Re: FLOATING-POINT CONSISTENCY, -FFLOAT-STORE, AND X86

[Marc Lehmann <pcg at goof dot com>]

On Sat, Dec 19, 1998 at 02:07:02AM -0500, N8TM@aol.com wrote:
> In a message dated 12/18/98 10:36:15 PM Pacific Standard Time, rth@cygnus.com
> writes:
> 
> << I have not tried quantifing the change.  I would want to examine
>  things more closely, however, because 25% seems low to me. >>
> 
> Some proponents of the idea felt that spills were so rare that no difference
> would be seen regardless of the efficiency of an 80-bit spilling
> implementation. My 25% figure is for a complete execution of the application;
> certainly there must be sections of this application where the 80-bit spills
> are doubling the time spent.  That means the 80-bit spills, a majority of them
> mis-aligned, are taking several times as long as the 32-bit spills.

Ok, _some_ data: if everything is in the cache, on my p-ii,

        fldl %0
        fxam
        fstpl %0
        fwait

takes 3 cycles regardless of how the memory is aligned.

The code sequence:

        fldt %0
        fxam
        fstpt %0
	fwait

takes 6 cycles.

I have no idea how valid these results are (I'm probably not measuring the
fst), but xfmode spills seem to be expensive.

I still think its a much better solution than -ffloat-store (slow) and
64 bit precision (changing too many things we have no control over).

PS: I'm not subscribed at the moment ;)

--
Happy New Year, I'll be away from 21. Dec to 7. Jan

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