[Bug fortran/84867] Wrong code generated, except at -O0, with inappropriate Warning: iteration 1 invokes undefined behavior [-Waggressive-loop-optimizations]

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84867

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |RESOLVED
   Last reconfirmed|2018-03-14 00:00:00         |
                 CC|                            |jakub at gcc dot gnu.org
         Resolution|---                         |INVALID

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
That is undefined behavior, so the warning correctly tells you your code will
not really work properly.
GCC has -funconstrained-commons option you can use to make that sort of
defined.
'-funconstrained-commons'
     This option tells the compiler that variables declared in common
     blocks (e.g.  Fortran) may later be overridden with longer trailing
     arrays.  This prevents certain optimizations that depend on knowing
     the array bounds.
Or you can use -fno-aggressive-loop-optimizations, though the hack is still UB
and it still might cause problems.