llvm dev - Feb 2020 - [flang-dev] About OpenMP dialect in MLIR

Reply to Kiran Chandramohan:
> You are welcome to participate, provide feedback and criticism to change
the design as well as to contribute to the implementation.

Thank you Kiran.
> But the latest is what is there in the RFC in discourse.
I have used this as reference for the response.
> We did a study of a few constructs and clauses which was shared as mails
to flang-dev and the RFC. As we make progress and before implementation, we
will share further details.
> “ Yes, parallel and flush would be the next two constructs that we will
do.” -- from a comment in latest RFC

For the above mentioned reasons, I will try to restrict my reply to how the
“parallel (do)” construct would be lowered.
> If it is OK we can have further discussions in discourse as River points
out.

Given that the multiple components of the LLVM project, namely clang,
flang, MLIR and LLVM are involved, llvm-dev is probably a better place,
with a much wider audience, until it is clear how different components must
interact.
> It is the review for translation to LLVM IR that is in progress.
> “If we decide that the OpenMP construct (for e.g. collapse) can be
handled fully in MLIR and  that is the best place to do it (based on
experiments) then we will not use the OpenMP IRBuilder for these constructs.”
-- latest RFC in discourse

If it is not finalized that the OpenMPIRBuilder will be used for all the
constructs, wouldn’t it be better to delay the submission of “translation
to LLVM IR” patch in MLIR? Lowering code will become inconsistent if the
OpenMPIRBuilder is used only for a few constructs and not for others.

Also, the patch does OpenMP dialect lowering *alongside* LLVM Dialect to
LLVM IR. This is different from most dialects which get directly lowered to
LLVM Dialect. I think lowering to LLVM Dialect would be a cleaner way if
OpenMPIRBuilder is not being considered for all constructs.

Mehdi also seems to have the same suggestion: “I agree that having dialect
lowering would be cleaner” in https://reviews.llvm.org/D72962
> Yes, the design has mildly changed over time to incorporate feedback.
But the latest is what is there in the RFC in discourse.

RFC fails to discuss the following (I have also mentioned some of them in
my reply to Johannes):
> The proposed plan involves a) lowering F18 AST with OpenMP directly to a
mix of OpenMP and FIR dialects. b) converting this finally to a mix of
OpenMP and LLVM dialects.

It is unclear in the RFC what other dialects are considered as supported
for OpenMP dialect  (std, affine, vector, loop, etc) and how it would be
transformed, used and lowered from FIR to LLVM.

It becomes important to list down the various dialects / operations / types
supported for OpenMP (which is mainly defined for C, C++ and Fortran
programs. MLIR has a much wider scope.

It wouldn’t add much value for the proposed OpenMP dialect to be in the
MLIR tree if it cannot support at least the relevant standard dialect types
/ operations.
> We would like to take advantage of the transformations in cases that are
possible. FIR loops will be converted to affine/loop dialect. So the loop
inside an omp.do can be in these dialects as clarified in the discussion in
discourse and also shown in slide 20 of the fosdem presentation (links to
both below).

https://llvm.discourse.group/t/rfc-openmp-dialect-in-mlir/397/7?u=kiranchandramohan

https://fosdem.org/2020/schedule/event/llvm_flang/attachments/slides/3839/export/events/attachments/llvm_flang/slides/3839/flang_llvm_frontend.pdf

Although it is mentioned that the affine/ loop.for is used, following
things are unclear:

I am assuming that there will be lowering / conversion code in f18 repo
dialect from fir.do to loop.for / affine.for. Is it the case? If so, I
think it is worth mentioning it in the “sequential code flow
representation” in the RFC.

This raises the following questions.



   1.

   Which types are supported? Standard dialect types and FIR types?


For example, what types are used for Fortran arrays used inside OpenMP
regions? Is it std.memref OR Fortran array representation in FIR dialect
(fir.array?) OR both?  Note that Fortran has support for column major
arrays. std.memref supports custom memory layouts. What custom layouts are
supported?


How would different non-scalar types in standard dialect  be lowered to
LLVM IR and passed to OpenMP runtime calls? Can you please elaborate on
this?

The example provided in slide 20 of the fosdem presentation contains

“loop.for %j = %lb2 to %ub2 : !integer {“

But loop.for accepts “index” type. Not sure what type “!integer” represents
here.


   1.

   What are the different memory access operations which are supported
   inside the OpenMP region and lowered to proper OpenMP runtime calls in LLVM
   IR?


The possibilities are:

   1.

   affine.load / affine.store
   2.

   std.load / std.store
   3.

   FIR dialect memory access operations.

> I must also point out that the question of where to do loop
transformations is a topic we have not fully converged on. See the
following thread for discussions.
http://lists.llvm.org/pipermail/flang-dev/2019-September/000042.html

Looks like placement (MLIR / LLVM) of various transformations related to
OpenMP has not been finalized, from what I could infer from Johannes’s
reply and the below text in the latest RFC in discourse:

“So there exist some questions regarding where the optimisations should be
carried out. We will decide on which framework to choose only after some
experimentation.”
> i) we need to keep the loops separately so as to take advantage of
transformations that other dialects like affine/loop would provide.

1) Keeping the loops separate from the OpenMP operations will expose them
to the “regular” transformations passes in MLIR inside the OpenMP region.
Most of them are invalid or in-efficient for OpenMP operations.

Examples:

   1.

   Constant propagation example mentioned by Johannes in this thread. (omp
   task shared(x))
   2.

   Loop (nest) transformations (permute / split / fuse / tile, etc) will
   happen ignoring the surrounding OpenMP operations.
   3.

   Hoisting and sinking of various memory/ SSA values inside the OpenMP
   region. This goes against the likes of “map”, “firstprivate”, shared, etc
   clauses and more.


2) Various loop operations (loop.for, affine.for, fir.do) have (or will
have) different transformations/ optimization passes which are different
from one another.

Example:

   1.

   AffineLoopInvariantCodeMotion.cpp is different from
   LoopInvariantCodeMotion.cpp.
   2.

   Other Loop transformation passes for affine.for


These loops also use different Types and memory access operations in
general for transformations. Example, most Affine dialect transformations
(if not all) work on affine.load and affine.store operations.

Supporting different loop operations means that there would be *OpenMP
specific transformations* for each one of them and also requires a way to
restrict each of them from existing transformations (when nested in OpenMP
constructs).

There would be different lowerings for different loop operations as well.
Example, affine.for and loop.for would have to be lowered to omp.do in
different ways.
>From slide 20 of fosdem presentation you mentioned, the LLVM + OpenMP
dialect representation is as follows:

------------------------------

Mlir.region(…) {

   omp.parallel  {

     %ub3 = …

     omp.do %i = 0 to %ub3 : !integer  {

     …

     }

  }

}

-------------------------------

Currently, the LLVM Dialect doesn’t contain a high level loop operation. It
is all based on CFG implementation.

Will omp.do follow the same structure (SizedRegion<1>) as loop.for? OR
there would be CFG for LLVM Dialect based loop operation?

Can you please mention how the OpenMP + LLVM dialect will look like for the
below parallel do construct?

integer :: i=1, k=10

integer :: a(10), b(10), c(10)

...

 !$omp parallel do

  do i = 1, k

    if (i .ne. 1) *cycle*

    c(i) = a(i) + b(i)

  end do

  !$omp end parallel do

print *,c


Thanks,

Vinay

On Fri, Feb 14, 2020 at 6:52 AM Kiran Chandramohan via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hello Vinay,
>
> Thanks for your mail about the OpenMP dialect in MLIR. Happy to know that
> you and several other groups are interested in the OpenMP dialect. At the
> outset, I must point out that the design is not set in stone and will
> change as we make progress. You are welcome to participate, provide
> feedback and criticism to change the design as well as to contribute to the
> implementation. I provide some clarifications and replies to your comments
> below. If it is OK we can have further discussions in discourse as River
> points out.
>
> 1. [May 2019] An OpenMPIRBuilder in LLVM was proposed for flang and clang
> frontends. Note that this proposal was before considering MLIR for FIR.
>
> A correction here. The proposal for OpenMPIRBuilder was made when MLIR was
> being considered for FIR.
> (i) Gary Klimowicz's minutes for Flang call in April 2019 mentions
> considering MLIR for FIR.
>
>
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-April/000194.html
> (ii) My reply to Johaness's proposal in May 2019 mentions MLIR for FIR.
>
>
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-May/000220.html
>
> b. Review of barrier construct is in progress:
> https://reviews.llvm.org/D72962
>
> Minor correction here. The addition of barrier construct was accepted and
> has landed (https://reviews.llvm.org/D7240
> <https://reviews.llvm.org/D72400>). It is the review for translation
to
> LLVM IR that is in progress.
>
> It looks like the design has evolved over time and there is no one place
> which contains the latest design decisions that fits all the different
> pieces of the puzzle. I will try to deduce it from the above mentioned
> references. Please correct me If I am referring to anything which has
> changed.
>
> Yes, the design has mildly changed over time to incorporate feedback. But
> the latest is what is there in the RFC in discourse.
>
> For most OpenMP design discussions, FIR examples are used (as seen in (2)
> and (3)). The MLIR examples mentioned in the design only talks about FIR
> dialect and LLVM dialect.
>
> Our initial concern was how will all these pieces (FIR, LLVM Dialect,
> OpenMPIRBuilder, LLVM IR) fit together. Hence you see the prominence of FIR
> and LLVM dialect and more information about lowering/translation than
> transformations/optimisations.
>
> This completely ignores the likes of standard, affine (where most loop
> transformations are supposed to happen) and loop dialects.
>
> Adding to the reply above. We would like to take advantage of the
> transformations in cases that are possible. FIR loops will be converted to
> affine/loop dialect. So the loop inside an omp.do can be in these dialects
> as clarified in the discussion in discourse and also shown in slide 20 of
> the fosdem presentation (links to both below).
>
>
https://llvm.discourse.group/t/rfc-openmp-dialect-in-mlir/397/7?u=kiranchandramohan
>
>
https://fosdem.org/2020/schedule/event/llvm_flang/attachments/slides/3839/export/events/attachments/llvm_flang/slides/3839/flang_llvm_frontend.pdf
>
> I must also point out that the question of where to do loop
> transformations is a topic we have not fully converged on. See the
> following thread for discussions.
> http://lists.llvm.org/pipermail/flang-dev/2019-September/000042.html
>
> Is it the same omp.do operation which now contains the bounds and
> induction variables of the loop after the LLVM conversion?
>
> The point here is that i) we need to keep the loops separately so as to
> take advantage of transformations that other dialects like affine/loop
> would provide. ii) We will need the loop information while lowering the
> OpenMP do operation. For implementation, if reusing the same operation (in
> different contexts) is difficult then we can add a new operation.
>
> It is also not mentioned how clauses like firstprivate, shared, private,
> reduce, map, etc are lowered to OpenMP dialect.
>
> Yes, it is not mentioned. We did a study of a few constructs and clauses
> which was shared as mails to flang-dev and the RFC. As we make progress and
> before implementation, we will share further details.
>
> it would be beneficial to have an omp.parallel_do operation which has
> semantics similar to other loop structures (may not be LoopLikeInterface)
> in MLIR.
>
> I am not against adding parallel_do if it can help with transformations or
> reduce the complexity of lowering. Please share the details in discourse as
> a reply to the RFC or a separate thread.
>
> it looks like having OpenMP operations based on standard MLIR types and
> operations (scalars and memrefs mainly) is the right way to go.
>
> This will definitely be the first version that we implement. But I do not
> understand why we should restrict to only the standard types and
> operations. To ease lowering and translation and to avoid adding OpenMP
> operations to other dialects, I believe OpenMP dialect should also be able
> to exist with other dialects like FIR and LLVM.
>
> E. Lowering of target constructs mentioned in ( 2(d) ) specifies direct
> lowering to LLVM IR ignoring all the advantages that MLIR provides.
>
> Also, OpenMP codegen will automatically benefit from the GPU dialect based
> optimizations. For example, it would be way easier to hoist a memory
> reference out of GPU kernel in MLIR than in LLVM IR.
>
> I might not have fully understood you here. But the dialect lives
> independently of the translation to LLVM IR. If there are optimisations
> (like hoisting that you mention here) I believe they can be performed as
> transformation passes on the dialect. It is not ruled out.
>
> --Kiran
> ------------------------------
> *From:* flang-dev <flang-dev-bounces at lists.llvm.org> on behalf of
Vinay
> Madhusudan via flang-dev <flang-dev at lists.llvm.org>
> *Sent:* 13 February 2020 16:33
> *To:* llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>;
> flang-dev at lists.llvm.org <flang-dev at lists.llvm.org>
> *Subject:* [flang-dev] About OpenMP dialect in MLIR
>
>
> Hi,
>
> I have few questions / concerns regarding the design of OpenMP dialect in
> MLIR that is currently being implemented, mainly for the f18 compiler.
> Below, I summarize the current state of various efforts in clang / f18 /
> MLIR / LLVM regarding this. Feel free to add to the list in case I have
> missed something.
>
> 1. [May 2019] An OpenMPIRBuilder in LLVM was proposed for flang and clang
> frontends. Note that this proposal was before considering MLIR for FIR.
>
> a. llvm-dev proposal :
>
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-May/000197.html
>
> b. Patches in review: https://reviews.llvm.org/D70290. This also includes
> the clang codegen changes.
>
> 2.  [July - September 2019] OpenMP dialect for MLIR was discussed /
> proposed with respect to the f18 compilation stack (keeping FIR in mind).
>
> a. flang-dev discussion link:
> https://lists.llvm.org/pipermail/flang-dev/2019-September/000020.html
>
> b. Design decisions captured in PPT:
> https://drive.google.com/file/d/1vU6LsblsUYGA35B_3y9PmBvtKOTXj1Fu/view
>
> c. MLIR google groups discussion:
> https://groups.google.com/a/tensorflow.org/forum/#!topic/mlir/4Aj_eawdHiw
>
> d. Target constructs  design:
>
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-September/000285.html
>
> e. SIMD constructs design:
>
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-September/000278.html
>
> 3.  [Jan 2020] OpenMP dialect RFC in llvm discourse :
> https://llvm.discourse.group/t/rfc-openmp-dialect-in-mlir/397
>
> 4.  [Jan- Feb 2020] Implementation of OpenMP dialect in MLIR:
>
> a. The first patch which introduces the OpenMP dialect was pushed.
>
> b. Review of barrier construct is in progress:
> https://reviews.llvm.org/D72962
> https://reviews.llvm.org/D72400
>
> I have tried to list below different topics of interest (to different
> people) around this work. Most of these are in the design phase (or very
> new) and multiple parties are interested with different sets of goals in
> mind.
>
> I.  Flang frontend and its integration
>
> II. Fortran representation in MLIR / FIR development
>
> III. OpenMP development for flang,  OpenMP builder in LLVM.
>
> IV. Loop Transformations in MLIR / LLVM with respect to OpenMP.
>
> It looks like the design has evolved over time and there is no one place
> which contains the latest design decisions that fits all the different
> pieces of the puzzle. I will try to deduce it from the above mentioned
> references. Please correct me If I am referring to anything which has
> changed.
>
> A. For most OpenMP design discussions, FIR examples are used (as seen in
> (2) and (3)). The MLIR examples mentioned in the design only talks about
> FIR dialect and LLVM dialect.
>
> This completely ignores the likes of standard, affine (where most loop
> transformations are supposed to happen) and loop dialects. I think it is
> critical to decouple the OpenMP dialect development in MLIR from the
> current flang / FIR effort. It would be useful if someone can mention these
> examples using existing dialects in MLIR and also how the different
> transformations / lowerings are planned.
>
> B. In latest RFC(3), it is mentioned that the initial OpenMP dialect
> version will be as follows,
>
>   omp.parallel {
>
>     omp.do {
>
>        fir.do %i = 0 to %ub3 : !fir.integer {
>
>         ...
>
>        }
>
>     }
>
>   }
>
> and then after the "LLVM conversion" it is converted as follows:
>
>   omp.parallel {
>
>     %ub3 >
>     omp.do %i = 0 to %ub3 : !llvm.integer {
>
>     ...
>
>     }
>
>   }
>
>
> a. Is it the same omp.do operation which now contains the bounds and
> induction variables of the loop after the LLVM conversion? If so, will the
> same operation have two different semantics during a single compilation?
>
> b. Will there be different lowerings for various loop operations from
> different dialects? loop.for and affine.for under omp operations would need
> different OpenMP / LLVM lowerings. Currently, both of them are lowered to
> the CFG based loops during the LLVM dialect conversion (which is much
> before the proposed OpenMP dialect lowering).
>
> There would be no standard way to represent OpenMP operations (especially
> the ones which involve loops) in MLIR. This would drastically complicate
> lowering.
>
> C. It is also not mentioned how clauses like firstprivate, shared,
> private, reduce, map, etc are lowered to OpenMP dialect. The example in
> the RFC contains FIR and LLVM types and nothing about std dialect types.
> Consider the below example:
>
> #pragma omp parallel for reduction(+:x)
>
> for (int i = 0; i < N; ++i)
>
>   x += a[i];
>
> How would the above be represented in OpenMP dialect? and What type would
> "x" be in MLIR?  It is not mentioned in the design as to how the
various
> SSA values for various OpenMP clauses are passed around in OpenMP
> operations.
>
> D. Because of (A), (B) and (C), it would be beneficial to have an omp.
> parallel_do operation which has semantics similar to other loop
> structures (may not be LoopLikeInterface) in MLIR. To me, it looks like
> having OpenMP operations based on standard MLIR types and operations
> (scalars and memrefs mainly) is the right way to go.
>
> Why not have omp.parallel_do operation with AffineMap based bounds, so as
> to decouple it from Value/Type similar to affine.for?
>
> 1. With the current design, the number of transformations / optimizations
> that one can write on OpenMP constructs would become limited as there can
> be any custom loop structure with custom operations / types inside it.
>
> 2. It would also be easier to transform the Loop nests containing OpenMP
> constructs if the body of the OpenMP operations is well defined (i.e., does
> not accept arbitrary loop structures). Having nested redundant
"parallel" ,
> "target" and "do" regions seems unnecessary.
>
> 3. There would also be new sets of loop structures in new dialects when
> C/C++ is compiled to MLIR. It would complicate the number of possible
> combinations inside the OpenMP region.
>
> E. Lowering of target constructs mentioned in ( 2(d) ) specifies direct
> lowering to LLVM IR ignoring all the advantages that MLIR provides. Being
> able to compile the code for heterogeneous hardware is one of the biggest
> advantages that MLIR brings to the table. That is being completely missed
> here. This also requires solving the problem of handling target information
> in MLIR. But that is a problem which needs to be solved anyway. Using GPU
> dialect also gives us an opportunity to represent offloading semantics in
> MLIR.
>
> Given the ability to represent multiple ModuleOps and the existence of GPU
> dialect, couldn't higher level optimizations on offloaded code be done
at
> MLIR level?. The proposed design would lead us to the same problems that we
> are currently facing in LLVM IR.
>
> Also, OpenMP codegen will automatically benefit from the GPU dialect based
> optimizations. For example, it would be way easier to hoist a memory
> reference out of GPU kernel in MLIR than in LLVM IR.
>
> Thanks,
>
> Vinay
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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On Sat, Feb 15, 2020 at 10:42 AM Vinay Madhusudan via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Reply to Kiran Chandramohan:
>
> > You are welcome to participate, provide feedback and criticism to
change
> the design as well as to contribute to the implementation.
>
> Thank you Kiran.
>
> > But the latest is what is there in the RFC in discourse.
>
> I have used this as reference for the response.
>
> > We did a study of a few constructs and clauses which was shared as
mails
> to flang-dev and the RFC. As we make progress and before implementation, we
> will share further details.
>
> > “ Yes, parallel and flush would be the next two constructs that we
will
> do.” -- from a comment in latest RFC
>
> For the above mentioned reasons, I will try to restrict my reply to how
> the “parallel (do)” construct would be lowered.
>
> > If it is OK we can have further discussions in discourse as River
points
> out.
>
> Given that the multiple components of the LLVM project, namely clang,
> flang, MLIR and LLVM are involved, llvm-dev is probably a better place,
> with a much wider audience
>
Possibly wider, but maybe less focused about discussing MLIR dialect
design. In particular there is an RFC thread for this particular dialect on
Discourse, which is the canonical place to discuss its design.

