[Bug fortran/31593] New: Invariant DO loop variables (and I/O)

[tkoenig at gcc dot gnu dot org]

Compare this:

$ cat count.f90
program main
  implicit none
  integer, parameter :: value = 747
  integer :: p1, p2, p3, p4
  integer :: i

  do i=1, 10
     do p1 = 1, value-2
        do p2 = p1 + 1, value - p1
           do p3 = p2 + 1, value - p1 - p2
              p4 = value - p1 - p2 - p3
              if (p1 * p2 * p3 * p4 == value * 1000000) &
                   print '(4(I0,:" "))',p1,p2,p3,p4
           end do
        end do
     end do
  end do
end program main
$ gfortran -O3 count.f90
$ time ./a.out > /dev/null

real    0m0.634s
user    0m0.620s
sys     0m0.004s


with the equivalent

$ cat count.c
#include <stdio.h>

const int value = 747;

int main()
{
  int i, p1, p2, p3, p4;

  for (i=1; i<=10; i++)
    {
      for (p1 = 1; p1 <= value - 2 ; p1++)
        {
          for (p2 = p1 + 1; p2 <= value - p1; p2++)
            {
              for (p3 = p2 + 1; p3 <= value - p1 - p2; p3++)
                {
                  p4 = value - p1 - p2 - p3;
                  if (p1 * p2 * p3 * p4 == value * 1000000)
                    printf("%d %d %d %d\n", p1, p2, p3, p4);
                }
            }
        }
    }
}
$ gcc -O3 count.c
$ time ./a.out > /dev/null

real    0m0.396s
user    0m0.380s
sys     0m0.000s


We don't

- take advantage of the fact that p1, p2, p3 and p4 cannot
  be legally changed in the do loop

- take advantage of the fact that the I/O statemetns don't change
  the values

so we miss out on common subexpression elimination.  Here's a
part from the *.optimized file:

<L34>:;
  p2.3 = p2;
  p3.5 = p2.3 + 1;
  D.1014 = (747 - p1) - p2.3;
  p3 = p3.5;
  if (p3.5 <= D.1014) goto <L35>; else goto <L12>;

<L35>:;
  p1.58 = p1;
  p2.60 = p2;
  p3.61 = p3;
  p4.7 = ((747 - p1.58) - p2.60) - p3.61;
  p4 = p4.7;
  if (((p2.60 * p1.58) * p3.61) * p4.7 == 747000000) goto <L8>; else goto <L9>;


-- 
           Summary: Invariant DO loop variables (and I/O)
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: fortran
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: tkoenig at gcc dot gnu dot org


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