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From: "bugdal at aerifal dot cx" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/32667] block copy with exact overlap is expanded as
 memcpy
Date: Thu, 23 Nov 2023 18:55:59 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D32667

--- Comment #42 from Rich Felker <bugdal at aerifal dot cx> ---
> I'm not saying that such an implementation will be a good idea, but just =
a remark: You could, in fact, keep restrict for the arguments in this case,=
 because the object pointed to by src and dest is not accessed at all when =
src=3D=3Ddest. So this is correct code according to the standard. (The exac=
t semantics of restrict are a bit involved...)

Nope, UB is invoked as soon as you evaluate src=3D=3Ddest, even with no
dereferencing. The semantics of restrict are such that the behavior of the =
code
must be unchanged if the pointer were replaced to a pointer to a relocated =
copy
of the pointed-to object. Since this would alter the result of the =3D=3D o=
perator,
that constraint is not satisfied and thereby the behavior is undefined.=