Re: [PATCH 3/3] commit-slab: introduce a macro to define a slab for new type

[Junio C Hamano <gitster@pobox.com>]

Jeff King <peff@peff.net> writes:

> On Sat, Apr 13, 2013 at 11:04:49PM -0700, Junio C Hamano wrote:
>
>> Suppose you want to give one bit per existing ref and paint commits
>> down to find which refs are descendants of each commit. You find
>> that you have 320 refs only at runtime.
>> 
>> The code can declare a commit slab "struct flagbits"
>> 
>> 	define_commit_slab(flagbits, unsigned char);
>> 	struct flagbits flags;
>> 
>> and initialize it by:
>> 
>> 	nrefs = ... count number of refs that returns say 320 ...
>> 	init_flagbits_with_stride(&flags, (nrefs + 7) / 8);
>> 
>> so that
>> 
>> 	unsigned char *fp = flagbits_at(&flags, commit);
>> 
>> will return a pointer pointing at an array of 40 "unsigned char"s
>> associated with the commit.
>
> Thanks, I was thinking originally that we would want to break it down
> into "unsigned long" or something, but there is probably no real
> performance advantage to doing that over bytes.

The 320 came from writing "an array of 5 unsigned long long" in the
first draft ;-)

> I'd probably further wrap it with a flagbit_set and flagbit_tst to wrap
> the "figure out which byte, then which bit of that byte" logic, but that
> would be a wrapper around flagbits_at, anyway. It can come later.

Exactly. At that point, it is not about "what you could use commit
slab for" but is about "how you would implement unbounded set of
flag bits".

> We'd probably want the hot path of this (returning the actual pointer)
> to be inline, but not necessarily the parts about growing,...

Yeah, this was just a technology demonstration as your original.