Data Parallel Execution

The HPF data distribution directives merely tell the compiler how to distribute arrays amongst the available processors, they do not express parallelism.

Parallelism is expressed through the use of special (often Fortran 90) statements such as array assignments, intrinsic calls with array arguments or any of the new HPF features:

• FORALL statement -- flexible data parallel assignment statement.
• PURE procedures -- `side-effect free' user procedures for parallel execution.
• INDEPENDENT directive -- perform loop iterations in parallel.
• NEW variables -- achieve independence using new variables.

FORALL and INDEPENDENT / INDEPENDENT NEW DO loops allow parallel assignment to arrays without actually using an array assignment statement. They are very expressive language features and often replace the need for contorted and multiply nested calls to array valued Fortran 90 intrinsics. PURE procedures are simply procedures that can be invoked from within a FORALL statement or construct. They are side-effect free. All Fortran 90 intrinsic procedures are PURE.

Both FORALL and PURE are included in the new Fortran standard, informally known as Fortran 95.

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