OpenMP

This page contains information on GCC's implementation of the OpenMP specification and related functionality like the auto parallelizer (-ftree-parallelize-loops).

As of GCC 4.2, the compiler implements version 2.5 of the OpenMP specification, as of 4.4 it implements version 3.0 and since GCC 4.7 it supports the OpenMP 3.1 specification. GCC 4.9 supports OpenMP 4.0 for C/C++, GCC 4.9.1 also for Fortran. GCC 5 adds support for Offloading. OpenMP 4.5 is supported for C/C++ since GCC 6 and since GCC 7 for Fortran (with omissions, largest missing item is structure element mapping). Since GCC 9, there is initial OpenMP 5 support (essentially C/C++, only). GCC 10 added some more features, mainly for C/C++ but also for Fortran.

GCC 11 extended the Fortran compiler to fully support OpenMP 4.5 and support more OpenMP 5.0; additionally, nonrectangular loops are now supported. GCC 12 has a growing support of OpenMP 5.0 and first support of OpenMP 5.1 features.

GCC 13 has a still growing support of OpenMP 5.0, extends the support of OpenMP 5.1 features and very limited first support of OpenMP 5.2 features.

GCC 14 (current release) is extending OpenMP 5.x support, mostly by integrating features from the OG12/OG13 branches.

OG13 branch (branch: 'devel/omp/gcc-13': Based on the GCC 13 release branch, OG13 contains several commits extending the OpenMP, OpenACC, and offloading support. Those additions are either only in GCC mainline/14 or still have to be upstreamed. In particular, OG13 supports metadirectives and unified-shared memory (all scheduled for GCC 14 integration).

Release Notes

GCC 14 Changes OpenMP Implementation Status – work in progress (as of 2023)
GCC 13 Changes OpenMP Implementation Status for GCC 13
GCC 12 Changes OpenMP Implementation Status for GCC 12
GCC 11 Changes
GCC 10 Changes
GCC 9 Changes – contains a TODO list
GCC 8 Changes
GCC 7 Changes
GCC 6 Changes
GCC 5 Changes
GCC 4.9 Changes
GCC 4.8 Changes
GCC 4.7 Changes
GCC 4.6 Changes
GCC 4.5 Changes
GCC 4.4 Changes
GCC 4.3 Changes
GCC 4.2 Changes

OpenMP Documentation

Documentation on libgomp (GNU Offloading and Multi Processing Runtime Library).
OpenMP Specification page – includes the specifications linked below (also available as book) and links to the offical OpenMP example document and additional documentation:
OpenMP v2.5 specification
OpenMP v3.0 specification
OpenMP v3.1 specification (July 2011)
OpenMP v4.0 specification (July2013) (Official OpenMP 4.0.2 Examples, OpenMP 4.0 API forum)
OpenMP 4.5 specification (November 2015)
OpenMP 5 specification (as HTML, November 2018)
OpenMP 5.1 draft (= Technical Report 8) (November 2019)
OpenMP 5.1 draft (= Technical Report 9) (August 2020)
OpenMP 5.1 (November 2020)
OpenMP 5.2 draft (= Technical Report 10) (July 2021)
OpenMP 5.2 (November 2021)
Technical Report (TR) 11 (= first OpenMP 6.0 preview) (November 2022)

Automatic Parallelization

(-ftree-parallelize-loops)

Streamization

Test Suites and Benchmarks

OpenMP Validation Suite by HLRS, Univ. Stuttgart and Univ. of Houston (2007 version at UH: README, Download)
OpenMP task test suite by BSC
SPEC OMP
EPCC Microbenchmarks
NAS Benchmarks (unofficial C version)
LLNL benchmark
Rodinia Benchmark suite (2): OpenMP, OpenCL, CUDA benchmark

TODO List

(Outdated!)

Feel free to add new items to this list as you run into issues or features that would be interesting to add. Send mail to the list and/or the GCC OpenMP maintainers if any item in this list sounds interesting but is hard to understand.

Fine tune the auto scheduling feature for parallel loops.
Implement untied tasks (no compliance issue; needs to be well tuned to be actually faster; cf. page 53 of pdf) - see also next item
Tasks need some tuning in taskwait. Cf. GCC email, comparison. Algorithms: PDF 1, PDF 2, PDF 3
- A comparison for tasks between libgomp, another library (gcc+libKOMP) and Intel (cf. benchmark on p. 10); see also another IOMP 2012 comparison
taskyield is a stub and mergeable task clonning could be optimized
OpenMP 4.0 (specifications released on July 2013). Cf. slides at the IWOMP, the International Workshop for OpenMP (slides and tutorials) in June 2010 and the OWOMP 2010 proceedings. There is also a blog entry. See also IWOMP 2012's talks and the committee report. SC2011 (November 2011): OpenMP Lang Committee Report, CEO report
- And the 2012 slides: IWOMP program, Language Committee report
- And the OpenMP 4.0 release candidate documents: OpenMP v4.0rc2 specification (March 2013) (OpenMP 4.0 API forum, rc2 changes)
- And a comment prior to the final release: OpenMP 4.0 about to be released and IWOMP 2013, by Michael Wong (May 2013)