rfc: auto-casted vector types

[Aldy Hernandez]

>>>>> "Geoff" == Geoff Keating <geoffk@geoffk.org> writes:

 > Jim Wilson <wilson@redhat.com> writes:
 >> It might be useful to note in the comments in the rs6000 file that the
 >> current implementation has a known flaw.  It pretends that V2SI is the opaque
 >> vector type.  This means that it accidentally allows conversions to/from the
 >> V2SI vector type without explicit casts.  In order to fix this, we need to
 >> use a different otherwise unused vector type for the opaque vector type.
 >> Nick started on a patch to do this, but didn't have enough time to finish it.

 > This shouldn't be done at the RTL level at all; the right thing to do
 > is to create a special '__ev64_opaque__' type which is different from
 > 'vector int' and just happens to also have V2SImode (or DImode or
 > whatever is convenient---I'd vote for DImode since that's what GCC
 > uses for all-purpose "blob of data" types).

Actually, I agree with this because I just ran into a similar problem
this morning with an upcoming patch that I solved by looking at the
type instead.

I'll put it on my TODO list.

Aldy