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

Doerfert, Johannes Rudolf via cfe-dev cfe-dev at lists.llvm.org
Tue Jan 22 10:43:17 PST 2019

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.


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From: Alexey Bataev <a.bataev at outlook.com>
Sent: Tuesday, January 22, 2019 12:34:01 PM
To: Doerfert, Johannes Rudolf; cfe-dev at lists.llvm.org
Cc: 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".

     - 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.

    - 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

None of this was implemented yet but we plan to start in the immediate
future. Any comments, ideas, criticism is welcome!


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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