[Bug libfortran/19032] modulo generates wrong result for divisor 1 and -1

["Thomas dot Koenig at online dot de" <gcc-bugzilla@gcc.gnu.org>]

------- Additional Comments From Thomas dot Koenig at online dot de  2004-12-17 14:27 -------
I've adjusted the subject.

I've had a look at the real modulo and mod case,
but didn't quite understand it - it appeared to
be overly complicated, compared to the straightforward
formula a - floor(a/b)*b.  I'm probably missing something.

Also, here's a new test case.

$ cat mod-array.f90
program main
  integer, parameter :: n=16
  real, dimension(n) :: ar, br, modulo_result, floor_result
  integer, dimension(n) :: ai, bi , imodulo_result, ifloor_result

  ai(1:4) = 5
  ai(5:8) = -5
  ai(9:12) = 1
  ai(13:16) = -1
  bi(1:4) = (/ 3,-3, 1, -1/)
  bi(5:8) = bi(1:4)
  bi(9:12) = bi(1:4)
  bi(13:16) = bi(1:4)
  ar = ai
  br = bi
  modulo_result = modulo(ar,br)
  imodulo_result = modulo(ai,bi)
  floor_result = ar-floor(ar/br)*br
  ifloor_result = nint(real(ai-floor(real(ai)/real(bi))*bi))

  do i=1,n
     if (modulo_result(i) /= floor_result(i) ) then
        print "(A,4F5.0)" ,"real case failed: ", &
             ar(i),br(i), modulo_result(i), floor_result(i)
    end if
    if (imodulo_result(i) /= ifloor_result(i)) then
       print "(A,4I5)", "int case failed: ", &
            ai(i), bi(i), imodulo_result(i), ifloor_result(i)

     end if
  end do
end program main
$ gfortran mod-array.f90 && ./a.out
real case failed:    5.  -1.  -1.   0.
int case failed:     5   -1   -1    0
real case failed:   -5.   1.   1.   0.
int case failed:    -5    1    1    0
real case failed:    1.  -1.  -1.   0.
int case failed:     1   -1   -1    0
real case failed:   -1.   1.   1.   0.
int case failed:    -1    1    1    0


-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|modulo generates wrong      |modulo generates wrong
                   |result for divisor -1       |result for divisor 1 and -1


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19032