Comment by Dwedit
Because "sub eax,eax" looks stupid. (and also clears the carry flag, unlike "xor eax, eax")
Because "sub eax,eax" looks stupid. (and also clears the carry flag, unlike "xor eax, eax")
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.
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.
If I remember correctly, sub used to be slower than xor on some ancient architectures.
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.)