[Bug tree-optimization/128074] [16/17 Regression] wrong code at -O3 for a self-recursive call that passes a struct to a const struct parameter

[jmelcr at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128074

Josef Melcr <jmelcr at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at redhat dot com,
                   |                            |jamborm at gcc dot gnu.org,
                   |                            |jmelcr at gcc dot gnu.org
            Summary|wrong code at -O3 for a     |[16/17 Regression] wrong
                   |self-recursive call that    |code at -O3 for a
                   |passes a struct to a const  |self-recursive call that
                   |struct parameter            |passes a struct to a const
                   |                            |struct parameter

--- Comment #1 from Josef Melcr <jmelcr at gcc dot gnu.org> ---
r16-180-gb0120fa9838f8f

`f` is tail call optimized:

int f (const struct S a, int n)
{
  const int a$x;
  struct S t;
  int _2;
  int _3;

  <bb 5> [local count: 118111600]:

  <bb 2> [local count: 1073741824]:
  # n_4 = PHI <n_6(D)(5), _3(3)>
  a$x_8 = a.x;
  if (n_4 != 0)
    goto <bb 3>; [89.00%]
  else
    goto <bb 4>; [11.00%]

  <bb 3> [local count: 955630224]:
  _2 = a$x_8 + 1;
  t.x = _2;
  _3 = n_4 + -1;
  a = t;                      <-- assignment to const?
  goto <bb 2>; [100.00%]

  <bb 4> [local count: 118111600]:
  return a$x_8;
}

ipa-cp later propagates 0 into a.x. Since a$x_8 is never assigned again, hence
the body gets folded to return 0