Re: [PATCH v2 2/2] oidset: use khash

From: "René Scharfe" <l.s.r@web.de>
To: Jeff King <peff@peff.net>
Cc: Git List <git@vger.kernel.org>, Junio C Hamano <gitster@pobox.com>
Subject: Re: [PATCH v2 2/2] oidset: use khash
Date: Thu, 4 Oct 2018 17:05:09 +0200	[thread overview]
Message-ID: <413fc698-87f8-f1ee-810a-ec9b143db299@web.de> (raw)
In-Reply-To: <20181004064833.GA9051@sigill.intra.peff.net>

Am 04.10.2018 um 08:48 schrieb Jeff King:
> On Thu, Oct 04, 2018 at 07:56:44AM +0200, René Scharfe wrote:
> 
>>> As the comment above notes, I think we're really looking at the case
>>> where this gets populated on the first call, but not subsequent ones. It
>>> might be less hacky to use a "static int initialized" here. Or if we
>>> want to avoid hidden globals, put the logic into filter_refs() to decide
>>> when to populate.
>>
>> Right.  I'd prefer the latter, but was unable to find a nice way that
>> still populates the oidset lazily.  It's certainly worth another look,
>> and a separate series.
> 
> It's a little awkward because the lazy load happens in a conditional.
> You can fully encapsulate it like the patch below, but I actually don't
> think it's really helping readability.

*Shudder*

>>> It might be nice if these functions could hide inside oidset.c (and just
>>> declare the struct here). It looks like we might be able to do that with
>>> __KHASH_TYPE(), but the double-underscore implies that we're not
>>> supposed to. ;)
>>>
>>> I guess we also use a few of them in our inlines here. I'm not 100% sure
>>> that oidset_* needs to be inlined either, but this is at least a pretty
>>> faithful conversion of the original.
>>
>> We could inline all of the oidset functions, following the spirit of
>> klib/khash.h.
>>
>> Or we could uninline all of them and then may be able to clean up
>> oidset.h by using KHASH_DECLARE.  Perhaps we'd need to guard with an
>> "#ifndef THIS_IS_OIDSET_C" or similar to avoid a clash with KHASH_INIT.
>>
>> Not sure if any of that would be a worthwhile improvement..
> 
> Unless we know something is a performance win to inline, I'd generally
> prefer not to.

Agreed.

> For a case like this with auto-generated functions, I'm mostly worried
> about bloating the compiled code. Either with a bunch of inlined
> non-trivial functions, or cases where the compiler says "this is too big
> to inline" and generates an anonymous file-scope function, but we end up
> with a bunch of duplicates, because we're generating the same functions
> in a bunch of C files.

The _iter_ functions look harmless in this regard, as the only use small
functions that are not even type-specific.

oidset_init() would better be moved to oidset.c, as the code for resizing
is quite big.

René