On Sun, Dec 22, 2019 at 10:46 AM Greg Ewing <greg.ew...@canterbury.ac.nz> wrote:
>
> On 22/12/19 9:04 am, Soni L. wrote:
>
> > switch (op) {
> > [...]
> > case OP_TAILCALL: {
> > adjust_regs();
> > some_other_stuff();
> > /* fallthrough */
> > }
> > case OP_CALL: {
> > make_the_call_happen();
> > break;
> > }
> > }
>
> Relying on fall-through in a switch is a micro-optimisation that
> may help make things go faster in C, but not so much in Python,
> so trying to find a literal translation is probably wrongheaded.
>
> I would be inclined to separate it into two independent cases:
>
> if op == OP_TAILCALL:
> adjust_regs()
> some_other_stuff()
> elif op == OP_CALL:
> adjust_regs()
> some_other_stuff()
> make_the_call_happen()
>
> If the parts being duplicated are more than just one or two
> calls as above, I would factor them out into a separate function.
(You have the translation backwards here; a tail call should do three
things while a non-tail call should do one.)
> Decoupling the different branches makes the code easier to read
> and maintain, and in Python probably doesn't cost anything
> significant in performance.
>
It's not just about performance. It's also about expressing a concept.
A tail call IS a call, and the correct way to write this is to do
stuff, and then do whatever a call does. (Based on this
implementation, of course.) "Refactor the common code into a function"
isn't a solution to all problems, because if you have to go look
elsewhere for the meaning of something, that stops you understanding
it. It's hard to see in a trivial example, but the cost is very real
in real code - unless you can give the refactored function a name that
completely represents its purpose, all you've done is trade one form
of hard-to-read code for another.
ChrisA
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