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Subject: Re: [boost] BOOST::SIMD - handling double : precision vs speed
From: Frank Mori Hess (frank.hess_at_[hidden])
Date: 2009-04-02 13:51:32

Hash: SHA1

On Thursday 02 April 2009, Joel Falcou wrote:
> SIMD algorithms for double precision seem to be rather hard to do right.
> It's difficult to get the right precision with respect to the scalar
> reference as scalar algorithm take advantages of the internal 80 bits
> floating points register, thus leading comparison between our
> implementation and the reference to yields things like 3000 ulp (ie
> 10^-13 RMS instead of 10^16).
> Fixing this is difficult and even if it's possible for some algorithms,
> the average speed-up then drop to less than 10% - ie as fast as an
> unrolled scalar call over the SIMD vector elements.
> What should we enforce : precision or speed ? Or is the 10^-13 RMS enough ?

Why would you want your 64 bit SIMD floating point calculations to act like
they are going through an x86 processor's 80 bit floating point unit? I
think the important thing is just conformance to the ieee floating point
standards. My impression was some of the early generations of SIMD
instructions were not compliant, but the newer versions all are. If you're
just worried about comparison against some non-SIMD reference code, maybe it
would help to use compiler flags to disable internal 80 bit rounding when
compiling the reference code (I think you can do this on gcc at least) .
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