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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug ipa/99309] [10/11/12 Regression] Segmentation fault with
 __builtin_constant_p usage at -O2
Date: Thu, 24 Mar 2022 13:59:20 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99309

--- Comment #10 from Richard Biener <rguenth at gcc dot gnu.org> ---
The issue for FRE is that we have

  <bb 2> [local count: 10737416]:
  step.val =3D 610334368;
  value.val =3D 1;
  goto <bb 6>; [100.00%]

  <bb 6> [local count: 1073741824]:
  # __1 =3D PHI <0(2), __11(4)>
  if (__1 !=3D 100)
    goto <bb 3>; [99.00%]
  else
    goto <bb 7>; [1.00%]

  <bb 3> [local count: 1063004409]:
  __builtin_printf ("%d %d\n", 1, 610334368);
  m =3D step;
  _12 =3D m.val;
  _13 =3D __builtin_constant_p (_12);
  if (_13 !=3D 0)
    goto <bb 4>; [100.00%]
  else
    goto <bb 5>; [0.00%]

  <bb 4> [local count: 1063004409]:
  __builtin_puts (&"normal"[0]);
  m =3D{v} {CLOBBER(eol)};
  __builtin_printf ("%d %d\n", 1, 610334368);
  __11 =3D __1 + 1;
  goto <bb 6>; [100.00%]

  <bb 5> [count: 0]:
  __builtin_unreachable ();

and while the first FRE iteration correctly determines that m.val is 610334=
368
we then make the backedge of the loop executable and because we translated
the lookup expression from m.val to step.val we are refusing to handle loops
(the abort_on_visited argument to get_continuation_for_phi, set from
translated in walk_non_aliased_vuses).

I don't remember why I disregarded the fix in comment#3 of PR54498, a fix
along that line would enable CSE here.  The concern was probably
compile-time (but we limit the amount of alias queries done).

It's a bit late to do such change for GCC 12 I think.  I will queue this
for GCC 13.=