[Bug fortran/31593] Invariant DO loop variables and subroutines

[tkoenig at gcc dot gnu dot org]

------- Comment #6 from tkoenig at gcc dot gnu dot org  2009-08-12 17:53 -------
We get a dramatic speedup when enclosing the arguments to
print in parentheses:

This is something we can do for

- write and print statements
- intent(in) arguments
- any do variable passed as an argument within the loop

$ gfortran -O2 count-2.f90 && time ./a.out > /dev/null

real    0m11.395s
user    0m11.361s
sys     0m0.004s
$ gfortran -O2 count-3.f90 && time ./a.out > /dev/null

real    0m4.607s
user    0m4.560s
sys     0m0.040s

$ cat count-2.f90
program main     
  implicit none  
  integer :: value
  integer :: p1, p2, p3, p4
  integer :: i             

  do value = 750,800
     do i=1, 10     
        do p1 = 1, value-2
           do p2 = p1 + 1, value - p1
              do p3 = p2 + 1, (value - (p1 + p2))/2
                 p4 = value - p1 - p2 - p3         
                 if (p1 * p2 * p3 * p4 == value * 1000000) &
                      print '(5(I0,:" "))',value,p1,p2,p3,p4
              end do                                        
           end do
        end do
     end do
  end do
end program main
$ cat count-3.f90
program main
  implicit none
  integer :: value
  integer :: p1, p2, p3, p4
  integer :: i

  do value = 750,800
     do i=1, 10
        do p1 = 1, value-2
           do p2 = p1 + 1, value - p1
              do p3 = p2 + 1, (value - (p1 + p2))/2
                 p4 = value - p1 - p2 - p3
                 if (p1 * p2 * p3 * p4 == value * 1000000) &
                      print '(5(I0,:" "))',(value),(p1),(p2),(p3),(p4)
              end do
           end do
        end do
     end do
  end do
end program main


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31593