[llvm-dev] [RFC] Late (OpenMP) GPU code "SPMD-zation"

Guray Ozen via llvm-dev
Wed, 23 Jan 2019 09:14:57 -0000

We are working on OpenMP target offloading for GPUs in Flang, and adopting the
same code generation strategy. The proposal is affecting us. It would be nice to
know more details about the proposal. So we can prepare ourselves to adapt flang
(if everything goes on the way).

Have you find and a solution for data sharing? How are you going to manage data
sharing for SPMD and non-SPMD?

From: cfe-dev <cfe-dev-bounces at lists.llvm.org> On Behalf Of Doerfert,
Johannes Rudolf via cfe-dev
Sent: Wednesday, January 23, 2019 12:50 AM
To: Alexey Bataev <a.bataev at outlook.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>; cfe-dev at lists.llvm.org;
openmp-dev at lists.llvm.org
Subject: Re: [cfe-dev] [RFC] Late (OpenMP) GPU code "SPMD-zation"


After an IRC discussion, I think Alexey and I are pretty much in agreement (on
the general feasibility at least).



I try to sketch the proposed idea again below, as the initial RFC was simply not
descriptive enough.

After that, I shortly summarize how I see these changes being developed and
committed so that we

 - never have any regressions,

 - can make an educated decision before removing any existing code.





What we want to do:

The intermediate goal is that the code generated by clang for the SPMD and
non-SPMD (earlier denoted as "guarded") case

is conceptually/structurally very similar. The current non-SPMD code is,
however, a state machine generated into the user code

module. This state machine is very hard to analyze and optimize. If the code
would look as the SPMD code but *behave the same

way it does now*, we could "easily" switch from non-SPMD to SPMD
version after a (late) analysis determined legality. To make

the code look the same but behave differently, we propose to hide the semantic
difference in the runtime library calls. That is,

the runtime calls emitted in the two modes are (slightly) different, or there is
a flag which indicates the (initial) mode. If that mode
is SPMD, the runtime behavior does not change compared to the way it is now. If
that mode is non-SPMD, the runtime would

separate the master and worker threads, as we do it now in the user code module,
and keep the workers in an internal state machine

waiting for the master to provide them with work. Only the master would return
from the runtime call and the mechanism

to distribute work to the worker threads would (for now) stay the same.









Preliminary implementation (and integration) steps:

1) Design and implement the necessary runtime extensions and determine
feasibility.

2) Allow to Clang codegen to use the new runtime extensions if explicitly chosen
by the user.

2b) Performance comparison unoptimized new code path vs. original code path on
test cases and real use cases.

3) Implement the middle-end pass to analyze and optimize the code using the
runtime extensions.

3b) Performance comparison optimized new code path vs. original code path on
real use cases.

4) If no conceptual problem was found and 2b)/3b) determined that the new code
path is superior, switch to the

    new code path by default.

5) If no regressions/complaints are reported after a grace period, remove the
old code path from the clang front-end.






Again, this is an early design RFC for which I welcome any feedback!



Thanks,

  Johannes

________________________________
From: Doerfert, Johannes Rudolf
Sent: Tuesday, January 22, 2019 1:50:51 PM
To: Alexey Bataev
Cc: cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>;
openmp-dev at lists.llvm.org<mailto:openmp-dev at lists.llvm.org>
Subject: Re: [RFC] Late (OpenMP) GPU code "SPMD-zation"


What do you refer to with: "No, we don't".



Again, I do not propose to remove the SPMD "detection" in Clang. We
will still identify SPMD mode based on the syntactic criteria we have now.

The Clang analysis is also not affected.  Thus, we will globalize/localize the
same variables as we do now. I don't see why this should be any different.





