The MIL-STD-1753 features have been implemented so widely that many users have forgotten that they are not part of FORTRAN 77. They are included in the HPF subset.
The biggest addition to FORTRAN 77 in the HPF subset language is the inclusion of the array language. A number of vendors have identified the usefulness of array operations for concise expression of parallelism and already support these features. However, the character array language is not part of the subset.
The new storage classes such as allocatable, automatic, and assumed-shape objects are included in the subset. They provide an important alternative to the use of storage association features such as EQUIVALENCE for memory management.
Interface blocks have been added to the subset in order to facilitate use of the HPF directives across subroutine boundaries. The interface blocks provide a mechanism to specify the expected mapping of data, in addition to the types and intents of the arguments.
There were other Fortran 90 features considered for the subset. Some features such as CASE or NAMELIST were recognized as popular features of Fortran 90, but had no direct bearing on high performance. Other features such as support for double precision complex (via KIND) or procedureless MODULES were rejected because of the perception that the additional implementation complexity might delay release of subset compilers. It was not a goal of HPFF to define an ``ideal'' subset of Fortran 90 for all purposes.
Additional syntactic improvements are included, such as long names and the ``!'' form of comments, because of their general usefulness in program documentation, including the description of HPF itself. (End of rationale.)