Another language issue with HPF was limited support for task parallelism. Users wanted more powerful strategies beyond the parallel loop feature offered in HPF. This was corrected in HPF 2.0 but too late for a revival of HPF. Including features like the OpenMP set of directives could have helped.

Immature compiler technology. HPF was defined on top of Fortran 90, the first major upgrade to the Fortran standard since 1977. Among the new features of Fortran 90 were explicit interfaces requiring array descriptors, recursion, dynamic storage allocation, and pointer-based data structures. Building a Fortran 90 compiler meant a substantial implementation effort compared with FORTRAN 77—a huge obstacle on the way to HPF.

Moreover, the features in HPF, including the HPF library, demanded new compilation strategies that in 1993 at the time of the release of HPF 1.0 had been implemented only in research compilers and the CM Fortran compiler. Proper compilation of HPF requires extensive global analysis of distributions, partitioning of computation, generation of communication, and optimizations (such as overlapping communication and computation).

Finally, efficient implementation of HPF programs required special attention to locality on individual processors. Since many of the processors used in distributed-memory systems had complex cache hierarchies, advanced transformation strategies (such as loop tiling) were becoming essential to generate efficient code. At the time of the release of HPF 1.0, these techniques were beginning to be understood by compiler developers, but most commercial compilers had not yet incorporated them. As a result the first compilers were immature, providing disappointing performance on many codes.

Barriers to portable performance. A key goal of HPF was enabling a single version of a parallel program to achieve a significant fraction of the possible performance on a variety of architectures but was difficult to achieve for two main reasons:

Different optimizations. Different
vendors focused on different optimiza-
tions in their implementations, caus-
ing a single HPF application to deliver
dramatically different performance on
machines from different vendors. In
turn, programmers were forced to re-
code their applications repeatedly to
take advantage of the strengths (and
avoid the weaknesses) of each vendor’s
implementation, thwarting the original
goal of portability.

significant influence

HPF also had significant influence on development of high-level parallel languages. In 2007, the CiteSeer database listed 827 citations for the language definition, 12 making it the 21st most-cited document in computer science at the time. In addition, more than 1,500 publications in CiteSeer refer to “High Performance Fortran,” many including approaches to implementing or improving the language, reflecting a great deal of intellectual activity within the academic community.

Fortran and its variants. Fortran 95. While the meetings leading to HPF 1. 1 were under way, the X3J3 committee of