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

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Tue, Dec 10, 2013 at 10:44 AM, Richard Sandiford
<rdsandiford@googlemail.com> wrote:
> "H.J. Lu" <hjl.tools@gmail.com> writes:
>> On Tue, Dec 10, 2013 at 10:26 AM, Richard Sandiford
>> <rdsandiford@googlemail.com> wrote:
>>> "H.J. Lu" <hjl.tools@gmail.com> writes:
>>>> On Tue, Dec 10, 2013 at 9:57 AM, Richard Sandiford
>>>> <rdsandiford@googlemail.com> wrote:
>>>>> "H.J. Lu" <hjl.tools@gmail.com> writes:
>>>>>> On Tue, Dec 10, 2013 at 8:05 AM, Kirill Yukhin
>>>>>> <kirill.yukhin@gmail.com> wrote:
>>>>>>> On 09 Dec 14:08, H.J. Lu wrote:
>>>>>>>>
>>>>>>>> There are no regressions on Linux/x86-64 with -m32 and -m64.
>>>>>>>> Can you check if it improves code quality on x886?
>>>>>>>
>>>>>>> As second thought. If Tejas and Richard are right and it is simply
>>>>>>> incorrect
>>>>>>> to check any offsets in this hook, may be we can end up with patch in the
>>>>>>> bottom?
>>>>>>
>>>>>> What is wrong to pass the correct offset to
>>>>>> CANNOT_CHANGE_MODE_CLASS?  Backends are free to
>>>>>> ignore it.
>>>>>
>>>>> The point is that:
>>>>>
>>>>>>> - /* Vector registers do not support subreg with nonzero offsets,
>>>>>>> which
>>>>>>> -        are otherwise valid for integer registers.  Since we can't see
>>>>>>> -        whether we have a nonzero offset from here, prohibit all
>>>>>>> -         nonparadoxical subregs changing size.  */
>>>>>>> -      if (GET_MODE_SIZE (to) < GET_MODE_SIZE (from))
>>>>>>> -       return true;
>>>>>
>>>>> seems to be trying to reject things like (subreg:SF (reg:V4SF X) 1),
>>>>> which is always invalid for a single-register V4SF.  See:
>>>>
>>>> That is correct.
>>>
>>> Sorry, what I mean is: that subreg is always invalid for single-
>>> register V4SFs regardless of the target.  This isn't something that
>>> CANNOT_CHANGE_MODE_CLASS should be expected to check.
>>>
>>
>> Why is
>>
>> (define_insn "*movv4sfdi_subreg"
>>  [(set (match_operand:DI 0 "nonimmediate_operand"           "=rxm")
>>        (subreg:DI (match_operand:V4SF 1 "register_operand" "x") 0))]
>>
>> invalid?
>
> Sorry, I don't understand.  I never said it was invalid.  I said
> (subreg:SF (reg:V4SF X) 1) was invalid if (reg:V4SF X) represents
> a single register.  On a little-endian target, the offset cannot be
> anything other than 0 in that case.
>
> So the CANNOT_CHANGE_MODE_CLASS code above seems to be checking for
> something that is always invalid, regardless of the target.  That kind
> of situation should be rejected by target-independent code instead.
>
> In other words I'm arguing against the idea of passing the offset to
> CANNOT_CHANGE_MODE_CLASS (which you seemed to be supporting in the
> quote above).  I think Kirill's patch to remove the i386.c check was
> the right way to go.
>
> There's no need for a separate insn though.  Once you allow the subregs
> (as per Kirill's patch), the normal move patterns will handle them.
>

We will wait for Kirill's results.


-- 
H.J.