fortran - Alignment of multi-dimensional array for omp simd -

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if understand aligned clause of omp simd construct, refers alignment of whole array.

how used multi-dimensional arrays? assume

ni = 131; nj = 137; nk = 127  !allocates  arr(1:131,1:137,1:127) aligned 64-bytes call somehow_allocate_aligned(arr, [ni,nj,nk], 64)  !$omp parallel collapse(2) k = 1, nk   j = 1, nj      call some_complicated_subroutine(arr(:,j,k))      !$omp simd aligned(arr:64)     = 1, ni       arr(i,j,k) = arithmetic expression involving arr(i,j,k)     end   end end !$omp end parallel

is correct way indicate alignment of array although iteration of inner loop starts @ arr(1,j,k)?

how compiler use information infer alignment of inner loop subarray?

does matter performance if run-time sizes nicer (say 128, 128, 128)?

it explained here, slides 160-165 : http://irpf90.ups-tlse.fr/files/parallel_programming.pdf

you should

1) align array

2) use padding force columns aligned : first dimension (specified in allocate statement) should multiple of number of elements reach 16, 32 or 64 -byte boundary depending on instruction set.

for example, 99x29x200 matrix avx instruction set (32 bytes alignment) in double precision (8 bytes/element), should do

n = 99 l = 29 m=200  delta_n = mod(n,32/8) if (delta_n == 0)   n_pad = n else   n_pad = n-delta_n+32/8 end if  allocate( a(n_pad,l,m) ) !dir$ attributes align : 32 ::  k=1,m   j=1,l     !$omp simd     i=1,n       a(i,j,k) = ...     end   end end

you can use c preprocessor make portable code replacing 32 , 8 in previous example.

note : careful using statements such b=a arrays, physical dimensions not correspond logical dimensions. practice set boundaries b(1:n,1:l,1:m) = a(1:n,1:l,1:m) still work if change physical dimensions.


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