Re: [PATCH] handle "invisible" reference in -Wdangling-pointer (PR104436)

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

On Tue, Feb 8, 2022 at 11:38 PM Jason Merrill via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> On 2/8/22 16:59, Martin Sebor wrote:
> > Transforming a by-value arguments to by-reference as GCC does for some
> > class types can trigger -Wdangling-pointer when the argument is used
> > to store the address of a local variable.  Since the stored value is
> > not accessible in the caller the warning is a false positive.
> >
> > The attached patch handles this case by excluding PARM_DECLs with
> > the DECL_BY_REFERENCE bit set from consideration.
> >
> > While testing the patch I noticed some instances of the warning are
> > uninitentionally duplicated as the pass runs more than once.  To avoid
> > that, I also introduce warning suppression into the handler for this
> > instance of the warning.  (There might still be others.)
>
> The second test should verify that we do warn about returning 't' from a
> function; we don't want to ignore the DECL_BY_REFERENCE RESULT_DECL.
>
> > +       tree var = SSA_NAME_VAR (lhs_ref.ref);
> > +       if (DECL_BY_REFERENCE (var))

I think you need to test var && TREE_CODE (var) == PARM_DECL here since
for DECL_BY_REFERENCE RESULT_DECL we _do_ escape to the caller.  Also
SSA_NAME_VAR var might be NULL.

> > +         /* Avoid by-value arguments transformed into by-reference.  */
> > +         continue;
>
> I wonder if we can we express this property of invisiref parms somewhere
> more general?  I imagine optimizations would find it useful as well.
> Could pointer_query somehow treat the reference as pointing to a
> function-local object?

I think points-to analysis got this correct when the reference was marked
restrict but now it also fails at this, making DSE fail to eliminate the
store in

struct A { A(); ~A(); int *p; };

void foo (struct A a, int *p)
{
  a.p = p;
}

> I previously tried to express this by marking the reference as
> 'restrict', but that was wrong
> (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97474).
>
> Jason
>