Parallel Programming Guide for HP-UX Systems

Troubleshooting

Misused directives and pragmas

Chapter 9 189

values of the elements of JA never coincide with the values of I.

Assuming that they might collide, the compiler conservatively avoids

parallelizing the loop.

DO I = 1,100

JA(I) = I + 10

ENDDO

DO I = 1, 100

DO J = I, 100

A(I) = A(I) + B(J) * C(J) !LINE 6

A(JA(J)) = B(J) + C(J) !LINE 7

ENDDO

NOTE In this example, as well as the examples that follow, the

apparent dependence becomes real if any of the values of

the elements of JA are equal to the values iterated over by

A no_loop_dependence directive or pragma placed before the J loop tells

the compiler that the indirect subscript does not cause a true

dependence. Because reductions are a form of dependence, this directive

also tells the compiler to ignore the reduction on A(I), which it would

normally handle. Ignoring this reduction causes the compiler to generate

incorrect code for the assignment on line 6. The apparent dependence on

line 7 is properly handled because of the directive. The resulting code

runs fast but produces incorrect answers.

To solve this problem, distribute the J loop, isolating the reduction from

the other statements, as shown in the following Fortran example:

DO I = 1, 100

DO J = I, 100

A(I) = A(I) + B(J) * C(J)

ENDDO

C$DIR NO_LOOP_DEPENDENCE(A)

DO I = 1, 100

DO J = I, 100

A(JA(J)) = B(J) + C(J)

ENDDO