Three Types of IVDEP Directive

• The IVDEP directive to the compiler is not part of any language and its interpretation is not standardized.

• • Default interpretation:
    • A and B and C are independent, that breaks both, lexically forward (i+k) and backward (i-k) dependencies.
      • index(1,i) != index(1,j)
      • index(2,i) != index(2,j)
      • But for some I: index(1,*) == index(2,*)
    • The default interpretation can be changed with the -OPT: compiler option. Possible other interpretations:
      • break only lexically backward dependencies (Cray IVDEP), I.e. assume only index(*,i)!=index(*,i-k) (cray_ivdep=on)
      • there are no dependencies what so ever (Liberal IVDEP, enable with -OPT:liberal_ivdep=on)

CDIR$ IVDEP

DO I=1,N

A(INDEX(1,I)) = B(I)

A(INDEX(2,I)) = C(I)

ENDDO

SGI default behaviour:

A and B and C are independent,

i.e. index(*,i) != index(*,j)

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