> , until it is clear how different components must interact.
>
They don't need to interact so closely: they are very loosely related:
flang will use MLIR but clang won't (in the foreseeable future) and LLVM
has many other frontends.

>
> > It is the review for translation to LLVM IR that is in progress.
>
> > “If we decide that the OpenMP construct (for e.g. collapse) can be
> handled fully in MLIR and  that is the best place to do it (based on
> experiments) then we will not use the OpenMP IRBuilder for these
constructs.”
> -- latest RFC in discourse
>
> If it is not finalized that the OpenMPIRBuilder will be used for all the
> constructs, wouldn’t it be better to delay the submission of “translation
> to LLVM IR” patch in MLIR? Lowering code will become inconsistent if the
> OpenMPIRBuilder is used only for a few constructs and not for others.
>
> Also, the patch does OpenMP dialect lowering *alongside* LLVM Dialect to
> LLVM IR. This is different from most dialects which get directly lowered to
> LLVM Dialect. I think lowering to LLVM Dialect would be a cleaner way if
> OpenMPIRBuilder is not being considered for all constructs.
>

I don't disagree, but there are a lot of speculation here: your quote
starts with "If we decide that the OpenMP construct (for e.g. collapse) can
be handled fully in MLIR", are you thinking that we need to first decide
this once and for all before making progress on building this path?
What disadvantages do you perceive to an approach where we would bring up
this dialect using the OpenMPIRBuilders for exporting to LLVM IR until we
gain enough experience? Do you think starting like this will make it
significantly harder to transition away from the builders if this is what
we want?
It seemed to me like it wouldn't, and that's why I'm supportive of
this
path: the omp dialect design, implementation, and the
transformation/analysis that will be performed there seems entirely
disjoint from the LLVM lowering, I'd hope we can swap the LLVM lowering at
a later time (if this is what we'd want).

>
> Mehdi also seems to have the same suggestion: “I agree that having
> dialect lowering would be cleaner” in https://reviews.llvm.org/D72962
>
Since you're calling me out: yes it would be cleaner from a pure MLIR point
of view, I don't think there is much disagreement on this (I think?).
However we already have the OpenMP builders available and they will
continue to be maintained/evolved to support OpenMP in clang.
Duplicating them entirely in MLIR for the sake of purity seems like a lack
of pragmatism here, so I support the current approach with the current
tradeoffs.

>
> > Yes, the design has mildly changed over time to incorporate feedback.
> But the latest is what is there in the RFC in discourse.
>
> RFC fails to discuss the following (I have also mentioned some of them in
> my reply to Johannes):
>
> > The proposed plan involves a) lowering F18 AST with OpenMP directly to
a
> mix of OpenMP and FIR dialects. b) converting this finally to a mix of
> OpenMP and LLVM dialects.
>
> It is unclear in the RFC what other dialects are considered as supported
> for OpenMP dialect  (std, affine, vector, loop, etc) and how it would be
> transformed, used and lowered from FIR to LLVM.
>
> It becomes important to list down the various dialects / operations /
> types supported for OpenMP (which is mainly defined for C, C++ and Fortran
> programs. MLIR has a much wider scope.
>
> It wouldn’t add much value for the proposed OpenMP dialect to be in the
> MLIR tree if it cannot support at least the relevant standard dialect types
> / operations.
>
I agree, and I think this was something I called out as important in the
RFC: "It seems that the dialect can be orthogonal to FIR and its type
system, which the most important thing to me to integrate MLIR (favor
reusability across other frontends / compiler frameworks)".
If you don't think that this is the case, then please raise this in the RFC!
I think it is perfectly fair to ask for more examples from the author and
digging a bit deeper if you're unconvinced that the proposed modeling can
be applicable outside of FIR. This is exactly why we ask such proposal to
go through RFC by the way: to allow people like you to point at the
blindspot and ask the right questions.

