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

[Daniel Berlin via llvm-dev]

On Mon, Feb 27, 2017 at 11:29 AM, Adam Nemet <anemet at apple.com> wrote:

>
> On Feb 27, 2017, at 10:11 AM, Hal Finkel <hfinkel at anl.gov> wrote:
>
>
> On 02/27/2017 11:47 AM, Adam Nemet wrote:
>
>
> On Feb 27, 2017, at 9:39 AM, Daniel Berlin <dberlin at dberlin.org> wrote:
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>
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> 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:
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>> 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.
>
>
> OK.  Another data point is that I’ve seen cases in the past where the
> alias checks required for the loop passes could enable GVN to remove
> redundant loads/stores.  Currently we can only pick these up with LTO when
> GVN is rerun.
>

This is just GVN brokenness, newgvn should not have this problem.
If it does, i'd love to see it.

(I'm working on the last few parts of turning it on by default, but it
requires a new getModRefInfo interface to be able to get the last few
testcases)
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