CPUs already special case xor reg,reg as zeroing out the register, breaking any data dependency on it. If zeroing bits of the stack were common enough, I'd beli

CPUs already special case xor reg,reg as zeroing out the register, breaking any data dependency on it. If zeroing bits of the stack were common enough, I'd believe CPUs could be made that handled it efficiently (they already special case the stack; push/pop)