Best,

-- 
Mehdi


> > We would like to take advantage of the transformations in cases that
are
> possible. FIR loops will be converted to affine/loop dialect. So the loop
> inside an omp.do can be in these dialects as clarified in the discussion in
> discourse and also shown in slide 20 of the fosdem presentation (links to
> both below).
>
>
>
https://llvm.discourse.group/t/rfc-openmp-dialect-in-mlir/397/7?u=kiranchandramohan
>
>
>
https://fosdem.org/2020/schedule/event/llvm_flang/attachments/slides/3839/export/events/attachments/llvm_flang/slides/3839/flang_llvm_frontend.pdf
>
> Although it is mentioned that the affine/ loop.for is used, following
> things are unclear:
>
> I am assuming that there will be lowering / conversion code in f18 repo
> dialect from fir.do to loop.for / affine.for. Is it the case? If so, I
> think it is worth mentioning it in the “sequential code flow
> representation” in the RFC.
>
> This raises the following questions.
>
>
>
>    1.
>
>    Which types are supported? Standard dialect types and FIR types?
>
>
> For example, what types are used for Fortran arrays used inside OpenMP
> regions? Is it std.memref OR Fortran array representation in FIR dialect
> (fir.array?) OR both?  Note that Fortran has support for column major
> arrays. std.memref supports custom memory layouts. What custom layouts are
> supported?
>
>
> How would different non-scalar types in standard dialect  be lowered to
> LLVM IR and passed to OpenMP runtime calls? Can you please elaborate on
> this?
>
> The example provided in slide 20 of the fosdem presentation contains
>
> “loop.for %j = %lb2 to %ub2 : !integer {“
>
> But loop.for accepts “index” type. Not sure what type “!integer”
> represents here.
>
>
>    1.
>
>    What are the different memory access operations which are supported
>    inside the OpenMP region and lowered to proper OpenMP runtime calls in
LLVM
>    IR?
>
>
> The possibilities are:
>
>    1.
>
>    affine.load / affine.store
>    2.
>
>    std.load / std.store
>    3.
>
>    FIR dialect memory access operations.
>
>
> > I must also point out that the question of where to do loop
> transformations is a topic we have not fully converged on. See the
> following thread for discussions.
> http://lists.llvm.org/pipermail/flang-dev/2019-September/000042.html
>
> Looks like placement (MLIR / LLVM) of various transformations related to
> OpenMP has not been finalized, from what I could infer from Johannes’s
> reply and the below text in the latest RFC in discourse:
>
> “So there exist some questions regarding where the optimisations should
> be carried out. We will decide on which framework to choose only after some
> experimentation.”
>
> > i) we need to keep the loops separately so as to take advantage of
> transformations that other dialects like affine/loop would provide.
>
> 1) Keeping the loops separate from the OpenMP operations will expose them
> to the “regular” transformations passes in MLIR inside the OpenMP region.
> Most of them are invalid or in-efficient for OpenMP operations.
>
> Examples:
>
>    1.
>
>    Constant propagation example mentioned by Johannes in this thread.
>    (omp task shared(x))
>    2.
>
>    Loop (nest) transformations (permute / split / fuse / tile, etc) will
>    happen ignoring the surrounding OpenMP operations.
>    3.
>
>    Hoisting and sinking of various memory/ SSA values inside the OpenMP
>    region. This goes against the likes of “map”, “firstprivate”, shared,
etc
>    clauses and more.
>
>
> 2) Various loop operations (loop.for, affine.for, fir.do) have (or will
> have) different transformations/ optimization passes which are different
> from one another.
>
> Example:
>
>    1.
>
>    AffineLoopInvariantCodeMotion.cpp is different from
>    LoopInvariantCodeMotion.cpp.
>    2.
>
>    Other Loop transformation passes for affine.for
>
>
> These loops also use different Types and memory access operations in
> general for transformations. Example, most Affine dialect transformations
> (if not all) work on affine.load and affine.store operations.
>
> Supporting different loop operations means that there would be *OpenMP
> specific transformations* for each one of them and also requires a way to
> restrict each of them from existing transformations (when nested in OpenMP
> constructs).
>
> There would be different lowerings for different loop operations as well.
> Example, affine.for and loop.for would have to be lowered to omp.do in
> different ways.
>
> From slide 20 of fosdem presentation you mentioned, the LLVM + OpenMP
> dialect representation is as follows:
>
> ------------------------------
>
> Mlir.region(…) {
>
>    omp.parallel  {
>
>      %ub3 = …
>
>      omp.do %i = 0 to %ub3 : !integer  {
>
>      …
>
>      }
>
>   }
>
> }
>
> -------------------------------
>
> Currently, the LLVM Dialect doesn’t contain a high level loop operation.
> It is all based on CFG implementation.
>
> Will omp.do follow the same structure (SizedRegion<1>) as loop.for?
OR
> there would be CFG for LLVM Dialect based loop operation?
>
> Can you please mention how the OpenMP + LLVM dialect will look like for
> the below parallel do construct?
>
> integer :: i=1, k=10
>
> integer :: a(10), b(10), c(10)
>
> ...
>
>  !$omp parallel do
>
>   do i = 1, k
>
>     if (i .ne. 1) *cycle*
>
>     c(i) = a(i) + b(i)
>
>   end do
>
>   !$omp end parallel do
>
> print *,c
>
> Thanks,
>
> Vinay
>
> On Fri, Feb 14, 2020 at 6:52 AM Kiran Chandramohan via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hello Vinay,
>>
>> Thanks for your mail about the OpenMP dialect in MLIR. Happy to know
that
>> you and several other groups are interested in the OpenMP dialect. At
the
>> outset, I must point out that the design is not set in stone and will
>> change as we make progress. You are welcome to participate, provide
>> feedback and criticism to change the design as well as to contribute to
the
>> implementation. I provide some clarifications and replies to your
comments
>> below. If it is OK we can have further discussions in discourse as
River
>> points out.
>>
>> 1. [May 2019] An OpenMPIRBuilder in LLVM was proposed for flang and
clang
>> frontends. Note that this proposal was before considering MLIR for FIR.
>>
>> A correction here. The proposal for OpenMPIRBuilder was made when MLIR
>> was being considered for FIR.
>> (i) Gary Klimowicz's minutes for Flang call in April 2019 mentions
>> considering MLIR for FIR.
>>
>>
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-April/000194.html
>> (ii) My reply to Johaness's proposal in May 2019 mentions MLIR for
FIR.
>>
>>
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-May/000220.html
>>
>> b. Review of barrier construct is in progress:
>> https://reviews.llvm.org/D72962
>>
>> Minor correction here. The addition of barrier construct was accepted
and
>> has landed (https://reviews.llvm.org/D7240
>> <https://reviews.llvm.org/D72400>). It is the review for
translation to
>> LLVM IR that is in progress.
>>
>> It looks like the design has evolved over time and there is no one
place
>> which contains the latest design decisions that fits all the different
>> pieces of the puzzle. I will try to deduce it from the above mentioned
>> references. Please correct me If I am referring to anything which has
>> changed.
>>
>> Yes, the design has mildly changed over time to incorporate feedback.
But
>> the latest is what is there in the RFC in discourse.
>>
>> For most OpenMP design discussions, FIR examples are used (as seen in
(2)
>> and (3)). The MLIR examples mentioned in the design only talks about
FIR
>> dialect and LLVM dialect.
>>
>> Our initial concern was how will all these pieces (FIR, LLVM Dialect,
>> OpenMPIRBuilder, LLVM IR) fit together. Hence you see the prominence of
FIR
>> and LLVM dialect and more information about lowering/translation than
>> transformations/optimisations.
>>
>> This completely ignores the likes of standard, affine (where most loop
>> transformations are supposed to happen) and loop dialects.
>>
>> Adding to the reply above. We would like to take advantage of the
>> transformations in cases that are possible. FIR loops will be converted
to
>> affine/loop dialect. So the loop inside an omp.do can be in these
dialects
>> as clarified in the discussion in discourse and also shown in slide 20
of
>> the fosdem presentation (links to both below).
>>
>>
https://llvm.discourse.group/t/rfc-openmp-dialect-in-mlir/397/7?u=kiranchandramohan
>>
>>
https://fosdem.org/2020/schedule/event/llvm_flang/attachments/slides/3839/export/events/attachments/llvm_flang/slides/3839/flang_llvm_frontend.pdf
>>
>> I must also point out that the question of where to do loop
>> transformations is a topic we have not fully converged on. See the
>> following thread for discussions.
>> http://lists.llvm.org/pipermail/flang-dev/2019-September/000042.html
>>
>> Is it the same omp.do operation which now contains the bounds and
>> induction variables of the loop after the LLVM conversion?
>>
>> The point here is that i) we need to keep the loops separately so as to
>> take advantage of transformations that other dialects like affine/loop
>> would provide. ii) We will need the loop information while lowering the
>> OpenMP do operation. For implementation, if reusing the same operation
(in
>> different contexts) is difficult then we can add a new operation.
>>
>> It is also not mentioned how clauses like firstprivate, shared,
private,
>> reduce, map, etc are lowered to OpenMP dialect.
>>
>> Yes, it is not mentioned. We did a study of a few constructs and
clauses
>> which was shared as mails to flang-dev and the RFC. As we make progress
and
>> before implementation, we will share further details.
>>
>> it would be beneficial to have an omp.parallel_do operation which has
>> semantics similar to other loop structures (may not be
LoopLikeInterface)
>> in MLIR.
>>
>> I am not against adding parallel_do if it can help with transformations
>> or reduce the complexity of lowering. Please share the details in
discourse
>> as a reply to the RFC or a separate thread.
>>
>> it looks like having OpenMP operations based on standard MLIR types and
>> operations (scalars and memrefs mainly) is the right way to go.
>>
>> This will definitely be the first version that we implement. But I do
not
>> understand why we should restrict to only the standard types and
>> operations. To ease lowering and translation and to avoid adding OpenMP
>> operations to other dialects, I believe OpenMP dialect should also be
able
>> to exist with other dialects like FIR and LLVM.
>>
>> E. Lowering of target constructs mentioned in ( 2(d) ) specifies direct
>> lowering to LLVM IR ignoring all the advantages that MLIR provides.
>>
>> Also, OpenMP codegen will automatically benefit from the GPU dialect
>> based optimizations. For example, it would be way easier to hoist a
memory
>> reference out of GPU kernel in MLIR than in LLVM IR.
>>
>> I might not have fully understood you here. But the dialect lives
>> independently of the translation to LLVM IR. If there are optimisations
>> (like hoisting that you mention here) I believe they can be performed
as
>> transformation passes on the dialect. It is not ruled out.
>>
>> --Kiran
>> ------------------------------
>> *From:* flang-dev <flang-dev-bounces at lists.llvm.org> on behalf
of Vinay
>> Madhusudan via flang-dev <flang-dev at lists.llvm.org>
>> *Sent:* 13 February 2020 16:33
>> *To:* llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>;
>> flang-dev at lists.llvm.org <flang-dev at lists.llvm.org>
>> *Subject:* [flang-dev] About OpenMP dialect in MLIR
>>
>>
>> Hi,
>>
>> I have few questions / concerns regarding the design of OpenMP dialect
in
>> MLIR that is currently being implemented, mainly for the f18 compiler.
>> Below, I summarize the current state of various efforts in clang / f18
/
>> MLIR / LLVM regarding this. Feel free to add to the list in case I have
>> missed something.
>>
>> 1. [May 2019] An OpenMPIRBuilder in LLVM was proposed for flang and
clang
>> frontends. Note that this proposal was before considering MLIR for FIR.
>>
>> a. llvm-dev proposal :
>>
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-May/000197.html
>>
>> b. Patches in review: https://reviews.llvm.org/D70290. This also
>> includes the clang codegen changes.
>>
>> 2.  [July - September 2019] OpenMP dialect for MLIR was discussed /
>> proposed with respect to the f18 compilation stack (keeping FIR in
mind).
>>
>> a. flang-dev discussion link:
>> https://lists.llvm.org/pipermail/flang-dev/2019-September/000020.html
>>
>> b. Design decisions captured in PPT:
>> https://drive.google.com/file/d/1vU6LsblsUYGA35B_3y9PmBvtKOTXj1Fu/view
>>
>> c. MLIR google groups discussion:
>>
https://groups.google.com/a/tensorflow.org/forum/#!topic/mlir/4Aj_eawdHiw
>>
>> d. Target constructs  design:
>>
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-September/000285.html
>>
>> e. SIMD constructs design:
>>
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-September/000278.html
>>
>> 3.  [Jan 2020] OpenMP dialect RFC in llvm discourse :
>> https://llvm.discourse.group/t/rfc-openmp-dialect-in-mlir/397
>>
>> 4.  [Jan- Feb 2020] Implementation of OpenMP dialect in MLIR:
>>
>> a. The first patch which introduces the OpenMP dialect was pushed.
>>
>> b. Review of barrier construct is in progress:
>> https://reviews.llvm.org/D72962
>> https://reviews.llvm.org/D72400
>>
>> I have tried to list below different topics of interest (to different
>> people) around this work. Most of these are in the design phase (or
very
>> new) and multiple parties are interested with different sets of goals
in
>> mind.
>>
>> I.  Flang frontend and its integration
>>
>> II. Fortran representation in MLIR / FIR development
>>
>> III. OpenMP development for flang,  OpenMP builder in LLVM.
>>
>> IV. Loop Transformations in MLIR / LLVM with respect to OpenMP.
>>
>> It looks like the design has evolved over time and there is no one
place
>> which contains the latest design decisions that fits all the different
>> pieces of the puzzle. I will try to deduce it from the above mentioned
>> references. Please correct me If I am referring to anything which has
>> changed.
>>
>> A. For most OpenMP design discussions, FIR examples are used (as seen
in
>> (2) and (3)). The MLIR examples mentioned in the design only talks
about
>> FIR dialect and LLVM dialect.
>>
>> This completely ignores the likes of standard, affine (where most loop
>> transformations are supposed to happen) and loop dialects. I think it
is
>> critical to decouple the OpenMP dialect development in MLIR from the
>> current flang / FIR effort. It would be useful if someone can mention
these
>> examples using existing dialects in MLIR and also how the different
>> transformations / lowerings are planned.
>>
>> B. In latest RFC(3), it is mentioned that the initial OpenMP dialect
>> version will be as follows,
>>
>>   omp.parallel {
>>
>>     omp.do {
>>
>>        fir.do %i = 0 to %ub3 : !fir.integer {
>>
>>         ...
>>
>>        }
>>
>>     }
>>
>>   }
>>
>> and then after the "LLVM conversion" it is converted as
follows:
>>
>>   omp.parallel {
>>
>>     %ub3 >>
>>     omp.do %i = 0 to %ub3 : !llvm.integer {
>>
>>     ...
>>
>>     }
>>
>>   }
>>
>>
>> a. Is it the same omp.do operation which now contains the bounds and
>> induction variables of the loop after the LLVM conversion? If so, will
the
>> same operation have two different semantics during a single
compilation?
>>
>> b. Will there be different lowerings for various loop operations from
>> different dialects? loop.for and affine.for under omp operations would
need
>> different OpenMP / LLVM lowerings. Currently, both of them are lowered
>> to the CFG based loops during the LLVM dialect conversion (which is
much
>> before the proposed OpenMP dialect lowering).
>>
>> There would be no standard way to represent OpenMP operations
(especially
>> the ones which involve loops) in MLIR. This would drastically
complicate
>> lowering.
>>
>> C. It is also not mentioned how clauses like firstprivate, shared,
>> private, reduce, map, etc are lowered to OpenMP dialect. The example in
>> the RFC contains FIR and LLVM types and nothing about std dialect
types.
>> Consider the below example:
>>
>> #pragma omp parallel for reduction(+:x)
>>
>> for (int i = 0; i < N; ++i)
>>
>>   x += a[i];
>>
>> How would the above be represented in OpenMP dialect? and What type
would
>> "x" be in MLIR?  It is not mentioned in the design as to how
the various
>> SSA values for various OpenMP clauses are passed around in OpenMP
>> operations.
>>
>> D. Because of (A), (B) and (C), it would be beneficial to have an omp.
>> parallel_do operation which has semantics similar to other loop
>> structures (may not be LoopLikeInterface) in MLIR. To me, it looks like
>> having OpenMP operations based on standard MLIR types and operations
>> (scalars and memrefs mainly) is the right way to go.
>>
>> Why not have omp.parallel_do operation with AffineMap based bounds, so
as
>> to decouple it from Value/Type similar to affine.for?
>>
>> 1. With the current design, the number of transformations /
optimizations
>> that one can write on OpenMP constructs would become limited as there
can
>> be any custom loop structure with custom operations / types inside it.
>>
>> 2. It would also be easier to transform the Loop nests containing
OpenMP
>> constructs if the body of the OpenMP operations is well defined (i.e.,
does
>> not accept arbitrary loop structures). Having nested redundant
"parallel" ,
>> "target" and "do" regions seems unnecessary.
>>
>> 3. There would also be new sets of loop structures in new dialects when
>> C/C++ is compiled to MLIR. It would complicate the number of possible
>> combinations inside the OpenMP region.
>>
>> E. Lowering of target constructs mentioned in ( 2(d) ) specifies direct
>> lowering to LLVM IR ignoring all the advantages that MLIR provides.
Being
>> able to compile the code for heterogeneous hardware is one of the
biggest
>> advantages that MLIR brings to the table. That is being completely
missed
>> here. This also requires solving the problem of handling target
information
>> in MLIR. But that is a problem which needs to be solved anyway. Using
GPU
>> dialect also gives us an opportunity to represent offloading semantics
in
>> MLIR.
>>
>> Given the ability to represent multiple ModuleOps and the existence of
>> GPU dialect, couldn't higher level optimizations on offloaded code
be done
>> at MLIR level?. The proposed design would lead us to the same problems
that
>> we are currently facing in LLVM IR.
>>
>> Also, OpenMP codegen will automatically benefit from the GPU dialect
>> based optimizations. For example, it would be way easier to hoist a
memory
>> reference out of GPU kernel in MLIR than in LLVM IR.
>>
>> Thanks,
>>
>> Vinay
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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Thanks, Vinay for further details and discussion.
> “If we decide that the OpenMP construct (for e.g. collapse) can be handled
fully in MLIR and  that is the best place to do it (based on experiments) then
we will not use the OpenMP IRBuilder for these constructs.” -- latest RFC in
discourse

If it is not finalized that the OpenMPIRBuilder will be used for all the
constructs, wouldn’t it be better to delay the submission of “translation to
LLVM IR” patch in MLIR? Lowering code will become inconsistent if the
OpenMPIRBuilder is used only for a few constructs and not for others.

I was hoping that we can identify a set of constructs/clauses which can be fully
handled inside the MLIR layer itself. As an example, I provided the collapse
clause. This will include constructs/clauses which do not generate runtime API
calls, which pass metadata to LLVM to do some optimisation etc. Yes, the list is
not finalized. The point here is that we do not want to use MLIR just as a
pass-through layer because MLIR has a lot of strengths. Would it be OK to
continue with only constructs that use OpenMPIRBuilder before the final list is
made?


Also, the patch does OpenMP dialect lowering *alongside* LLVM Dialect to LLVM
IR. This is different from most dialects which get directly lowered to LLVM
Dialect. I think lowering to LLVM Dialect would be a cleaner way if
OpenMPIRBuilder is not being considered for all constructs.
Mehdi also seems to have the same suggestion: “I agree that having dialect
lowering would be cleaner” in https://reviews.llvm.org/D72962

The point here is that if we lower to LLVM dialect, we will not be able to reuse
OpenMP codegen & optimisation code from Clang/LLVM.
It was pointed out to me early on by the MLIR developers that there are a few
dialects (like NVVM) which are lowered along with LLVM dialect.


You ask a lot of specific questions about which types, dialects, memory access
operations will be supported and also the lowering for parallel do. Yes, the RFC
does not provide all this information. This will become clear only as we make
progress with the OpenMP dialect. I would like and I am interested to provide
answers to all your questions in the following weeks. Please allow some time.

