3 ms·

Because "sub eax,eax" looks stupid. (and also clears the carry flag, unlike "xor eax, eax")

• tom_ 11mo ago
  xor clears the carry as well? In fact, looks like xor and sub affect the same set of flags! xor: > The OF and CF flags are cleared; the SF, ZF, and PF flags are set according to the result. The state of the AF flag is undefined. sub: > The OF, SF, ZF, AF, PF, and CF flags are set according to the result. (I don't have an x64 system handy, but hopefully the reference manual can be trusted. I dimly remembered this, or something like it, tripping me up after coming from programming for the 6502.)
  • trollbridge 11mo ago
    This is a good thing since the pipeline now doesn’t have to track the state of the flags since they all got zero’d.
  • sfink 11mo ago
    Strangely, the only difference on the flags is that AF (auxiliary carry) is undefined for `xor eax, eax` but guaranteed to be zeroed for `sub eax, eax`. I don't know what that means in practice, though I'm guessing that at the very least the hardware would not treat it as a dependency on the previous value.
• HackerThemAll 11mo ago
  If I remember correctly, sub used to be slower than xor on some ancient architectures.