Re: [PATCH/RFC v3 08/12] pack-objects: refer to delta objects by index instead of pointer

From: Junio C Hamano <gitster@pobox.com>
Date: Mar 14, 2018, 16:18 UTC
Message-ID: <xmqqy3iuzl3i.fsf@gitster-ct.c.googlers.com>
In-Reply-To: <20180308114232.10508-9-pclouds@gmail.com>

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

> Notice that packing_data::nr_objects is uint32_t, we could only handle
> maximum 4G objects and can address all of them with an uint32_t. If we
> use a pointer here, we waste 4 bytes on 64 bit architecture.

Some things are left unsaid or left unclear and make readers stutter a bit while reading this paragraph. We can address them with uint32_t only because we happen to have a linear array of all objects involved already, i.e. the pack->objects[] array. The readers are forced to rephrase the above in their mind

	... and each of them can be identified with an uint32_t.
	Because we have all of these objects in pack->objects[], we
	can replace the "delta" field in each object entry that
	points at its delta base object with uint32_t index into
	this array to save memory (on 64-bit arch, 8-byte pointer
	gets shrunk to 4-byte uint).

or something like that before understanding why this is a valid memory footprint optimization.