Re: [gentoo-user] USE="mmx mmxext sse sse2 ssse3 3dnow 3dnowext"

[Mike Edenfield <kutulu@kutulu.org>]

Stroller<stroller@stellar.eclipse.co.uk>  writes:
> But, surely "-march=" also instructs gcc to support the additional
> instructions.  Suggest you re-read Daniel's post that I was replying
> to.
>
> What's the difference between supporting the "certain set of
> instructions" with "-march=" and doing so with USEs?

One is for telling gcc how to compile C code, the other is for telling 
packages what inline assembler code is legal.

In the case of mplayer, just setting mmx/sse/etc does absolutely 
nothing.  You also have to enable custom-cpuopts, which then tells 
mplayer to ignore its own build-time detection of CPU features and only 
use the ones you tell it.  Either way, if mplayer does it automatically 
or you pass in USE flags, all they do is enable selected asm files in 
the codecs that use those opcodes.

--K