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>

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;

revision.c

@@ -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;