Specifying the Dependency Rules

In the following example:

if K>N no dependency; if K<N there is a dependency.

The value of K is unknown to the compiler , thus the

compiler will assume dependencies.

The ivdep directive can be used to

communicate to the compiler the

data dependency rules.

IVDEP = Ignore Vector DEPendency

SUBROUTINE DAXPYI(N,X,K,A)

INTEGER N,K

REAL*8 X(N),A

DO I=1,N

X(K+I) = X(K+I) + A*X(I)

ENDDO

END

Compiler schedules:

K<N (dependence) 14% peak

K>N (no dependence) 33% peak

SUBROUTINE DAXPYI(N,X,K,A)

INTEGER N,K

REAL*8 X(N),A

cdir$ ivdep

DO I=1,N

X(K+I) = X(K+I) + A*X(I)

ENDDO

END

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