llvm dev - Feb 2017 - [Proposal][RFC] Epilog loop vectorization

> 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?

I see this as a problem that needs to be solved regardless when loop-versioning
is performed by multiple passes [1].  We need to be able to come up with the
minimum sets of checks:

* remove redundant checks in favor of identical dominating checks
* share (i.e. factor out) identical checks required by consecutive loops 

Ashutosh, I suggest that you try rerunning the vectorizer on the relevant
testcases and file PRs about the inefficiencies you encounter.

Adam

[1] http://lists.llvm.org/pipermail/llvm-dev/2015-May/085922.html
> 
>  -Hal
> 
>>  
>> 2) Best & worst case for epilog vectorization.
>> Epilog vectorization incurs additional cost of checks which decides to
execute either epilog vector loop or scalar loop.
>> These checks are:
>> a)      Min trip count check for epilog vector loop.
>> b)      Alias result check (only if alias check is generated for first
vector version)
>>  
>> Where the epilog vector trip count is large epilog vectorization with
profitable width likely to give gain. Worst case probably after these additional
checks executing the scalar loop.
>>  
>> 3) Benchmarking & results:
>> We have observed 4% improvement in one of our internal benchmark.
>> Tried CPU2006 and didn’t found any degrade & improvements.
>> On code size increase I have not yet verified yet will check and get
back.
>>  
>> 4)  What should be the minimum first vector loop width to enable epilog
vectorization.
>> This count should not be small as it may degrade the performance, with
my limited tests I have observed 16 is a point it shows gains with one of our
internal benchmark. This require more experiments & testing to decide what
should be the minimum width.
>>  
>> 5) Unrolling issues:
>> As Ayal mentioned with large unroll factor the next profitable EpilogVF
could be equal to VF.
>> With the same reason the current patch enforces UF=1, as unrolling can
minimize the possibility of executing epilog vector loop.
>>  
>> Example to understand the new layout:
>>  
>> void foo (char *A, char *B, char *C, int len) {
>>   int i = 0;
>>   for (i=0 ; i< len; i++)
>>     A[i] = B[i] + C[i];
>> }
>>  
>> This loop get vectorize with width 32 and epilog loop get vectorized
with width 8.
>> a)      Please check attached IR(test.ll)
>> b)      To understand how alias check result got used, check temporary
“acr.val” usage.
>>  
>> Regards,
>> Ashutosh
>>  
>> From: Zaks, Ayal [mailto:ayal.zaks at intel.com <mailto:ayal.zaks at
intel.com>]
>> Sent: Sunday, February 26, 2017 10:53 PM
>> To: Hal Finkel <hfinkel at anl.gov> <mailto:hfinkel at
anl.gov>; Adam Nemet <anemet at apple.com> <mailto:anemet at
apple.com>; Nema, Ashutosh <Ashutosh.Nema at amd.com>
<mailto:Ashutosh.Nema at amd.com>
>> Cc: llvm-dev <llvm-dev at lists.llvm.org> <mailto:llvm-dev at
lists.llvm.org>
>> Subject: RE: [llvm-dev] [Proposal][RFC] Epilog loop vectorization
>>  
>> +1  for “just rerun the vectorizer” on the scalar remainder loop, as
the proposed decision process is broken down into “first determine best VF for
main loop, then determine best next EpilogVF for remainder loop” anyhow:
>>  
>>    const LoopVectorizationCostModel::VectorizationFactor EpilogVF
>>           CM.identifyNextProfitableVF(VF.Width);
>>  <> 
>> Raising some aspects:
>>  
>> o The unroll factor may also affect the best EpilogVF. For instance, if
UF=1 then EpilogVF < VF, as the patch currently enforces. But if UF is larger
the next profitable EpilogVF could be equal to VF.
>> 
>> o The scalar loop serves two purposes, as determined by its two
pre-headers: either as a default in case runtime dependence checks fail, or as a
remainder loop in which case it is known to be vectorizable with trip count
less-than VF*UF (or equal-to it*). Would be good to keep this in mind when
rerunning.
>>  
>> (*) Note that if original loop requiresScalarEpilogue(), the trip count
of the remainder loop may be equal to VF*UF, and/or the vectorized remainder
loop may too require a scalar epilogue.
>>  
>> o It should be interesting to see how a single masked iteration that
uses the original VF, say for UF=1, works. LV should hopefully already support
most of what’s needed.
>>  
>> o The original Trip Count clearly affects the profitability of
vectorizing the remainder loop. Would be good to leverage any information that
can be derived about TC either statically or based on profiling, when
determining EpilogVF. Potential speedups and overheads/slowdowns could possibly
be demonstrated using extreme cases; what would the best and worst cases be?
Perhaps TinyTripCountVectorThreshold is also affected?
>>  
>> o Finally, VPlan is currently modeling how to vectorize the loop body