--Kiran


________________________________
From: Vinay Madhusudan <vinay at compilertree.com>
Sent: 15 February 2020 16:22
To: Kiran Chandramohan <Kiran.Chandramohan at arm.com>
Cc: Vinay Madhusudan via flang-dev <flang-dev at lists.llvm.org>; llvm-dev
at lists.llvm.org <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] [flang-dev] About OpenMP dialect in MLIR


Reply to Kiran Chandramohan:

> You are welcome to participate, provide feedback and criticism to change
the design as well as to contribute to the implementation.

Thank you Kiran.

> But the latest is what is there in the RFC in discourse.

I have used this as reference for the response.

> We did a study of a few constructs and clauses which was shared as mails to
flang-dev and the RFC. As we make progress and before implementation, we will
share further details.
> “ Yes, parallel and flush would be the next two constructs that we will
do.” -- from a comment in latest RFC

For the above mentioned reasons, I will try to restrict my reply to how the
“parallel (do)” construct would be lowered.

> If it is OK we can have further discussions in discourse as River points
out.

Given that the multiple components of the LLVM project, namely clang, flang,
MLIR and LLVM are involved, llvm-dev is probably a better place, with a much
wider audience, until it is clear how different components must interact.

> It is the review for translation to LLVM IR that is in progress.
> “If we decide that the OpenMP construct (for e.g. collapse) can be handled
fully in MLIR and  that is the best place to do it (based on experiments) then
we will not use the OpenMP IRBuilder for these constructs.” -- latest RFC in
discourse

If it is not finalized that the OpenMPIRBuilder will be used for all the
constructs, wouldn’t it be better to delay the submission of “translation to
LLVM IR” patch in MLIR? Lowering code will become inconsistent if the
OpenMPIRBuilder is used only for a few constructs and not for others.


Also, the patch does OpenMP dialect lowering *alongside* LLVM Dialect to LLVM
IR. This is different from most dialects which get directly lowered to LLVM
Dialect. I think lowering to LLVM Dialect would be a cleaner way if
OpenMPIRBuilder is not being considered for all constructs.


Mehdi also seems to have the same suggestion: “I agree that having dialect
lowering would be cleaner” in https://reviews.llvm.org/D72962

> Yes, the design has mildly changed over time to incorporate feedback.  But
the latest is what is there in the RFC in discourse.

RFC fails to discuss the following (I have also mentioned some of them in my
reply to Johannes):

> The proposed plan involves a) lowering F18 AST with OpenMP directly to a
mix of OpenMP and FIR dialects. b) converting this finally to a mix of OpenMP
and LLVM dialects.

It is unclear in the RFC what other dialects are considered as supported for
OpenMP dialect  (std, affine, vector, loop, etc) and how it would be
transformed, used and lowered from FIR to LLVM.


It becomes important to list down the various dialects / operations / types
supported for OpenMP (which is mainly defined for C, C++ and Fortran programs.
MLIR has a much wider scope.


It wouldn’t add much value for the proposed OpenMP dialect to be in the MLIR
tree if it cannot support at least the relevant standard dialect types /
operations.

> We would like to take advantage of the transformations in cases that are
possible. FIR loops will be converted to affine/loop dialect. So the loop inside
an omp.do can be in these dialects as clarified in the discussion in discourse
and also shown in slide 20 of the fosdem presentation (links to both below).
https://llvm.discourse.group/t/rfc-openmp-dialect-in-mlir/397/7?u=kiranchandramohan

https://fosdem.org/2020/schedule/event/llvm_flang/attachments/slides/3839/export/events/attachments/llvm_flang/slides/3839/flang_llvm_frontend.pdf


Although it is mentioned that the affine/ loop.for is used, following things are
unclear:


I am assuming that there will be lowering / conversion code in f18 repo dialect
from fir.do to loop.for / affine.for. Is it the case? If so, I think it is worth
mentioning it in the “sequential code flow representation” in the RFC.


This raises the following questions.


  1.  Which types are supported? Standard dialect types and FIR types?


For example, what types are used for Fortran arrays used inside OpenMP regions?
Is it std.memref OR Fortran array representation in FIR dialect (fir.array?) OR
both?  Note that Fortran has support for column major arrays. std.memref
supports custom memory layouts. What custom layouts are supported?


How would different non-scalar types in standard dialect  be lowered to LLVM IR
and passed to OpenMP runtime calls? Can you please elaborate on this?


The example provided in slide 20 of the fosdem presentation contains


“loop.for %j = %lb2 to %ub2 : !integer {“


But loop.for accepts “index” type. Not sure what type “!integer” represents
here.


  1.  What are the different memory access operations which are supported inside
the OpenMP region and lowered to proper OpenMP runtime calls in LLVM IR?


The possibilities are:

  1.  affine.load / affine.store

  2.  std.load / std.store

  3.  FIR dialect memory access operations.

> I must also point out that the question of where to do loop transformations
is a topic we have not fully converged on. See the following thread for
discussions.
http://lists.llvm.org/pipermail/flang-dev/2019-September/000042.html

Looks like placement (MLIR / LLVM) of various transformations related to OpenMP
has not been finalized, from what I could infer from Johannes’s reply and the
below text in the latest RFC in discourse:


“So there exist some questions regarding where the optimisations should be
carried out. We will decide on which framework to choose only after some
experimentation.”

> i) we need to keep the loops separately so as to take advantage of
transformations that other dialects like affine/loop would provide.

1) Keeping the loops separate from the OpenMP operations will expose them to the
“regular” transformations passes in MLIR inside the OpenMP region. Most of them
are invalid or in-efficient for OpenMP operations.

Examples:

  1.  Constant propagation example mentioned by Johannes in this thread. (omp
task shared(x))

  2.  Loop (nest) transformations (permute / split / fuse / tile, etc) will
happen ignoring the surrounding OpenMP operations.

  3.  Hoisting and sinking of various memory/ SSA values inside the OpenMP
region. This goes against the likes of “map”, “firstprivate”, shared, etc
clauses and more.


2) Various loop operations (loop.for, affine.for, fir.do) have (or will have)
different transformations/ optimization passes which are different from one
another.

Example:

  1.  AffineLoopInvariantCodeMotion.cpp is different from
LoopInvariantCodeMotion.cpp.

  2.  Other Loop transformation passes for affine.for


These loops also use different Types and memory access operations in general for
transformations. Example, most Affine dialect transformations (if not all) work
on affine.load and affine.store operations.


Supporting different loop operations means that there would be *OpenMP specific
transformations* for each one of them and also requires a way to restrict each
of them from existing transformations (when nested in OpenMP constructs).


There would be different lowerings for different loop operations as well.
Example, affine.for and loop.for would have to be lowered to omp.do in different
ways.

>From slide 20 of fosdem presentation you mentioned, the LLVM + OpenMP
dialect representation is as follows:

------------------------------

Mlir.region(…) {

   omp.parallel  {

     %ub3 = …

     omp.do %i = 0 to %ub3 : !integer  {

     …

     }

  }

}

-------------------------------


Currently, the LLVM Dialect doesn’t contain a high level loop operation. It is
all based on CFG implementation.


Will omp.do follow the same structure (SizedRegion<1>) as loop.for? OR
there would be CFG for LLVM Dialect based loop operation?


Can you please mention how the OpenMP + LLVM dialect will look like for the
below parallel do construct?


integer :: i=1, k=10

integer :: a(10), b(10), c(10)

...

 !$omp parallel do

  do i = 1, k

    if (i .ne. 1) *cycle*

    c(i) = a(i) + b(i)

  end do

  !$omp end parallel do


print *,c


Thanks,

Vinay

On Fri, Feb 14, 2020 at 6:52 AM Kiran Chandramohan via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Hello Vinay,

Thanks for your mail about the OpenMP dialect in MLIR. Happy to know that you
and several other groups are interested in the OpenMP dialect. At the outset, I
must point out that the design is not set in stone and will change as we make
progress. You are welcome to participate, provide feedback and criticism to
change the design as well as to contribute to the implementation. I provide some
clarifications and replies to your comments below. If it is OK we can have
further discussions in discourse as River points out.
1. [May 2019] An OpenMPIRBuilder in LLVM was proposed for flang and clang
frontends. Note that this proposal was before considering MLIR for FIR.
A correction here. The proposal for OpenMPIRBuilder was made when MLIR was being
considered for FIR.
(i) Gary Klimowicz's minutes for Flang call in April 2019 mentions
considering MLIR for FIR.
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-April/000194.html
(ii) My reply to Johaness's proposal in May 2019 mentions MLIR for FIR.
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-May/000220.html


b. Review of barrier construct is in progress: https://reviews.llvm.org/D72962

Minor correction here. The addition of barrier construct was accepted and has
landed (https://reviews.llvm.org/D7240<https://reviews.llvm.org/D72400>).
It is the review for translation to LLVM IR that is in progress.

It looks like the design has evolved over time and there is no one place which
contains the latest design decisions that fits all the different pieces of the
puzzle. I will try to deduce it from the above mentioned references. Please
correct me If I am referring to anything which has changed.
Yes, the design has mildly changed over time to incorporate feedback. But the
latest is what is there in the RFC in discourse.

For most OpenMP design discussions, FIR examples are used (as seen in (2) and
(3)). The MLIR examples mentioned in the design only talks about FIR dialect and
LLVM dialect.
Our initial concern was how will all these pieces (FIR, LLVM Dialect,
OpenMPIRBuilder, LLVM IR) fit together. Hence you see the prominence of FIR and
LLVM dialect and more information about lowering/translation than
transformations/optimisations.

This completely ignores the likes of standard, affine (where most loop
transformations are supposed to happen) and loop dialects.
Adding to the reply above. We would like to take advantage of the
transformations in cases that are possible. FIR loops will be converted to
affine/loop dialect. So the loop inside an omp.do can be in these dialects as
clarified in the discussion in discourse and also shown in slide 20 of the
fosdem presentation (links to both below).
https://llvm.discourse.group/t/rfc-openmp-dialect-in-mlir/397/7?u=kiranchandramohan
https://fosdem.org/2020/schedule/event/llvm_flang/attachments/slides/3839/export/events/attachments/llvm_flang/slides/3839/flang_llvm_frontend.pdf

I must also point out that the question of where to do loop transformations is a
topic we have not fully converged on. See the following thread for discussions.
http://lists.llvm.org/pipermail/flang-dev/2019-September/000042.html

Is it the same omp.do operation which now contains the bounds and induction
variables of the loop after the LLVM conversion?
The point here is that i) we need to keep the loops separately so as to take
advantage of transformations that other dialects like affine/loop would provide.
ii) We will need the loop information while lowering the OpenMP do operation.
For implementation, if reusing the same operation (in different contexts) is
difficult then we can add a new operation.
It is also not mentioned how clauses like firstprivate, shared, private, reduce,
map, etc are lowered to OpenMP dialect.
Yes, it is not mentioned. We did a study of a few constructs and clauses which
was shared as mails to flang-dev and the RFC. As we make progress and before
implementation, we will share further details.

it would be beneficial to have an omp.parallel_do operation which has semantics
similar to other loop structures (may not be LoopLikeInterface) in MLIR.
I am not against adding parallel_do if it can help with transformations or
reduce the complexity of lowering. Please share the details in discourse as a
reply to the RFC or a separate thread.
it looks like having OpenMP operations based on standard MLIR types and
operations (scalars and memrefs mainly) is the right way to go.
This will definitely be the first version that we implement. But I do not
understand why we should restrict to only the standard types and operations. To
ease lowering and translation and to avoid adding OpenMP operations to other
dialects, I believe OpenMP dialect should also be able to exist with other
dialects like FIR and LLVM.
E. Lowering of target constructs mentioned in ( 2(d) ) specifies direct lowering
to LLVM IR ignoring all the advantages that MLIR provides.
Also, OpenMP codegen will automatically benefit from the GPU dialect based
optimizations. For example, it would be way easier to hoist a memory reference
out of GPU kernel in MLIR than in LLVM IR.
I might not have fully understood you here. But the dialect lives independently
of the translation to LLVM IR. If there are optimisations (like hoisting that
you mention here) I believe they can be performed as transformation passes on
the dialect. It is not ruled out.

--Kiran
________________________________
From: flang-dev <flang-dev-bounces at
lists.llvm.org<mailto:flang-dev-bounces at lists.llvm.org>> on behalf
of Vinay Madhusudan via flang-dev <flang-dev at
lists.llvm.org<mailto:flang-dev at lists.llvm.org>>
Sent: 13 February 2020 16:33
To: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
<llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>;
flang-dev at lists.llvm.org<mailto:flang-dev at lists.llvm.org>
<flang-dev at lists.llvm.org<mailto:flang-dev at lists.llvm.org>>
Subject: [flang-dev] About OpenMP dialect in MLIR


Hi,


I have few questions / concerns regarding the design of OpenMP dialect in MLIR
that is currently being implemented, mainly for the f18 compiler. Below, I
summarize the current state of various efforts in clang / f18 / MLIR / LLVM
regarding this. Feel free to add to the list in case I have missed something.


1. [May 2019] An OpenMPIRBuilder in LLVM was proposed for flang and clang
frontends. Note that this proposal was before considering MLIR for FIR.

a. llvm-dev proposal :
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-May/000197.html

b. Patches in review: https://reviews.llvm.org/D70290. This also includes the
clang codegen changes.


2.  [July - September 2019] OpenMP dialect for MLIR was discussed / proposed
with respect to the f18 compilation stack (keeping FIR in mind).

a. flang-dev discussion link:
https://lists.llvm.org/pipermail/flang-dev/2019-September/000020.html

b. Design decisions captured in PPT:
https://drive.google.com/file/d/1vU6LsblsUYGA35B_3y9PmBvtKOTXj1Fu/view

c. MLIR google groups discussion:
https://groups.google.com/a/tensorflow.org/forum/#!topic/mlir/4Aj_eawdHiw

d. Target constructs  design:
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-September/000285.html

e. SIMD constructs design:
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-September/000278.html


3.  [Jan 2020] OpenMP dialect RFC in llvm discourse :
https://llvm.discourse.group/t/rfc-openmp-dialect-in-mlir/397


4.  [Jan- Feb 2020] Implementation of OpenMP dialect in MLIR:

a. The first patch which introduces the OpenMP dialect was pushed.

b. Review of barrier construct is in progress: https://reviews.llvm.org/D72962

https://reviews.llvm.org/D72400

I have tried to list below different topics of interest (to different people)
around this work. Most of these are in the design phase (or very new) and
multiple parties are interested with different sets of goals in mind.

I.  Flang frontend and its integration

II. Fortran representation in MLIR / FIR development

III. OpenMP development for flang,  OpenMP builder in LLVM.

IV. Loop Transformations in MLIR / LLVM with respect to OpenMP.


It looks like the design has evolved over time and there is no one place which
contains the latest design decisions that fits all the different pieces of the
puzzle. I will try to deduce it from the above mentioned references. Please
correct me If I am referring to anything which has changed.


