On 2005-05-26 20:49:13 +0200, Allan Sandfeld Jensen wrote:
> I can't speak of all compilers, only the ones I've tried. ICC
> enables it always, Sun CC, Dec CXX, and HP CC at certain levels of
> optimizations (equivalent to -O2).
I've tried with "cc: Sun WorkShop 6 2000/04/07 C 5.1", and there
doesn't seem to be unsafe optimizations. Here's an example:
#include <stdio.h>
int main (void)
{
int i;
for (i = 2; i >= 0; i--)
{
double x = (double) i;
x = -x;
printf ("%2g %2g\n", x, x + 0.0);
}
for (i = 0; i < 100; i += 30)
{
long double x = (long double) i;
printf ("%.20Lg\n", x / 30.0);
}
return 0;
}
craffe:~/src/fp> cc -xO5 -o tst tst.c -lsunmath
craffe:~/src/fp> ./tst
-2 -2
-1 -1
-0 0
0
1
2
3
But with gcc, using -ffast-math:
craffe:~/src/fp> gcc -O -ffast-math -o tst tst.c
craffe:~/src/fp> ./tst
-2 -2
-1 -1
-0 -0
0
1
2
3
See the third line: -0 -0.
And indeed, icc does some unsafe optimizations by default, when
assuming IEEE-754, but nothing buggy...
latour:~/src/fp> /localdisk/icc/opt/intel_cc_80/bin/icc -o tst tst.c
latour:~/src/fp> ./tst
-2 -2
-1 -1
-0 -0
0
1
2
3
contrary to gcc:
latour:~/src/fp> gcc -O -ffast-math -o tst tst.c
latour:~/src/fp> ./tst
-2 -2
-1 -1
-0 -0
0
1
2
3.0000000000000000002
So, yes, -ffast-math by default would really be a bad idea and would
make gcc much worse than other compilers.
> > Standard should be the default.
> >
> > (Is this a troll or what?)
>
> So why isn't -ansi or -pendantic default?
-ansi is broken.
--
Vincent Lefèvre <[EMAIL PROTECTED]> - Web: <http://www.vinc17.org/>
100% accessible validated (X)HTML - Blog: <http://www.vinc17.org/blog/>
Work: CR INRIA - computer arithmetic / SPACES project at LORIA
