Memory alignment for multidimensional arrays
Multi-dimensional arrays need to be padded in thefastest-moving dimension, to ensure array sec9ons to be aligned at the desired byte boundaries : Fortran: first array dimension C/C++: last array dimension npadded= ((n +veclen– 1) /veclen) *veclen No alignment requested:veclen= 1 16-byte alignment (SSE):veclen= 4 (sp) or 2 (dp) 32-byte alignment (AVX2):veclen= 8 (sp) or 4 (dp) 64-byte alignment (AVX-512):veclen= 16 (sp) or 8 (dp) Example: real, allocatable:: a(:,:), b(:,:), c(:,:) !dir$ attributes align : 32 :: a,b,c ... allocate (a(npadded,n)) allocate (b(npadded,n)) allocate (c(npadded,n)) ... do j=1,n do k=1,n !dir$ vector aligned do i=1,npadded c(i,j) = c(i,j) &+ a(i,k) * b(k,j) end do end do end do Source