Re: R-alpha: seq

Peter Dalgaard BSA (pd@kubism.ku.dk)
04 Dec 1996 13:36:36 +0100


To: r-testers@stat.math.ethz.ch (r-testers)
Subject: Re: R-alpha: seq
From: Peter Dalgaard BSA <pd@kubism.ku.dk>
Date: 04 Dec 1996 13:36:36 +0100
In-Reply-To: Kurt Hornik's message of Wed, 4 Dec 1996 13:09:09 +0100
Message-Id: <x2wwuy7agb.fsf@bush.kubism.ku.dk>

Kurt Hornik <Kurt.Hornik@ci.tuwien.ac.at> writes:

> R> .Machine$double.eps
> [1] 2.220446e-16
> R> .Machine$double.max
> [1] 1024
> 
> for the version with -ffloat-store with
> 
> R> .Machine$double.eps
> [1] 1.084202e-19
> R> .Machine$double.max
> [1] -2
> 
> (wow, no positive numbers on my machine?) for the version without.
> 
> Hence, I strongly recommend to ALWAYS add -ffloat-store to CFLAGS (or
> remove any -Ox) when compiling on Linux/Intel.

Well, that is obviously buggy and we need to find a fix for it before
something will actually rely on .Machine$whatever. 

However, -ffloat-store will (a) reduce precision in computations and
(b) remove all efficiency gained by keeping variables in FPU
registers. So I think that your recommendation is throwing the baby
out with the bathing water. Of course, 90% of what we do is not really
FPU bound anyway, but there are important procedures spending a lot of
their time doing e.g. large-matrix inversion and they would certainly
suffer from having forced stores to RAM in their inner loops.

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard@biostat.ku.dk)             FAX: (+45) 35327907
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