Re: tail-call elimination

[Stefan Monnier <monnier@IRO.UMontreal.CA>]

> Based on feedback by Stefan, I have updated the patch.  This is a
> slightly less "pure" version of TCO than the original patch, but it
> should be faster.  (And really, the difference between this and the
> "pure" version is pretty much academic.)

It's looking pretty good.  I have a few remaining questions/comments/nitpicks:

> +  stack.next = byte_stack_list;
> +  byte_stack_list = &stack;
> +
>    CHECK_STRING (bytestr);
>    CHECK_VECTOR (vector);
>    CHECK_NATNUM (maxdepth);

IIUC These checks are "runtime redundant" with the checks you added
right after the "tail_call:" label.  Try to restructure the code to
remove this redundancy.

> +            /* If the next op is return, maybe we can eliminate the tail call */

Please add a "." at the end of that sentence and make sure it is
followed by 2 spaces.

> +                if (SYMBOLP (fun) && !EQ (fun, Qunbound)
> +                    && (fun = XSYMBOL (fun)->function, SYMBOLP (fun)))

You can drop the !EQ (fun, Qunbound) test.

> +                  fun = indirect_function (fun);
> +                if (COMPILEDP(fun))

Please add a space between COMPILEDP and the following open parenthesis.

> +                        int prev_maxdepth = XFASTINT(maxdepth);

Add the same space here too.

> +                        args_template = syms_left;

Not sure why you named the var "syms_left".

> +                        else
> +                          {
> +                            top = bottom;
> +                          }

You don't need that braces (tho they don't hurt), but you do need to add
a comment explaining that that elements that were on top are now
referenced by `args' and will be moved back to `top' and that this move
needs to be careful not to overwrite data before it reads it since the
two areas may (and often will) overlap.

One more thing: have you performed some performance tests?
I'd recommend you compare something like

   cd lisp; rm **/*.elc; time make compile


-- Stefan