A. For most OpenMP design discussions, FIR examples are used (as seen in (2) and
(3)). The MLIR examples mentioned in the design only talks about FIR dialect and
LLVM dialect.


This completely ignores the likes of standard, affine (where most loop
transformations are supposed to happen) and loop dialects. I think it is
critical to decouple the OpenMP dialect development in MLIR from the current
flang / FIR effort. It would be useful if someone can mention these examples
using existing dialects in MLIR and also how the different transformations /
lowerings are planned.


B. In latest RFC(3), it is mentioned that the initial OpenMP dialect version
will be as follows,


  omp.parallel {

    omp.do {

       fir.do %i = 0 to %ub3 : !fir.integer {

        ...

       }

    }

  }


and then after the "LLVM conversion" it is converted as follows:


  omp.parallel {

    %ub3 
    omp.do %i = 0 to %ub3 : !llvm.integer {

    ...

    }

  }



a. Is it the same omp.do operation which now contains the bounds and induction
variables of the loop after the LLVM conversion? If so, will the same operation
have two different semantics during a single compilation?


b. Will there be different lowerings for various loop operations from different
dialects? loop.for and affine.for under omp operations would need different
OpenMP / LLVM lowerings. Currently, both of them are lowered to the CFG based
loops during the LLVM dialect conversion (which is much before the proposed
OpenMP dialect lowering).


There would be no standard way to represent OpenMP operations (especially the
ones which involve loops) in MLIR. This would drastically complicate lowering.


C. It is also not mentioned how clauses like firstprivate, shared, private,
reduce, map, etc are lowered to OpenMP dialect. The example in the RFC contains
FIR and LLVM types and nothing about std dialect types. Consider the below
example:


#pragma omp parallel for reduction(+:x)

for (int i = 0; i < N; ++i)

  x += a[i];


How would the above be represented in OpenMP dialect? and What type would
"x" be in MLIR?  It is not mentioned in the design as to how the
various SSA values for various OpenMP clauses are passed around in OpenMP
operations.


D. Because of (A), (B) and (C), it would be beneficial to have an
omp.parallel_do operation which has semantics similar to other loop structures
(may not be LoopLikeInterface) in MLIR. To me, it looks like having OpenMP
operations based on standard MLIR types and operations (scalars and memrefs
mainly) is the right way to go.


Why not have omp.parallel_do operation with AffineMap based bounds, so as to
decouple it from Value/Type similar to affine.for?


1. With the current design, the number of transformations / optimizations that
one can write on OpenMP constructs would become limited as there can be any
custom loop structure with custom operations / types inside it.


2. It would also be easier to transform the Loop nests containing OpenMP
constructs if the body of the OpenMP operations is well defined (i.e., does not
accept arbitrary loop structures). Having nested redundant "parallel"
, "target" and "do" regions seems unnecessary.


3. There would also be new sets of loop structures in new dialects when C/C++ is
compiled to MLIR. It would complicate the number of possible combinations inside
the OpenMP region.


E. Lowering of target constructs mentioned in ( 2(d) ) specifies direct lowering
to LLVM IR ignoring all the advantages that MLIR provides. Being able to compile
the code for heterogeneous hardware is one of the biggest advantages that MLIR
brings to the table. That is being completely missed here. This also requires
solving the problem of handling target information in MLIR. But that is a
problem which needs to be solved anyway. Using GPU dialect also gives us an
opportunity to represent offloading semantics in MLIR.


Given the ability to represent multiple ModuleOps and the existence of GPU
dialect, couldn't higher level optimizations on offloaded code be done at
MLIR level?. The proposed design would lead us to the same problems that we are
currently facing in LLVM IR.


Also, OpenMP codegen will automatically benefit from the GPU dialect based
optimizations. For example, it would be way easier to hoist a memory reference
out of GPU kernel in MLIR than in LLVM IR.


Thanks,

Vinay

_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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On 02/15, Mehdi AMINI via llvm-dev wrote:
> > Mehdi also seems to have the same suggestion: “I agree that having
> > dialect lowering would be cleaner” in https://reviews.llvm.org/D72962
> >
> 
> Since you're calling me out: yes it would be cleaner from a pure MLIR
point
> of view, I don't think there is much disagreement on this (I think?).
> However we already have the OpenMP builders available and they will
> continue to be maintained/evolved to support OpenMP in clang.
> Duplicating them entirely in MLIR for the sake of purity seems like a lack
> of pragmatism here, so I support the current approach with the current
> tradeoffs.
What benefit would we hope to achieve by representing runtime API calls
in the LLVM dialect? I mean, as far as I understand, the central idea of
MLIR is to retain and exploit "high-level" information. Translating to
LLVM dialect for the sake of it seems in any context like a suboptimal
choice.

> > > Yes, the design has mildly changed over time to incorporate
feedback.
> > But the latest is what is there in the RFC in discourse.
> >
> > RFC fails to discuss the following (I have also mentioned some of them
in
> > my reply to Johannes):
> >
> > > The proposed plan involves a) lowering F18 AST with OpenMP
directly to a
> > mix of OpenMP and FIR dialects. b) converting this finally to a mix of
> > OpenMP and LLVM dialects.
> >
> > It is unclear in the RFC what other dialects are considered as
supported
> > for OpenMP dialect  (std, affine, vector, loop, etc) and how it would
be
> > transformed, used and lowered from FIR to LLVM.
> >
> > It becomes important to list down the various dialects / operations /
> > types supported for OpenMP (which is mainly defined for C, C++ and
Fortran
> > programs. MLIR has a much wider scope.
> >
> > It wouldn’t add much value for the proposed OpenMP dialect to be in
the
> > MLIR tree if it cannot support at least the relevant standard dialect
types
> > / operations.
> >
> 
> I agree, and I think this was something I called out as important in the
> RFC: "It seems that the dialect can be orthogonal to FIR and its type
> system, which the most important thing to me to integrate MLIR (favor
> reusability across other frontends / compiler frameworks)".
> If you don't think that this is the case, then please raise this in the
RFC!
> I think it is perfectly fair to ask for more examples from the author and
> digging a bit deeper if you're unconvinced that the proposed modeling
can
> be applicable outside of FIR. This is exactly why we ask such proposal to
> go through RFC by the way: to allow people like you to point at the
> blindspot and ask the right questions.
I was told that there is no technical downside of having FIR in
`/flang/`. If that is the case, why doesn't the OpenMP dialect live
there as well? There is also `/openmp/`, which arguably would make
sense, but that would introduce a dependence we yet not have.

Long story short, does it make much of a difference? It seems we are in
agreement the dialect will live in `/llvm-project/`, which subproject
seems less important (to me).

> > > We would like to take advantage of the transformations in cases
that are
> > possible. FIR loops will be converted to affine/loop dialect. So the
loop
> > inside an omp.do can be in these dialects as clarified in the
discussion in
> > discourse and also shown in slide 20 of the fosdem presentation (links
to
> > both below).
> >
> >
> >
https://llvm.discourse.group/t/rfc-openmp-dialect-in-mlir/397/7?u=kiranchandramohan
> >
> >
> >
https://fosdem.org/2020/schedule/event/llvm_flang/attachments/slides/3839/export/events/attachments/llvm_flang/slides/3839/flang_llvm_frontend.pdf
> >
> > Although it is mentioned that the affine/ loop.for is used, following
> > things are unclear:
> >
> > I am assuming that there will be lowering / conversion code in f18
repo
> > dialect from fir.do to loop.for / affine.for. Is it the case? If so, I
> > think it is worth mentioning it in the “sequential code flow
> > representation” in the RFC.
Off topic:
Do we have a counter for the different number of "loop-like"
constructs
in and around MLIR? I think plotting it over time will eventually make a
great XKCD comic ;)
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Please find the reply inline below

On Sun, Feb 16, 2020 at 12:59 AM Mehdi AMINI <joker.eph at gmail.com>
wrote:
>
>
> On Sat, Feb 15, 2020 at 10:42 AM Vinay Madhusudan via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Reply to Kiran Chandramohan:
>>
>> > You are welcome to participate, provide feedback and criticism to
>> change the design as well as to contribute to the implementation.
>>
>> Thank you Kiran.
>>
>> > But the latest is what is there in the RFC in discourse.
>>
>> I have used this as reference for the response.
>>
>> > We did a study of a few constructs and clauses which was shared as
>> mails to flang-dev and the RFC. As we make progress and before
>> implementation, we will share further details.
>>
>> > “ Yes, parallel and flush would be the next two constructs that we
>> will do.” -- from a comment in latest RFC
>>
>> For the above mentioned reasons, I will try to restrict my reply to how
>> the “parallel (do)” construct would be lowered.
>>
>> > If it is OK we can have further discussions in discourse as River
>> points out.
>>
>> Given that the multiple components of the LLVM project, namely clang,
>> flang, MLIR and LLVM are involved, llvm-dev is probably a better place,
>> with a much wider audience
>>
>
> Possibly wider, but maybe less focused about discussing MLIR dialect
> design. In particular there is an RFC thread for this particular dialect on
> Discourse, which is the canonical place to discuss its design.
>
>
>> , until it is clear how different components must interact.
>>
>
> They don't need to interact so closely: they are very loosely related:
> flang will use MLIR but clang won't (in the foreseeable future) and
LLVM
> has many other frontends.
>
>
>>
>> > It is the review for translation to LLVM IR that is in progress.
>>
>> > “If we decide that the OpenMP construct (for e.g. collapse) can be
>> handled fully in MLIR and  that is the best place to do it (based on
>> experiments) then we will not use the OpenMP IRBuilder for these
constructs.”
>> -- latest RFC in discourse
>>
>> If it is not finalized that the OpenMPIRBuilder will be used for all
the
>> constructs, wouldn’t it be better to delay the submission of
“translation
>> to LLVM IR” patch in MLIR? Lowering code will become inconsistent if
the
>> OpenMPIRBuilder is used only for a few constructs and not for others.
>>
>
>> Also, the patch does OpenMP dialect lowering *alongside* LLVM Dialect
to
>> LLVM IR. This is different from most dialects which get directly
lowered to
>> LLVM Dialect. I think lowering to LLVM Dialect would be a cleaner way
if
>> OpenMPIRBuilder is not being considered for all constructs.
>>
>
>
> I don't disagree, but there are a lot of speculation here: your quote
> starts with "If we decide that the OpenMP construct (for e.g.
collapse) can
> be handled fully in MLIR", are you thinking that we need to first
decide
> this once and for all before making progress on building this path?
> What disadvantages do you perceive to an approach where we would bring up
> this dialect using the OpenMPIRBuilders for exporting to LLVM IR until we
> gain enough experience? Do you think starting like this will make it
> significantly harder to transition away from the builders if this is what
> we want?
> It seemed to me like it wouldn't, and that's why I'm supportive
of this
> path: the omp dialect design, implementation, and the
> transformation/analysis that will be performed there seems entirely
> disjoint from the LLVM lowering, I'd hope we can swap the LLVM lowering
at
> a later time (if this is what we'd want).
>
>
>>
>>
The statement you quoted is from the RFC in discourse by Kiran. It is
actually unclear to whom you are referring to here. I am assuming that it
is for him to answer.

The below details would cover some of your questions as well.

About Clang / MLIR / LLVM being loosely related and not being relevant in
llvm-dev:

With the introduction of the OpenMPIRBuilder in MLIR (from this review :
https://reviews.llvm.org/D72962), Clang and MLIR would now have the common
code for building OpenMP constructs. I do not think it is so loosely
related anymore. Note that MLIR and Clang frontends for LLVM are very
different. Clang emits LLVM IR with almost no optimizations and MLIR
already supports considerable amount of optimizations.

Decision of using the OpenMPIRBuilder for MLIR was discussed in the
following flang-dev threads (Please correct me If I am missing some newer
discussions on the below topics)


   1.

   [May 2019] h
  
ttp://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-May/000197.html
  
<http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-May/000197.html>
   2.

   [June 2019]
  
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-June/000251.html


However I could not find any conclusions for the concerns raised by Kiran:


   1.

   Early outlining  (in MLIR) vs. Late outlining (in LLVM)
   2.

   Handling of target constructions: high-level transformations for GPUs
   and  CPUs (offloading in LLVM vs. MLIR?)


Kiran seems to suggest the early outlining (version 2) would be better(
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-May/000224.html
). But currently, the late outlining has been implemented in LLVM (version
1) (
https://github.com/llvm/llvm-project/blob/master/llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp
).

Early outlining in MLIR would have the following benefits as suggested in
the thread:



   1.

   Enables more optimization in MLIR (intra-procedural because of regions).
   2.

   Offloading in MLIR (which is designed for heterogenous hardware
   compilation support)
   3.

   Direct LLVM Dialect lowering of OpenMP operations (no LLVM IR lowering)


MLIR google groups discussion (*h*
ttps://groups.google.com/a/tensorflow.org/forum/#!topic/mlir/4Aj_eawdHiw
<https://groups.google.com/a/tensorflow.org/forum/#!topic/mlir/4Aj_eawdHiw>)
regarding the use of OpenMPIRBuilder doesn’t seem to discuss the above
concerns and also about how the various design decisions in OpenMPIRBuilder
affects MLIR in general.

Also,

 > “The point here is that we do not want to use MLIR just as a
pass-through layer because MLIR has a lot of strengths”

  ....

  > “The point here is that if we lower to LLVM dialect, we will not be
able to reuse OpenMP codegen & optimisation code from Clang/LLVM.”

     --- by Kiran in
https://lists.llvm.org/pipermail/llvm-dev/2020-February/139181.html

In the latest reply from Kiran (quoted above) to this thread, Kiran seems
to suggest that lowering to LLVM Dialect (instead of LLVM IR) would
restrict the use of OpenMP Optimization code from LLVM and also MLIR will
just be a pass-through to the OpenMPIRBuilder.

