1 of 1

Clacc: OpenACC Support for C/C++

Scientific Achievement

Traditional OpenACC compiler and runtime support built on Clang/LLVM
Integration with profiling tools like TAU via OpenACC profiling interface
Kokkos application support via Kokkos’s OpenACC backend
Reuse of OpenMP implementation for OpenACC applications
Device support inherited from Clang/LLVM (e.g., AMD and NVIDIA GPUs)

Significance and Impact

OpenACC facilitates GPU/accelerator programming in HPC apps & beyond
Clang/LLVM provide widely popular platform for compiler R&D
Clacc is the only operational OpenACC support based on Clang
Clacc is the only OpenACC support for C/C++ deployed on ORNL’s Frontier
Clacc is the only AMD GPU support for Kokkos’s OpenACC backend

Clacc Compiler Design

Clacc’s design decision to translate OpenACC to OpenMP enables two OpenACC compilation modes: traditional compilation mode and source-to-source mode. Clacc allows existing OpenACC applications to be ported to heterogeneous devices using the underlying OpenMP support available on the target system.

PI(s): Robert Ross (ANL) Project POC: Joel E. Denny (ORNL)

ASCR Program: [SciDAC5 RAPIDS2] ASCR PM: Kalyan Perumalla

Publication for this work: Joel E. Denny, et al., “Clacc: OpenACC for C/C++ in Clang”, International Journal of High Performance Computing Applications (accepted).

DOI: 10.1109/HUSTProtools51951.2020.00012

Technical Approach

A key Clacc design decision is to translate OpenACC to OpenMP in order to build upon the OpenMP support provided by Clang/LLVM
Source-to-source mode enables targeting other OpenMP compilers

Joel E. Denny, Seyong Lee, Pedro Valero Lara, Marc Gonzalez-Tallada, Keita Teranishi, and Jeffrey S. Vetter, “Clacc: OpenACC for C/C++ in Clang”, International Journal of High Performance Computing Applications (accepted).

LOCAL LAB POC:

Project PI: Rob Ross (ANL)
ORNL PI: Scott Klasky
Project POC: Joel E. Denny

TALKING POINTS:

OpenACC and LLVM

Since its launch in 2010, OpenACC has evolved into one of the most well-known portable programming models for GPUs and other accelerators on HPC systems.
A variety of OpenACC implementations exist, but the only production, open-source implementation appearing in openacc.org’s list of OpenACC tools is GCC.
Especially within the DOE, the LLVM compiler and tool chain infrastructure has gained favor for compiler R&D efforts, but OpenACC support in LLVM is lacking.

Clacc

Under ECP funding, Clacc has developed the only operational OpenACC support based on LLVM’s Clang to date, and Clacc provides the only OpenACC support for C/C++ deployed on ORNL’s Frontier system.
Clacc’s design enables it to build OpenACC support on the OpenMP offload support developed by many institutions: traditional compilation mode leverages Clang/LLVM’s OpenMP support, and source-to-source mode offers a path to leverage other OpenMP implementations.
Under SciDAC RAPIDS2 funding, we continue to extend Clacc’s support for C++ as an OpenACC base language and for Kokko’s OpenACC backend, and we continue to develop publications and presentations to describe our work.

METADATA:

Name of the associated awarded project: SciDAC5 RAPIDS2

PI name(s): Rob Ross (ANL), Scott Klasky (ORNL)

Name of the program manager: Supporting technology supported by both ECP and ASCR research (Kalyan Perumalla)

CITATIONS:

Joel E. Denny, Seyong Lee, Pedro Valero Lara, Marc Gonzalez-Tallada, Keita Teranishi, and Jeffrey S. Vetter, “Clacc: OpenACC for C/C++ in Clang”, International Journal of High Performance Computing Applications (accepted).
Aaron Jarmusch, Aaron Liu, Christian Munley, Daniel Horta, Vaidhyanathan Ravichandran, Joel Denny, and Sunita Chandrasekaran, “Analysis of Validating and Verifying OpenACC Compilers 3.0 and Above”, WACCPD, Dallas, TX, USA (November, 2022). https://doi.org/10.1109/WACCPD56842.2022.00006
Camille Coti, Joel E. Denny, Kevin Huck, Seyong Lee, Allen D. Malony, Sameer Shende, and Jeffrey S. Vetter, “OpenACC Profiling Support for Clang and LLVM using Clacc and TAU”, ProTools, GA, USA (November 2020). http://doi.org/10.1109/HUSTProtools51951.2020.00012
Joel E. Denny, Seyong Lee, and Jeffrey S. Vetter, “Clacc: Translating OpenACC to OpenMP in Clang,” 2018 IEEE/ACM 5th Workshop on the LLVM Compiler Infrastructure in HPC (LLVM-HPC), Dallas, TX, USA (November 2018). https://doi.org/10.1109/LLVM-HPC.2018.8639349