I just tried this, and GCC6 does indeed optimise this correctly: 0000000000400560 <maybe_overflow>: 400560: 01 f7 add %esi,%edi

I just tried this, and GCC6 does indeed optimise this correctly: 0000000000400560 <maybe_overflow>: 400560: 01 f7 add %esi,%edi 400562: 70 03 jo 400567 <maybe_overflow+0x7> 400564: 89 f8 mov %edi,%eax 400566: c3 retq 400567: 31 c0 xor %eax,%eax 400569: c3 retq