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SubjectRe: nasm over gas?
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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* Please avoid sending me Word/PowerPoint/Excel attachments.
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There should be some SCO's source code in Linux -
my servers sometimes are crashing. -- People

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