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Next: Intrinsic and Library Up: The INDEPENDENT Directive Previous: Examples of INDEPENDENT

Visualization of INDEPENDENT Directives

Graphically, the INDEPENDENT directive can be visualized as eliminating edges from a precedence graph representing the program. Figure shows some of the dependences that may normally be present in a DO and a FORALL. (Most of the transitive dependences are not shown.) An arrow from a left-hand side node (for example, ``lhsa(1)'') to a right-hand side node (``rhsb(1)'') means that the right-hand side computation might use values assigned in the left-hand side node; thus the right-hand side must be computed after the left-hand side completes its store. Similarly, an arrow from a right-hand side node to a left-hand side node means that the left-hand side may overwrite a value needed by the right-hand side computation, again forcing an ordering. Edges from the ``BEGIN'' and to the ``END'' nodes represent control dependences. The INDEPENDENT directive asserts that the only dependences that a compiler need enforce are those in Figure . That is, the programmer who uses INDEPENDENT is certifying that if the compiler enforces only these edges, then the resulting program will be equivalent to the one in which all the edges are present. Note that the set of asserted dependences is identical for INDEPENDENT DO and FORALL constructs.

The compiler is justified in producing a warning if it can prove that one of these assertions is incorrect. It is not required to do so, however. A program containing any false assertion of this type is not HPF-conforming, thus is not defined by HPF, and the compiler may take any action it deems appropriate.