________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org<mailto:llvm-dev-bounces
at lists.llvm.org>> on behalf of Alexey Bataev via llvm-dev <llvm-dev
at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Sent: Tuesday, January 22, 2019 1:46:39 PM
To: Doerfert, Johannes Rudolf
Cc: llvm-dev; cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>;
openmp-dev at lists.llvm.org<mailto:openmp-dev at lists.llvm.org>
Subject: Re: [llvm-dev] [RFC] Late (OpenMP) GPU code "SPMD-zation"


No, we don't. We need to perform the different kind of the analysis for SPMD
mode constructs and Non-SPMD.

For SPMD mode we need to globalize only reduction/lastprivate variables. For
Non-SPMD mode, we need to globalize all the private/local variables, that may
escape their declaration context in the construct.

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

Best regards,

Alexey Bataev
22.01.2019 14:29, Doerfert, Johannes Rudolf пишет:
We would still know that. We can do exactly the same reasoning as we do now.

I think the important question is, how different is the code generated for
either mode and can we hide (most of) the differences in the runtime.



If I understand you correctly, you say the data sharing code looks very
different and the differences cannot be hidden, correct?

It would be helpful for me to understand your point if you could give me a piece
of OpenMP for which the data sharing in SPMD mode and "guarded"

mode are as different as possible. I can compile it in both modes myself so
high-level OpenMP is fine (I will disable SPMD mode manually in the source if
necessary).



Thanks,

  Johannes



________________________________
From: llvm-dev <llvm-dev-bounces at
lists.llvm.org><mailto:llvm-dev-bounces at lists.llvm.org> on behalf of
Alexey Bataev via llvm-dev <llvm-dev at lists.llvm.org><mailto:llvm-dev
at lists.llvm.org>
Sent: Tuesday, January 22, 2019 13:10
To: Doerfert, Johannes Rudolf
Cc: Alexey Bataev; LLVM-Dev; Arpith Chacko Jacob; openmp-dev at
lists.llvm.org<mailto:openmp-dev at lists.llvm.org>; cfe-dev at
lists.llvm.org<mailto:cfe-dev at lists.llvm.org>
Subject: Re: [llvm-dev] [RFC] Late (OpenMP) GPU code "SPMD-zation"

But we need to know the execution mode, SPMD or "guarded"

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

Best regards,

Alexey Bataev
22.01.2019 13:54, Doerfert, Johannes Rudolf пишет:
We could still do that in clang, couldn't we?
Get Outlook for Android<https://aka.ms/ghei36>

________________________________
From: Alexey Bataev <a.bataev at outlook.com><mailto:a.bataev at
outlook.com>
Sent: Tuesday, January 22, 2019 12:52:42 PM
To: Doerfert, Johannes Rudolf; cfe-dev at lists.llvm.org<mailto:cfe-dev at
lists.llvm.org>
Cc: openmp-dev at lists.llvm.org<mailto:openmp-dev at lists.llvm.org>;
LLVM-Dev; Finkel, Hal J.; Alexey Bataev; Arpith Chacko Jacob
Subject: Re: [RFC] Late (OpenMP) GPU code "SPMD-zation"


The globalization for the local variables, for example. It must be implemented
in the compiler to get the good performance, not in the runtime.



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

Best regards,

Alexey Bataev
22.01.2019 13:43, Doerfert, Johannes Rudolf пишет:
Could you elaborate on what you refer to wrt data sharing. What do we currently
do in the clang code generation that we could not effectively implement in the
runtime, potentially with support of an llvm pass.
Thanks,
  James
Get Outlook for Android<https://aka.ms/ghei36>

________________________________
From: Alexey Bataev <a.bataev at outlook.com><mailto:a.bataev at
outlook.com>
Sent: Tuesday, January 22, 2019 12:34:01 PM
To: Doerfert, Johannes Rudolf; cfe-dev at lists.llvm.org<mailto:cfe-dev at
lists.llvm.org>
Cc: openmp-dev at lists.llvm.org<mailto:openmp-dev at lists.llvm.org>;
LLVM-Dev; Finkel, Hal J.; Alexey Bataev; Arpith Chacko Jacob
Subject: Re: [RFC] Late (OpenMP) GPU code "SPMD-zation"




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

Best regards,

Alexey Bataev
22.01.2019 13:17, Doerfert, Johannes Rudolf пишет:

Where we are

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



Currently, when we generate OpenMP target offloading code for GPUs, we

use sufficient syntactic criteria to decide between two execution modes:

  1)      SPMD -- All target threads (in an OpenMP team) run all the code.

