Re: [boost] BOOST::SIMD - handling double : precision vs speed

Subject: Re: [boost] BOOST::SIMD - handling double : precision vs speed
From: dherring_at_[hidden]
Date: 2009-04-02 13:46:56

On Thu, 2 Apr 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).
...
> Discussions welcome.

My understanding is that that the problem lies with Intel's 80-bit 
"internal" precision.  I've seen people force a copy out of the FP 
registers to counteract this, but I forget the full logic behind why. 
Maybe just to achieve cross-platform repeatability.

For your purposes, it might be best to have "slow, IEEE-compliant" scalar 
ops for checking results and "fast, Intel-specific" scalars for comparing 
timings.

- Daniel