Re: [PATCH 1/4] strbuf_split_buf(): use ALLOC_GROW()

On Tue, Nov 06, 2012 at 08:54:07AM +0100, Michael Haggerty wrote:

quoted

I suspect this was not used originally because ALLOC_GROW relies on
alloc_nr, which does fast growth early on. At (x+16)*3/2, we end up with
24 slots for the first allocation. We are typically splitting 1 or 2
values.

It probably doesn't make a big difference in practice, though, as we're
talking about wasting less than 200 bytes on a 64-bit platform, and we
do not tend to keep large numbers of split lists around.

I did a little bit of archeology, and found out that

* ALLOC_GROW() did indeed exist when this code was developed, so it
  *could have* been used.

* OTOH, I didn't find any indication on the mailing list that the
  choice not to use ALLOC_GROW() was a conscious decision.

So history doesn't give us much guidance.

Thanks for digging.

If the size of the initial allocation is a concern, then I would suggest
adding a macro like ALLOC_SET_SIZE(ary,nr,alloc) that could be called to
initialize the size to some number less than 24.  Such a macro might be
useful elsewhere, too.  It wouldn't, of course, slow the growth rate
*after* the first allocation.

I think we are getting into premature optimization territory. Let's
take your series as a cleanup, and we can worry about micro-optimizing
the allocation if and when it ever becomes an issue.

-Peff