, May 17, 2021 at 6:41 AM Derrick Stolee <stolee@xxxxxxxxx> wrote:
>
> On 5/3/21 10:12 PM, Elijah Newren via GitGitGadget wrote:
> > From: Elijah Newren <newren@xxxxxxxxx>
> >
> > When there are many renames between the old base of a series of commits
> > and the new base for a series of commits, the sequence of merges
> > employed to transplant those commits (from a cherry-pick or rebase
> > operation) will repeatedly detect the exact same renames. This is
> > wasted effort.
> >
> > Add data structures which will be used to cache rename detection
> > results, along with the initialization and deallocation of these data
> > structures. Future commits will populate these caches, detect the
> > appropriate circumstances when they can be used, and employ them to
> > avoid re-detecting the same renames repeatedly.
>
> I appreciate the definitions and boilerplate for these data
> structures being isolated to their own patch.
>
> > @@ -140,6 +140,37 @@ struct rename_info {
> > int callback_data_nr, callback_data_alloc;
> > char *callback_data_traverse_path;
> >
> > + /*
> > + * cached_pairs: Caching of renames and deletions.
> > + *
> > + * These are mappings recording renames and deletions of individual
> > + * files (not directories). They are thus a map from an old
> > + * filename to either NULL (for deletions) or a new filename (for
> > + * renames).
> > + */
> > + struct strmap cached_pairs[3];
> > +
> > + /*
> > + * cached_target_names: just the destinations from cached_pairs
> > + *
> > + * We sometimes want a fast lookup to determine if a given filename
> > + * is one of the destinations in cached_pairs. cached_target_names
> > + * is thus duplicative information, but it provides a fast lookup.
> > + */
> > + struct strset cached_target_names[3];
>
> These two work well together. Very clear.
>
> > + /*
> > + * cached_irrelevant: Caching of rename_sources that aren't relevant.
> > + *
> > + * cached_pairs records both renames and deletes. Sometimes we
> > + * do not know if a path is a rename or a delete because we pass
> > + * RELEVANT_LOCATION to diffcore_rename_extended() and based on
> > + * various optimizations it returns without detecting whether that
> > + * path is actually a rename or a delete. We need to cache such
> > + * paths too, but separately from cached_pairs.
> > + */
> > + struct strset cached_irrelevant[3];
>
> I'm having a hard time parsing what these "irrelevant" paths will be.
> It seems like diffcore_rename_extended() will report something other
> than "rename" or "delete" for some paths. Could we explicitly mark
> that state as "irrelevant"?
The state is better known as RELEVANT_NO_MORE, yes.
> /*
> * cached_irrelevant: Caching of rename_sources that aren't relevant.
> *
> * cached_pairs records both renames and deletes. Sometimes we
> * do not know if a path is a rename or a delete because we pass
> * RELEVANT_LOCATION to diffcore_rename_extended() which might
> * describe a path as "irrelevant" instead of as a "rename" or "delete".
> * We need to cache such paths too, but separately from cached_pairs.
> */
>
> Does this make sense? diffcore_rename_extended() might need an update
> to match this extra, explicit state.
Hmm, let's flesh out the description a bit and try to be more
explicit. How about:
/*
* cached_irrelevant: Caching of rename_sources that aren't relevant.
*
* If we try to detect a rename for a source path and succeed, it's
* part of a rename. If we try to detect a rename for a source path
* and fail, then it's a delete. If we do not try to detect a rename
* for a path, then we don't know if it's a rename or a delete. If
* merge-ort doesn't think the path is relevant, then we just won't
* cache anything for that path. But there's a slight problem in
* that merge-ort can think a path is RELEVANT_LOCATION, but due to
* commit 9bd342137e ("diffcore-rename: determine which
* relevant_sources are no longer relevant", 2021-03-13),
* diffcore-rename can downgrade the path to RELEVANT_NO_MORE. To
* avoid excessive calls to diffcore_rename_extended() we still need
* to cache such paths, though we cannot record them as either
* renames or deletes. So we cache them here as a "turned out to be
* irrelevant *for this commit*" as they are often also irrelevant
* for subsequent commits, though we will have to do some extra
* checking to see whether such paths become relevant for rename
* detection when cherry-picking/rebasing subsequent commits.
*/
