This is not a NOP; it explicitly clears the upper 32 bits of EDI since the compiler does not know that they are zero in this situation. If you change cc from a

This is not a NOP; it explicitly clears the upper 32 bits of EDI since the compiler does not know that they are zero in this situation. If you change cc from an int to size_t (long on x86-64) the compiler will generate: mov eax, OFFSET FLAT:.LC0 cmp rdi, 258 ja .L1 mov rax, QWORD PTR CSWTCH.1[0+rdi*8] Note that in some cases the compiler can do this automatically via lifetime analysis but not in this freestanding example.