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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 submission deadline is August 12, 2019 at 11:59PM PDT.
In this talk we will describe the new language mode that has been added into Clang for using functionality of C++17 in the OpenCL kernel language - C++ for OpenCL. As this language mode is fully backwards compatible with OpenCL C 2.0, existing OpenCL applications can gradually switch to using C++ features without any major modifications.
During the implementation the strategy was chosen to generalize features that exist in a range of accelerator devices to C++. For example, address space support was improved in C++ to be used as a language extension and OpenCL functionality was built on top of it. This was done to take advantage of common logic in some language features among multiple C++ dialects and extensions that are available in Clang.
At the end of the talk we will describe the future roadmap. Some documentation has been started in Clang. There is also discussion with the Khronos Group about wider adoption of this language mode and possibly more formal documentation to appear in the future. Additionally we would like to highlight our positive experience of community engagement and the help we have received with early testing and feature evaluation from the users of Clang.
OpenCL provides about 13000 different function overloads that can be used by OpenCL code. Clang provides a header file containing all declarations, but unfortunately parsing this header file takes several seconds per OpenCL program. Ameliorating the parsing time by precompiling the header file has several drawbacks: the resulting PCH file is several megabytes large and requires special handling for macros and conditional compilation.
We present a new TableGen driven approach to support all OpenCL builtins in a fast and compact manner. TableGen generates a trie and tables for recognising and constructing function declarations. From this, Clang constructs the necessary builtin function declarations when a regular name lookup fails and retries the lookup. This approach avoids the need to parse the opencl-c.h header or populate the symbol table with 1000s of declarations. The generated tables and functions together take less than 250 kilobytes which is significantly smaller than the PCH approach. The initial patches of this work have been committed (e.g. r362371) and the remainder of the implementation is currently being upstreamed and reviewed.