[msla]

> It turns out that xor eax, eax is assigned to opcode 0x90, which is better known to most people as NOP.

This can't be true, since xoring a register with itself zeroes that register, and zeroing a register can't possibly be a general NOP instruction.

XOR also sets flags, another thing NOPs can't do.

[simcop2387]

That's because it's actually xchg eax, eax not xor eax, eax.

[CalChris]

xor eax,eax is a zero (or dependency breaking) idiom. Zero idioms are detected and removed by the renamer. So they have no execution latency.