OpenACC

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Programming standard for parallel computing

title: "OpenACC" type: doc version: 1 created: 2026-02-28 author: "Wikipedia contributors" status: active scope: public tags: ["application-programming-interfaces", "c-programming-language-family", "fortran", "parallel-computing", "standards"] description: "Programming standard for parallel computing" topic_path: "technology/programming-languages" source: "https://en.wikipedia.org/wiki/OpenACC" license: "CC BY-SA 4.0" wikipedia_page_id: 0 wikipedia_revision_id: 0

::summary Programming standard for parallel computing ::

::data[format=table title="Infobox software"]

Field	Value
name	OpenACC
developer	The OpenACC Organization
platform	Cross-platform
genre	API
latest_release_version	3.3
latest_release_date
programming_language	C, C++, and Fortran
operating_system	Cross-platform
website
::

As in OpenMP, the programmer can annotate C, C++ and Fortran source code to identify the areas that should be accelerated using compiler directives and additional functions. Like OpenMP 4.0 and newer, OpenACC can target both the CPU and GPU architectures and launch computational code on them.

OpenACC members have worked as members of the OpenMP standard group to merge into OpenMP specification to create a common specification which extends OpenMP to support accelerators in a future release of OpenMP. These efforts resulted in a technical report for comment and discussion timed to include the annual Supercomputing Conference (November 2012, Salt Lake City) and to address non-Nvidia accelerator support with input from hardware vendors who participate in OpenMP.

At ISC’12 OpenACC was demonstrated to work on Nvidia, AMD and Intel accelerators, without performance data.

On November 12, 2012, at the SC12 conference, a draft of the OpenACC version 2.0 specification was presented. New suggested capabilities include new controls over data movement (such as better handling of unstructured data and improvements in support for non-contiguous memory), and support for explicit function calls and separate compilation (allowing the creation and reuse of libraries of accelerated code). OpenACC 2.0 was officially released in June 2013.

Version 2.5 of the specification was released in October 2015, while version 2.6 was released in November 2017. Subsequently, version 2.7 was released in November 2018.

The latest version is version 3.3, which was released in November 2022.

Compiler support

Support of OpenACC is available in commercial compilers from PGI (from version 12.6), and (for Cray hardware only) Cray.

OpenUH is an Open64 based open source OpenACC compiler supporting C and FORTRAN, developed by HPCTools group from University of Houston.

OpenARC is an open source C compiler developed at Oak Ridge National Laboratory to support all features in the OpenACC 1.0 specification. An experimental open source compiler, accULL, is developed by the University of La Laguna (C language only).

Omni Compiler is an open source compiler developed at HPCS Laboratory of University of Tsukuba and Programming Environment Research Team of RIKEN Center for Computational Science, Japan, supported OpenACC, and combining XcalableMP and OpenACC.

IPMACC is an open source C compiler developed by University of Victoria that translates OpenACC to CUDA, OpenCL, and ISPC. Currently, only following directives are supported: data, kernels, loop, and cache.

GCC support for OpenACC was slow in coming. A GPU-targeting implementation from Samsung was announced in September 2013; this translated OpenACC 1.1-annotated code to OpenCL. The announcement of a "real" implementation followed two months later, this time from NVIDIA and based on OpenACC 2.0. This sparked some controversy, as the implementation would only target NVIDIA's own PTX assembly language, for which no open source assembler or runtime was available. Experimental support for OpenACC/PTX did end up in GCC as of version 5.1. GCC6 and GCC7 release series include a much improved implementation of the OpenACC 2.0a specification. GCC 9.1 offers nearly complete OpenACC 2.5 support.

Usage

In a way similar to OpenMP 3.x on homogeneous system or the earlier OpenHMPP, the primary mode of programming in OpenACC is directives. The specifications also include a runtime library defining several support functions. To exploit them, user should include "openacc.h" in C or "openacc_lib.h" in Fortran; and then call acc_init() function.

Directives

OpenACC defines an extensive list of pragmas (directives), for example: ::code[lang=c] #pragma acc parallel #pragma acc kernels ::

Both are used to define parallel computation kernels to be executed on the accelerator, using distinct semantics

::code[lang=c] #pragma acc data ::

Is the main directive to define and copy data to and from the accelerator.

::code[lang=c] #pragma acc loop ::

Is used to define the type of parallelism in a parallel or kernels region.

::code[lang=c] #pragma acc cache #pragma acc update #pragma acc declare #pragma acc wait ::

Runtime API

There are some runtime API functions defined too: acc_get_num_devices(), acc_set_device_type(), acc_get_device_type(), acc_set_device_num(), acc_get_device_num(), acc_async_test(), acc_async_test_all(), acc_async_wait(), acc_async_wait_all(), acc_init(), acc_shutdown(), acc_on_device(), acc_malloc(), acc_free().

OpenACC generally takes care of work organisation for the target device however this can be overridden through the use of gangs and workers. A gang consists of workers and operates over a number of processing elements (as with a workgroup in OpenCL).

References

(4 November 2011). "Nvidia, Cray, PGI, and CAPS launch 'OpenACC' programming standard for parallel computing".
"OpenACC standard version 2.5".
"How does the OpenACC API relate to the OpenMP API?".
"How did the OpenACC specifications originate?".
(5 November 2012). "The OpenMP Consortium Releases First Technical Report".
(29 August 2012). "OpenMP at SC12".
(20 June 2012). "OpenACC Group Reports Expanding Support for Accelerator Programming Standard".
(12 November 2012). "OpenACC Version 2.0 Posted for Comment".
"OpenACC 2.0 Spec {{!}} www.openacc.org".
"OpenACC Standards Group Announces Release of the 2.5 Specification; Member Vendors Add Support for ARM & x86 as Parallel Devices {{!}} www.openacc.org".
"What's new in OpenACC 2.6? {{!}} OpenACC".
"What's new in OpenACC 2.7! {{!}} OpenACC".
(16 November 2011). "OpenACC Standard to Help Developers to Take Advantage of GPU Compute Accelerators".
"OpenUH Compiler".
"OpenARC Compiler".
"accULL The OpenACC research implementation".
"Omni Compiler".
(2019-10-17). "Omni Compiler for C and Fortran programs with XcalableMP and OpenACC directives: omni-compiler/omni-compiler". omni-compiler.
"IPMACC Compiler".
Larabel, Michael. (4 December 2012). "OpenACC Still Not Loved By Open Compilers".
Larabel, Michael. (30 September 2013). "GCC Support Published For OpenACC On The GPU".
Larabel, Michael. (14 November 2013). "OpenACC 2.0 With GPU Support Coming To GCC".
Larabel, Michael. (15 November 2013). "NVIDIA, Mentor Graphics May Harm GCC".
Larabel, Michael. (21 November 2013). "In-Fighting Continues Over OpenACC In GCC".
"OpenACC - GCC Wiki".
Schwinge, Thomas. (15 January 2015). "Merge current set of OpenACC changes from gomp-4_0-branch".
Jelinek, Jakub. (3 May 2019). "GCC 9.1 Released".
(11 June 2012). "Easy GPU Parallelism with OpenACC".
(November 2011). "OpenACC API QuickReference Card, version 1.0".
"OpenACC standard version 2.0".
(August 2012). "OpenACC Kernels and Parallel Constructs".
(3 January 2013). "OpenACC parallel section VS kernels".

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