https://gcc.gnu.org/bugzilla/show_bug.cgi?id=32667
--- 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 that __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] = "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); }
