revision: quit pruning diff more quickly when possible

When the revision traversal machinery is given a pathspec,
we must compute the parent-diff for each commit to determine
which ones are TREESAME. We set the QUICK diff flag to avoid
looking at more entries than we need; we really just care
whether there are any changes at all.

But there is one case where we want to know a bit more: if
--remove-empty is set, we care about finding cases where the
change consists only of added entries (in which case we may
prune the parent in try_to_simplify_commit()). To cover that
case, our file_add_remove() callback does not quit the diff
upon seeing an added entry; it keeps looking for other types
of entries.

But this means when --remove-empty is not set (and it is not
by default), we compute more of the diff than is necessary.
You can see this in a pathological case where a commit adds
a very large number of entries, and we limit based on a
broad pathspec. E.g.:

  perl -e '
    chomp(my $blob = `git hash-object -w --stdin </dev/null`);
    for my $a (1..1000) {
      for my $b (1..1000) {
        print "100644 $blob\t$a/$b\n";
      }
    }
  ' | git update-index --index-info
  git commit -qm add

  git rev-list HEAD -- .

This case takes about 100ms now, but after this patch only
needs 6ms. That's not a huge improvement, but it's easy to
get and it protects us against even more pathological cases
(e.g., going from 1 million to 10 million files would take
ten times as long with the current code, but not increase at
all after this patch).

This is reported to minorly speed-up pathspec limiting in
real world repositories (like the 100-million-file Windows
repository), but probably won't make a noticeable difference
outside of pathological setups.

This patch actually covers the case without --remove-empty,
and the case where we see only deletions. See the in-code
comment for details.

Note that we have to add a new member to the diff_options
struct so that our callback can see the value of
revs->remove_empty_trees. This callback parameter could be
passed to the "add_remove" and "change" callbacks, but
there's not much point. They already receive the
diff_options struct, and doing it this way avoids having to
update the function signature of the other callbacks
(arguably the format_callback and output_prefix functions
could benefit from the same simplification).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Jeff King 2017-10-13 11:27:45 -04:00 коммит произвёл Junio C Hamano
Родитель 9752ad0bb7
Коммит a937b37e76
2 изменённых файлов: 14 добавлений и 3 удалений

1
diff.h
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@ -180,6 +180,7 @@ struct diff_options {
pathchange_fn_t pathchange;
change_fn_t change;
add_remove_fn_t add_remove;
void *change_fn_data;
diff_format_fn_t format_callback;
void *format_callback_data;
diff_prefix_fn_t output_prefix;

Просмотреть файл

@ -394,8 +394,16 @@ static struct commit *one_relevant_parent(const struct rev_info *revs,
* if the whole diff is removal of old data, and otherwise
* REV_TREE_DIFFERENT (of course if the trees are the same we
* want REV_TREE_SAME).
* That means that once we get to REV_TREE_DIFFERENT, we do not
* have to look any further.
*
* The only time we care about the distinction is when
* remove_empty_trees is in effect, in which case we care only about
* whether the whole change is REV_TREE_NEW, or if there's another type
* of change. Which means we can stop the diff early in either of these
* cases:
*
* 1. We're not using remove_empty_trees at all.
*
* 2. We saw anything except REV_TREE_NEW.
*/
static int tree_difference = REV_TREE_SAME;
@ -406,9 +414,10 @@ static void file_add_remove(struct diff_options *options,
const char *fullpath, unsigned dirty_submodule)
{
int diff = addremove == '+' ? REV_TREE_NEW : REV_TREE_OLD;
struct rev_info *revs = options->change_fn_data;
tree_difference |= diff;
if (tree_difference == REV_TREE_DIFFERENT)
if (!revs->remove_empty_trees || tree_difference != REV_TREE_NEW)
DIFF_OPT_SET(options, HAS_CHANGES);
}
@ -1346,6 +1355,7 @@ void init_revisions(struct rev_info *revs, const char *prefix)
DIFF_OPT_SET(&revs->pruning, QUICK);
revs->pruning.add_remove = file_add_remove;
revs->pruning.change = file_change;
revs->pruning.change_fn_data = revs;
revs->sort_order = REV_SORT_IN_GRAPH_ORDER;
revs->dense = 1;
revs->prefix = prefix;