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

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

On Fri, Feb 11, 2022 at 12:05 AM Martin Sebor via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> On 2/8/22 15:37, Jason Merrill 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.
>
> The indirect aggregate case isn't handled and needs more work but
> since you brought it up I thought I should look into finishing it.
> The attached patch #2 adds support for it.  It also incorporates
> Richard's suggestion to test PARM_DECL.  Patch #2 applies on top
> of patch #1 which is unchanged from the first revision.

patch #1 would be OK if you'd do the PARM_DECL check there - I'd
rather defer #2 to stage1.

Richard.

>
> I have retested it on x86_64-linux and by building Glibc and
> Binutils + GDB.
>
> If now is too late for the aggregate enhancement I'm okay with
> deferring it until stage 1.
>
> >
> >> +      tree var = SSA_NAME_VAR (lhs_ref.ref);
> >> +      if (DECL_BY_REFERENCE (var))
> >> +        /* 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 don't quite see where in the pointer_query class this would be
> useful (the class also isn't used for optimization).  I could add
> a helper to the access_ref class to query this property in warning
> code but as this is the only potential client I'm also not sure
> that's quite what you had in mind.  I'd need some guidance as to
> what you're thinking of here.
>
> Martin
>
>
> >
> > 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
> >