[RTL, i386] Use subreg instead of UNSPEC_CAST

Marc Glisse marc.glisse@inria.fr
Wed Mar 20 15:54:00 GMT 2013

On Wed, 20 Mar 2013, Richard Biener wrote:

> On Wed, Mar 20, 2013 at 4:29 PM, Marc Glisse <marc.glisse@inria.fr> wrote:
>> On Wed, 20 Mar 2013, Richard Henderson 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?
>> I am mostly interested in the reverse, a paradoxical subreg, since
>> vec_select can only model one direction (and only rvalues, but that's a
>> different question).
> vec_duplicate?

There is already some of that in various places, and there may be even 
more vec_merge+vec_duplicate patterns soon, but you want to make sure you 
don't actually do the duplication.

> Honestly, what semantics should  _mm256_castpd128_pd256 have if
> it is supposed to cast a v2df to a v4df?

NOP. We don't care what is in the high part of the vector.

> Or what use?

Many vector operations are defined as taking 2 vectors and merging them 
somehow. I didn't check if this case works, but for instance if you want 
to copy a V2DF to the bottom part of a V4DF using Intel's intrinsics, you 
will probably have to cast the V2DF to a V4DF and then use an intrinsic 
that takes 2 V4DF. (there are many issues with those intrinsics, but we 
don't control them)

Marc Glisse

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