[Bug middle-end/66375] [4.8/4.9/5 Regression] wrong code at -O2 and -O3 on x86_64-linux-gnu

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66375

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
Indeed as we just feed the initial condition to chrec_convert it happily just
fold_convert()s the zero to signed char and then back to int ...

So

              res = follow_ssa_edge
                (loop, SSA_NAME_DEF_STMT (rhs0), halting_phi,
                 evolution_of_loop, limit);
              if (res == t_true)
                *evolution_of_loop = add_to_evolution
                  (loop->num, chrec_convert (type, *evolution_of_loop,
                                             at_stmt),
                   code, rhs1, at_stmt);

doesn't work at all (the follow_ssa_edge call just picking up the initial
value converted to the type of the add).

static tree
analyze_evolution_in_loop (gphi *loop_phi_node,
                           tree init_cond)
{
...
          /* Pass in the initial condition to the follow edge function.  */
          ev_fn = init_cond;
          res = follow_ssa_edge (loop, ssa_chain, loop_phi_node, &ev_fn, 0);

I suppose a trick would be to always pass a symbolic initial condition
but then this would likely pessimize code.  With that we get

(add_to_evolution
  (loop_nb = 1)
  (chrec_before = (int) (signed char) D.1876)
  (to_add = -11)
  (res = {(int) (signed char) D.1876, +, -11}_1))
  (evolution_function = {(int) (signed char) 0, +, -11}_1))

Hmm, but that isn't correct either.  We want (int)(signed char){(int)0, +,
-11)_1


Sorter testcase pin-pointing the issue in VRP/SCEV:


int a;
extern void abort (void);
int main ()
{
  int c = 0;
  for (; a < 13; ++a)
    c = (signed char)c - 11;
  if (c != 113)
    abort ();
  return c;
}