Because of the above reasons, it seems to me that design considerations of
using OpenMPIRBuilder for MLIR should also be mentioned (and discussed)
before commiting LLVM IR lowering part for OpenMP dialect in
https://reviews.llvm.org/D72962


> Mehdi also seems to have the same suggestion: “I agree that having
>> dialect lowering would be cleaner” in https://reviews.llvm.org/D72962
>>
>
> Since you're calling me out: yes it would be cleaner from a pure MLIR
> point of view, I don't think there is much disagreement on this (I
think?).
> However we already have the OpenMP builders available and they will
> continue to be maintained/evolved to support OpenMP in clang.
> Duplicating them entirely in MLIR for the sake of purity seems like a lack
> of pragmatism here, so I support the current approach with the current
> tradeoffs.
>
>
>>
>> > Yes, the design has mildly changed over time to incorporate
feedback.
>> But the latest is what is there in the RFC in discourse.
>>
>> RFC fails to discuss the following (I have also mentioned some of them
in
>> my reply to Johannes):
>>
>> > The proposed plan involves a) lowering F18 AST with OpenMP
directly to
>> a mix of OpenMP and FIR dialects. b) converting this finally to a mix
of
>> OpenMP and LLVM dialects.
>>
>> It is unclear in the RFC what other dialects are considered as
supported
>> for OpenMP dialect  (std, affine, vector, loop, etc) and how it would
be
>> transformed, used and lowered from FIR to LLVM.
>>
>> It becomes important to list down the various dialects / operations /
>> types supported for OpenMP (which is mainly defined for C, C++ and
Fortran
>> programs. MLIR has a much wider scope.
>>
>> It wouldn’t add much value for the proposed OpenMP dialect to be in the
>> MLIR tree if it cannot support at least the relevant standard dialect
types
>> / operations.
>>
>
> I agree, and I think this was something I called out as important in the
> RFC: "It seems that the dialect can be orthogonal to FIR and its type
> system, which the most important thing to me to integrate MLIR (favor
> reusability across other frontends / compiler frameworks)".
> If you don't think that this is the case, then please raise this in the
> RFC!
> I think it is perfectly fair to ask for more examples from the author and
> digging a bit deeper if you're unconvinced that the proposed modeling
can
> be applicable outside of FIR. This is exactly why we ask such proposal to
> go through RFC by the way: to allow people like you to point at the
> blindspot and ask the right questions.
>
> Best,
>
> --
> Mehdi
>
>
>
>> > We would like to take advantage of the transformations in cases
that
>> are possible. FIR loops will be converted to affine/loop dialect. So
the
>> loop inside an omp.do can be in these dialects as clarified in the
>> discussion in discourse and also shown in slide 20 of the fosdem
>> presentation (links to both below).
>>
>>
>>
https://llvm.discourse.group/t/rfc-openmp-dialect-in-mlir/397/7?u=kiranchandramohan
>>
>>
>>
https://fosdem.org/2020/schedule/event/llvm_flang/attachments/slides/3839/export/events/attachments/llvm_flang/slides/3839/flang_llvm_frontend.pdf
>>
>> Although it is mentioned that the affine/ loop.for is used, following
>> things are unclear:
>>
>> I am assuming that there will be lowering / conversion code in f18 repo
>> dialect from fir.do to loop.for / affine.for. Is it the case? If so, I
>> think it is worth mentioning it in the “sequential code flow
>> representation” in the RFC.
>>
>> This raises the following questions.
>>
>>
>>
>>    1.
>>
>>    Which types are supported? Standard dialect types and FIR types?
>>
>>
>> For example, what types are used for Fortran arrays used inside OpenMP
>> regions? Is it std.memref OR Fortran array representation in FIR
dialect
>> (fir.array?) OR both?  Note that Fortran has support for column major
>> arrays. std.memref supports custom memory layouts. What custom layouts
are
>> supported?
>>
>>
>> How would different non-scalar types in standard dialect  be lowered to
>> LLVM IR and passed to OpenMP runtime calls? Can you please elaborate on
>> this?
>>
>> The example provided in slide 20 of the fosdem presentation contains
>>
>> “loop.for %j = %lb2 to %ub2 : !integer {“
>>
>> But loop.for accepts “index” type. Not sure what type “!integer”
>> represents here.
>>
>>
>>    1.
>>
>>    What are the different memory access operations which are supported
>>    inside the OpenMP region and lowered to proper OpenMP runtime calls
in LLVM
>>    IR?
>>
>>
>> The possibilities are:
>>
>>    1.
>>
>>    affine.load / affine.store
>>    2.
>>
>>    std.load / std.store
>>    3.
>>
>>    FIR dialect memory access operations.
>>
>>
>> > I must also point out that the question of where to do loop
>> transformations is a topic we have not fully converged on. See the
>> following thread for discussions.
>> http://lists.llvm.org/pipermail/flang-dev/2019-September/000042.html
>>
>> Looks like placement (MLIR / LLVM) of various transformations related
to
>> OpenMP has not been finalized, from what I could infer from Johannes’s
>> reply and the below text in the latest RFC in discourse:
>>
>> “So there exist some questions regarding where the optimisations should
>> be carried out. We will decide on which framework to choose only after
some
>> experimentation.”
>>
>> > i) we need to keep the loops separately so as to take advantage of
>> transformations that other dialects like affine/loop would provide.
>>
>> 1) Keeping the loops separate from the OpenMP operations will expose
them
>> to the “regular” transformations passes in MLIR inside the OpenMP
region.
>> Most of them are invalid or in-efficient for OpenMP operations.
>>
>> Examples:
>>
>>    1.
>>
>>    Constant propagation example mentioned by Johannes in this thread.
>>    (omp task shared(x))
>>    2.
>>
>>    Loop (nest) transformations (permute / split / fuse / tile, etc)
will
>>    happen ignoring the surrounding OpenMP operations.
>>    3.
>>
>>    Hoisting and sinking of various memory/ SSA values inside the OpenMP
>>    region. This goes against the likes of “map”, “firstprivate”,
shared, etc
>>    clauses and more.
>>
>>
>> 2) Various loop operations (loop.for, affine.for, fir.do) have (or will
>> have) different transformations/ optimization passes which are
different
>> from one another.
>>
>> Example:
>>
>>    1.
>>
>>    AffineLoopInvariantCodeMotion.cpp is different from
>>    LoopInvariantCodeMotion.cpp.
>>    2.
>>
>>    Other Loop transformation passes for affine.for
>>
>>
>> These loops also use different Types and memory access operations in
>> general for transformations. Example, most Affine dialect
transformations
>> (if not all) work on affine.load and affine.store operations.
>>
>> Supporting different loop operations means that there would be *OpenMP
>> specific transformations* for each one of them and also requires a way
to
>> restrict each of them from existing transformations (when nested in
OpenMP
>> constructs).
>>
>> There would be different lowerings for different loop operations as
well.
>> Example, affine.for and loop.for would have to be lowered to omp.do in
>> different ways.
>>
>> From slide 20 of fosdem presentation you mentioned, the LLVM + OpenMP
>> dialect representation is as follows:
>>
>> ------------------------------
>>
>> Mlir.region(…) {
>>
>>    omp.parallel  {
>>
>>      %ub3 = …
>>
>>      omp.do %i = 0 to %ub3 : !integer  {
>>
>>      …
>>
>>      }
>>
>>   }
>>
>> }
>>
>> -------------------------------
>>
>> Currently, the LLVM Dialect doesn’t contain a high level loop
operation.
>> It is all based on CFG implementation.
>>
>> Will omp.do follow the same structure (SizedRegion<1>) as
loop.for? OR
>> there would be CFG for LLVM Dialect based loop operation?
>>
>> Can you please mention how the OpenMP + LLVM dialect will look like for
>> the below parallel do construct?
>>
>> integer :: i=1, k=10
>>
>> integer :: a(10), b(10), c(10)
>>
>> ...
>>
>>  !$omp parallel do
>>
>>   do i = 1, k
>>
>>     if (i .ne. 1) *cycle*
>>
>>     c(i) = a(i) + b(i)
>>
>>   end do
>>
>>   !$omp end parallel do
>>
>> print *,c
>>
>
>> Thanks,
>>
>> Vinay
>>
>> On Fri, Feb 14, 2020 at 6:52 AM Kiran Chandramohan via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> Hello Vinay,
>>>
>>> Thanks for your mail about the OpenMP dialect in MLIR. Happy to
know
>>> that you and several other groups are interested in the OpenMP
dialect. At
>>> the outset, I must point out that the design is not set in stone
and will
>>> change as we make progress. You are welcome to participate, provide
>>> feedback and criticism to change the design as well as to
contribute to the
>>> implementation. I provide some clarifications and replies to your
comments
>>> below. If it is OK we can have further discussions in discourse as
River
>>> points out.
>>>
>>> 1. [May 2019] An OpenMPIRBuilder in LLVM was proposed for flang and
>>> clang frontends. Note that this proposal was before considering
MLIR for
>>> FIR.
>>>
>>> A correction here. The proposal for OpenMPIRBuilder was made when
MLIR
>>> was being considered for FIR.
>>> (i) Gary Klimowicz's minutes for Flang call in April 2019
mentions
>>> considering MLIR for FIR.
>>>
>>>
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-April/000194.html
>>> (ii) My reply to Johaness's proposal in May 2019 mentions MLIR
for FIR.
>>>
>>>
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-May/000220.html
>>>
>>> b. Review of barrier construct is in progress:
>>> https://reviews.llvm.org/D72962
>>>
>>> Minor correction here. The addition of barrier construct was
accepted
>>> and has landed (https://reviews.llvm.org/D7240
>>> <https://reviews.llvm.org/D72400>). It is the review for
translation to
>>> LLVM IR that is in progress.
>>>
>>> It looks like the design has evolved over time and there is no one
place
>>> which contains the latest design decisions that fits all the
different
>>> pieces of the puzzle. I will try to deduce it from the above
mentioned
>>> references. Please correct me If I am referring to anything which
has
>>> changed.
>>>
>>> Yes, the design has mildly changed over time to incorporate
feedback.
>>> But the latest is what is there in the RFC in discourse.
>>>
>>> For most OpenMP design discussions, FIR examples are used (as seen
in
>>> (2) and (3)). The MLIR examples mentioned in the design only talks
about
>>> FIR dialect and LLVM dialect.
>>>
>>> Our initial concern was how will all these pieces (FIR, LLVM
Dialect,
>>> OpenMPIRBuilder, LLVM IR) fit together. Hence you see the
prominence of FIR
>>> and LLVM dialect and more information about lowering/translation
than
>>> transformations/optimisations.
>>>
>>> This completely ignores the likes of standard, affine (where most
loop
>>> transformations are supposed to happen) and loop dialects.
>>>
>>> Adding to the reply above. We would like to take advantage of the
>>> transformations in cases that are possible. FIR loops will be
converted to
>>> affine/loop dialect. So the loop inside an omp.do can be in these
dialects
>>> as clarified in the discussion in discourse and also shown in slide
20 of
>>> the fosdem presentation (links to both below).
>>>
>>>
https://llvm.discourse.group/t/rfc-openmp-dialect-in-mlir/397/7?u=kiranchandramohan
>>>
>>>
https://fosdem.org/2020/schedule/event/llvm_flang/attachments/slides/3839/export/events/attachments/llvm_flang/slides/3839/flang_llvm_frontend.pdf
>>>
>>> I must also point out that the question of where to do loop
>>> transformations is a topic we have not fully converged on. See the
>>> following thread for discussions.
>>>
http://lists.llvm.org/pipermail/flang-dev/2019-September/000042.html
>>>
>>> Is it the same omp.do operation which now contains the bounds and
>>> induction variables of the loop after the LLVM conversion?
>>>
>>> The point here is that i) we need to keep the loops separately so
as to
>>> take advantage of transformations that other dialects like
affine/loop
>>> would provide. ii) We will need the loop information while lowering
the
>>> OpenMP do operation. For implementation, if reusing the same
operation (in
>>> different contexts) is difficult then we can add a new operation.
>>>
>>> It is also not mentioned how clauses like firstprivate, shared,
>>> private, reduce, map, etc are lowered to OpenMP dialect.
>>>
>>> Yes, it is not mentioned. We did a study of a few constructs and
clauses
>>> which was shared as mails to flang-dev and the RFC. As we make
progress and
>>> before implementation, we will share further details.
>>>
>>> it would be beneficial to have an omp.parallel_do operation which
has
>>> semantics similar to other loop structures (may not be
LoopLikeInterface)
>>> in MLIR.
>>>
>>> I am not against adding parallel_do if it can help with
transformations
>>> or reduce the complexity of lowering. Please share the details in
discourse
>>> as a reply to the RFC or a separate thread.
>>>
>>> it looks like having OpenMP operations based on standard MLIR types
and
>>> operations (scalars and memrefs mainly) is the right way to go.
>>>
>>> This will definitely be the first version that we implement. But I
do
>>> not understand why we should restrict to only the standard types
and
>>> operations. To ease lowering and translation and to avoid adding
OpenMP
>>> operations to other dialects, I believe OpenMP dialect should also
be able
>>> to exist with other dialects like FIR and LLVM.
>>>
>>> E. Lowering of target constructs mentioned in ( 2(d) ) specifies
direct
>>> lowering to LLVM IR ignoring all the advantages that MLIR provides.
>>>
>>> Also, OpenMP codegen will automatically benefit from the GPU
dialect
>>> based optimizations. For example, it would be way easier to hoist a
memory
>>> reference out of GPU kernel in MLIR than in LLVM IR.
>>>
>>> I might not have fully understood you here. But the dialect lives
>>> independently of the translation to LLVM IR. If there are
optimisations
>>> (like hoisting that you mention here) I believe they can be
performed as
>>> transformation passes on the dialect. It is not ruled out.
>>>
>>> --Kiran
>>> ------------------------------
>>> *From:* flang-dev <flang-dev-bounces at lists.llvm.org> on
behalf of Vinay
>>> Madhusudan via flang-dev <flang-dev at lists.llvm.org>
>>> *Sent:* 13 February 2020 16:33
>>> *To:* llvm-dev at lists.llvm.org <llvm-dev at
lists.llvm.org>;
>>> flang-dev at lists.llvm.org <flang-dev at lists.llvm.org>
>>> *Subject:* [flang-dev] About OpenMP dialect in MLIR
>>>
>>>
>>> Hi,
>>>
>>> I have few questions / concerns regarding the design of OpenMP
dialect
>>> in MLIR that is currently being implemented, mainly for the f18
compiler.
>>> Below, I summarize the current state of various efforts in clang /
f18 /
>>> MLIR / LLVM regarding this. Feel free to add to the list in case I
have
>>> missed something.
>>>
>>> 1. [May 2019] An OpenMPIRBuilder in LLVM was proposed for flang and
>>> clang frontends. Note that this proposal was before considering
MLIR for
>>> FIR.
>>>
>>> a. llvm-dev proposal :
>>>
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-May/000197.html
>>>
>>> b. Patches in review: https://reviews.llvm.org/D70290. This also
>>> includes the clang codegen changes.
>>>
>>> 2.  [July - September 2019] OpenMP dialect for MLIR was discussed /
>>> proposed with respect to the f18 compilation stack (keeping FIR in
mind).
>>>
>>> a. flang-dev discussion link:
>>>
https://lists.llvm.org/pipermail/flang-dev/2019-September/000020.html
>>>
>>> b. Design decisions captured in PPT:
>>>
https://drive.google.com/file/d/1vU6LsblsUYGA35B_3y9PmBvtKOTXj1Fu/view
>>>
>>> c. MLIR google groups discussion:
>>>
https://groups.google.com/a/tensorflow.org/forum/#!topic/mlir/4Aj_eawdHiw
>>>
>>> d. Target constructs  design:
>>>
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-September/000285.html
>>>
>>> e. SIMD constructs design:
>>>
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-September/000278.html
>>>
>>> 3.  [Jan 2020] OpenMP dialect RFC in llvm discourse :
>>> https://llvm.discourse.group/t/rfc-openmp-dialect-in-mlir/397
>>>
>>> 4.  [Jan- Feb 2020] Implementation of OpenMP dialect in MLIR:
>>>
>>> a. The first patch which introduces the OpenMP dialect was pushed.
>>>
>>> b. Review of barrier construct is in progress:
>>> https://reviews.llvm.org/D72962
>>> https://reviews.llvm.org/D72400
>>>
>>> I have tried to list below different topics of interest (to
different
>>> people) around this work. Most of these are in the design phase (or
very
>>> new) and multiple parties are interested with different sets of
goals in
>>> mind.
>>>
>>> I.  Flang frontend and its integration
>>>
>>> II. Fortran representation in MLIR / FIR development
>>>
>>> III. OpenMP development for flang,  OpenMP builder in LLVM.
>>>
>>> IV. Loop Transformations in MLIR / LLVM with respect to OpenMP.
>>>
>>> It looks like the design has evolved over time and there is no one
place
>>> which contains the latest design decisions that fits all the
different
>>> pieces of the puzzle. I will try to deduce it from the above
mentioned
>>> references. Please correct me If I am referring to anything which
has
>>> changed.
>>>
>>> A. For most OpenMP design discussions, FIR examples are used (as
seen
>>> in (2) and (3)). The MLIR examples mentioned in the design only
talks about
>>> FIR dialect and LLVM dialect.
>>>
>>> This completely ignores the likes of standard, affine (where most
loop
>>> transformations are supposed to happen) and loop dialects. I think
it is
>>> critical to decouple the OpenMP dialect development in MLIR from
the
>>> current flang / FIR effort. It would be useful if someone can
mention these
>>> examples using existing dialects in MLIR and also how the different
>>> transformations / lowerings are planned.
>>>
>>> B. In latest RFC(3), it is mentioned that the initial OpenMP
dialect
>>> version will be as follows,
>>>
>>>   omp.parallel {
>>>
>>>     omp.do {
>>>
>>>        fir.do %i = 0 to %ub3 : !fir.integer {
>>>
>>>         ...
>>>
>>>        }
>>>
>>>     }
>>>
>>>   }
>>>
>>> and then after the "LLVM conversion" it is converted as
follows:
>>>
>>>   omp.parallel {
>>>
>>>     %ub3 >>>
>>>     omp.do %i = 0 to %ub3 : !llvm.integer {
>>>
>>>     ...
>>>
>>>     }
>>>
>>>   }
>>>
>>>
>>> a. Is it the same omp.do operation which now contains the bounds
and
>>> induction variables of the loop after the LLVM conversion? If so,
will the
>>> same operation have two different semantics during a single
compilation?
>>>
>>> b. Will there be different lowerings for various loop operations
from
>>> different dialects? loop.for and affine.for under omp operations
would need
>>> different OpenMP / LLVM lowerings. Currently, both of them are
lowered
>>> to the CFG based loops during the LLVM dialect conversion (which is
much
>>> before the proposed OpenMP dialect lowering).
>>>
>>> There would be no standard way to represent OpenMP operations
>>> (especially the ones which involve loops) in MLIR. This would
drastically
>>> complicate lowering.
>>>
>>> C. It is also not mentioned how clauses like firstprivate, shared,
>>> private, reduce, map, etc are lowered to OpenMP dialect. The
example in
>>> the RFC contains FIR and LLVM types and nothing about std dialect
types.
>>> Consider the below example:
>>>
>>> #pragma omp parallel for reduction(+:x)
>>>
>>> for (int i = 0; i < N; ++i)
>>>
>>>   x += a[i];
>>>
>>> How would the above be represented in OpenMP dialect? and What type
>>> would "x" be in MLIR?  It is not mentioned in the design
as to how the
>>> various SSA values for various OpenMP clauses are passed around in
OpenMP
>>> operations.
>>>
>>> D. Because of (A), (B) and (C), it would be beneficial to have an
omp.
>>> parallel_do operation which has semantics similar to other loop
>>> structures (may not be LoopLikeInterface) in MLIR. To me, it looks
like
>>> having OpenMP operations based on standard MLIR types and
operations
>>> (scalars and memrefs mainly) is the right way to go.
>>>
>>> Why not have omp.parallel_do operation with AffineMap based bounds,
so
>>> as to decouple it from Value/Type similar to affine.for?
>>>
>>> 1. With the current design, the number of transformations /
>>> optimizations that one can write on OpenMP constructs would become
limited
>>> as there can be any custom loop structure with custom operations /
types
>>> inside it.
>>>
>>> 2. It would also be easier to transform the Loop nests containing
OpenMP
>>> constructs if the body of the OpenMP operations is well defined
(i.e., does
>>> not accept arbitrary loop structures). Having nested redundant
"parallel" ,
>>> "target" and "do" regions seems unnecessary.
>>>
>>> 3. There would also be new sets of loop structures in new dialects
when
>>> C/C++ is compiled to MLIR. It would complicate the number of
possible
>>> combinations inside the OpenMP region.
>>>
>>> E. Lowering of target constructs mentioned in ( 2(d) ) specifies
direct
>>> lowering to LLVM IR ignoring all the advantages that MLIR provides.
Being
>>> able to compile the code for heterogeneous hardware is one of the
biggest
>>> advantages that MLIR brings to the table. That is being completely
missed
>>> here. This also requires solving the problem of handling target
information
>>> in MLIR. But that is a problem which needs to be solved anyway.
Using GPU
>>> dialect also gives us an opportunity to represent offloading
semantics in
>>> MLIR.
>>>
>>> Given the ability to represent multiple ModuleOps and the existence
of
>>> GPU dialect, couldn't higher level optimizations on offloaded
code be done
>>> at MLIR level?. The proposed design would lead us to the same
problems that
>>> we are currently facing in LLVM IR.
>>>
>>> Also, OpenMP codegen will automatically benefit from the GPU
dialect
>>> based optimizations. For example, it would be way easier to hoist a
memory
>>> reference out of GPU kernel in MLIR than in LLVM IR.
>>>
>>> Thanks,
>>>
>>> Vinay
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
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> The point here is that we do not want to use MLIR just as a pass-through
layer because MLIR has a lot of strengths
> The point here is that if we lower to LLVM dialect, we will not be able
to reuse OpenMP codegen & optimisation code from Clang/LLVM.

