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

[Daniel Berlin via llvm-dev]

On Mon, Feb 27, 2017 at 9:47 AM, Adam Nemet <anemet at apple.com> wrote:

>
> On Feb 27, 2017, at 9:39 AM, Daniel Berlin <dberlin at dberlin.org> wrote:
>
>
>
> On Mon, Feb 27, 2017 at 9:29 AM, Adam Nemet <anemet at apple.com> wrote:
>
>>
>> On Feb 27, 2017, at 7:27 AM, Hal Finkel <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.
>
> I guess the pipeline to experiment with for now is opt -loop-vectorize
> -loop-vectorize -newgvn.
>
> Adam
>

Right, and if you guarantee the conditions involve scalars (IE the pointer
is not from a load ), it can be made evenfaster by turning off the memory
handling (IE not building memoryssa)..

NewGVN will still give correct answers if you value number any instruction
you like as "unknown expression".
I have a patch to add a debug counter that does just that.
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