[Openmp-dev] [RFC] Late (OpenMP) GPU code "SPMD-zation"
Doerfert, Johannes via Openmp-dev
openmp-dev at lists.llvm.org
Wed Mar 13 12:35:59 PDT 2019
thank you for your quick feedback.
> There are tooooooo(!) many changes, I don't who's going to review sooooo big patch.
I can for sure split it in the three components/repositories that are touched, clang, llvm, and openmp.
I feared it will then be harder to navigate the code in order to see the connection points.
I am a bit amazed by your hyperbolism though given the complexity is not that height
due to the absence of modified or removed lines. Anyway, you seem to have very strong feelings about
this so I am open to suggestion on how to split it up.
> Also, I don't like the idea adding of one more class for NVPTX codegen. All your changes should be on top of the eixisting solution.
Could you please explain to me why? This will only make everything more complicated and entangled.
Also, the new class is supposed to be "target agnostic" so a new offloading target, e.g., AMD GPUs, could easily reuse
the new code while the old code is sprinkled with NVPTX specific details, e.g., function calls, constants, etc.
From: Alexey Bataev <a.bataev at outlook.com>
Sent: Wednesday, March 13, 2019 2:15:39 PM
To: Doerfert, Johannes; 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"
There are tooooooo(!) many changes, I don't who's going to review sooooo big patch. You definitely need to split it into several smaller patches. Also, I don't like the idea adding of one more class for NVPTX codegen. All your changes should be on top of the eixisting solution.
13.03.2019 15:08, Doerfert, Johannes пишет:
Please consider reviewing the code for the proposed approach here:
Initial tests, e.g., on the nw (needleman-wunsch) benchmark in the
rodinia 3.1 benchmark suite, showed 30% improvement after SPMD mode was
enabled automatically. The code in nw is conceptually equivalent to the
first example in the "to_SPMD_mode.ll" test case that can be found here:
The implementation is missing key features but one should be able to see
the overall design by now. Once accepted, the missing features and more
optimizations will be added.
On 01/22, Johannes Doerfert wrote:
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 , 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.
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.
Argonne National Laboratory
Lemont, IL 60439, USA
jdoerfert at anl.gov<mailto:jdoerfert at anl.gov>
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the Openmp-dev