Re: [PATCH, vec-tails 07/10] Support loop epilogue combining

[Richard Biener <richard dot guenther at gmail dot com>]

On Tue, Jul 26, 2016 at 3:03 PM, Ilya Enkovich <enkovich.gnu@gmail.com> wrote:
> 2016-07-26 14:51 GMT+03:00 Richard Biener <richard.guenther@gmail.com>:
>> On Tue, Jul 26, 2016 at 11:57 AM, Ilya Enkovich <enkovich.gnu@gmail.com> wrote:
>>> 2016-07-26 0:08 GMT+03:00 Jeff Law <law@redhat.com>:
>>>> On 07/25/2016 12:32 PM, Richard Biener wrote:
>>>>>
>>>>> On July 25, 2016 8:01:17 PM GMT+02:00, Jeff Law <law@redhat.com> wrote:
>>>>>>
>>>>>> On 07/22/2016 05:36 AM, Richard Biener wrote:
>>>>>>>
>>>>>>> The thing that needs work I think is re-running of if-conversion.
>>>>>>
>>>>>> I wonder if we could revamp if-conversion to work on a subset of the
>>>>>> CFG?   I can see that potentially being useful in other contexts.
>>>>>> Would
>>>>>> that work for you Richi?
>>>>>
>>>>>
>>>>> Well, you need to make it not need post-dominators or preserve them (or
>>>>> compute "post-dominators" on SESE regions).
>>>>
>>>> Oh, but it'd be so nice to have DOMs and/or PDOMs on regions.  But that's
>>>> probably out of scope for gcc-7.
>>>>
>>>>
>>>>>
>>>>> What doesn't work with the idea to clone the epilogue using
>>>>> __built-in_vectorized()
>>>>> For the if- vs. Not if-converted loop?
>>>>
>>>> I must be missing something.   I don't see how builtin_vectorized_function
>>>> helps, but maybe I've got the wrong built-in or don't understand what you're
>>>> suggesting.
>>>>
>>>> It sounds like this is the biggest impediment to moving forward.  So let's
>>>> reset and make sure we're all on the same page here.
>>>>
>>>> Ilya, what's the fundamental reason why we need to run if-conversion again?
>>>> Yes, I know you want to if-convert the epilogue, but why?
>>>>
>>>> What are the consequences of not doing if-conversion on the epilogue?
>>>> Presumably we miss a vectorization opportunity on the tail.  But that may be
>>>> a reasonable limitation to allow the existing work to move forward while you
>>>> go back and revamp things a little.
>>>
>>> If we have some control-flow in a loop then we have to if-convert it
>>> for vectorizer.
>>> We need to preserve both versions: if-converted one for vectorizer and
>>> the original
>>> one to be used if vectorization fails.  For epilogues we have similar
>>> situation and
>>> need two versions.  I do it by running if-conversion on a copy of original loop.
>>> Note that it doesn't run full if-conversion pass. If-conversion is
>>> called for epilogue
>>> loop only.
>>
>> But it will still compute post-dominators for the full function for example.
>>
>> You have the if-converted loop available already - it's the loop we are going
>> to vectorize.  If if-conversion generated if (__builtin_vectorized_p ()) style
>> loop copies then you can simply create the epilogue in the same way.
>> If it didn't then the loop is already if-converted anyway.
>>
>
> Agree.  Calling if-conversion is just much simpler in implementation.

Agreed, but it's also quadratic in the number of vectorized loops in a function.
Not sure if it is really very much simpler either.

Richard.

> Thanks,
> Ilya
>
>> I see no need to re-run if-conversion here.
>>
>> Richard.
>>
>>> Thanks,
>>> Ilya
>>>
>>>>
>>>> Jeff