Re: [PATCH v3 0/21] pack bitmaps

[Jeff King <peff@peff.net>]

On Thu, Nov 14, 2013 at 07:19:38PM +0000, Ramsay Jones wrote:

> Unfortunately, I didn't find time this weekend to finish the msvc build
> fixes. However, after a quick squint at these patches, I think you have
> almost done it for me! :-D
> 
> I must have misunderstood the previous discussion, because my patch was
> written on the assumption that the ewah directory wouldn't be "git-ified"
> (e.g. #include git-compat-util.h).

I think it was up for debate at some point, but we did decide to go
ahead and git-ify. Please feel free to submit further fixups if you need
them.

> >   - the ewah code used gcc's __builtin_ctzll, but did not provide a
> >     suitable fallback. We now provide a fallback in C.
> 
> ... here.
> 
> I was messing around with several implementations (including the use of
> msvc compiler intrinsics) with the intention of doing some timing tests
> etc. [I suspected my C fallback function (a different implementation to
> yours) would be slightly faster.]

Yeah, I looked around for several implementations, and ultimately wrote
one that was the most readable to me. The one I found shortest and most
inscrutable was:

  return popcount((x & -x) - 1);

the details of which I still haven't worked through in my head. ;)

I do think on most platforms that intrinsics or inline assembler are the
way to go. My main goal was to get something correct that would let it
compile everywhere, and then people can use that as a base for
optimizing. Patches welcome. :)

-Peff