在 Fortran 中将数组传递给子例程：假定形状与显式形状

问：

当将数组传递给过程时，在 （1） 速度和 （2） 内存、假定形状或显式形状方面，什么最好？前段时间在这个论坛上也问过一个类似的问题，但不是用这些术语：在 Fortran 过程中将大小作为参数传递 VS 假设形状

我提供了一个简单的程序来说明我的意思

! Compile with
! ifort /O3 main.f90 -o run_win.exe

module mymod
    USE iso_Fortran_env, ONLY: dp => real64
    implicit none
    private
    public :: dp, sub_trace, sub_trace_es
    
    contains
    
    subroutine sub_trace(mat,trace)
    ! Assumed shape
        implicit none
        real(dp), intent(in) :: mat(:,:)
        real(dp), intent(out) :: trace
        real(dp) :: V(size(mat,dim=1))
        integer :: i,N
        
        if (size(mat,dim=1) /= size(mat,dim=2)) then
            error stop "Input matrix is not square!"
        endif
        
        N = size(mat,dim=1)
        do i=1,N
            V(i) = mat(i,i)
        enddo
        trace = sum(V)
    
    end subroutine sub_trace
    
    subroutine sub_trace_es(n,mat,trace)
    ! Passing array explicit shape
        implicit none
        integer, intent(in) :: n
        real(dp), intent(in) :: mat(n,n)
        real(dp), intent(out) :: trace
        real(dp) :: V(n)
        integer :: i
        
        do i=1,n
            V(i) = mat(i,i)
        enddo
        trace = sum(V)
    
    end subroutine sub_trace_es
    
end module mymod    
    
program main
    use mymod, only: dp, sub_trace,sub_trace_es
    implicit none
    integer, parameter :: nn=2
    real(dp) :: mat(nn,nn)
    real(dp), allocatable :: mat4(:,:)
    real(dp) :: trace1,trace2,trace3,trace4
    
    write(*,*) "Passing arrays to subroutines:"
    write(*,*) "Assumed-shape vs explicit shape"
    
    mat(1,:) = [2_dp,3_dp]
    mat(2,:) = [4_dp,5_dp]
    
    call sub_trace(mat,trace1)
    
    write(*,*) "trace1 = ", trace1
    
    call sub_trace_es(nn,mat,trace2)
    
    write(*,*) "trace2 = ", trace2
    
    ! First example offered by francescalus:
    call sub_trace_es(2,real([1,2,3,4,5,6,7,8,9],dp), trace3)
    
    write(*,*) "trace3 = ", trace3
    
    ! Second example
    mat4 = reshape(real([1,2,3,4,5,6,7,8,9],dp),[3,3])
    call sub_trace(mat4, trace4)
    
    write(*,*) "trace4 = ", trace4
        
    pause

end program