CODEWITHSUNDEEP
optimizationfortranlapackblas
sponce
sponce

Reputation: 1399

Optimization of conditional Fortran loop

I would like to optimize for speed the following block of code:

DO i=1, dim1
  DO j=1, dim2
    DO k=1, dim3
      IF (A(k,j,i)>0) &
        B(k,j,i) = exp(C(k))/A(k,j,i)
    ENDDO
  ENDDO
ENDDO

Very importantly, A is an INTEGER and B and C are COMPLEX!

There are two issues: 1) How to replace this by a BLAS/LAPACK call? The issue is the condition. 2) Calculation of the exp is slow. How to speed that up?

Upvotes: 0

Views: 293

Answers (2)

RussF
RussF

Reputation: 721

I ran a couple of tests with idim[1-3] being various permutation of [40,40,1000] and found that using a temporary array for the exponential and keeping the original loop ordering to be fastest by a factor of 2 or more than the other answer supplied. You milage may vary with compiler etc.

d=exp(c)
DO i=1, dim1
  DO j=1, dim2
    DO k=1, dim3
      IF (A(k,j,i)>0) &
        B(k,j,i) = d(k)/A(k,j,i)
    ENDDO
  ENDDO
ENDDO

Upvotes: 3

user1196549
user1196549

Reputation: 

DO k=1, dim3
  expCk= exp(C(k))
  DO i=1, dim1
    DO j=1, dim2
      IF (A(k,j,i)>0) &
        B(k,j,i) = expCk/A(k,j,i)
    ENDDO
  ENDDO
ENDDO

I don't think that any BLAS/LAPACK function can be of help here. Inversion of matrix elements is not an operation encountered in linear algebra problems.

Upvotes: 2

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