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From: mitchalsup@aol.com (MitchAlsup1)
Newsgroups: comp.arch
Subject: Re: Continuations
Date: Wed, 17 Jul 2024 17:41:49 +0000
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On Wed, 17 Jul 2024 16:50:27 +0000, Thomas Koenig wrote:

> MitchAlsup1 <mitchalsup@aol.com> schrieb:
>
>> What I am talking about is to improve their performance until a
>> sin() takes about the same number of cycles of FDIV, not 10× more.
>
> Maybe time for a little story.
>
> Some unspecified time ago, a colleague did CFD calculations which
> included fluid flow (including turbulence modelling and diffusion)
> and quite a few chemical reactions together.  So, he evaluated a
> huge number of Arrhenius equations,
>
> k = A * exp(-E_a/(R*T))
>
> and because some of the reactions he looked at were highly
> exothermic or endothermic, he needed tiny relaxation factors (aka
> small steps).  His calculaiton spent most of the time evaluating
> the Arrhenius equation above many, many, many, many times.
>
> A single calculation took _months_, and he didn't use weak hardware.
>
> A fully pipelined evaluation of, let's say, four parallel exp and
> four parallel fdiv instructions would have reduced his calculation
> time by orders of magnitude, and allowed him to explore the design
> space instead of just scratching the surface.
>
> (By the way, if I had found a reasonable way to incorporate the
> Arrhenius equation into your ISA, I would have done so already :-)

     FMUL     Rt,RR,RT
     FDIV     Rt,-RE,Rt
     EXP      Rt,Rt
     FMUL     Rk,RA,Rt

Does not look "all that bad" to me.