Consider the following Fortran 90 program:
PROGRAM Warty
IMPLICIT NONE
REAL, DIMENSION(4) :: C
REAL, DIMENSION(8) :: D
REAL, DIMENSION(2) :: E
C = 1; D = 2
E = D(::4) + C(::2)
END PROGRAM Warty
Given the patterns of array references in the program Warty, we would make the decision to align the arrays as stated on the slide. Our main concern is to minimise the communications in a program.
The main assignment combines E(1) with D(1) and C(1); E(2) with D(5) and C(3); E(3) with D(9) and C(5) and so on. We can use the subscript-triplet expressions in the assignment as the basis for our alignment specification:
should be given these HPF directives to ensure minimal (zero) communications:
!HPF$ ALIGN C(:) WITH D(::2) !HPF$ ALIGN E(:) WITH D(::4) !HPF$ DISTRIBUTE (BLOCK) :: D
This alignment that is given will result in zero communications.
(Clearly this is an artificial example. In the real world, there will be many different assignments and it will be much tougher to choose a good alignment. This is one of the most difficult area of HPF programming.)
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