Fwd: [GSOC] [QUESTION] ref-filter: can %(raw) implement reuse oi.content?

From: ZheNing Hu <adlternative@gmail.com>
Date: Aug 17, 2021, 14:34 UTC
Message-ID: <CAOLTT8QRrHp4OmCS7-sV481cMPK2nmVuiaQpqzv_cM5T0HKbRw@mail.gmail.com>
In-Reply-To: <CAOLTT8TJ_VBh8mreExXHdCFb1e6hB1-Exsn6OFdo2sZNkVFJkA@mail.gmail.com>

Hi,

Because this email was accidentally sent to everyone’s mail instead of CC everyone, so this email was re-sent again.

In the implementation of %(raw) atom (bd0708c7 ref-filter: add %(raw) atom), we use xmemdupz() to copy the content of the object. But if we can reuse the content of the object?

Since git cat-file --batch needs to use ref-filter as the backend, if the object buffer can be reused correctly here, we can save a lot of copies and improve its performance by about 6%.

Tracing back to the source, the object buffer is allocated from oid_object_info_extended(), but in parse_object_buffer() we may lose the ownership of the buffer (maybe the buffer is eaten), but I browsed the source code of for-each-ref.c or cat-file.c, and I don’t seem to find that the buffers which have been eaten are released by the program.

So can we reuse it? (Or can we free() it freely?)

This is what I want to do:

diff --git a/ref-filter.c b/ref-filter.c
index 93ce2a6ef2..1f6c1daabd 100644
--- a/ref-filter.c
+++ b/ref-filter.c
@@ -1443,7 +1443,7 @@ static void grab_sub_body_contents(struct
atom_value *val, int deref, struct exp
                        unsigned long buf_size = data->size;

                        if (atom->u.raw_data.option == RAW_BARE) {
-                               v->s = xmemdupz(buf, buf_size);
+                               v->s = buf;
                                v->s_size = buf_size;
                        } else if (atom->u.raw_data.option == RAW_LENGTH) {
                                v->s = xstrfmt("%"PRIuMAX, (uintmax_t)buf_size);
@@ -1768,8 +1768,6 @@ static int get_object(struct ref_array_item
*ref, int deref, struct object **obj
        }

        grab_common_values(ref->value, deref, oi);
-       if (!eaten)
-               free(oi->content);
        return 0;
 }

Thanks.
--
ZheNing Hu