diff: avoid useless filespec population

builtin_diff calls fill_mmfile fairly early, which in turn
calls diff_populate_filespec, which actually retrieves the
file's blob contents into a buffer. Long ago, this was
sensible as we would need to look at the blobs eventually.

These days, however, we may not ever want those blobs if we
end up using a textconv cache, and for large binary files
(exactly the sort for which you might have a textconv
cache), just retrieving the objects can be costly.

This patch just pushes the fill_mmfile call a bit later, so
we can avoid populating the filespec in some cases.  There
is one thing to note that looks like a bug but isn't. We
push the fill_mmfile down into the first branch of a
conditional. It seems like we would need it on the other
branch, too, but we don't; fill_textconv does it for us (in
fact, before this, we were just writing over the results of
the fill_mmfile on that branch).

Here's a timing sample on a commit with 45 changed jpgs and
avis. The result is fully textconv cached, but we still
wasted a lot of time just pulling the blobs from storage.
The total size of the blobs (source and dest) is about
180M.

[before]
$ time git show >/dev/null
real    0m0.352s
user    0m0.148s
sys     0m0.200s

[after]
$ time git show >/dev/null
real    0m0.009s
user    0m0.004s
sys     0m0.004s

And that's on a warm cache. On a cold cache, the "after"
case is not much worse, but the "before" case has to do an
extra 180M of I/O.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/diff.c b/diff.c
index 72d8503d89ec99a278e9307090a440c8279cd266..4cb6d9a9e8e80bc834e14f15c07e67069e449a89 100644 (file)
--- a/diff.c
+++ b/diff.c
@@ -1680,12 +1680,11 @@ static void builtin_diff(const char *name_a,
                }
        }
 
-       if (fill_mmfile(&mf1, one) < 0 || fill_mmfile(&mf2, two) < 0)
-               die("unable to read files to diff");
-
        if (!DIFF_OPT_TST(o, TEXT) &&
-           ( (diff_filespec_is_binary(one) && !textconv_one) ||
-             (diff_filespec_is_binary(two) && !textconv_two) )) {
+           ( (!textconv_one && diff_filespec_is_binary(one)) ||
+             (!textconv_two && diff_filespec_is_binary(two)) )) {
+               if (fill_mmfile(&mf1, one) < 0 || fill_mmfile(&mf2, two) < 0)
+                       die("unable to read files to diff");
                /* Quite common confusing case */
                if (mf1.size == mf2.size &&
                    !memcmp(mf1.ptr, mf2.ptr, mf1.size))