On Thu, Jun 23, 2016 at 9:39 PM, Jason Merrill <ja...@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.
