Re: [PATCH] Don't fold away division by zero (PR middle-end/66127)

[Marek Polacek <polacek@redhat.com>]

On Wed, May 13, 2015 at 03:55:10PM +0200, Jakub Jelinek wrote:
> On Wed, May 13, 2015 at 03:41:11PM +0200, Marek Polacek wrote:
> > As discussed in the PR, match.pd happily folds 0 * whatever into 0.  That
> > is undesirable from the C/C++ FE POV, since it can make us accept invalid
> > initializers.
> > 
> > So fixed in match.pd -- I'd hope there's a better way to do this, but this
> > seems to work.  There was some fallout, but nothing unexpected or surprising.
> 
> Will it handle cases 0 * (int) (1 / 0) etc., when perhaps the division by
> zero isn't immediately the operand of mult, but somewhere deeper?

It won't handle e.g. 0 * (unsigned) (1 / 0).

> Also, can't the divisor be optimized into 0 only later on, so your code
> would still see !integer_zerop there and fold into 0?
> Perhaps the answer is that in both cases we'd have simplified those already
> into a division by zero.

Yes, it's a dumb attempt.

I don't know how to reliably fix this :(.  We really want 
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66127#c1>...

	Marek