[Rd] NA_real_ <op> NaN -> NA or NaN, should we care?

[William Dunlap]

> From: Martin Maechler [mailto:maechler at stat.math.ethz.ch] 
> Sent: Friday, May 01, 2009 5:15 AM
> To: William Dunlap
> Cc: r-devel at r-project.org
> Subject: Re: [Rd] NA_real_ <op> NaN -> NA or NaN, should we care?
> 
> >>>>> William Dunlap <wdunlap at tibco.com>
> >>>>>     on Thu, 30 Apr 2009 10:51:43 -0700 writes:
> 
>     > On Linux when I compile R 2.10.0(devel) 
> (src/main/arithmetic.c in
>     > particular)
>     > with gcc 3.4.5 using the flags -g -O2 I get 
> noncommutative behavior when
> 
> is this really gcc 3.4.5  (which is quite old) ?

Yes, it was 3.4.5, but here is a self-contained example of the same
issue using gcc 4.1.3 on an Ubuntu Linux machine:

% gcc -O2 t.c -o a.out ; ./a.out
NA : 7ff00000000007a2
NaN: fff8000000000000
NA+NaN: 7ff80000000007a2
NaN+NA: fff8000000000000
% gcc  t.c -o a.out ; ./a.out
NA : 7ff00000000007a2
NaN: fff8000000000000
NA+NaN: 7ff80000000007a2
NaN+NA: 7ff80000000007a2
% gcc -v
Using built-in specs.
Target: i486-linux-gnu
Configured with: ../src/configure -v --enable-languages=c,c++,fortran,objc,obj-c++,treelang --prefix=/usr --enable-shared --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --enable-nls --with-gxx-include-dir=/usr/include/c++/4.1.3 --program-suffix=-4.1 --enable-__cxa_atexit --enable-clocale=gnu --enable-libstdcxx-debug --enable-mpfr --enable-checking=release i486-linux-gnu
Thread model: posix
gcc version 4.1.3 20070929 (prerelease) (Ubuntu 4.1.2-16ubuntu2)
% cat t.c
#include <stdio.h>
#include <stdint.h>
#include <string.h>

int main(int argc, char *argv[])
{
    int64_t NA_int64 = 0x7ff00000000007a2LL ;
    int64_t NaN_int64 = 0xfff8000000000000LL ;
    int64_t sum_int64 ;
    double NA_double, NaN_double, sum_double ;

    memcpy((void*)&NA_double, (void*)&NA_int64, 8) ;
    memcpy((void*)&NaN_double, (void*)&NaN_int64, 8) ;

    NaN_double = 1/0.0 - 1/0.0 ;

    printf("NA : %Lx\n", *(int64_t*)&NA_double);
    printf("NaN: %Lx\n", *(int64_t*)&NaN_double);
    sum_double = NA_double + NaN_double ;
    memcpy((void*)&sum_int64, (void*)&sum_double, 8) ;
    printf("NA+NaN: %Lx\n", sum_int64) ;
    sum_double = NaN_double + NA_double ;
    memcpy((void*)&sum_int64, (void*)&sum_double, 8) ;
    printf("NaN+NA: %Lx\n", sum_int64);
    return 0 ;
}

When I add -Wall to the -O2 then it gives me some warnings about the
*(int64_t)&doubleVal in the printf statements for the inputs, but I used
memcpy() to avoid the warnings when printing the outputs.

% gcc -Wall -O2 t.c -o a.out ; ./a.out
t.c: In function âmainâ:
t.c:17: warning: dereferencing type-punned pointer will break strict-aliasing rules
t.c:18: warning: dereferencing type-punned pointer will break strict-aliasing rules
NA : 7ff00000000007a2
NaN: fff8000000000000
NA+NaN: 7ff80000000007a2
NaN+NA: fff8000000000000

Bill Dunlap
TIBCO Software Inc - Spotfire Division
wdunlap tibco.com