Co-arrays in the next Fortran Standard

Issue title: Fortran Programming Language and Scientific Programming: 50 Years of Mutual Growth

Article type: Research Article

Authors: Reid, John | Numrich, Robert W.

Affiliations: JKR Associates, Convener, ISO/IEC Fortran Committee WG5, UK. E-mail: j.k.reid@rl.ac.uk | Minnesota Supercomputing Institute, University of Minnesota, Minneapolis, MN, USA

Abstract: The WG5 committee, at its meeting in Delft, May 2005, decided to include co-arrays in the next Fortran Standard. A Fortran program containing co-arrays is interpreted as if it were replicated a fixed number of times and all copies were executed asynchronously. Each copy has its own set of data objects and is called an image. The array syntax of Fortran is extended with additional trailing subscripts in square brackets to give a clear and straightforward representation of access to data on other images. References without square brackets are to local data, so code that can run independently is uncluttered. Any occurrence of square brackets is a warning about communication between images. The additional syntax requires support in the compiler, but it has been designed to be easy to implement and to give the compiler scope both to apply its optimizations within each image and to optimize the communication between images. The extension includes execution control statements for synchronizing images and intrinsic procedures to return the number of images, to return the index of the current image, and to perform collective operations. The paper does not attempt to describe the full details of the feature as it now appears in the draft of the new standard. Instead, we describe a subset and demonstrate the use of this subset with examples.

Journal: Scientific Programming, vol. 15, no. 1, pp. 9-26, 2007