Re: GCC and Floating-Point

[chris jefferson <caj at cs dot york dot ac dot uk>]

Vincent Lefevre wrote:

> On 2005-05-24 09:04:11 +0200, Uros Bizjak wrote:
>  
>
> >  I would like to point out that for applications that crunch data
> > from real world (no infinites or nans, where precision is not
> > critical) such as various simulations, -ffast-math is something that
> > can speed up application a lot.
> >    
>
> But note that even when precision is not critical, you may get
> consistency problems or annoying unintuitive side effects. For
> instance, see
>
>  http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15134
>
> where
>
> float x = 30.0;
> int main()
> {
>  if ( 90.0/x != 3.0)
>    abort();
>  return 0;
> }
>
> fails with -ffast-math (on x86). I would not recommend it, unless
> the user knows all the consequences.
>
>  
On the other hand, in general using != and == on floating point numbers 
is always dangerous if you do not know all the consequences. For 
example, on your above program if I use 30.1 and 90.3, the program fails 
without -ffast-math.

Chris