Index Reversal

- Index reversal on B: i.e. B(I,J) replaced by B(J,I) must be done everywhere in the program
- This has to be done manually, there is no compiler optimization that does index reversal.

Original loop:

DO I=1,N

DO J=1,M

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

ENDDO ENDDO

The access is poor for A and C,

while it is optimal for B

Interchanged loops + Index reversal:

DO J=1,M

DO I=1,N

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

ENDDO ENDDO

interchange will be good for A and C,

it will be bad for B