[R] build on Irix failed reg-tests

[Thomas Lumley]

On Wed, 17 Mar 2004, Liaw, Andy wrote:

> Sorry for being dense:  So What is the likely source of the problem, then?
> Is it the compiler, the OS, R itself, or some combination of these?  Any
> suggestion on how to resolve this?

In a sense it is R itself, making assumptions that hold on many, but not
all systems.  There are many different NaN values, all of which are
guaranteed to give NaN results in arithmetic observations.  On many
systems, if one operand is NaN and the other is a valid number the result
is not only NaN, but *the same NaN* as the operand.

In R we call one particular one of these NaN values "NA".  The standards
guarantee that any operation on NA gives some NaN, but we have assumed it
gives the right NaN, the NA one.  On Irix this seems not to be the case.

The solution is to change arithmetic.c not to make this assumption, which
will be slower. It would be nice to detect at build time whether the
assumption holds, so as to keep the current behaviour on other systems.

It should be rare for the error to make any difference, since is.na() is
still true for NaN.  AFAICT the only problem would be that is.nan(0+NA)
can be TRUE when it should be FALSE.

	-thomas