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From: "msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/103534] [12 regression] Spurious -Wstringop-overflow
 warning with std::string concatencation
Date: Thu, 02 Dec 2021 23:19:46 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103534

--- Comment #4 from Martin Sebor <msebor at gcc dot gnu.org> ---
Yes, the warning does disappear when malloc() and free() are used instead of
operator new and delete.  foo() also ends up much better optimized, even at
-O1:

__attribute__((abi_tag ("cxx11")))
struct string foo ()
{
  struct string & _7(D);
  char * _69;

  <bb 2> [local count: 1073741824]:
  _69 =3D __builtin_malloc (17);
  __builtin_memcpy (_69, "1234567890123456", 16);
  MEM[(char_type &)_69 + 16] =3D 0;
  MEM[(struct basic_string *)_7(D)]._M_dataplus._M_p =3D _69;
  MEM[(struct basic_string *)_7(D)].D.33183._M_allocated_capacity =3D 16;
  MEM[(struct basic_string *)_7(D)]._M_string_length =3D 16;
  return _7(D);

}

The -O1 dump in comment #1 doesn't look right, I may have messed something =
up.=20
The same issue happens at -O2 where the dump is as follows:

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D BB 3 =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
Imports: n_5(D)=20=20
Exports: _1  n_5(D)=20=20
         _1 : n_5(D)(I)=20=20
n_5(D)  int [-INF, -1][1, +INF]
    <bb 3> [local count: 536870913]:
    _1 =3D (sizetype) n_5(D);
    if (_1 =3D=3D 1)
      goto <bb 4>; [51.12%]
    else
      goto <bb 5>; [48.88%]

_1 : sizetype [1, 2147483647][18446744071562067968, +INF]
3->4  (T) _1 :  sizetype [1, 1]
3->4  (T) n_5(D) :      int [1, 1]
3->5  (F) _1 :  sizetype [2, 2147483647][18446744071562067968, +INF]
3->5  (F) n_5(D) :      int [-INF, -1][2, +INF]

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D BB 4 =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
    <bb 4> [local count: 274448412]:
    MEM[(char *)&b] =3D 0;
    goto <bb 6>; [100.00%]


=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D BB 5 =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
    <bb 5> [local count: 262422500]:
    __builtin_memcpy (&b, &a, _1);=