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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug ipa/98834] [10/11 Regression] Code path incorrectly determined
 to be unreachable
Date: Tue, 26 Jan 2021 09:18:16 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98834

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2021-01-26
           Keywords|                            |needs-bisection, wrong-code
                 CC|                            |jamborm at gcc dot gnu.org,
                   |                            |marxin at gcc dot gnu.org
     Ever confirmed|0                           |1
          Component|tree-optimization           |ipa
   Target Milestone|---                         |10.3
           Priority|P3                          |P2
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed, the __builtin_unreachable is introduced by inlining:

Considering _S_multiplies.isra/23 with 12 size
 to be inlined into int main()/13 in t.C:54
 Estimated badness is -inf, frequency 1.00.
    Badness calculation for int main()/13 -> _S_multiplies.isra/23
      size growth -2, time 2.000000 unspec 10.000000 IPA hints: declared_in=
line
big_speedup
      -inf: Growth -2 <=3D 0
      Adjusted by hints -inf
                Accounting size:2.00, time:2.00 on predicate exec:(true)
Introduced new external node (void __builtin_unreachable()/24).
Processing frequency _S_multiplies.isra/23
  Called by int main()/13 that is executed once
                Accounting size:-4.00, time:-13.00 on predicate exec:(true)
t.C:54:62: optimized:  Inlined _S_multiplies.isra/23 into int main()/13 whi=
ch
now has time 8.000000 and size 9, net change of -14.

which points to IPA SRA.  VRP then simplifies __builtin_constant_p seen as

  _12 =3D __builtin_constant_p (__xx_11);
  if (_12 !=3D 0)
    goto <bb 4>; [100.00%]
  else
    goto <bb 3>; [0.00%]

  <bb 3> [count: 0]:
  __builtin_unreachable ();

  <bb 4> [local count: 1073741824]:
  MEM <int> [(struct _Tuple *)&D.2557] =3D 0;

and uses conservatively false but that runs into __builtin_unreachable () .=
..

The IPA inline transform ends us with

  _12 =3D __builtin_constant_p (__xx_11);
  if (_12 !=3D 0)
    goto <bb 4>; [100.00%]
  else
    goto <bb 3>; [0.00%]

  <bb 3> [count: 0]:
  _13 =3D _16(D);
  __builtin_unreachable ();

  <bb 4> [local count: 1073741824]:
  # SR.30_15 =3D PHI <__y_10(2)>

which eventually shows the reason (the unused argument?)=