Re: [PATCH] Optimize nested permutation to single VEC_PERM_EXPR [PR54346]

["Xu, Liwei" <liwei.xu@intel.com>]

Hi Richard

I just found that modifying match.pd may be a better way since for forwprop-19.c, VEC_PERM_EXPR exists in all gimple until 235t.optimize with current trunk code, that leave us no pass to ‘scan-tree-dump-not’.

Best Regards
Levy

> On 14 Oct 2022, at 2:19 pm, Richard Biener <richard.guenther@gmail.com> wrote:
> 
> On Fri, Oct 14, 2022 at 3:49 AM Lulu Cheng <chenglulu@loongson.cn> wrote:
>> 
>> 
>>> 在 2022/10/13 下午7:10, Richard Biener 写道:
>>> On Thu, Oct 13, 2022 at 10:16 AM Lulu Cheng <chenglulu@loongson.cn> wrote:
>>>> 
>>>> 在 2022/10/13 下午2:44, Xi Ruoyao 写道:
>>>>> On Thu, 2022-10-13 at 14:15 +0800, Levy wrote:
>>>>>> Hi RuoYao
>>>>>> 
>>>>>> It’s probably because loongarch64 doesn’t support
>>>>>> can_vec_perm_const_p(result_mode, op_mode, sel2, false)
>>>>>> 
>>>>>> I’m not sure whether if loongarch will support it or should I just
>>>>>> limit the test target for pr54346.c?
>>>>> I'm not sure if we can add TARGET_VECTORIZE_VEC_PERM_CONST when we don't
>>>>> actually support vector.  (LoongArch has SIMD instructions but the
>>>>> support in GCC won't be added in a very recent future.)
>>>>> 
>>>> If what I understand is correct, I think this might be a better solution.
>>>> 
>>>>   /* { dg-do compile } */
>>>> 
>>>> +/* { dg-require-effective-target vect_perm } */
>>>>   /* { dg-options "-O -fdump-tree-dse1" } */
>>> Btw, what forwprop does is check whether any of the original permutations are
>>> not supported and then elide the supportability check for the result.
>>> The reasoning
>>> is that the original permute(s) would be lowered during vectlower so we can as
>>> well do that for the result.  We should just never turn a supported permutation
>>> sequence into a not supported one.
>>> 
>>> Richard.
>>> 
>> Hi Richard:
>> 
>> I'm very sorry. I don't fully understand what you mean.
>> 
>> Could you give me some more details?
> 
> The argument is that if Loongarch64 doesn't support the VEC_PERM_EXPR in the IL
> before the transform then it doesn't matter if the transform introduces another
> VEC_PERM_EXPR that isn't supported.  Both are later (in the veclower pass)
> lowered to scalar operations.
> 
> So instead of
> 
>  if (can_vec_perm_const_p (result_mode, op_mode, sel2, false))
>   ..
> 
> you'd do
> 
>  if (can_vec_perm_const_p (result_mode, op_mode, sel2, false)
>      || !can_vec_perm_const_p ( ... original vec_perm1 ...)
>      || !can_vec_perm_const_p ( ... original vec_perm2 ...))
> 
> Richard.
> 
>> 
>> Thanks!
>> 
>> Lulu Cheng
>>