Re: nasm over gas?

[Ihar 'Philips' Filipau]

Jamie Lokier wrote:
> Ihar 'Philips' Filipau wrote:
>
> >  It will depend on arch CPU only in case if you have unlimited i$ size.
> >  Servers with 8MB of cache - yes it is faster.
> >  Celeron with 128k of cache - +4bytes == higher probability of i$ miss 
> > == lower performance.
>
> Higher probability != optimal performance.
>
> It depends on your execution context.  If it's part of a tight loop
> which is executed often, then saving a cycle in the loop gains more
> performance than saving icache, even on a 128k Celeron.

  You think as system-programmer.
  Every bit of i$ waste - hit user space applications too.
  128k of $ - is for every app.

  If you gained one cycle by polluting one more cache line - do not 
forget that this cache line probably contained some info, which was able 
to avoid cache miss for another application. So you gained cycle here - 
and lost it immediately in another app. Not good.

  If you can improve performance by NOT polluting cache - it would be 
another story :-)))

> The execution context can depend on the input to the program, in which
> case the faster of the two code sequences can depend on the program's
> input too.  Then, for optimal performance, you need to profile the
> "expected" inputs.
>
>
> You obviously have not read the GAS documentation.
>
> It has quite a good macro facility built in.

  Indeed. RTFM quickly shown some good examples.

  But still I never saw this kind of thing being used in kernel. 
Instead of writing normal asm we have something like i386/mmx.c. And 
i386/checksum.S not the best sample of asm in kernel too. Sad.

--
Ihar 'Philips' Filipau  / with best regards from Saarbruecken.
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