[RTL, i386] Use subreg instead of UNSPEC_CAST

[Richard Biener]

On Wed, Mar 20, 2013 at 4:13 PM, Richard Henderson <rth@redhat.com> wrote:
> On 03/20/2013 08:00 AM, Marc Glisse wrote:
>> Do you at least agree that vector-vector subregs make sense, or is that part
>> wrong as well?
>
> You mean a V4SImode subreg of a V8SImode register, not just same-size casting?
> It makes logical sense, but I'm fairly sure you'll need a lot more surgery
> throughout the compiler to make that happen.
>
> I'm curious how a define_expand can fail in LRA, but your define_insn succeeds?
> Is the failure because of ix86_cannot_change_mode_class?  Because that hook
> fairly well defines what subregs are valid.  And if that says it isn't valid,
> then even having a define_insn that uses such is wrong.

Don't we have vec_select to get a V4SImode out of a V8SImode?  So you
only need a define_insn that special-cases the subreg-like ones?

Richard.

>
> r~