Mailinglist Archive: opensuse-programming (27 mails)

[David Haller <dnh@xxxxxxxxxxxx>]

Hello,

On Tue, 12 Jan 2010, Per Jessen wrote:
> David Haller wrote:
> > On Tue, 12 Jan 2010, Per Jessen wrote:
[..]
> > > I'm processing a dataset of 262176 bytes.  Not a lot, but still about
> > > 2million bits.  The computation is 99.9% CPU-bound.
> > >
> > > In my first version, I used a div instruction in the code, and the
> > > computation took 151minutes (wall-time).
> > > In the second, I removed the div, and probably removed one or two
> > > other instructions.  New computation took 62minutes.
> >
> > ... and replaced it by what? And have you compared to C-Code compiled
> > with gcc or icc? ;)
>
> The div was a divide by 10 followed by examination of remainder and
> quotient.  Under the circumstances, the number being divided would
> always be in the range 0-19, which meant I could easily substitute with
> a subtract 10 and a check of the result. The div was effectively
> replaced by 3 other instructions, and I think I managed to do away with
> one more instruction along the way. (I really only looked at the
> timing).
>
> As for gcc code, I did start out with C-code for a quick prototype. I
> also had a quick look at what was generated - it would have been
> horrendously slow in comparison.  After all, neither gcc nor icc could
> have been aware of the special circumstances that I knew about. 
> Hand-optimized assembler will _always_ beat the compiler, but the
> trade-off is in speed vs maintainability. The latter is (in this
> project) not a concern for me.

Definitely not. Generated code can be surprisingly efficient today
(with proper options). But, in special cases, of course, *nothing*
beats hand-crafted assembler[1] ;) But one should check that it
actually does. I'm glad you found a good solution.

Have Fun!

-dnh, more of a perlmonger, and I do run benchmarks there, with at
    times surprising results :)

[1] just have a look at my /bin/true ;) 45 Bytes hand- + nasm-crafted
    bytes of ELF Binary. Not by me, mind you.
    http://www.muppetlabs.com/%7Ebreadbox/software/tiny/true.asm.txt

-- 
Lies halt mal dclp.*, da faellt dir nix mehr ein. Wenn man ein
Guerteltier ueber die Tastatur abrollt, kommt besserer PHP Code
raus als da gepostet wird.             -- R. Huebenthal in darw
-- 
To unsubscribe, e-mail: opensuse-programming+unsubscribe@xxxxxxxxxxxx
For additional commands, e-mail: opensuse-programming+help@xxxxxxxxxxxx