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From: "aldyh at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/101186] predictable comparison of integer
 variables not folded
Date: Sun, 27 Jun 2021 15:30:22 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101186

Aldy Hernandez <aldyh at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |aldyh at gcc dot gnu.org,
                   |                            |jeffreyalaw at gmail dot c=
om

--- Comment #4 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
To give a little more context, the IL at thread1 looks like this:

  <bb 3> [local count: 59055800]:
  goto <bb 9>; [100.00%]

  <bb 4> [local count: 955630225]:
  if (a_5(D) !=3D 0)
    goto <bb 5>; [67.00%]
  else
    goto <bb 7>; [33.00%]

  <bb 5> [local count: 640272253]:
  if (x_8(D) < c_12)
    goto <bb 6>; [33.00%]
  else
    goto <bb 8>; [67.00%]

  <bb 6> [local count: 211289842]:
  unreachable ();
  goto <bb 8>; [100.00%]

  <bb 7> [local count: 315357972]:
  blah ();

  <bb 8> [local count: 955630227]:
  b_11 =3D b_1 + 1;
  goto <bb 10>; [100.00%]

  <bb 9> [local count: 118111600]:
  # c_12 =3D PHI <x_8(D)(3), y_4(D)(2)>

  <bb 10> [local count: 1073741824]:
  # b_1 =3D PHI <b_6(D)(9), b_11(8)>
  if (b_1 <=3D 999)
    goto <bb 4>; [89.00%]
  else
    goto <bb 11>; [11.00%]

The path from 3->9->10->4->5 could *theoretically* resolve the conditional =
in
BB5.  If this were a permissible jump threading path, the threader would
replace the conditional in BB5 into a jump to BB8, thus eliding the
unreachable.

However, BB9->BB10 would cross loop boundaries which is not permitted in the
backwards threader.  Specifically, the threading candidate conditional is B=
B5,
which is in a loop:

;; Loop 1
;;  header 10, latch 8
;;  depth 1, outer 0
;;  nodes: 10 8 6 7 5 4 (<-- note BB5 & BB10 in here)

And BB9 is outside said loop.

See path_crosses_loops in tree-ssa-threadbackward.c as well as handle_phi()
where we specifically skip PHI edges pointing outside the current loop.

Note that even though my upcoming rewrite of the backward threader does not=
 use
the relational oracle, Andrew says it would be trivial to do so.  In which
case, we *could* resolve the above path, but would still fail to thread it
because it crosses a loop boundary.

Perhaps Jeff can opine on whether the path duplicator in the threader back-=
end,
could be taught to thread through loops.=