[cfe-dev] [RFC] Upstreaming PACXX (Programing Accelerators with C++)

[Jeff Hammond via cfe-dev]

This is cool.  I'm very glad to see your PhD research was done in a
production environment and that you are open-sourcing it.

PACXX looks a lot like SYCL.  Have you considered whether it can be evolved
into a high-quality implementation of the SYCL standard API?  There's a lot
of value in implementing a standardized API with multiple implementations.
I've used ComputeCpp, triSYCL, and sycl-gtx recently, and each has
limitations that could be overcome by having SYCL support in Clang/LLVM.

Your contribution of a good SYCL implementation would be really valuable,
since there appears to be no implementation that both performs well and is
open-source.  I completely understand if you have no time in turn PACXX
into SYCL, but if you have technical arguments against doing so even if
time permitted, I think they'd be useful to share, although perhaps not in
this forum.

On a practical level, PACXX seems to require some software hardening.  I
tried to follow the docs and build it locally, but had some issues (GitHub
issues were created, so I'll omit details here) and ultimately failed to
compile your example program.  I'd love to be able to try it out, since
I've recently evaluated SYCL and Boost.Compute using
https://github.com/ParRes/Kernels, and it seems like PACXX is a peer of
these.

Best,

Jeff


On Sun, Feb 4, 2018 at 11:50 PM, Haidl, Michael via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> HI LLVM comunity,
>
> after 3 years of development, various talks on LLVM-HPC and EuroLLVM and
> other scientific conferences I want to present my PhD research topic to the
> lists.
>
> The main goal for my research was to develop a single-source programming
> model equal to CUDA or SYCL for accelerators supported by LLVM (e.g.,
> Nvidia GPUs). PACXX uses Clang as front-end for code generation and comes
> with a runtime library (PACXX-RT) to execute kernels on the available
> hardware. Currently, PACXX supports Nvidia GPUs through the NVPTX Target
> and CUDA, CPUs through MCJIT (including whole function vectorization thanks
> to RV [1]) and has an experimental back-end for AMD GPUs using the AMDGPU
> Target and ROCm.
>
> The main idea behind PACXX is the use of the LLVM IR as kernel code
> representation which is integrated into the executable together with the
> PACXX-RT. At runtime of the program the PACXX-RT compiles the IR to the
> final MC level and hands it over to the device. Since, PACXX does currently
> not enforce any major restrictions on the C++ code we managed to run
> (almost) arbitrary C++ code on GPUs including range-v3 [2, 3].
>
> A short vector addition example using PACXX:
>
> using namespace pacxx::v2;
> int main(int argc, char *argv[]) {
>    // get the default executor
>    auto &exec = Executor::get();
>     size_t size = 128;
>     std::vector<int> a(size, 1);
>     std::vector<int> b(size, 2);
>     std::vector<int> c(size, 0);
>
>     // allocate device side memory
>     auto &da = exec.allocate<int>(a.size());
>     auto &db = exec.allocate<int>(b.size());
>     auto &dc = exec.allocate<int>(c.size());
>     // copy data to the accelerator
>     da.upload(a);
>     db.upload(b);
>     dc.upload(c);
>     // get the raw pointer
>     auto pa = da.get();
>     auto pb = db.get();
>     auto pc = dc.get();
>
>     // define the computation
>     auto vadd = [=](auto &config) {
>       auto i = config.get_global(0);
>       if (i < size)
>        pc[i] = pa[i] + pb[i];
>     };
>
>     // launch and synchronize
>     std::promise<void> promise;
>     auto future = exec.launch(vadd, {{1}, {128}}, promise);
>     future.wait();
>     // copy back the data
>     dc.download(c);
> }
>
> Recently, I open sourced PACXX on github [3] under the same license LLVM
> is currently using.
> Since my PhD is now in its final stage I wanted to ask if there is
> interest in having such an SPMD programming model upstreamed.
> PACXX is currently on par with release_60 and only requires minor
> modifications to Clang, e.g., a command line switch, C++ attributes, some
> diagnostics and metadata generation during code gen.
> The PACXX-RT can be integrated into the LLVM build system and may remain a
> standalone project. (BTW, may I ask to add PACXX to the LLVM projects?).
>
> Looking forward for your feedback.
>
> Cheers,
> Michael Haidl
>
> [1] https://github.com/cdl-saarland/rv
> [2] https://github.com/ericniebler/range-v3
> [3] https://dl.acm.org/authorize?N20051
> [4] https://github.com/pacxx/pacxx-llvm
> _______________________________________________
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> cfe-dev at lists.llvm.org
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>



-- 
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/
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