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

[Ilya Enkovich <enkovich.gnu@gmail.com>]

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.

Thanks,
Ilya

>
> Jeff