Fortran Programming Language :: Vector-values functions slow?


	Vector-values functions slow? Dear newsgroup, I encountered some problems with vector-valued functions in F90/F95. Here, is a simple demo-program: program quick implicit none integer, parameter :: dp=8 real(dp), dimension(:), pointer :: u,v,r real :: tt0,tt1 integer :: i,n n=1000000 allocate(u(n),v(n),r(n)) u=2; v=2 call cpu_time(tt0) do i=1,1000 r=feval(u,v) end do call cpu_time(tt1) print , tt1-tt0 call cpu_time(tt0) do i=1,1000 call seval(u,v,r) end do call cpu_time(tt1) print , tt1-tt0 deallocate(u,v,r) contains function feval(u,v) result(r) real(dp), dimension(:), intent(in) :: u,v real(dp), dimension(size(u)) :: r r=uv end function feval subroutine seval(u,v,r) real(dp), dimension(:), intent(in) :: u,v real(dp), dimension(:), intent(out) :: r r=uv end subroutine seval end program quick I compiled it both with Ifort (9.1) and G95 (0.91) and see, that functions are much slower. My question is why? Nothing needs to be allocated on-the-fly. Shouldn't the code perform similar in both cases? Here are the timing: G95 function: 19.70523 subroutine: 5.620352 Ifort function: 9.824615 subroutine: 5.768361 Many thanks in advance, Matthias Moeller Matthias Mller wrote: > I compiled it both with Ifort (9.1) and G95 (0.91) and see, that > functions are much slower. My question is why? Nothing needs to be > allocated on-the-fly. Shouldn't the code perform similar in both cases? The function result needs to be allocated on the fly, and depending on compiler options and its size that may be on the heap or on the stack. Perhaps more importantly, the function result also needs to be copied from the internal variable to the the parent r on assignment , whereas the assignment takes place once in the case of the subroutine. You didn't indicate what optimization options you used, but I'd be interested to see if ifort didn't do a lot better in the function case with inter-procedural optimization enabled -- with inlining, it's possible the compiler would be able to eliminate the extra assignment. Matthias Mller wrote: > Dear newsgroup, > I encountered some problems with vector-valued functions in F90/F95. > Here, is a simple demo-program: > program quick > implicit none > integer, parameter :: dp=8 > real(dp), dimension(:), pointer :: u,v,r these really should use allocatable (not pointer).