  2) "Guarded" -- The master thread (of an OpenMP team) runs the user

                  code. If an OpenMP distribute region is encountered, thus

                  if all threads (in the OpenMP team) are supposed to

                  execute the region, the master wakes up the idling

                  worker threads and points them to the correct piece of

                  code for distributed execution.



For a variety of reasons we (generally) prefer the first execution mode.

However, depending on the code, that might not be valid, or we might

just not know if it is in the Clang code generation phase.



The implementation of the "guarded" execution mode follows roughly the

state machine description in [1], though the implementation is different

(more general) nowadays.





What we want

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



Increase the amount of code executed in SPMD mode and the use of

lightweight "guarding" schemes where appropriate.





How we get (could) there

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



We propose the following two modifications in order:



  1) Move the state machine logic into the OpenMP runtime library. That

     means in SPMD mode all device threads will start the execution of

     the user code, thus emerge from the runtime, while in guarded mode

     only the master will escape the runtime and the other threads will

     idle in their state machine code that is now just "hidden".



     Why:

     - The state machine code cannot be (reasonably) optimized anyway,

       moving it into the library shouldn't hurt runtime but might even

       improve compile time a little bit.

     - The change should also simplify the Clang code generation as we

       would generate structurally the same code for both execution modes

       but only the runtime library calls, or their arguments, would

       differ between them.

     - The reason we should not "just start in SPMD mode" and
"repair"

       it later is simple, this way we always have semantically correct

       and executable code.

     - Finally, and most importantly, there is now only little

       difference (see above) between the two modes in the code

       generated by clang. If we later analyze the code trying to decide

       if we can use SPMD mode instead of guarded mode the analysis and

       transformation becomes much simpler.

The last item is wrong, unfortunately. A lot of things in the codegen depend on
the execution mode, e.g. correct support of the data-sharing. Of course, we can
try to generalize the codegen and rely completely on the runtime, but the
performance is going to be very poor.

We still need static analysis in the compiler. I agree, that it is better to
move this analysis to the backend, at least after the inlining, but at the
moment it is not possible. We need the support for the late outlining, which
will allow to implement better detection of the SPMD constructs + improve
performance.

 2) Implement a middle-end LLVM-IR pass that detects the guarded mode,

    e.g., through the runtime library calls used, and that tries to

    convert it into the SPMD mode potentially by introducing lightweight

    guards in the process.



    Why:

    - After the inliner, and the canonicalizations, we have a clearer

      picture of the code that is actually executed in the target

      region and all the side effects it contains. Thus, we can make an

      educated decision on the required amount of guards that prevent

      unwanted side effects from happening after a move to SPMD mode.

    - At this point we can more easily introduce different schemes to

      avoid side effects by threads that were not supposed to run. We

      can decide if a state machine is needed, conditionals should be

      employed, masked instructions are appropriate, or "dummy" local

      storage can be used to hide the side effect from the outside

      world.





None of this was implemented yet but we plan to start in the immediate

future. Any comments, ideas, criticism is welcome!





Cheers,

  Johannes





P.S. [2-4] Provide further information on implementation and features.



[1] https://ieeexplore.ieee.org/document/7069297

[2] https://dl.acm.org/citation.cfm?id=2833161

[3] https://dl.acm.org/citation.cfm?id=3018870

[4] https://dl.acm.org/citation.cfm?id=3148189





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