Re: Resurrecting -Wunreachable

[Florian Weimer <fweimer at redhat dot com>]

On 05/07/2014 02:11 PM, Richard Biener wrote:

> > Precisely.  But optimizing this:
> >
> >
> > int main()
> > {
> >     if (0)
> >       foo ();
> >     else
> >       throw std::logic_error ("error");
> >     bar ();
> > }
> >
> > to:
> >
> > int main()
> > {
> >     throw std::logic_error ("error");
> >     bar ();
> > }
> >
> > would cause the code to issue such unwanted warnings.  That's why I need
> > access to these trivial if statements.
>
> I don't think it would.  bar () would be gone already as throw doesn't return.

Fair point.  So here's a different example:

int main()
{
  if (0)
    foo ();
  else
    throw std::logic_error ("always reachable");
  throw std::logic_error ("maybe reachable (no warning)");
  throw std::logic_error ("not reachable, warning required");
}

I think this also applies to other warnings such as missing null pointer 
checks.  We still want to report them for code unreachable after (some) 
optimization, just like we report type errors there.

The more challenging issue with early GIMPLE is that loops have already 
been lowered to gotos, so adopting the syntax-based Java reachability 
rules is impossible.  Oh dear.

--
Florian Weimer / Red Hat Product Security Team