[Bug fortran/31711] rhs array is changed while assiging to same lhs array

[burnus at gcc dot gnu dot org]

------- Comment #5 from burnus at gcc dot gnu dot org  2007-04-26 20:50 -------
> This can be seen in the file (result.dat) generated by the program.

I missed that part. Reduced test:

program laplsolv
  IMPLICIT NONE
  integer, parameter                  :: n=10
  double precision,dimension(0:n+1,0:n+1) :: T
  double precision,dimension(n)       :: tmp1,tmp2
  integer                             :: i

  T=0.0
  T(0:n+1 , 0)     = 1.0
  T(0:n+1 , n+1)   = 1.0
  T(n+1   , 0:n+1) = 2.0

  tmp1=T(1:n,0)
  T(1:n,1)=(T(0:n-1,1)+T(1:n,1+1)+tmp1)

  do i = 1, n
    print '(20f4.1)', T(i,1)
  end do
end program laplsolv

The "T(1:n,1)=(T(0:n-1,1)+T(1:n,1+1)+tmp1)" get wrongly translated as:

  {
    int8 S.4;
    S.4 = 1;
    while (1)
      {
        if (S.4 > 10) goto L.5;
        t[S.4 + 12] = (t[S.4 + 11] + t[S.4 + 24]) + tmp1[S.4 + -1];
        S.4 = S.4 + 1;
      }
    L.5:;
  }

It should be something like:
  for(int i = 1; i < 10; i++) {
    t.1[i + 12 ] = (t[S.4 + 11] + t[S.4 + 24]) + tmp1[S.4 + -1];
  for(int i = 1; i < 10; i++) {
    t[i + 12] = t.1[i + 12]

That is: First the right-hand side needs to be evaluated before the lhs is
written.


-- 

burnus at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |burnus at gcc dot gnu dot
                   |                            |org
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
           Keywords|                            |wrong-code
   Last reconfirmed|0000-00-00 00:00:00         |2007-04-26 20:50:32
               date|                            |
            Summary|Data parallel code gets     |rhs array is changed while
                   |transformed to serial       |assiging to same lhs array


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31711