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

[Ilya Enkovich]

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.

Thanks,
Ilya

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