Re: [Gcl-devel] Re: [Maxima] Re: m68k gcl/maxima: Minor Floating point errors
Greetings! I think its a matter of when the registers are written to
memory. Apparently the m68k can optimize in such a way that many
intermediate results are held in registers before having to be written
out. There is no -ffloat-store on x86. The difference is perhaps
related to the paucity of registers on x86, and the fact that m68k
registers are *96* bits wide, AFAICR, instead of 80. But in general,
one does get more precision on x86 FPU calculations than the strictly
64bit SSE2, for example.
Take care,
Raymond Toy <toy@rtp.ericsson.se> writes:
> >>>>> "Camm" == Camm Maguire <camm@enhanced.com> writes:
>
> Camm> Greetings!
> Camm> Rick Younie <rick@def.debian.net> writes:
>
> >> Camm Maguire wrote:
> >> > Greetings! I have one Debian machine (m68k) which is producing very
> >> > small numerical discrepancies on the results of the two floating point
> >> > intensive tests in rtest8.mac:
> >>
> >> Hi,
> >>
> >> That rang a bell. There's this in the gcc man page:
> >>
> >> -ffloat-store
> >> Do not store floating point variables in registers. This pre-
> >> vents undesirable excess precision on machines such as the 68000
> >> where the floating registers (of the 68881) keep more precision
> >> than a double is supposed to have.
> >>
> >> For most programs, the excess precision does only good, but a
> >> few programs rely on the precise definition of IEEE floating
> >> point. Use `-ffloat-store' for such programs.
>
> Camm> That did it -- thanks! As Richard Fateman observed, its quite
> Camm> surprising the magnitude of the difference was so large. Apparently
> Camm> m68k has the more accurate answer, right? In any case, I'm adding
> Camm> -ffloat-store to gcl's compile flags on m68k to avoid testing
> Camm> problems.
>
> But does gcl on x86 also do a -ffloat-store? x86 also keeps extra
> precision, so I would have expected similar issues on x86.
>
> Ray
>
>
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>
--
Camm Maguire camm@enhanced.com
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