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Re: [Patch, RTL] Eliminate redundant vec_select moves.


On 05 Dec 16:40, Kirill Yukhin wrote:
> On 05 Dec 05:30, H.J. Lu wrote:
> > Kirill, can you take a look why it doesn't work for x86?
> Okay, I'll look at this.

I've looked at this. It seems that `CANNOT_CHANGE_MODE_CLASS'
is too conservative for x86.

In rtlanal.c we have `simplify_subreg_regno' which call target
hook `REG_CANNOT_CHANGE_MODE_P'. It takes only 3 arguments:
from mode, to mode and regclass.

Hook in x86 called `ix86_cannot_change_mode_class' and comment
says that we cannot change mode for nonzero offsets, which sounds
quite reasonable. That is why this hook returns `true' for this
tuple <V4SF, SF, FIRST_SSE_REG> and `simplify_subreg_regno'
prohibits simplification of that:
  (set (reg:SF 21 xmm0 [orig:86 D.1816 ] [86])
       (vec_select:SF (reg:V4SF 21 xmm0 [87])
          (parallel [(const_int 0 [0])])))

I think we can extend the hook and add `offset in frommode' to it.
We may set it to -1 for the cases where it is unknown and work
conservatively in the target hook.
For most cases offset is known and we could pass it to the hook.
This will require changes throughout all targets though.

Alternatively, we may introduce another target hook, say
`CANNOT_CHANGE_MODE_CLASS' + offset and which will be defaulted to it.
For x86 (and possibly other targets) we'll implement this hook, which
will checko ffset.

What do you think?

Thanks, K

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