[Bug middle-end/79720] floating point result different at compile time / runtime

["tkoenig at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79720

--- Comment #4 from Thomas Koenig <tkoenig at gcc dot gnu.org> ---
(In reply to Dominique d'Humieres from comment #3)
> Where is computed 1./a? AFAICT the roundoff errors difference with
> optimization is restricted to this computation.

Yep, you're right... seems that constant of complex
division is off.

So here is a new, much shorter test case:

$ cat r3.f90
program main
  complex :: c1, c2, r1, r2
  character(len=50) :: inp
  c1 =   (12e10, -3.2)
  inp = '(12e10, -3.2)'
  read (unit=inp, fmt=*) c2 ! Hide the assignment from the optimizer
  print *,'c1 - c2 = ', c1-c2 ! Should be zero
  r1 = 1/c1 ! Compile-time assignment
  r2 = 1/c2 ! Run-time assignment
  print *,'r1 = ', r1
  print *,'r2 = ', r2
  print *,'r1 - r2 = ', r1-r2 ! Should be zero, but is not.
end program main
$ gfortran -O r3.f90
$ ./a.out
 c1 - c2 =              (0.00000000,0.00000000)
 r1 =    (8.333333155E-12,2.222222180E-22)
 r2 =    (8.333333155E-12,2.222221927E-22)
 r1 - r2 =         (0.00000000,2.524354897E-29)