Re: "SSE instruction set disabled?"

[Jan Hubicka <hubicka at ucw dot cz>]

> On Wed, 4 Dec 2002, R. Kelley Cook wrote:
> 
> > This makes absolute complete sense.
> > 
> > "-mmmx", "-msse", "-msse2", and "-m3dnow" (and their coresponding 
> > -mno-xxx) should just go away.
> > 
> > They already are implied by -march=blahblah that we have.  If someone 
> > wants code tuned for a pentium4, but without any SSE or MMX support then 
> > they can type "-march=pentiumpro -mcpu=pentium4"
I am not agains this.  What I think does not make sense is
-march=pentiumpro -msse2 -mfpmath=sse
(ie I have imaginary pentiumpro with SSE2 supported)
Perhaps more sense would make -march=pentium4 -mnosse, but similary we
may want -march=pentium4 -mno-cmov or any other extension...
> 
> Don't some CPU types exist in versions with and without some of these
> features, or should this be allowed for by more fine-grained -march?

This should be dealt with the -march.  Situation is somewhat complicated
for Athlons that exists in SSE and non-SSE version and additionally some
CPUs have SSE support enabled by feature register by BIOS so on some
notebooks SSE is not available and appears after BIOS update as long as
I am informed.

There are also operating systems not supporting SSE context switch so
one can't use SSE on these... So I am not quite sure how to deal with
all these dead ends.
> 
> There's also the question of allowing people to specify the options that
> are the closest approximation to right if they have some clone CPU that
> doesn't have a specific GCC option.
> 
> > Although, in my opinion "-mcpu=" should be renamed "-mtune=" like was 
> > debated ages ago: http://gcc.gnu.org/ml/gcc/1999-01n/msg01004.html
> 
> I think the last proposal was for consistently named --arch / --tune
> options across all targets (with corresponding ability for --help to list
> the supported types with descriptions) ("gcc/config/mips/mips.h reorg"
> thread on gcc-patches, June 2001).

-mtune is more clear name for the feature than -mcpu, so I would not
object against adding it.  We can support both as well to avoid people
from rewriting their makefiles once again...

Honza
> 
> -- 
> Joseph S. Myers
> jsm28@cam.ac.uk