Re: [PATCH] Add a new type attribute always_alias (PR79671)

[Jakub Jelinek <jakub@redhat.com>]

On Wed, Apr 05, 2017 at 09:46:09AM +0000, Bernd Edlinger wrote:
> this is related to the P1 codegen bug PR79671:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79671
> 
> Therefore I would like to add this now although
> the trunk is already at in stage 4.
> 
> I propose to add a new type attribute always_alias that
> works like may_alias but unlike may_alias it should
> also make instances alias anything.  It is also exposed
> in C/C++ as __attribute__((always_alias)).

I have just two small comments, for review IMHO want agreement
between Jason and Richard on this.

One thing is whether it is a good idea to keep this info in the IL
as an attribute, especially when you add it automatically in the
C++ FE.  I see e.g. 25 spare bits in tree_type_common.  Don't say we
need to waste them, but I'd say in C++ unsigned char arrays are fairly
common and thus not having to create the attribute for them each time
and look it up might be beneficial.

Also, wonder if you need to mark all types containing such arrays,
if you couldn't just set that flag in C++ on unsigned char/std::byte
arrays (and on anything with that attribute), have that imply alias set
0 on it and then let the rest of alias machinery handle aggregate types
containing such fields.

	Jakub