Re: [9fans] integer width on AMD64 (was: Re: AMD64 system)

[steve <steve@quintile.net>]

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sorry for being vague.

treating pixels as 64bit on amd64 as that is the natural size for the machine,
vs using 32bits per pixel - 10 bits of r, g, and b or y, u, and v plus 2 spare
leads to a significant speedup; where significant is a number lost in the mists of time.

i believe this speedup is due to the reduction in the rate of cache line refills,
as forsyth described.

-Steve


On 6 May 2012, at 12:43 PM, Comeau At9Fans <comeauat9fans@gmail.com> wrote:

> I've heard that 64-bit is not an immediate win over 32 for graphics and such, but then again also heard that 32 bit is not the pits, and that it more or less becomes a wash.  So, in your case, what is is about 32 that makes it a *significant* win, given that the space would have been used in either case (I don't know if that's true, but assuming it is)?   (And sorry if I've connected "graphics and such" with image processing since it may be you're doing something I'm not realizing and so we might be talking about different things.)
> 
> On Sun, May 6, 2012 at 5:20 AM, steve <steve@quintile.net> wrote:
> i think this is an often misunderstood fact, 32bit ints are, in my experience,
> a significant win compared with 64bit when doing memory intensive work
> - image processing in my case.
> 
> -Steve
> 
> 
> On 5 May 2012, at 06:48 PM, Charles Forsyth <charles.forsyth@gmail.com>
> .
> > if it's performance you're worried about, for programs that don't care about width, i'd expect 32 bits at least
> > to match performance with 64 bits (if there's a measurable difference). for one thing, cache lines will contain
> > more values, and several will be fetched at once when cache lines are filled.
> >
> 
> 
> 
> 
> -- 
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