XL Fortran for AIX 8.1

Language Reference



When the CNCALL directive is placed before a DO loop, you are explicitly declaring to the compiler that no loop-carried dependencies exist within any procedure called from the DO loop.

This directive only takes effect if you specify either the -qsmp or -qhot compiler option.




The first noncomment line (not including other directives) that is following the CNCALL directive must be a DO loop. This line cannot be an infinite DO or DO WHILE loop. The CNCALL directive applies only to the DO loop that is immediately following the directive and not to any nested DO loops.

When specifying the CNCALL directive, you are explicitly declaring to the compiler that no procedures invoked within the DO loop have any loop-carried dependencies. If the DO loop invokes a procedure, separate iterations of the loop must be able to concurrently call that procedure. The CNCALL directive does not assert that other operations in the loop do not have dependencies, it is only an assertion about procedure references.

A loop-carried dependency occurs when two iterations within a DO loop interfere with one another. See ASSERT for the definition of interference.


! An example of CNCALL where the procedure invoked has
! no loop-carried dependency but the code within the
! DO loop itself has a loop-carried dependency.
         PROGRAM EX3
           INTEGER A(100)
           DO I = 1, N
             A(I) = A(I) * FNC3(I)
             A(I) = A(I) + A(I-1)    ! This has loop-carried dependency
           END DO
         END PROGRAM EX3
         FUNCTION FNC3 (I)
           FNC3 = I * I

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