CODEWITHSUNDEEP
fortranvectorizationgfortran
bestrong
bestrong

Reputation: 151

Could the compiler vectorize the looping with an array which consists of an array inside?

I would like to vectorize this code below (just for an example), just assume somehow I should write an array inside an array. 

PROGRAM TEST

  IMPLICIT NONE
  REAL, DIMENSION(2000):: A,B,C  !100000
  INTEGER, DIMENSION(2000):: E
  REAL(KIND=8):: TIME1,TIME2
  INTEGER::I

  DO I=1, 2000       !Actually only this loop could be vectorized
     B(I)=100.00     !by the compiler 
     C(I)=200.00
     E(I)=I  
  END DO

  !Computing computer's running time (start)
  CALL CPU_TIME (TIME1)

  DO I=1, 2000               !This is the problem, somehow I should put
     A(E(I))=B(E(I))*C(E(I)) !an integer array E(I) inside an array 
  END DO                     !I would like to vectorize this loop also, but it didn't work

  PRINT *, 'Results  =', A(2000)
  PRINT *, '   '

  !Computing computer's running time (finish)
  CALL CPU_TIME (TIME2)

  PRINT *, 'Elapsed real time = ', TIME2-TIME1, 'second(s)'

END PROGRAM TEST

I thought at first time, that compiler could understand what I want which somehow be vectorized like this:

DO I=1, 2000, 4   !Unrolled 4 times
   A(E(I))=B(E(I))*C(E(I))
   A(E(I+1))=B(E(I+1))*C(E(I+1))
   A(E(I+2))=B(E(I+2))*C(E(I+2))
   A(E(I+3))=B(E(I+3))*C(E(I+3))
END DO

but I was wrong. I used: gfortran -Ofast -o -fopt-info-optimized Tes.F95 and I got the information that only the first looping was successfully to be vectorized.

Do you have any idea how I could vectorize it? Or can't it be vectorized at all?

Upvotes: 0

Views: 123

Answers (2)

Jonatan Öström
Jonatan Öström

Reputation: 2609

If E hase equal values for different I, then you would be manipulating the same elements of A multiple times, in which case the order could matter. (Though not in your case.) Also, if you have multiple index arrays, like E1, E2 and E3, and

DO I=1, 2000              
   A(E3(I))=B(E1(I))*C(E2(I)) 
END DO

the order could matter too. So I think this kind of indexing is not in general allowed in parallel loops.

Upvotes: 1

Holmz
Holmz

Reputation: 724

With ifort one can use !DIR$ IVDEP which is "ignore Vector dependence". It only works when E(I) is linear as in the example...

Assuming that one wants to do all the indexes then just replace (E(i)) with (I) and work out the obvious E(I) order later...

Upvotes: 0

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