On Wed, Jul 19, 2017 at 12:45:12PM +0200, Richard Biener wrote:
> On Tue, Jul 18, 2017 at 6:05 PM, Marek Polacek <pola...@redhat.com> wrote:
> > We ended up in infinite recursion between extract_muldiv_1 and
> > fold_plusminus_mult_expr, because one turns this expression into the other
> > and the other does the reverse:
> >
> > ((2147483648 / 0) * 2) + 2 <-> 2 * (2147483648 / 0 + 1)
> >
> > I tried (unsuccessfully) to fix it in either extract_muldiv_1 or
> > fold_plusminus_mult_expr, but in the end I went with just turning (x / 0) +
> > A
> > to x / 0 (and similarly for %), because with that undefined division we can
> > do
> > anything and this fixes the issue. Any better ideas?
>
> Heh - I looked at this at least twice as well with no conclusive fix...
>
> My final thought was to fold division/modulo by zero to __builtin_trap () but
> I didn't get to implement that. I'm not sure if we need to preserve
> the behavior
> of raising SIGFPE as I think at least the C standard makes it undefined.
> OTOH other languages with non-call-exceptions might want to catch division
> by zero.
It's definitely undefined in C, so there we can do anything we see fit, but not
sure about the rest
> Did you see why the oscillation doesn't happen for
>
> ((2147483648 / A) * 2) + 2 <-> 2 * (2147483648 / A + 1)
>
> ? What's special for the zero constant as divisor?
I think it comes down to how split_tree splits the expression. For the above
we never call associate_trees, i.e., this condition is never true:
9647 if (ok
9648 && (2 < ((var0 != 0) + (var1 != 0)
9649 + (con0 != 0) + (con1 != 0)
9650 + (lit0 != 0) + (lit1 != 0)
9651 + (minus_lit0 != 0) + (minus_lit1 != 0))))
because var0 = so, lit1 = 2, and the rest is null.
We also don't go into infinite recursion with x / 0 instead of 2147483648 / 0,
because split_tree will put "x / 0 + 1" into var0, whereas it will put
2147483648 / 0 into con1, because it's TREE_CONSTANT - and so we have more than
2 exprs that are non-null and we end up looping.
One thing I pondered was to set TREE_OVERFLOW for division-by-zero, and avoid
folding such expressions (e.g. in extract_muldiv), but that probably wouldn't
help if the division-by-zero was nested in an expression.
Also, a funny thing, the original testcase
uint32_t
ls (uint32_t so)
{
return (so + so) * (0x80000000 / 0 + 1);
}
will compile just fine if we change the parameter type to unsigned int. Even
though uint32_t _is_ unsigned int!
Marek
