Re: FWD: FLOATING-POINT CONSISTENCY, -FFLOAT-STORE, AND X86

[Toon Moene <toon at moene dot indiv dot nluug dot nl>]

> Using 80-bit spills is a quick approximation to 
> extending the FP stack into memory, and it should give > some of the benefit with very little  (hopefully) 
> hassle.  Of course, 80 bits is wider than the normal 
> spill, so it eats more memory bandwidth, cache space, 
> etc.  Anyone who's that concerned will bend over 
> backwards to avoid spills anyways.

Am I the only one - apart from Harvey J. Stein and Tim Prince - who
finds this whole discussion unreal ?  Surely, 80 bit temporaries might
seem a neat hack to a numerical analyst like Dr. Kahan, but the ordinary
computational physicist or chemist knows better than to choose "poorly
conditioned" algorithms.

My main concern is that there is a grid spacing that will render the
basic equation of geostrophy badly approximated in 32-bit arithmetic:

	 1  dp
	--- -- = f v
	rho dx

p	pressure
rho	mass of air per unit volume (1 kg / m^3)
x	distance
f	Coriolis parameter (10^-4 s^-1)
v	wind speed

You can do the math (p ~ 10^5 kg m^-1 s^-2, v ~ 10 m/s, what dx will
make dp < 10^-3 p ?)

If that's the case we have to rethink our finite difference code for the
first time in 13 years and use a trick like subtracting a basic state
from the equations - big deal.

The last thing I need is to have egcs slowed down to a crawl by having
it spill unaligned 80-bit temporaries for something that shouldn't be
larger than 32 bits in the first place.

Please make this and other "accuracy" options a "-pedantic-numerics"
one.

-- 
Toon Moene (toon@moene.indiv.nluug.nl)
Saturnushof 14, 3738 XG  Maartensdijk, The Netherlands
Phone: +31 346 214290; Fax: +31 346 214286
g77 Support: fortran@gnu.org; egcs: egcs-bugs@cygnus.com