[Bug tree-optimization/19723] [4.0 Regression] A side effect is missed in 0 % a++.

[Jeffrey A Law]

On Mon, 2005-01-31 at 22:36 -0800, Richard Henderson wrote:
> On Mon, Jan 31, 2005 at 08:45:34PM -0700, Jeffrey A Law wrote:
> > +       /* X % 0, return X % 0 unchanged so that we can get the
> > + 	 proper warnings and errors.  */
> >         if (integer_zerop (arg1))
> >   	return t;
> >   
> > +       /* 0 % X is always zero, but be sure to preserve any side
> > + 	 effects in X.  Place this after checking for X == 0.  */
> > +       if (integer_zerop (arg0))
> > + 	return omit_one_operand (type, integer_zero_node, arg1);
> 
> Not ok yet.  You have to *know* that arg1 is not zero.  Otherwise
> you're still potentially removing a division-by-zero.
Would would mean that we really can't do anything with DIV/MOD when
the numerator is a constant and the denominator is an unknown.

What I think we really want is to know whether or not the language
requires trapping on a DIV/MOD by zero. If so, then disable this class
of optimizations, otherwise we have a non-conformant program.

jeff