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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/107138] [12/13 regression] std::variant<std::string, ...>
 triggers false-positive 'may be used uninitialized' warning
Date: Thu, 22 Dec 2022 13:42:02 +0000
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X-Bugzilla-Keywords: diagnostic, missed-optimization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107138

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
   Last reconfirmed|                            |2022-12-22
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
           Priority|P3                          |P2

--- Comment #11 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.  We're diagnosing the read

# VUSE <.MEM_106>
_60 =3D MEM[(const struct basic_string *)&D.48280]._M_string_length;

which happens conditional after the lifetime of D.48280 starts:

MEM[(struct __as_base  &)&D.48280] =3D{v} {CLOBBER};
MEM[(union _Variadic_union *)&D.48280] =3D{v} {CLOBBER};
MEM[(union _Variadic_union *)&D.48280] =3D{v} {CLOBBER};
MEM[(struct _Uninitialized *)&D.48280] =3D{v} {CLOBBER};
_128 =3D (void *) ivtmp.494_2;
_19 =3D MEM[(const type &)_128];
MEM[(struct _Uninitialized *)&D.48280]._M_storage =3D _19;
MEM[(struct _Variant_storage *)&D.48280]._M_index =3D 2;
...

the condition chains is

<bb 7> [local count: 668941184]:
MEM[(struct _Variant_storage *)_35]._M_index =3D 255;
_45 =3D D.48280.D.48091.D.47570.D.47488.D.47356.D.47242.D.47146._M_index;
_46 =3D (signed char) _45;
_47 =3D (long unsigned int) _46;
if (_47 > 4)
  goto <bb 40>; [16.67%]
else
  goto <bb 8>; [83.33%]

<bb 8> [local count: 557450988]:
switch (_45) <default: <L7> [20.00%], case 1: <L8> [20.00%], case 2: <L9>
[20.00%], case 3: <L10> [20.00%], case 4: <L14> [20.00%]>

<bb 9> [local count: 111490196]:
<L7>:
_57 =3D &MEM[(struct basic_string *)_35].D.36033._M_local_buf;
MEM[(struct _Alloc_hider *)_35]._M_p =3D _57;
_58 =3D MEM[(const struct basic_string *)&D.48280]._M_dataplus._M_p;
if (&MEM[(const struct basic_string *)&D.48280].D.36033._M_local_buf =3D=3D=
 _58)
  goto <bb 10>; [30.00%]
else
  goto <bb 12>; [70.00%]

<bb 10> [local count: 33447058]:
_60 =3D MEM[(const struct basic_string *)&D.48280]._M_string_length;


in this case we have other stores possibly clobbering the _M_index store
so we fail to statically optimize these branches (from the =3D 2 initializer
which should be still there?).=