Did you use gcc or clang or...? When I ran this locally I get: 0x7ffeee2a4808 0x7ffeee2a4810 0 0x7ffeee2a4808 0x7ffeee2a4810 0 With -O0 I get 1/1 ins

Did you use gcc or clang or...? When I ran this locally I get: 0x7ffeee2a4808 0x7ffeee2a4810 0 0x7ffeee2a4808 0x7ffeee2a4810 0 With -O0 I get 1/1 instead of 0/0 so this appears to be a compiler optimization.