[R] 64-bit programming

[Prof Brian Ripley]

On Tue, 11 Sep 2001 helgito at hi.is wrote:

> Has R been ported to  64-bit platforms?  E.g. as in FORTRAN when you
> define
> double precision number as:
>
> REAL*16
>
> I have heard that some SUN computers already offer 64-bit architecture
> but I am unsure
> about the GNU support  for that hardware.  I am seeking increased
> accuracy  in floating
> point operations.

We run R in 64-bit mode on Suns under the native compiler. However,
the double type in 64-bit mode is 8 bytes, not 16 bytes, and that's
part of the agreement between manufacturers for 64-but platforms as we
understand it:

int - 4 bytes
long - 8 bytes
double - 8 bytes

The 64-bit refers to the pointer size and perhaps to buses and to the
preferred size for integer arithmetic. It does not refer to the FPU,
and FPUs have been 64 bits or greater (e.g. i686) for about 15 years.

Why do you think REAL*16 will give you increased accuracy?  Accuracy has a
lot more to do with the algorithms used and their convergence criteria.
As R is a statistical package, numerical accuracy is almost always
secondary to data uncertainty.

At least the build of gcc 3.0.1 we have does not run in -m64 mode.
The potential benefit of 64-but mode is to address large amounts of
virtual memory from a single process, but R has internal limits there
still, I believe.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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