gcc, march and sse

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

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) ?

TIA,
    Jo