The optimizer in gcc 4.2 (but not 4.1) on Linux seems to get this
issue "right" on one of my test machines (a 32-bit Ubuntu box).
4.1 fails on a 64-bit Linux but I don't have a newer gcc on any
64-bit Linux machine I can find.
A cleaner version of the test is:
% cat t.c
#include <stdio.h>
#include <stdint.h>
#include <string.h>
void print_double(char *text, double x_double)
{
int64_t x_int64 ;
memcpy((void*)&x_int64, (void*)&x_double, 8) ;
printf("%s: %Lx (%g)\n", text, x_int64, x_double) ;
}
int main(int argc, char *argv[])
{
int64_t NA_int64 = 0x7ff00000000007a2LL ;
int64_t NaN_int64 = 0xfff8000000000000LL ;
double NA_double, NaN_double, sum_double ;
memcpy((void*)&NA_double, (void*)&NA_int64, 8) ;
memcpy((void*)&NaN_double, (void*)&NaN_int64, 8) ;
print_double(" NA", NA_double);
print_double("NaN", NaN_double);
sum_double = NA_double + NaN_double ;
print_double("NA+NaN", sum_double) ;
sum_double = NaN_double + NA_double ;
print_double("NaN+NA", sum_double);
return 0 ;
}
Bill Dunlap
TIBCO Software Inc - Spotfire Division
wdunlap tibco.com
> -----Original Message-----
> From: r-devel-boun...@r-project.org
> [mailto:r-devel-boun...@r-project.org] On Behalf Of William Dunlap
> Sent: Friday, May 01, 2009 7:50 AM
> To: Martin Maechler
> Cc: r-devel@r-project.org
> Subject: Re: [Rd] NA_real_ <op> NaN -> NA or NaN, should we care?
>
> > From: Martin Maechler [mailto:maech...@stat.math.ethz.ch]
> > Sent: Friday, May 01, 2009 5:15 AM
> > To: William Dunlap
> > Cc: r-devel@r-project.org
> > Subject: Re: [Rd] NA_real_ <op> NaN -> NA or NaN, should we care?
> >
> > >>>>> William Dunlap <wdun...@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
>
> ______________________________________________
> R-devel@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
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