Re: [PATCH] Improved folding of virtual calls (gimple and C++ review requested)

[Richard Guenther <richard dot guenther at gmail dot com>]

On Mon, May 10, 2010 at 11:17 PM, Jason Merrill <jason@redhat.com> wrote:
> On 04/23/2010 11:48 AM, Martin Jambor wrote:
>>
>> On Wed, Apr 21, 2010 at 08:48:26PM -0400, Jason Merrill wrote:
>
>>> I don't really see this in the patch. ?If the BINFOs have the wrong
>>> offsets, that sounds like a front end bug...
>
> Indeed, it seems that if we have a class with a primary base (that is, a
> base at offset 0 with which we share a vtable) which itself has a primary
> base--such as B, A_2 and A in otr-fold-2.C--we update BINFO_VIRTUALS in B
> and A_2-in-B but not in A-in-A2-in-B. ?This seems like a bug in the front
> end, but I suppose nothing in the front end is looking at the list of
> virtuals in that BINFO. ?Indeed, I see this comment in dfs_modify_vtables:
>
> /* There's no need to modify the vtable for a non-virtual primary base;
> we're not going to use that vtable anyhow. ?We do still need to do this for
> virtual primary bases, as they could become non-primary in a construction
> vtable. ?*/
>
>>> Is this supposed to be testing whether CLASSTYPE_AS_BASE for the base
>>> class
>>> is different from the class itself? ?That's the effect I would expect,
>>> and
>>> that's different from whether multiple inheritance is involved.
>
> I was confused about this; the base field type is changed to be the normal
> type when class layout is done.
>
> Your algorithm seems sound; as you mentioned, the BINFO_VIRTUALS for the
> derived class will be a superset of those for the primary base. ?My only
> remaining concern is that this is assuming G++ layout rules; do other front
> ends use the BINFO system?

I don't think any other frontend uses them.  If they want to we now have
middle-end rules to follow ;)

Richard.

> Jason
>