[Numpy-discussion] Different results from repeated calculation

Keith Goodman kwgoodman at gmail.com
Sun Jan 28 12:06:32 CST 2007


On 1/28/07, Fernando Perez <fperez.net at gmail.com> wrote:
> On 1/28/07, Charles R Harris <charlesr.harris at gmail.com> wrote:
>
> > > The problem goes away if I remove atlas (atlas3-sse2 for me). But that
> > > just introduces another problem: slowness.
> > >
> > > So is this something to report to Clint Whaley? Or does it have to do
> > > with how numpy uses atlas?
> >
> >
> > Interesting, I wonder if ATLAS is resetting the FPU flags and changing the
> > rounding mode? It is just the LSB of the mantissa that  looks to be
> > changing. Before reporting the problem it might be good to pin it down a bit
> > more if possible.
>
> Well, the fact that I don't see the problem on a PentiumIII (with
> atlas-sse) but I see it on my desktop (atlas-sse2) should tell us
> something.  The test code uses double arrays, and SSE2 has double
> precision support but it's purely 64-bit doubles.  SSE is
> single-precision only, which means that for a double computation,
> ATLAS isn't used and the Intel FPU does the computation instead.
> Intel FPUs use 80 bits internally for intermediate operations (even
> though they only return a normal 64-bit double result), so it's fairly
> common to see this kind of thing.

Removing ATLAS SSE2 does fix the problem. But why does test1 pass when
ATLAS SSE2 is present?


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