according to the potentially profitable VF’s. It’s modelling could be used to
generate vector code for both body and remainder, and to consider their
combined, overall cost.
>>  
>> Ayal.
>>  
>>  
>>  <>From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org
<mailto:llvm-dev-bounces at lists.llvm.org>] On Behalf Of Hal Finkel via
llvm-dev
>> Sent: Friday, February 24, 2017 00:14
>> To: Adam Nemet <anemet at apple.com <mailto:anemet at
apple.com>>; Nema, Ashutosh <Ashutosh.Nema at amd.com
<mailto:Ashutosh.Nema at amd.com>>
>> Cc: llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>>
>> Subject: Re: [llvm-dev] [Proposal][RFC] Epilog loop vectorization
>>  
>>  
>> 
>> On 02/22/2017 11:52 AM, Adam Nemet via llvm-dev wrote:
>> Hi Ashutosh,
>>  
>> On Feb 22, 2017, at 1:57 AM, Nema, Ashutosh via llvm-dev <llvm-dev
at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>  
>> Hi,
>>  
>> This is a proposal about epilog loop vectorization.
>>  
>> Currently Loop Vectorizer inserts an epilogue loop for handling loops
that don’t have known iteration counts.
>>  
>> The Loop Vectorizer supports loops with an unknown trip count, unknown
trip count may not be a multiple of the vector width, and the vectorizer has to
execute the last few iterations as scalar code. It keeps a scalar copy of the
loop for the remaining iterations.
>>  
>> Loop with the large width has a high possibility of executing many
scalar iterations.
>> i.e. i8 data type with 256bits target register can vectorize with
vector width 32, with that maximum trip count possibility for scalar(epilog)
loop is 31, which is significant & worth vectorizing.
>>  
>> Large vector factor has following challenges:
>> 1)    Possibility of remainder iteration is substantial.
>> 2)    Actual trip count at runtime is substantial but not meeting
minimum trip count to execute vector loop.
>>  
>> These challenges can be addressed by mask instructions, but these
instructions are limited and may not be available to all targets.
>>  
>> By epilog vectorization our aim to vectorize epilog loop where original
loop is vectorized with large vector factor and has a high possibility of
executing scalar iterations.
>>  
>> This require following changes:
>> 1)    Costing: Preserve all profitable vector factor.
>> 2)    Transform: Create an additional vector loop with next profitable
vector factor.
>>  
>> Is this something that you propose to be on by default for wide VPU
architectures without masking support? I.e. how widely is this applicable?   If
not then perhaps a better strategy would be to just annotate the remainder loop
with some metadata to limit the vectorization factor and just rerun the
vectorizer.
>> 
>> Why would this solution (annotating the remainder loop to limit
vectorization and rerunning the vectorization process) not be preferred
regardless?
>> 
>> One issue that might be relevant here are runtime aliasing checks,
which are probably going to be redundant, or partially redundant, between the
different vectorized versions. Will we be able to do any necessary cleanup after
the fact? Moreover, we often don't hoist (unswitch) these checks out of
inner loops (perhaps because they're inside the trip-count checks?); I
wonder if the proposed block structure will make this situation better or worse
(or have no overall effect).
>> 
>> Thanks again,
>> Hal
>> 
>> 
>>  
>> Adam
>>  
>> 
>>  
>> Please refer attached file (BlockLayout.png) for the details about
transformed block layout.
>>  
>> Patch is available at: https://reviews.llvm.org/D30247
<https://reviews.llvm.org/D30247>
>>  
>> Regards,
>> Ashutosh
>>  
>> <BlockLayout.png>_______________________________________________
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<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
>> 
>> 
>> 
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>>  
>> 
>> -- 
>> Hal Finkel
>> Lead, Compiler Technology and Programming Languages
>> Leadership Computing Facility
>> Argonne National Laboratory
>> ---------------------------------------------------------------------
>> Intel Israel (74) Limited
>> 
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> 
> -- 
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
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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:
>
> 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.
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.
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> 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
<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.

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.
> 
> 
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