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From: "aldyh at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/104475] [12 Regression] Wstringop-overflow +
 atomics incorrect warning on dynamic object
Date: Thu, 03 Mar 2022 14:00:53 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104475

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |amacleod at redhat dot com

--- Comment #3 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
This isn't the threader but VRP/ranger.

What happens is that the threader isolates the path, making it easier for V=
RP
to see the equivalence, and then CCP4 folds the constant into the problemat=
ic
call.  This is from the .ccp4 pass:

Folding statement: __atomic_or_fetch_4 (pretmp_29, 64, 0);
Folded into: __atomic_or_fetch_4 (184B, 64, 0);

In VRP2 the ranger is folding:

Folding statement: pretmp_29 =3D &MEM[(struct __atomic_base *)_1 + 184B]._M=
_i;
Folded into: pretmp_29 =3D 184B;

The ranger is determining that _1 is 0 because it has determined that since=
 _2
is 0 on the 2->3 edge, so is _1, as m_mutex is the first field of _1:

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D BB 2 =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
Imports: _1=20=20
Exports: _1  _2=20=20
         _2 : _1(I)=20=20
    <bb 2> [local count: 1073741824]:
    _1 =3D this_10(D)->d;
    _2 =3D &_1->m_mutex;
    MEM[(struct __as_base  &)&lock] =3D{v} {CLOBBER};
    if (_2 !=3D 0B)
      goto <bb 5>; [90.00%]
    else
      goto <bb 3>; [10.00%]

2->5  (T) _1 :  struct QFutureInterfaceBasePrivate * [1B, +INF]
2->5  (T) _2 :  struct QMutex * [1B, +INF]
2->3  (F) _1 :  struct QFutureInterfaceBasePrivate * [0B, 0B]
2->3  (F) _2 :  struct QMutex * [0B, 0B]

Andrew, how/where is that we relate _1 and _2 here?  I can't seem to find i=
t.

My gut feeling is that special casing anything in the ranger for this is wr=
ong.=