Assumed shape array discussion in Fortran Wiki

What are the performance considerations of assumed shape arrays?
* [Sun f90 bad optimization for assumed-shape array](http://groups.google.com/group/comp.lang.fortran/browse_thread/thread/bf8f615e331a81c0/) on [[comp.lang.fortran]]
* [Pros and cons of assumed-shape arrays](http://groups.google.com/group/comp.lang.fortran/browse_frm/thread/ee1ca98c4930c9e7/) on [[comp.lang.fortran]]
* [Why is Fortran9x slower than Fortran77?](http://softwarecommunity.intel.com/isn/Community/en-US/forums/thread/30231353.aspx)
* [LAPACK95 performance statement](http://www.netlib.org/lapack95/lug95/node66.html)
and [tables](http://www.netlib.org/lapack95/lug95/node67.html)
* [Fortran Arguments Passing Test Sets](http://ftp.aset.psu.edu/pub/ger/fortran/test/FortranArgPassing/)
* [Using Arrays Efficiently](http://www.intel.com/software/products/compilers/flin/docs/for_ug2/prg_arrs.htm) -
 from the [[Intel Fortran compiler|Intel Fortran Compiler]] for Linux User's Guide, Volume 2,
 Optimizing Applications.
* [efficiency of explicit vs. assumed shape arrays](http://groups.google.com/group/comp.lang.fortran/browse_frm/thread/d66e3e3ad31effa9/) on [[comp.lang.fortran]].
* [Passing Arrays to Subprograms - Adjustable and Assumed Size Arrays](http://www.ibiblio.org/pub/languages/fortran/ch2-4.html) in User Notes on Fortran Programming.
Examples of the two primary alternatives:
~~~~~~~~~~ {: lang=fortran }
! 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_auto
~~~~~~~~~~
One 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.