[llvm-dev] [flang-dev] About OpenMP dialect in MLIR
Kiran Chandramohan via llvm-dev
llvm-dev at lists.llvm.org
Sat Feb 15 17:18:57 PST 2020
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
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