[PATCH 2/2] Simplify subreg of vec_merge of vec_duplicate

[Richard Sandiford]

"H.J. Lu" <hjl.tools@gmail.com> writes:
> On 10/17/18, Marc Glisse <marc.glisse@inria.fr> wrote:
>> On Wed, 17 Oct 2018, H.J. Lu wrote:
>>
>>> We may simplify
>>>
>>>  (subreg (vec_merge (vec_duplicate X) (vector) (const_int 1)) 0)
>>>
>>> to X when mode of X is the same as of mode of subreg.
>>
>> Hello,
>>
>> we already have code to simplify vec_select(vec_merge):
>>
>>      /* If we select elements in a vec_merge that all come from the same
>>         operand, select from that operand directly.  */
>>
>> It would make sense to me to make the subreg transform as similar to it as
>> possible, in particular you don't need to special case vec_duplicate, the
>> transformation would see that everything comes from the first vector,
>> produce (subreg (vec_duplicate X) 0), and let another transformation
>> optimize that.

Sorry, didn't see this before the OK.

> What do you mean by another transformation? If simplify_subreg doesn't
> return X for
>
>   (subreg (vec_merge (vec_duplicate X)
> 		     (vector)
> 		     (const_int ((1 << N) | M)))
> 	  (N * sizeof (X)))
>
>
> no further transformation will be done.

I think the point was that we should transform:

  (subreg (vec_merge X
		     (vector)
		     (const_int ((1 << N) | M)))
	  (N * sizeof (X)))

into:

  simplify_gen_subreg (outermode, X, innermode, byte)

which should further simplify when X is a vec_duplicate.

Thanks,
Richard