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

[Alexey Bataev via llvm-dev]

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