The lightning documentation states: "Note that arguments in registers are very cheap, but will be overwritten at any moment, including on some operations, for example division, that on several ports is implemented as a function call."
For writing portable code, it is important to know, which instructions actually overwrite (register) arguments, or rather, which instructions preserve them. Obviously, function calls do not preserve the arguments, and the above quote says that using division also does not guarantee that arguments are preserved.
makes it clear, that simple operations like subtraction actually do preserve arguments.