Just to be clear, are you suggesting that if OpenMPIRBuilder is used, MLIR
will have to be used as pass-through without the optimizations?


On Sun, Feb 16, 2020 at 6:49 AM Kiran Chandramohan <
Kiran.Chandramohan at arm.com> wrote:
> Thanks, Vinay for further details and discussion.
>
> > “If we decide that the OpenMP construct (for e.g. collapse) can be
> handled fully in MLIR and  that is the best place to do it (based on
> experiments) then we will not use the OpenMP IRBuilder for these
constructs.”
> -- latest RFC in discourse
>
> If it is not finalized that the OpenMPIRBuilder will be used for all the
> constructs, wouldn’t it be better to delay the submission of “translation
> to LLVM IR” patch in MLIR? Lowering code will become inconsistent if the
> OpenMPIRBuilder is used only for a few constructs and not for others.
>
>
> I was hoping that we can identify a set of constructs/clauses which can be
> fully handled inside the MLIR layer itself. As an example, I provided the
> collapse clause. This will include constructs/clauses which do not generate
> runtime API calls, which pass metadata to LLVM to do some optimisation etc.
> Yes, the list is not finalized. The point here is that we do not want to
> use MLIR just as a pass-through layer because MLIR has a lot of strengths.
> Would it be OK to continue with only constructs that use OpenMPIRBuilder
> before the final list is made?
>
> Also, the patch does OpenMP dialect lowering *alongside* LLVM Dialect to
> LLVM IR. This is different from most dialects which get directly lowered to
> LLVM Dialect. I think lowering to LLVM Dialect would be a cleaner way if
> OpenMPIRBuilder is not being considered for all constructs.
> Mehdi also seems to have the same suggestion: “I agree that having
> dialect lowering would be cleaner” in https://reviews.llvm.org/D72962
>
>
> The point here is that if we lower to LLVM dialect, we will not be able to
> reuse OpenMP codegen & optimisation code from Clang/LLVM.
> It was pointed out to me early on by the MLIR developers that there are a
> few dialects (like NVVM) which are lowered along with LLVM dialect.
>
>
> You ask a lot of specific questions about which types, dialects, memory
> access operations will be supported and also the lowering for parallel do.
Yes,
> the RFC does not provide all this information. This will become clear only
> as we make progress with the OpenMP dialect. I would like and I am
> interested to provide answers to all your questions in the following weeks.
> Please allow some time.
>
>
Thank you all for your responses so far. Awaiting your further responses.

