Re: [patch] Fix segfault caused by spurious constant overflow

[Richard Biener <richard.guenther@gmail.com>]

On Mon, Jun 3, 2019 at 12:38 PM Eric Botcazou <ebotcazou@adacore.com> wrote:
>
> > Hmm, ISTR we had such mitigations in place (or have) elsewhere keying
> > on the most significant bit set instead of power-of-two.  But your case
> > likely recurses and runs into the extract_multiv limiting to eventually
> > stop, even for (N + 4) * 8, right?  If so shouldn't we prevent this
> > even for !TYPE_OVERFLOW_WRAPS?  Also
> >
> > +         && !(tree_fits_shwi_p (c)
> > +              && exact_log2 (absu_hwi (tree_to_shwi (c))) > 0))
> >
> > is better written as
> >
> >            && exact_log2 (wi::to_wide (c)) > 0
>
> It turns out that pow2p_hwi can be used instead and is cheaper, so I have
> changed both extract_muldiv_1 and fold_plusminus_mult_expr to using it.

OK, thanks.
Richard.

>
>         * fold-const.c (extract_muldiv_1) <PLUS_EXPR>: Do not distribute a
>         multiplication by a power-of-two value.
>         (fold_plusminus_mult_expr): Use pow2p_hwi to detect a power-of-two value
>         and turn the modulo operation into a masking operation.
>
> --
> Eric Botcazou