[Numpy-discussion] Different results from repeated calculation

Keith Goodman kwgoodman at gmail.com
Sun Jan 28 13:53:34 EST 2007

On 1/28/07, Fernando Perez <fperez.net at gmail.com> wrote:
> On 1/28/07, Keith Goodman <kwgoodman at gmail.com> wrote:
> > On 1/28/07, Fernando Perez <fperez.net at gmail.com> wrote:
> > > [snip]  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.
> >
> > So since I use N.float64, ATLAS SSE won't help me?
> Well, the SSE part won't, but you're still better off with ATLAS than
> with the default reference BLAS implementation.  I think even an ATLAS
> SSE has special code for double (not using any SSE-type engine) that's
> faster than the reference BLAS which is pure generic Fortran.  Someone
> who knows the ATLAS internals please correct me if that's not the
> case.

That makes sense.

Unfortunately my simulation gives different results with and without
ATLAS SSE even though the test script I made doesn't detect the

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