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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/98465] [11 Regression] Bogus -Wstringop-overread
 with -std=gnu++20 -O2 and std::string::insert
Date: Thu, 04 Feb 2021 21:09:33 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98465

--- Comment #27 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Martin Sebor from comment #26)
> I tried the __builtin_object_size patch and while it avoids the warning in
> the reported test case it's not effective in others derived from it, such=
 as:
>=20
> #include <string>
>=20
> const char constantString[] =3D {42, 53};
>=20
> void f(std::string& s, int n)
> {
>   if (n < 2) n =3D 2;
>   s.insert(0, static_cast<const char*>(constantString), n);
> }

That could be handled by doing:
__builtin_constant_p (__builtin_object_size (__s, 0) < (2 * __len2 - __len1=
) *
sizeof(_CharT))
&& __builtin_object_size (__s, 0) < (2 * __len2 - __len1) * sizeof(_CharT)
assuming VRP is able to determine the comparison is constant even when one =
of
the lengths are non-constant.=