Comment by gaul

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.