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From: "nyh at math dot technion.ac.il" <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, 05 Jan 2023 09:19:49 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D32667

--- Comment #21 from Nadav Har'El <nyh at math dot technion.ac.il> ---
This old problem has become a real problem in gcc 12 with a real effect on
incorrect code generation, where code that copies an object was incorrectly
"optimized" to use __builtin_memcpy() instead of __builtin_memmove() even
though the source and destination objects may overlap - and it turns out th=
at
__builtin_memcpy() may produce incorrect results for overlapping addresses.

This bug was discovered in the OSv project
https://github.com/cloudius-systems/osv/issues/1212 with code that doesn't
(obviously) call __builtin_memcpy() directly, but rather had a 27-character
type being copied and the compiler implemented this copy with a call to
__builtin_memcpy(). Here is an example of code which generates the wrong
results (note the missing "c" in the result and unexpectedly doubled "g"):

#include <cstdio>
int main(){
    char buf[128] =3D "0123456789abcdefghijklmnopqrstuvwxyz";
    struct [[gnu::packed]] data {
        char x[27];
    };
    void *p0 =3D buf;
    void *p1 =3D &buf[1];
    *static_cast<data*>(p0) =3D *static_cast<const data*>(p1);
    printf("%s", buf);
}=