Re: RFC (attributes): PATCH for c++/50800 to set affects_type_identity for may_alias

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

On Thu, Jun 23, 2016 at 9:39 PM, Jason Merrill <jason@redhat.com> wrote:
> My earlier patch for 50800 fixed the ICE by consistently stripping
> non-mangled attributes from template arguments, and mangling those that
> affect type identity.  At the C++ meeting this week someone pointed out to
> me that this is a real problem for x86 vector code, which relies on
> may_alias semantics: if may_alias is stripped from __m128, users can't use
> templates with vectors.
>
> So, it seems that the solution is to mangle may_alias by saying that it
> affects type identity.  But since we still want to be able to convert back
> and forth, I thought that it would make sense to treat the may_alias version
> of a type as a variant, rather than a new distinct type.  So the first patch
> creates a new category of attributes that are treated as type variants.
>
> An alternative patch just sets affects_type_identity and adjusts the C++
> front end to allow conversion between pointers to add or discard may_alias.
>
> Thoughts?

As may_alias purely affects semantics in the implementation of an API
but not the ABI it shouldn't effect mangling.  In the middle-end we use
TYPE_REF_CAN_ALIAS_ALL and that and the unqualified pointer
share the same canonical type (but it's not a variant type, pointer types
are chained via TYPE_POINTER_TO).

Not sure if you can make use of the canonicalness in the C++ FE
and maybe drop the attribute early there.

Richard.

> Tested x86_64-pc-linux-gnu.