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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/105884] A possible optimization bug when uint64_t is used
 with -O2/-O3.
Date: Wed, 08 Jun 2022 09:35:43 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105884

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Definitely an aliasing bug:

        ((uint64_t*)result->s6_addr)[0] =3D ((uint64_t*)a->s6_addr)[0] &
((uint64_t*)b->s6_addr)[0];
        ((uint64_t*)result->s6_addr)[1] =3D ((uint64_t*)a->s6_addr)[1] &
((uint64_t*)b->s6_addr)[1];

Even their "fix" to use uint32_t instead is wrong.

With glibc the s6_addr array has type uint8_t[16] so there are no uint64_t =
(or
uint32_t) objects there.

With _GNU_SOURCE or _DEFAULT_SOURCE (formerly _BSD_SOURCE) defined, there i=
s a
s6_addr32 member which can be used to type-pun the s6_addr array as 32-bit
values.

The portable solution is to memcpy from s6_addr to uint64_t[2], perform the
bitwise AND ops on the uint64_t objects, and then memcpy back to s6_addr.=