https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90027
--- Comment #2 from vfdff <zhongyunde at huawei dot com> --- for deja testcase: gcc.c-torture/execute/20010518-2.c as the struct a_struct define with __attribute__ ((packed)), so the member variable b also not aligned with 4 bytes, is this case undefined behavior ? typedef struct { short a; long b; /* Will not aligned with 4 bytes */ short c; short d; } __attribute__ ((packed)) a_struct; int main(void) { volatile a_struct *a; volatile a_struct b; a = &b; *a = (a_struct){1,2,3,4}; if (a->a != 1 || a->b != 2 || a->c != 3 || a->d != 4) abort (); exit (0); }