[llvm-dev] [Proposal][RFC] Epilog loop vectorization

[Hal Finkel via llvm-dev]

On 02/27/2017 11:47 AM, Adam Nemet wrote:
>
>> On Feb 27, 2017, at 9:39 AM, Daniel Berlin <dberlin at dberlin.org 
>> <mailto:dberlin at dberlin.org>> wrote:
>>
>>
>>
>> On Mon, Feb 27, 2017 at 9:29 AM, Adam Nemet <anemet at apple.com 
>> <mailto:anemet at apple.com>> wrote:
>>
>>
>>>     On Feb 27, 2017, at 7:27 AM, Hal Finkel <hfinkel at anl.gov
>>>     <mailto:hfinkel at anl.gov>> wrote:
>>>
>>>
>>>     On 02/27/2017 06:29 AM, Nema, Ashutosh wrote:
>>>>     Thanks for looking into this.
>>>>     1) Issues with re running vectorizer:
>>>>     Vectorizer might generate redundant alias checks while
>>>>     vectorizing epilog loop.
>>>>     Redundant alias checks are expensive, we like to reuse the
>>>>     results of already computed alias checks.
>>>>     With metadata we can limit the width of epilog loop, but not
>>>>     sure about reusing alias check result.
>>>>     Any thoughts on rerunning vectorizer with reusing the alias
>>>>     check result ?
>>>
>>>     One way of looking at this is: Reusing the alias-check result is
>>>     really just a conditional propagation problem; if we don't
>>>     already have an optimization that can combine these after the
>>>     fact, then we should.
>>
>>     +Danny
>>
>>     Isn’t Extended SSA supposed to help with this?
>>
>>
>> Yes, it will solve this with no issue already.  GVN probably does 
>> already too.
>>
>> even if if you have
>>
>> if (a == b)
>> if (a == c)
>>  if (a == d)
>>  if (a == e)
>>  if (a == g)
>>
>>
>> and  we can prove a ... g equivalent, newgvn will eliminate them all 
>> and set all the branches true.
>>
>> If you need a simpler clean up pass, we could run it on sub-graphs.
>
> Yes we probably don’t want to run a full GVN after the 
> “loop-scheduling” passes.

FWIW, we could, just without the memory-dependence analysis enabled 
(i.e. set the NoLoads constructor parameter to true). GVN is pretty fast 
in that mode.

  -Hal

>
> I guess the pipeline to experiment with for now is opt -loop-vectorize 
> -loop-vectorize -newgvn.
>
> Adam
>
>> The only thing you'd have to do is write some code to set "live on 
>> entry" subgraph variables in their own congruence classes.
>> We already do this for incoming arguments.
>>
>> Otherwise, it's trivial to make it only walk things in the subgraph.
>>
>>
>

-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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