Re: [PATCH v4 07/11] pack-objects: refer to delta objects by index instead of pointer

From: Junio C Hamano <gitster@pobox.com>
Date: Mar 16, 2018, 20:59 UTC
Message-ID: <xmqq7eqb20vd.fsf@gitster-ct.c.googlers.com>
In-Reply-To: <20180316183200.31014-8-pclouds@gmail.com>

Nguyễn Thái Ngọc Duy  <pclouds@gmail.com> writes:

> These delta pointers always point to elements in the objects[] array
> in packing_data struct. We can only hold maximum 4GB of those objects

4GB, as in "number of bytes"? Or "We can hold 4 billion or so of those objects"?

Show 12 quoted lines

> because the array length, nr_objects, is uint32_t. We could use
> uint32_t indexes to address these elements instead of pointers. On
> 64-bit architecture (8 bytes per pointer) this would save 4 bytes per
> pointer.
>
> Convert these delta pointers to indexes. Since we need to handle NULL
> pointers as well, the index is shifted by one [1].
>
> [1] This means we can only index 2^32-2 objects even though nr_objects
>     could contain 2^32-1 objects. It should not be a problem in
>     practice because when we grow objects[], nr_alloc would probably
>     blow up long before nr_objects hits the wall.