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Labelled DO loop and /opt /64 leads to optimisation bug

 
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mecej4



Joined: 31 Oct 2006
Posts: 1886

PostPosted: Sat Mar 09, 2024 6:35 pm    Post subject: Labelled DO loop and /opt /64 leads to optimisation bug Reply with quote

The following is what is left after trimming a 25,000 LOC Fortran 95 program to pin down a suspected optimisation bug.

Code:
program labDObug
   implicit none
   integer, parameter :: nx = 5, ny = 1, nz = 1
   integer i, j, k, ij, ik
   integer :: np(nx,ny), ktop(nx)

   np(:,1) = (/ (i, i=1,nx) /)
   ktop = 0
   do j = 1,ny
      iLoop: DO i = 1, nx
         ij = (j-1)*nx + i
         do k = nz, 1, -1
            ik = (k-1)*nx + i
            if (np(ik,j) /= 0) then
               ktop(ij) = k
               cycle iLoop
            endif
         end do !k
      end do iLoop
   end do !j
   print '(1x,10i5)',ktop
end program


The output when the options /opt /64 are used:

Code:
     1    0    0    0    0


The correct output, which is obtained with /64 alone, or in 32-bit compilations with or without /opt:

Code:
     1    1    1    1    1


In this tiny program, replacing "cycle iLoop" by "exit" makes the bug go away.
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JohnCampbell



Joined: 16 Feb 2006
Posts: 2554
Location: Sydney

PostPosted: Sun Mar 10, 2024 5:31 am    Post subject: Reply with quote

The following update identifies that ij is not being updated in the do i loop if /opt is selected
Code:
program labDObug
 use iso_fortran_env
   implicit none
   integer, parameter :: nx = 5, ny = 1, nz = 1
   integer i, j, k, ij, ik, n
   integer :: np(nx,ny), ktop(nx)

   write (*,*) 'Vern : ',compiler_version ()
   write (*,*) 'Opts : ',compiler_options ()

   np(:,1) = (/ (i, i=1,nx) /)
   ktop = 0
   n = 0
   do j = 1,ny
      iLoop: DO i = 1, nx
         ij = (j-1)*nx + i
         do k = nz, 1, -1
            n = n+1
            ik = (k-1)*nx + i
            if (np(ik,j) /= 0) then
               ktop(ij) = k
               cycle iLoop
            endif
         end do !k
      end do iLoop
   end do !j
   print '(1x,10i5)',ktop
   print '(1x,10i5)',n,ij

end program
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PaulLaidler
Site Admin


Joined: 21 Feb 2005
Posts: 7924
Location: Salford, UK

PostPosted: Mon Mar 18, 2024 10:28 am    Post subject: Reply with quote

mecej4 and John

Thank you for the feedback. This failure has now been fixed for the next release of FTN95. The optimisation in question is number 30.
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