Re: Division by zero

["Jie Zhang" <jzhang918 at gmail dot com>]

On 2/10/07, Robert Dewar <dewar@adacore.com> wrote:
> Ian Lance Taylor wrote:
> > "Jie Zhang" <jzhang918@gmail.com> writes:
> >
> >> But now gcc seems to optimize it away. For the following function:
> >>
> >> $ cat t.c
> >> #include <limits.h>
> >> void foo (int rc)
> >> {
> >>   int x = rc / INT_MAX;
> >>   x = 4 / x;
> >> }
> >
> > I believe we still keep division by zero in general.  In your example
> > it gets optimized away because it is dead code.  Nothing uses x.
>
> And it is certainly reasonable to do this optimization given that
> the result of the division is undefined in C. In Ada, such a
> division has well defined semantics (raise an exception), but
> it is interesting to note that the optimization is valid in
> Ada as well, since there is a special rule that basically says
> you don't need to evaluate an expression if the only reason for
> doing so is to see if it raises a predefined exception. That
> rule is precisely to deal with cases like this.

The code I posted in my first email is from libgloss/libnosys/_exit.c.
It's used to cause an exception deliberately. From your replies, it
seems it should find another way to do that.

Thanks,
Jie