[Bug tree-optimization/58122] loops are not evaluated at compile time if loop count > 17

[olegendo at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58122

--- Comment #3 from Oleg Endo <olegendo at gcc dot gnu.org> ---
Looking at the tree dump (-fdump-tree-all -fdump-tree-all-details), it seems
that this is related to loop unrolling (dump file *t.cunroll) and induction
variable optimization.

If the number of iterations is small enough the loop gets unrolled and then the
calculations on 'sum' are folded away.

For this one:

int test2 (void)
{
  int sum = 0;
  for (int x = 0; x < 5; x++)
    sum += x;

  return sum;
}


t.cunroll shows:

;; Function int test2() (_Z5test2v, funcdef_no=0, decl_uid=1593,
symbol_order=0)

int test2() ()
{
  int x;
  int sum;

  <bb 2>:
  return 10;
}

However, with this one:

int test2 (void)
{
  int sum = 0;
  for (int x = 0; x < 6; x++)
    sum += x;

  return sum;
}

The unrolled result is:

int test2() ()
{
  int x;
  int sum;
  unsigned int ivtmp_2;
  unsigned int ivtmp_14;
  unsigned int ivtmp_20;
  unsigned int ivtmp_26;
  unsigned int ivtmp_32;
  unsigned int ivtmp_38;

  <bb 2>:
  sum_12 = 0;
  x_13 = 1;
  ivtmp_14 = 5;
  sum_18 = sum_12 + x_13;
  x_19 = x_13 + 1;
  ivtmp_20 = ivtmp_14 + 4294967295;
  sum_24 = sum_18 + x_19;
  x_25 = x_19 + 1;
  ivtmp_26 = ivtmp_20 + 4294967295;
  sum_30 = sum_24 + x_25;
  x_31 = x_25 + 1;
  ivtmp_32 = ivtmp_26 + 4294967295;
  sum_36 = sum_30 + x_31;
  x_37 = x_31 + 1;
  ivtmp_38 = ivtmp_32 + 4294967295;
  sum_3 = sum_36 + x_37;
  x_4 = x_37 + 1;
  ivtmp_2 = ivtmp_38 + 4294967295;
  return sum_3;

}