> --Kiran
>
>
> ------------------------------
> *From:* Vinay Madhusudan <vinay at compilertree.com>
> *Sent:* 15 February 2020 16:22
> *To:* Kiran Chandramohan <Kiran.Chandramohan at arm.com>
> *Cc:* Vinay Madhusudan via flang-dev <flang-dev at lists.llvm.org>;
> llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>
> *Subject:* Re: [llvm-dev] [flang-dev] About OpenMP dialect in MLIR
>
>
> Reply to Kiran Chandramohan:
>
> > You are welcome to participate, provide feedback and criticism to
change
> the design as well as to contribute to the implementation.
>
> Thank you Kiran.
>
> > But the latest is what is there in the RFC in discourse.
>
> I have used this as reference for the response.
>
> > We did a study of a few constructs and clauses which was shared as
mails
> to flang-dev and the RFC. As we make progress and before implementation, we
> will share further details.
>
> > “ Yes, parallel and flush would be the next two constructs that we
will
> do.” -- from a comment in latest RFC
>
> For the above mentioned reasons, I will try to restrict my reply to how
> the “parallel (do)” construct would be lowered.
>
> > If it is OK we can have further discussions in discourse as River
points
> out.
>
> Given that the multiple components of the LLVM project, namely clang,
> flang, MLIR and LLVM are involved, llvm-dev is probably a better place,
> with a much wider audience, until it is clear how different components must
> interact.
>
> > It is the review for translation to LLVM IR that is in progress.
>
> > “If we decide that the OpenMP construct (for e.g. collapse) can be
> handled fully in MLIR and  that is the best place to do it (based on
> experiments) then we will not use the OpenMP IRBuilder for these
constructs.”
> -- latest RFC in discourse
>
> If it is not finalized that the OpenMPIRBuilder will be used for all the
> constructs, wouldn’t it be better to delay the submission of “translation
> to LLVM IR” patch in MLIR? Lowering code will become inconsistent if the
> OpenMPIRBuilder is used only for a few constructs and not for others.
>
> Also, the patch does OpenMP dialect lowering *alongside* LLVM Dialect to
> LLVM IR. This is different from most dialects which get directly lowered to
> LLVM Dialect. I think lowering to LLVM Dialect would be a cleaner way if
> OpenMPIRBuilder is not being considered for all constructs.
>
> Mehdi also seems to have the same suggestion: “I agree that having
> dialect lowering would be cleaner” in https://reviews.llvm.org/D72962
>
> > Yes, the design has mildly changed over time to incorporate feedback.
> But the latest is what is there in the RFC in discourse.
>
> RFC fails to discuss the following (I have also mentioned some of them in
> my reply to Johannes):
>
> > The proposed plan involves a) lowering F18 AST with OpenMP directly to
a
> mix of OpenMP and FIR dialects. b) converting this finally to a mix of
> OpenMP and LLVM dialects.
>
> It is unclear in the RFC what other dialects are considered as supported
> for OpenMP dialect  (std, affine, vector, loop, etc) and how it would be
> transformed, used and lowered from FIR to LLVM.
>
> It becomes important to list down the various dialects / operations /
> types supported for OpenMP (which is mainly defined for C, C++ and Fortran
> programs. MLIR has a much wider scope.
>
> It wouldn’t add much value for the proposed OpenMP dialect to be in the
> MLIR tree if it cannot support at least the relevant standard dialect types
> / operations.
>
> > We would like to take advantage of the transformations in cases that
are
> possible. FIR loops will be converted to affine/loop dialect. So the loop
> inside an omp.do can be in these dialects as clarified in the discussion in
> discourse and also shown in slide 20 of the fosdem presentation (links to
> both below).
>
>
>
https://llvm.discourse.group/t/rfc-openmp-dialect-in-mlir/397/7?u=kiranchandramohan
>
>
>
https://fosdem.org/2020/schedule/event/llvm_flang/attachments/slides/3839/export/events/attachments/llvm_flang/slides/3839/flang_llvm_frontend.pdf
>
> Although it is mentioned that the affine/ loop.for is used, following
> things are unclear:
>
> I am assuming that there will be lowering / conversion code in f18 repo
> dialect from fir.do to loop.for / affine.for. Is it the case? If so, I
> think it is worth mentioning it in the “sequential code flow
> representation” in the RFC.
>
> This raises the following questions.
>
>
>
>    1.
>
>    Which types are supported? Standard dialect types and FIR types?
>
>
> For example, what types are used for Fortran arrays used inside OpenMP
> regions? Is it std.memref OR Fortran array representation in FIR dialect
> (fir.array?) OR both?  Note that Fortran has support for column major
> arrays. std.memref supports custom memory layouts. What custom layouts are
> supported?
>
>
> How would different non-scalar types in standard dialect  be lowered to
> LLVM IR and passed to OpenMP runtime calls? Can you please elaborate on
> this?
>
> The example provided in slide 20 of the fosdem presentation contains
>
> “loop.for %j = %lb2 to %ub2 : !integer {“
>
> But loop.for accepts “index” type. Not sure what type “!integer”
> represents here.
>
>
>    1.
>
>    What are the different memory access operations which are supported
>    inside the OpenMP region and lowered to proper OpenMP runtime calls in
LLVM
>    IR?
>
>
> The possibilities are:
>
>    1.
>
>    affine.load / affine.store
>    2.
>
>    std.load / std.store
>    3.
>
>    FIR dialect memory access operations.
>
>
> > I must also point out that the question of where to do loop
> transformations is a topic we have not fully converged on. See the
> following thread for discussions.
> http://lists.llvm.org/pipermail/flang-dev/2019-September/000042.html
>
> Looks like placement (MLIR / LLVM) of various transformations related to
> OpenMP has not been finalized, from what I could infer from Johannes’s
> reply and the below text in the latest RFC in discourse:
>
> “So there exist some questions regarding where the optimisations should
> be carried out. We will decide on which framework to choose only after some
> experimentation.”
>
> > i) we need to keep the loops separately so as to take advantage of
> transformations that other dialects like affine/loop would provide.
>
> 1) Keeping the loops separate from the OpenMP operations will expose them
> to the “regular” transformations passes in MLIR inside the OpenMP region.
> Most of them are invalid or in-efficient for OpenMP operations.
>
> Examples:
>
>    1.
>
>    Constant propagation example mentioned by Johannes in this thread.
>    (omp task shared(x))
>    2.
>
>    Loop (nest) transformations (permute / split / fuse / tile, etc) will
>    happen ignoring the surrounding OpenMP operations.
>    3.
>
>    Hoisting and sinking of various memory/ SSA values inside the OpenMP
>    region. This goes against the likes of “map”, “firstprivate”, shared,
etc
>    clauses and more.
>
>
> 2) Various loop operations (loop.for, affine.for, fir.do) have (or will
> have) different transformations/ optimization passes which are different
> from one another.
>
> Example:
>
>    1.
>
>    AffineLoopInvariantCodeMotion.cpp is different from
>    LoopInvariantCodeMotion.cpp.
>    2.
>
>    Other Loop transformation passes for affine.for
>
>
> These loops also use different Types and memory access operations in
> general for transformations. Example, most Affine dialect transformations
> (if not all) work on affine.load and affine.store operations.
>
> Supporting different loop operations means that there would be *OpenMP
> specific transformations* for each one of them and also requires a way to
> restrict each of them from existing transformations (when nested in OpenMP
> constructs).
>
> There would be different lowerings for different loop operations as well.
> Example, affine.for and loop.for would have to be lowered to omp.do in
> different ways.
>
> From slide 20 of fosdem presentation you mentioned, the LLVM + OpenMP
> dialect representation is as follows:
>
> ------------------------------
>
> Mlir.region(…) {
>
>    omp.parallel  {
>
>      %ub3 = …
>
>      omp.do %i = 0 to %ub3 : !integer  {
>
>      …
>
>      }
>
>   }
>
> }
>
> -------------------------------
>
> Currently, the LLVM Dialect doesn’t contain a high level loop operation.
> It is all based on CFG implementation.
>
> Will omp.do follow the same structure (SizedRegion<1>) as loop.for?
OR
> there would be CFG for LLVM Dialect based loop operation?
>
> Can you please mention how the OpenMP + LLVM dialect will look like for
> the below parallel do construct?
>
> integer :: i=1, k=10
>
> integer :: a(10), b(10), c(10)
>
> ...
>
>  !$omp parallel do
>
>   do i = 1, k
>
>     if (i .ne. 1) *cycle*
>
>     c(i) = a(i) + b(i)
>
>   end do
>
>   !$omp end parallel do
>
> print *,c
>
>
> Thanks,
>
> Vinay
>
> On Fri, Feb 14, 2020 at 6:52 AM Kiran Chandramohan via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Hello Vinay,
>
> Thanks for your mail about the OpenMP dialect in MLIR. Happy to know that
> you and several other groups are interested in the OpenMP dialect. At the
> outset, I must point out that the design is not set in stone and will
> change as we make progress. You are welcome to participate, provide
> feedback and criticism to change the design as well as to contribute to the
> implementation. I provide some clarifications and replies to your comments
> below. If it is OK we can have further discussions in discourse as River
> points out.
>
> 1. [May 2019] An OpenMPIRBuilder in LLVM was proposed for flang and clang
> frontends. Note that this proposal was before considering MLIR for FIR.
>
> A correction here. The proposal for OpenMPIRBuilder was made when MLIR was
> being considered for FIR.
> (i) Gary Klimowicz's minutes for Flang call in April 2019 mentions
> considering MLIR for FIR.
>
>
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-April/000194.html
> (ii) My reply to Johaness's proposal in May 2019 mentions MLIR for FIR.
>
>
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-May/000220.html
>
> b. Review of barrier construct is in progress:
> https://reviews.llvm.org/D72962
>
> Minor correction here. The addition of barrier construct was accepted and
> has landed (https://reviews.llvm.org/D7240
> <https://reviews.llvm.org/D72400>). It is the review for translation
to
> LLVM IR that is in progress.
>
> It looks like the design has evolved over time and there is no one place
> which contains the latest design decisions that fits all the different
> pieces of the puzzle. I will try to deduce it from the above mentioned
> references. Please correct me If I am referring to anything which has
> changed.
>
> Yes, the design has mildly changed over time to incorporate feedback. But
> the latest is what is there in the RFC in discourse.
>
> For most OpenMP design discussions, FIR examples are used (as seen in (2)
> and (3)). The MLIR examples mentioned in the design only talks about FIR
> dialect and LLVM dialect.
>
> Our initial concern was how will all these pieces (FIR, LLVM Dialect,
> OpenMPIRBuilder, LLVM IR) fit together. Hence you see the prominence of FIR
> and LLVM dialect and more information about lowering/translation than
> transformations/optimisations.
>
> This completely ignores the likes of standard, affine (where most loop
> transformations are supposed to happen) and loop dialects.
>
> Adding to the reply above. We would like to take advantage of the
> transformations in cases that are possible. FIR loops will be converted to
> affine/loop dialect. So the loop inside an omp.do can be in these dialects
> as clarified in the discussion in discourse and also shown in slide 20 of
> the fosdem presentation (links to both below).
>
>
https://llvm.discourse.group/t/rfc-openmp-dialect-in-mlir/397/7?u=kiranchandramohan
>
>
https://fosdem.org/2020/schedule/event/llvm_flang/attachments/slides/3839/export/events/attachments/llvm_flang/slides/3839/flang_llvm_frontend.pdf
>
> I must also point out that the question of where to do loop
> transformations is a topic we have not fully converged on. See the
> following thread for discussions.
> http://lists.llvm.org/pipermail/flang-dev/2019-September/000042.html
>
> Is it the same omp.do operation which now contains the bounds and
> induction variables of the loop after the LLVM conversion?
>
> The point here is that i) we need to keep the loops separately so as to
> take advantage of transformations that other dialects like affine/loop
> would provide. ii) We will need the loop information while lowering the
> OpenMP do operation. For implementation, if reusing the same operation (in
> different contexts) is difficult then we can add a new operation.
>
> It is also not mentioned how clauses like firstprivate, shared, private,
> reduce, map, etc are lowered to OpenMP dialect.
>
> Yes, it is not mentioned. We did a study of a few constructs and clauses
> which was shared as mails to flang-dev and the RFC. As we make progress and
> before implementation, we will share further details.
>
> it would be beneficial to have an omp.parallel_do operation which has
> semantics similar to other loop structures (may not be LoopLikeInterface)
> in MLIR.
>
> I am not against adding parallel_do if it can help with transformations or
> reduce the complexity of lowering. Please share the details in discourse as
> a reply to the RFC or a separate thread.
>
> it looks like having OpenMP operations based on standard MLIR types and
> operations (scalars and memrefs mainly) is the right way to go.
>
> This will definitely be the first version that we implement. But I do not
> understand why we should restrict to only the standard types and
> operations. To ease lowering and translation and to avoid adding OpenMP
> operations to other dialects, I believe OpenMP dialect should also be able
> to exist with other dialects like FIR and LLVM.
>
> E. Lowering of target constructs mentioned in ( 2(d) ) specifies direct
> lowering to LLVM IR ignoring all the advantages that MLIR provides.
>
> Also, OpenMP codegen will automatically benefit from the GPU dialect based
> optimizations. For example, it would be way easier to hoist a memory
> reference out of GPU kernel in MLIR than in LLVM IR.
>
> I might not have fully understood you here. But the dialect lives
> independently of the translation to LLVM IR. If there are optimisations
> (like hoisting that you mention here) I believe they can be performed as
> transformation passes on the dialect. It is not ruled out.
>
> --Kiran
> ------------------------------
> *From:* flang-dev <flang-dev-bounces at lists.llvm.org> on behalf of
Vinay
> Madhusudan via flang-dev <flang-dev at lists.llvm.org>
> *Sent:* 13 February 2020 16:33
> *To:* llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>;
> flang-dev at lists.llvm.org <flang-dev at lists.llvm.org>
> *Subject:* [flang-dev] About OpenMP dialect in MLIR
>
>
> Hi,
>
> I have few questions / concerns regarding the design of OpenMP dialect in
> MLIR that is currently being implemented, mainly for the f18 compiler.
> Below, I summarize the current state of various efforts in clang / f18 /
> MLIR / LLVM regarding this. Feel free to add to the list in case I have
> missed something.
>
> 1. [May 2019] An OpenMPIRBuilder in LLVM was proposed for flang and clang
> frontends. Note that this proposal was before considering MLIR for FIR.
>
> a. llvm-dev proposal :
>
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-May/000197.html
>
> b. Patches in review: https://reviews.llvm.org/D70290. This also includes
> the clang codegen changes.
>
> 2.  [July - September 2019] OpenMP dialect for MLIR was discussed /
> proposed with respect to the f18 compilation stack (keeping FIR in mind).
>
> a. flang-dev discussion link:
> https://lists.llvm.org/pipermail/flang-dev/2019-September/000020.html
>
> b. Design decisions captured in PPT:
> https://drive.google.com/file/d/1vU6LsblsUYGA35B_3y9PmBvtKOTXj1Fu/view
>
> c. MLIR google groups discussion:
> https://groups.google.com/a/tensorflow.org/forum/#!topic/mlir/4Aj_eawdHiw
>
> d. Target constructs  design:
>
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-September/000285.html
>
> e. SIMD constructs design:
>
http://lists.flang-compiler.org/pipermail/flang-dev_lists.flang-compiler.org/2019-September/000278.html
>
> 3.  [Jan 2020] OpenMP dialect RFC in llvm discourse :
> https://llvm.discourse.group/t/rfc-openmp-dialect-in-mlir/397
>
> 4.  [Jan- Feb 2020] Implementation of OpenMP dialect in MLIR:
>
> a. The first patch which introduces the OpenMP dialect was pushed.
>
> b. Review of barrier construct is in progress:
> https://reviews.llvm.org/D72962
> https://reviews.llvm.org/D72400
>
> I have tried to list below different topics of interest (to different
> people) around this work. Most of these are in the design phase (or very
> new) and multiple parties are interested with different sets of goals in
> mind.
>
> I.  Flang frontend and its integration
>
> II. Fortran representation in MLIR / FIR development
>
> III. OpenMP development for flang,  OpenMP builder in LLVM.
>
> IV. Loop Transformations in MLIR / LLVM with respect to OpenMP.
>
> It looks like the design has evolved over time and there is no one place
> which contains the latest design decisions that fits all the different
> pieces of the puzzle. I will try to deduce it from the above mentioned
> references. Please correct me If I am referring to anything which has
> changed.
>
> A. For most OpenMP design discussions, FIR examples are used (as seen in
> (2) and (3)). The MLIR examples mentioned in the design only talks about
> FIR dialect and LLVM dialect.
>
> This completely ignores the likes of standard, affine (where most loop
> transformations are supposed to happen) and loop dialects. I think it is
> critical to decouple the OpenMP dialect development in MLIR from the
> current flang / FIR effort. It would be useful if someone can mention these
> examples using existing dialects in MLIR and also how the different
> transformations / lowerings are planned.
>
> B. In latest RFC(3), it is mentioned that the initial OpenMP dialect
> version will be as follows,
>
>   omp.parallel {
>
>     omp.do {
>
>        fir.do %i = 0 to %ub3 : !fir.integer {
>
>         ...
>
>        }
>
>     }
>
>   }
>
> and then after the "LLVM conversion" it is converted as follows:
>
>   omp.parallel {
>
>     %ub3 >
>     omp.do %i = 0 to %ub3 : !llvm.integer {
>
>     ...
>
>     }
>
>   }
>
>
> a. Is it the same omp.do operation which now contains the bounds and
> induction variables of the loop after the LLVM conversion? If so, will the
> same operation have two different semantics during a single compilation?
>
> b. Will there be different lowerings for various loop operations from
> different dialects? loop.for and affine.for under omp operations would need
> different OpenMP / LLVM lowerings. Currently, both of them are lowered to
> the CFG based loops during the LLVM dialect conversion (which is much
> before the proposed OpenMP dialect lowering).
>
> There would be no standard way to represent OpenMP operations (especially
> the ones which involve loops) in MLIR. This would drastically complicate
> lowering.
>
> C. It is also not mentioned how clauses like firstprivate, shared,
> private, reduce, map, etc are lowered to OpenMP dialect. The example in
> the RFC contains FIR and LLVM types and nothing about std dialect types.
> Consider the below example:
>
> #pragma omp parallel for reduction(+:x)
>
> for (int i = 0; i < N; ++i)
>
>   x += a[i];
>
> How would the above be represented in OpenMP dialect? and What type would
> "x" be in MLIR?  It is not mentioned in the design as to how the
various
> SSA values for various OpenMP clauses are passed around in OpenMP
> operations.
>
> D. Because of (A), (B) and (C), it would be beneficial to have an omp.
> parallel_do operation which has semantics similar to other loop
> structures (may not be LoopLikeInterface) in MLIR. To me, it looks like
> having OpenMP operations based on standard MLIR types and operations
> (scalars and memrefs mainly) is the right way to go.
>
> Why not have omp.parallel_do operation with AffineMap based bounds, so as
> to decouple it from Value/Type similar to affine.for?
>
> 1. With the current design, the number of transformations / optimizations
> that one can write on OpenMP constructs would become limited as there can
> be any custom loop structure with custom operations / types inside it.
>
> 2. It would also be easier to transform the Loop nests containing OpenMP
> constructs if the body of the OpenMP operations is well defined (i.e., does
> not accept arbitrary loop structures). Having nested redundant
"parallel" ,
> "target" and "do" regions seems unnecessary.
>
> 3. There would also be new sets of loop structures in new dialects when
> C/C++ is compiled to MLIR. It would complicate the number of possible
> combinations inside the OpenMP region.
>
> E. Lowering of target constructs mentioned in ( 2(d) ) specifies direct
> lowering to LLVM IR ignoring all the advantages that MLIR provides. Being
> able to compile the code for heterogeneous hardware is one of the biggest
> advantages that MLIR brings to the table. That is being completely missed
> here. This also requires solving the problem of handling target information
> in MLIR. But that is a problem which needs to be solved anyway. Using GPU
> dialect also gives us an opportunity to represent offloading semantics in
> MLIR.
>
> Given the ability to represent multiple ModuleOps and the existence of GPU
> dialect, couldn't higher level optimizations on offloaded code be done
at
> MLIR level?. The proposed design would lead us to the same problems that we
> are currently facing in LLVM IR.
>
> Also, OpenMP codegen will automatically benefit from the GPU dialect based
> optimizations. For example, it would be way easier to hoist a memory
> reference out of GPU kernel in MLIR than in LLVM IR.
>
> Thanks,
>
> Vinay
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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