PING [PATCH] avoid -Warray-bounds checks for vtable assignments (PR 98266)

[Jeff Law]

On 2/23/21 2:52 PM, Jason Merrill wrote:
>
> I don't see how the issue here depends on the artificiality of the vptr;
That's what I was trying to get at -- is DECL_ARTIFICIAL really a good
way to detect these kinds of cases.  It sounds like it likely isn't, but
it may also be the best we can do.  I'm not sure yet.




> I'd expect to see the same problem with a data member.  The problem is
> that a D base subobject is smaller than a complete D object, and in
> this case the base subobject is allocated such that if it were a full
> D object, it would overlap the end of E.  And we're checking the
> MEM_REF as though accessing a full D object, so we get a warning.
>
> The general issue about the confusion between complete and as-base
> types is PR 22488; I have various thoughts about a proper fix, but
> there have always been higher-priority things to work on.
>
> One possible approach to this PR is that we don't need to check an
> intermediate _REF; in
>
> MEM[(struct D *)&D.2651 + 24B]._vptr.D
>
> we aren't loading a whole D, we're only looking at the _vptr, which is
> within the bounds of E.
That raises a really interesting question.  What's the right bounds to
use with these kinds of constructs.  Should we be looking at D's type
(struct E) or struct D?  I'm not sure there's a one-size-fits-all answer
here and I don't think we have any other context go help guide us.

Jeff