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

["Bin.Cheng" <amker.cheng@gmail.com>]

On Fri, Sep 2, 2016 at 3:46 PM, Yuri Rumyantsev <ysrumyan@gmail.com> wrote:
> 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
Hi Yuri,
I didn't read the patch, so please correct me if I mis-understand
anything.  It might be better not to introduce versioning logic in
peeling stuff if possible.  The peeling part is complicated and
generates somehow inefficient CFG.  I am preparing patches rewriting
the peeling stuff.

Thanks,
bin
> 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