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Nema, Ashutosh via llvm-dev

2017-Feb-27 12:29 UTC

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

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 ?

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]
Sent: Sunday, February 26, 2017 10:53 PM
To: Hal Finkel <hfinkel at anl.gov>; Adam Nemet <anemet at
apple.com>; Nema, Ashutosh <Ashutosh.Nema at amd.com>
Cc: llvm-dev <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] 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

Regards,
Ashutosh

<BlockLayout.png>_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev




_______________________________________________

LLVM Developers mailing list

llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>

http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev


--

Hal Finkel

Lead, Compiler Technology and Programming Languages

Leadership Computing Facility

Argonne National Laboratory

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Hal Finkel via llvm-dev

2017-Feb-27 15:27 UTC

head link

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

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.

  -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]
> *Sent:* Sunday, February 26, 2017 10:53 PM
> *To:* Hal Finkel <hfinkel at anl.gov>; Adam Nemet <anemet at
apple.com>;
> Nema, Ashutosh <Ashutosh.Nema at amd.com>
> *Cc:* llvm-dev <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] *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
>
>         Regards,
>
>         Ashutosh
>
>        
<BlockLayout.png>_______________________________________________
>         LLVM Developers mailing list
>         llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>
>         http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
>
>
>     _______________________________________________
>
>     LLVM Developers mailing list
>
>     llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>
>     http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
> -- 
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
> ---------------------------------------------------------------------
> Intel Israel (74) Limited
>
> This e-mail and any attachments may contain confidential material for
> the sole use of the intended recipient(s). Any review or distribution
> by others is strictly prohibited. If you are not the intended
> recipient, please contact the sender and delete all copies.
>
-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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Adam Nemet via llvm-dev

2017-Feb-27 17:29 UTC

head link

[llvm-dev] [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>_______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
>> 
>> 
>> 
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
>>  
>> 
>> -- 
>> Hal Finkel
>> Lead, Compiler Technology and Programming Languages
>> Leadership Computing Facility
>> Argonne National Laboratory
>> ---------------------------------------------------------------------
>> Intel Israel (74) Limited
>> 
>> This e-mail and any attachments may contain confidential material for
>> the sole use of the intended recipient(s). Any review or distribution
>> by others is strictly prohibited. If you are not the intended
>> recipient, please contact the sender and delete all copies.
>> 
> 
> -- 
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
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Zaks, Ayal via llvm-dev

2017-Feb-28 09:10 UTC

head link

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

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 ?

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.

This minimum width is meant to be a lower bound on EpilogVF, right? The main
loop has no such lower bound, but instead uses its cost model to find the best
VF from among {2,4,8,…,maxVF}. Can the remainder loop rely on its cost model as
well, possibly improved to better consider short trip counts, w/o enforcing a
lower bound? OTOH, if there are obvious lower-bounds, using them for both loops
can save compile-time.

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.

On the contrary: unrolling should promote executing an epilog vector loop. Put
differently, if unrolling would have been applied to the vectorized loop after
vectorization, instead of doing the vectorization and unrolling together, we
would have had left-over vector iterations.

Ayal.

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]
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] 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

Regards,
Ashutosh

<BlockLayout.png>_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev



_______________________________________________

LLVM Developers mailing list

llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>

http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev


--

Hal Finkel

Lead, Compiler Technology and Programming Languages

Leadership Computing Facility

Argonne National Laboratory

---------------------------------------------------------------------
Intel Israel (74) Limited

This e-mail and any attachments may contain confidential material for
the sole use of the intended recipient(s). Any review or distribution
by others is strictly prohibited. If you are not the intended
recipient, please contact the sender and delete all copies.
---------------------------------------------------------------------
Intel Israel (74) Limited

This e-mail and any attachments may contain confidential material for
the sole use of the intended recipient(s). Any review or distribution
by others is strictly prohibited. If you are not the intended
recipient, please contact the sender and delete all copies.
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Nema, Ashutosh via llvm-dev

2017-Feb-28 11:22 UTC

head link

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

From: Zaks, Ayal [mailto:ayal.zaks at intel.com]
Sent: Tuesday, February 28, 2017 2:40 PM
To: Nema, Ashutosh <Ashutosh.Nema at amd.com>; Hal Finkel <hfinkel at
anl.gov>; Adam Nemet <anemet at apple.com>; Renato Golin
<renato.golin at linaro.org>; mkuper at google.com; Mehdi Amini
<mehdi.amini at apple.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>
Subject: RE: [llvm-dev] [Proposal][RFC] Epilog loop vectorization

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 ?

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.

This minimum width is meant to be a lower bound on EpilogVF, right? The main
loop has no such lower bound, but instead uses its cost model to find the best
VF from among {2,4,8,…,maxVF}. Can the remainder loop rely on its cost model as
well, possibly improved to better consider short trip counts, w/o enforcing a
lower bound? OTOH, if there are obvious lower-bounds, using them for both loops
can save compile-time.

Epilog vectorization is more likely to show gains when high possibility of
remainder iterations, this is only possible when the first vector version is
vectorized with large trip count. For this I have kept one threshold in the
implementation with default value 16, when first vector version width goes
beyond this threshold then only enable epilog vectorization. Epilog vector
version width will be less than first vector version. In the current
implementation both first & epilog vector loop profitability get decided by
the same costing step. Costing tries to find all profitable vector factor and
later for epilog vectorization it finds next profitable vector factor in already
computed profitable vectors.

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.

On the contrary: unrolling should promote executing an epilog vector loop. Put
differently, if unrolling would have been applied to the vectorized loop after
vectorization, instead of doing the vectorization and unrolling together, we
would have had left-over vector iterations.

My understanding might be wrong here, in the Loop vectorizer first it tries to
find the profitable vector factor from among {2,4,6…MaxVF}, then it tries to
find the unroll factor. Does it mean that even without unroll vector loop is
profitable with identified vector factor (in addition to it unrolling may give
more gains) ?
Epilog vector loop has very low iterations possibility, and unrolling can
further reduce this. i.e. if first loop is vectorized with VF16 then possibility
for epilog scalar loop iterations are 15, if vectorizer generates epilog vector
loop with width 4 then it can cater 4 to 15 scalar iterations, but if epilog
vector loop get unrolled by 2 then it can only cater 8-15 scalar iterations.

We have to little careful with unroll, not sure forcing UF=1 for epilog vector
loop is a good idea.

Ayal.

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]
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] 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

Regards,
Ashutosh

<BlockLayout.png>_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

_______________________________________________

LLVM Developers mailing list

llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>

http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

--

Hal Finkel

Lead, Compiler Technology and Programming Languages

Leadership Computing Facility

Argonne National Laboratory

---------------------------------------------------------------------
Intel Israel (74) Limited

This e-mail and any attachments may contain confidential material for
the sole use of the intended recipient(s). Any review or distribution
by others is strictly prohibited. If you are not the intended
recipient, please contact the sender and delete all copies.

---------------------------------------------------------------------
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the sole use of the intended recipient(s). Any review or distribution
by others is strictly prohibited. If you are not the intended
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