Re: Performance issue w/ `cl-loop`s `collect...into`

[Tianxiang Xiong <tianxiang.xiong@gmail.com>]

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One thing I don't understand is the common

(push `(progn (setq ...) t) cl--loop-body)

pattern found in the code. I'm not sure why the `(progn ... t)` is
necessary. If anyone could explain that I'd add it as a comment.

On Sun, Apr 8, 2018 at 2:13 PM, Stefan Monnier <monnier@iro.umontreal.ca>
wrote:

> > Avoid O(n^2) nconc-ing by keeping track of tail of collection.
>
> I took a quick look at your patch, and it looks pretty good.
> See comments below.
>
>
>         Stefan
>
>
> >       ((memq word '(collect collecting))
> > -      (let ((what (pop cl--loop-args))
> > -         (var (cl--loop-handle-accum nil 'nreverse)))
> > -     (if (eq var cl--loop-accum-var)
> > -         (push `(progn (push ,what ,var) t) cl--loop-body)
> > -       (push `(progn
> > -                   (setq ,var (nconc ,var (list ,what)))
> > -                   t)
> > -                cl--loop-body))))
> > +      (let ((what (pop cl--loop-args)))
> > +        (cl-multiple-value-bind (var var-tail)
>
> `cl-multiple-value-bind` is the "destructor" corresponding to the
> `cl-values` "constructor".   Since your code doesn't use `cl-values` it
> should not use `cl-multiple-value-bind` either (you probably meant to
> use cl-destructuring-bind instead).
>
> > +            (cl--loop-handle-accum nil 'nreverse)
> > +          (if (eq var cl--loop-accum-var)
> > +              (push `(progn (push ,what ,var) t) cl--loop-body)
> > +            (push `(progn
> > +                     (if (null ,var-tail)
> > +                         (setq ,var (list ,what) ,var-tail (last ,var))
> > +                       (setq ,var-tail (setcdr ,var-tail (list ,what))))
> > +                     t)
> > +                  cl--loop-body)))))
>
> The cl-loop macro's code lacks comments.  Could you take advantage of
> "being there" to try and add comments?  E.g. in the above code I see
> that depending on (eq var cl--loop-accum-var) we end up accumulating in
> the from or in the back.  Could you add a comments explaining why and
> mentioning where we correct this discrepancy?
>
> > +      (let ((what (pop cl--loop-args)))
> > +     (cl-destructuring-bind (var) (cl--loop-handle-accum nil 'nreverse)
> > +          (push `(progn
> > +                   (setq ,var
> > +                         ,(if (eq var cl--loop-accum-var)
> > +                              `(nconc
> > +                                (,(if (memq word '(nconc nconcing))
> > +                                      #'nreverse #'reverse)
> > +                                 ,what)
> > +                                ,var)
> > +                            `(,(if (memq word '(nconc nconcing))
> > +                                   #'nconc #'append)
> > +                              ,var ,what)))
> > +                   t)
> > +                cl--loop-body))))
>
> In the `nconc` case (when (eq var cl--loop-accum-var) is nil) we could
> also use the `var-tail` to speed up the `nconc`.
>
> Also, to avoid the N² behavior for the `append` case, maybe we
> could/should make it use `copy-sequence`, i.e.
>
>     `(nconc ,var-tail (copy-sequence ,what))
>
> > -(defun cl--loop-handle-accum (def &optional func) ; uses loop-*
> > -  (if (eq (car cl--loop-args) 'into)
> > +(defun cl--loop-handle-accum (def &optional func type) ; uses loop-*
> > +  (setq type (or type 'list))
>
> Please add a docstring explaining whatever you managed to understand of
> this code, and describing also what this new arg `type` does.
>
>
>

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