[Bug c/108296] New: __builtin_memcpy generating wrong code in some cases

["nyh at math dot technion.ac.il" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108296

            Bug ID: 108296
           Summary: __builtin_memcpy generating wrong code in some cases
           Product: gcc
           Version: 12.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nyh at math dot technion.ac.il
  Target Milestone: ---

The following trivial code, copying a string over itself moved by one byte,
shows wrong results for __builtin_memcpy() on gcc 12.2.1:

#include <cstring>
int main(){
    char bufa[128] = "0123456789abcdefghijklmnopqrstuvwxyz";
    char bufb[128] = "0123456789abcdefghijklmnopqrstuvwxyz";
    memcpy(bufa, bufa+1, 27);
    printf("          memcpy: %s\n", bufa);
    __builtin_memcpy(bufb, bufb+1, 27);
    printf("__builtin_memcpy: %s\n", bufb);
}

As you can see running it, memcpy() returned the right result,
123456789abcdefghijklmnopqrrstuvwxyz (the first 27 characters shifted back, so
"r" is double in the response), but __builtin_memcpy() returned the *wrong*
result - 123456789abdefgghijklmnopqrrstuvwxyz (the "c" character disappeared
and the "g" is  also doubled).


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(). The original miscompiling code in OSv  was something like
the following:

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

This appears to be a regression - this code did not miscompile in earlier gcc
releases.