Re: "SSE instruction set disabled?"

[Jan Hubicka <hubicka at ucw dot cz>]

> 
> >> This makes absolute complete sense.
> >>
> >> "-mmmx", "-msse", "-msse2", and "-m3dnow" (and their corresponding
> >> -mno-xxx) should just go away.
> 
> >So what do you do if you want your binaries or libraries run on any
> >CPU supporting SSE, ie. ATM pIII, p4, athlon-{4,xp,mp}?
> >-march=i686 -msse -mfpmath=sse is what you use now, using -march=pentium3
> >is not a good idea for the athlons and likewise -march=athlon-xp is
> >not a good idea for pentiums.
> 
> See this is why it is so confusing.  To GCC, what you wrote is the same
> thing since
> 
> "-march=pentium3" == "-march=i686 -msse"
> 
> To be even more anal, looking through specs
> 
>    this               implies
> -march=i386         -mcpu=i386
> -march=i486         -mcpu=i486
> -march=i586         -mcpu=i586
> -march=pentium      -mcpu=i586
> -march=pentium-mmx  -mcpu=i586 -mmmx
> -march=i686         -mcpu=i686
> -march=pentiumpro   -mcpu=i686
> -march=pentium2     -mcpu=i686 -mmmx
> -march=pentium3     -mcpu=i686 -msse
> -march=pentium4     -mcpu=pentium4 -msse2
> -march=athlon       -mcpu=athlon -m3dnow
> -march=athlon-tbird -mcpu=athlon -m3dnow
> -march=athlon-xp    -mcpu=athlon -m3dnow -msse
> -march=athlon-mp    -mcpu=athlon -m3dnow -msse
> -march=athlon-4     -mcpu=athlon -m3dnow -msse
> -march=k6           -mcpu=k6
> -march=k6-2         -mcpu=k6 -m3dnow
> -march=k6-2         -mcpu=k6 -m3dnow
> 
> ....
> 
> I agree with others "-msse" or "-msse2" should generally imply "
> -mfpmath=sse".  Its kind of silly to have to specify that twice.

It is not the same, unfortuantely.
-msse2 simply enables the instruction set extension.  The compiled
programs will have same behaviour as before just can be faster.
-mfpmath=sse changes behaviour of floating point execution by not using
80bit temporaries like x87 code does.  This breaks some things (glibc)
and makes other happy (those who want numberic stability).
I am inclined to argue to make -mfpmath=sse default when SSE is
available however last time I lost the battle.

Honza
> 
> ...
> 
> And depreciate "-mcpu" for "-mtune"; it is much more intuitive (at least
> for english speakers).
> 
> Choose the minimum ARCHitecture (capabilities) that you wish to run the
> binary on. Then, if you wish, choose a more recent model to TUNE (optimize)
> the binary for.
> 
> Kelley Cook
>