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

[Yuri Rumyantsev <ysrumyan@gmail.com>]

Hi Jeff,

I am trying to reduce cost of repeated call of if-conversion for
epilogue vectorization. I'd like to clarify your recommendation -
should I design additional support for versioning in
vect_do_peeling_for_loop_bound or lightweight version of if-conversion
is sufficient? Any help in clarification will be appreciated.

Thanks ahead.
Yuri.

2016-08-01 19:10 GMT+03:00 Jeff Law <law@redhat.com>:
> On 08/01/2016 03:09 AM, Ilya Enkovich wrote:
>>
>> 2016-07-26 18:38 GMT+03:00 Ilya Enkovich <enkovich.gnu@gmail.com>:
>>>
>>> 2016-07-26 18:26 GMT+03:00 Jeff Law <law@redhat.com>:
>>>>
>>>> On 07/26/2016 03:57 AM, Ilya Enkovich wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>> 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.
>>>>
>>>>
>>>> Right.  So what I think Richi wants you to try is to use the
>>>> if-converted
>>>> loop to construct the if-converted epilogue.  It seems conceptually
>>>> simple
>>>> and low cost -- the question is on the implementation side.  I have no
>>>> clue
>>>> how painful that would be.
>>>
>>>
>>> Probably another part of if-conversion may be re-used to build required
>>> epilogue.  I'll have a look.
>>
>>
>> Hi,
>>
>> Yuri will continue my work from this point.
>
> Understood.  I'm actually got some comments on #5 and Yuri is already on the
> CC list for that draft message.
>
> Jeff