Derived data type memory access

[Gordey_G_]

Hello everyone!

We have found strange behaviour of the following simple program that implements jacobi-like iteration with array of derived data type (block_t). This type consists of one-dimensional allocatable array. Pointer introduction (p1, p2) leads to the decrease in execution time from 3.2 seconds to 1.54 seconds on  Intel(R) Xeon(R) CPU X5355 @ 2.66GHz and compiler ifort 15.0.2 20150121 with -O3 option. Also we checked it on Intel i3-3220 3.3GHz and compiler ifort 18.0.0 20170811. It is remarkable that without if-condition in the loops, execution time becomes actually the same and equals 0.92 seconds.

Can anybody explain such a behaviour?

Thank you in advance.

Gordey.

program main
  implicit none
  
  type :: block_t
    real(kind=8), allocatable :: b(:)
  end type block_t
  
  type(block_t), allocatable, target :: var1(:), var2(:)
  real(kind=8), pointer, contiguous :: p1(:), p2(:)
  logical(kind=1), allocatable :: mask(:)
   
  integer(kind=4) :: nx, nb, nit
  integer(kind=4) :: i , ib, ni
  real   (kind=8) :: ts, te
  
  nx = 1000
  nit = 1000000
  nb = 1
  
  allocate(var1(nb),var2(nb))
  allocate(var1(nb).b(0:nx+1),var2(nb).b(0:nx+1))
  allocate(mask(0:nx+1))
  
  var1(1).b = 1
  var2(1).b = 2
  mask = .true.
  
  call CPU_time(ts)
  do ni = 1, nit
    do ib = 1, nb
      do i = 1, nx
	if (mask(i)) then
	  var2(ib).b(i) = 0.5_8 * (var1(ib).b(i+1) + var1(ib).b(i-1))
	end if
      end do
      var1(ib).b = var2(ib).b
    end do
  end do
  call CPU_time(te)
  print*, 'no pointer time', te - ts
  
  call CPU_time(ts)
  do ni = 1, nit
    do ib = 1, nb
      p1 => var1(ib).b
      p2 => var2(ib).b
      do i = 1, nx
	if (mask(i)) then
	  p2(i) = 0.5_8 * (p1(i+1) + p1(i-1))
	end if
      end do
      var1(ib).b = var2(ib).b
    end do
  end do
  call CPU_time(te)
  print*, 'pointer time', te - ts
  
end program main