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From: "Stephen Fuld" <SFuld@alumni.cmu.edu.invalid>
Newsgroups: comp.arch
Subject: Re: Continuations
Date: Wed, 17 Jul 2024 18:30:47 -0000 (UTC)
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MitchAlsup1 wrote:

> 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.

So for your GbOoO CPU, how many of the various FP operations, and the
EXP instruction can be done in parallel?



-- 
 - Stephen Fuld 
(e-mail address disguised to prevent spam)