Parallel Programming Guide for HP-UX Systems

Troubleshooting

Misused directives and pragmas

Chapter 9 187

Misused directives and pragmas

Misused directives and pragmas are a common cause of wrong answers.

Some of the more common misuses of directives and pragmas involve the

following:

• Loop-carried dependences

• Reductions

• Nondeterminism of parallel execution

Descriptions of and methods for avoiding the items listed above are

described in the sections below.

Loop-carried dependences

Forcing parallelization of a loop containing a call is safe only if the called

routine contains no dependences.

Do not assume that it is always safe to parallelize a loop whose data is

safe to localize. You can safely localize loop data in loops that do not

contain a loop-carried dependence (LCD) of the form shown in the

following Fortran loop:

DO I = 2, M

DO J = 1, N

A(I,J) = A(I+IADD,J+JADD) + B(I,J)

ENDDO

where one of IADD and JADD is negative and the other is positive.

You cannot safely parallelize a loop that contains any kind of LCD,

except by using ordered sections around the LCDs.

The MAIN section of the Fortran program below initializes A, calls CALC,

and outputs the new array values. In subroutine CALC, the indirect index

used in A(IN(I)) introduces a potential dependence that prevents the

compiler from parallelizing CALC’s I loop.

PROGRAM MAIN

REAL A(1025)

INTEGER IN(1025)

COMMON /DATA/ A