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What are the performance considerations of assumed shape arrays?
Sun f90 bad optimization for assumed-shape array on comp.lang.fortran
Using Arrays Efficiently - from the Intel Fortran Compiler for Linux User’s Guide, Volume 2, Optimizing Applications.
efficiency of explicit vs. assumed shape arrays on comp.lang.fortran.
Passing Arrays to Subprograms - Adjustable and Assumed Size Arrays in User Notes on Fortran Programming.
Examples of the two primary alternatives:
! This is a subroutine using assumed shape array
subroutine test_as(array)
real, intent(in), dimension(:) :: array
! ...
end subroutine test_as
! This is a subroutine using automatic arrays
subroutine test_auto(array, n)
real, intent(in), dimension(n) :: array
integer, intent(in) :: n
! ...
end subroutine test_autoOne of the major differences is that an assumed-shape array can be "strided". If you pass an array section, such as the third column in a 2D array, the caller can pass an array reference that points to strided (non-contiguous) data in memory. That can be an advantage, because the source array does not have to be packed into a contiguous temporary array. However, when the routine is called with contiguous arrays, the optimization can be hindered.
Another difference, especially for multi-dimensional arrays. is that a small, fixed inner dimension can be optimized more efficiently. If a coordinate array always has dimension (3,*), there is no need to implement a loop over the 3 inner elements. If declared (:,:), there is a good chance that a loop will be used.