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

[Jeff King <peff@peff.net>]

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.

> >> +KHASH_INIT(oid, struct object_id, int, 0, oid_hash, oid_equal)
> > 
> > This will declare these "static inline". Our other major "macros become
> > inline functions" code is commit-slab.h, and there we found it necessary
> > to add MAYBE_UNUSED. I wonder if we ought to be doing the same here (I
> > don't get any warnings, but I suspect sparse might complain).
> 
> I doubt it (but didn't check) because khash.h defines kh_clear_##name(),
> which we don't use it anywhere and there have been no complaints so far.
> And if we wanted to add MAYBE_UNUSED then the right place for that would
> be in KHASH_INIT, no?

Right, that would be the correct spot. I'm OK to leave it until somebody
complains. Looking at commit-slab again, its functions are straight
statics, not static inline. That's probably the important difference.

> > 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.

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.

I may be worried about nothing, though. I don't know how clever the
compiler and linker can be there.

-Peff