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>

1 files changed, 1 insertions, 0 deletions

diff --git a/diff.h b/diff.h
index e9ccb38..fe5c287 100644
--- a/diff.h
+++ b/diff.h
@@ -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;