Re: PATCH to fix bogus warning with -Wstringop-truncation -g (PR tree-optimization/84228)

[Jakub Jelinek <jakub at redhat dot com>]

On Fri, Feb 09, 2018 at 11:40:29AM +0100, Richard Biener wrote:
> > I.e., having to track all pointers to d between the call to
> > strncpy and the assignment of the nul and make sure none of
> > them ends up used in a string function.  It didn't seem
> > the additional complexity would have been worth the effort
> > (or the likely false negatives).
> 
> Well, I'd just walk immediate uses of the VDEF of the
> strncpy call, not of the pointer argument.  There will be exactly _one_ possible
> store (gimple_vdef () is non-NULL) that you need to verify (with using
> the current matching
> logic).  But it'll skip non-store statements for you.

Well, it should also punt on the immediate uses of the VDEF that
have NULL gimple_vdef and the alias oracle says that might alias with that,
i.e. warn about say
  strncpy (p, ...);
  foo (p);
  p[whatever] = '\0';
where foo is pure, because it might read the unterminated string, but don't
warn about
  strncpy (p, ...);
  x = *q;
  p[whatever] = '\0';
if q[0] can't alias with p.

Or just warn if there are any immediate uses of the strncpy VDEF that have
gimple_vdef NULL, or non-NULL and aren't the zero store you are looking for.

	Jakub