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2018 LLVM Developers’ Meeting
2018 LLVM Developers’ Meeting Banner
October 17-18, 2018
San Jose Convention Center, San Jose, CA

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The LLVM Developers' Meeting is a bi-annual 2 day gathering of the entire LLVM Project community. The conference is organized by the LLVM Foundation and many volunteers within the LLVM community. Developers and users of LLVM, Clang, and related subprojects will enjoy attending interesting talks, impromptu discussions, and networking with the many members of our community. Whether you are a new to the LLVM project or a long time member, there is something for each attendee.


The Future Direction of C++ and the Four Horsemen of Heterogeneous Computing
Speaker: Michael Wong, Codeplay
Date/Time: October 17 • 9:15am - 10:00am
View on LLVM Developers' Meeting Website

The C++ Direction Group has set a future direction for C++ and includes recommendation for C++ in the short and medium tem. It will have immediate impact on what will enter C++20, and beyond. First half of this talk will devote to the Directions Groups description of where future C++ is heading as a member of the DG.

It also includes a guidance towards Heterogeneous C++.

The introduction of the executors TS means for the first time in C++ there will be a standard platform for writing applications which can execute across a wide range of architectures including multi-core and many-core CPUs, GPUs, DSPs, and FPGAs.

The SYCL standard from the Khronos Group is a strong candidate to implement this upcoming C++ standard as are many other C++ frameworks from DOE, and HPX for the distributed case. One of the core ideas of this standard is that everything must be standard C++, the only exception being that some feature of C++ cannot be used in places that can be executed on an OpenCL device, often due to hardware limitation.

Implementing Heteorgeneous C++ is like battling the four Horsemen of the Apocalypse. These are:

  • Data movement
  • Data Locality
  • Data Layout
  • Data Affinity

The rest of this talk presents some of the challenges and solutions to implement a Heterogeneous C++ standard in clang based on our implementation of Khrono's SYCL language with Codeplay's ComputeCpp compiler, with the fast growth of C++ and clang being a platform of choice to prototype many of the new C++ features.

We describe the major issues with ABI for separate compilation tool chain that comes from non-standard layout type of lambdas, as well as the issues of data addressing that comes from non-flat and possibly non-coherent address space.

We also describe various papers which are being proposed to ISO C++ to move towards standardizing heterogeneous and distributed computing in C++. The introduction of a unified interface for execution across a wide range of different hardware, extensions to this to support concurrent exception handling and affinity queries, and an approach to improve the capability of the parallel algorithms through composability. All of this adds up to a future C++ which is much more aware of heterogeneity and capable of taking advantage of it to improve parallelism and performance.

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