Re: gcc, march and sse

["Joel Carnat" <joel%carnat.net@localhost>]

Le Mer 3 décembre 2008 16:40, Dennis den Brok a écrit :
> Joel Carnat <joel%carnat.net@localhost> schrieb:
>> Hello,
>>
>> I read that Slackware Linux compiles most of its packages with
>> "-march=i486 -mtune=i686". As far as I understood gcc's manpage, this
>> means "keep code compatible with 486 CPU but allow the use pentiumpro
>> special features", right ?
>>
>> Still from the man page, MMX is supported since "pentium2 (or
>> pentium-mmx
>> only)", SSE since "pentium3", SSE2 since "pentium-m", SSE3 since
>> "prescott".
>>
>> I tweaked my mk.conf to compile pkgsrc (on a domU) using "-O2 -march=486
>> -mtune=686" but I'm now wondering if any on those compiled programs will
>> use MMX or SSE when running on my laptop or intel-based servers (intel
>> core2duo).
>>
>> Then second question, in case the previous thought is correct, if I use
>> "-march=i486 -mtune=nocona", is there a way to see which instruction the
>> program uses ? For example, if I compile xorg/firefox/mplayer and then
>> run
>> it in a special manner, can I see that the core2duo uses MMX/SSE*
>> instruction while the C3 only use MMX instruction (from the same binary)
>> ?
>>
>
> -march specifies the instruction set the compiler may use; thus, neither
> MMX
> nor SSE will be used. There may be exceptions, however, because some
> pkgsrc

hum... then what would be the point in setting "mtune" ?

> applications, notably mplayer, choose which instruction set to use at
> runtime,
> so in that case, you wouldn't have to worry about losing a few CPU cycles.
>
> HTH,
> Dennis den Brok
>
>