This is the mail archive of the gcc-patches@gcc.gnu.org mailing list for the GCC project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: [patches] Re: recognize x86 CPU variants and default SSE/MMX/3dNOW support


> Jan Hubicka <jh@suse.cz> writes:
> 
> >> On Tue, Dec 11, 2001 at 11:30:25AM +0100, Jan Hubicka wrote:
> >> > this patch adds the support for the CPU variants as discussed earlier.
> >> 
> >> We can't do this until SSE math is moved to a different switch.
> >
> > What is wrong with defaulting to SSE math on these CPUs?  It is faster so I
> > believe it is similar as when we enable conditional moves or other use of given
> > architecture.  We probably can have switch for backward compatibility with
> > i386 80bit temporaries, but I would still like to see SSE math to default
> > on SSE, mainly SSE2 enabled chips.
> 
> We cannot switch to SSE math since for ia32 since it would violate
> some assumptions people make about floating point precission (all
> evolution done in 80-bit) on Linux.  This would give different
> numeric results depending on the switch.
Yes, but we already do give different results depending on -O switches
and gcc version, so I still don't think it is much worse.
Gcc is allowed to truncate the value anytime it wants to. Additionally
Windows and some other oses do use 64bit math, so I guess the default
fp i386 app to eighter not worry or be relativly portable in this respect.

We are speaking about 5-10% performance difference in average FP code
that can be shame to miss. One still can disable it if he wants to.

Honza
> 
> Andreas
> -- 
>  Andreas Jaeger
>   SuSE Labs aj@suse.de
>    private aj@arthur.inka.de
>     http://www.suse.de/~aj


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]