bug#48841: fido-mode is slower than ido-mode with similar settings

[Dmitry Gutov <dgutov@yandex.ru>]

On 12.06.2021 02:24, João Távora wrote:
> Dmitry Gutov <dgutov@yandex.ru> writes:
> 
>> Looking forward for your analysis of fido-vertical-mode's performance
>> improvement over the "normal" one.
> 
> So, I benchmarked before and after this patch to icomplete.el:
> 
>      diff --git a/lisp/icomplete.el b/lisp/icomplete.el
>      index 08b4ef2030..3561ebfa04 100644
>      --- a/lisp/icomplete.el
>      +++ b/lisp/icomplete.el
>      @@ -858,16 +858,8 @@ icomplete-completions
>                      ;; removing making `comps' a proper list.
>                      (base-size (prog1 (cdr last)
>                                   (if last (setcdr last nil))))
>      -               (most-try
>      -                (if (and base-size (> base-size 0))
>      -                    (completion-try-completion
>      -                     name candidates predicate (length name) md)
>      -                  ;; If the `comps' are 0-based, the result should be
>      -                  ;; the same with `comps'.
>      -                  (completion-try-completion
>      -                   name comps nil (length name) md)))
>      -               (most (if (consp most-try) (car most-try)
>      -                       (if most-try (car comps) "")))
>      +               (most-try nil)
>      +               (most "")
>                      ;; Compare name and most, so we can determine if name is
>                      ;; a prefix of most, or something else.
>                      (compare (compare-strings name nil nil

All right, so this is not about try-completion, it's about 
completion-try-completion. That makes sense.

> The patch itself nullifies the calculation of the 'determ' thing that I
> and presumably some other users don't value that much.  It doesn't
> affect fido-mode's basic funcionality.
> 
> How did I benchmark?  Well, to measure the delay the user experiences
> until all completions are presented I had to take out the
> `while-no-input` in icomplete-exhibit so that this test would work:
> 
>      ;; After the form, type C-u C-x C-e C-m in quick succession
>      (benchmark-run (completing-read "bla" obarray))
> 
> If I don't remove this `while-no-input`, icomplete will not lose time
> showing all the completions and will instead select just the first one.
> That's a very nice feature for actual use, but for this benchmark that
> is not what I want: I want to measure the time to show all the
> completions.

Did the same, can repro.

> Then, the times presented by benchmark-run are the same that the user
> sees if he waits to see the completions.  Now the values:
> 
> Before my patch:
> 
>      (1.802209488 5 1.3678843490000077)
>      (1.609066281 4 1.1170432569999775)
>      (1.878972079 5 1.3725165670000479)
>      (1.901952581 5 1.3979494059999524)
>      (1.820800064 5 1.3283940110000003)
> 
> After the patch:
> 
>      (0.552051921 1 0.3079724459999511)
>      (0.58396499 1 0.3038616050000087)
>      (0.861106587 2 0.6046198220000178)
>      (0.611551175 1 0.30275532399997473)
>      (0.62500199 1 0.3160454470000218)

I get

(0.377195885 10 0.24448539800000013)

before and

(0.245218061 6 0.1390041310000001)

after. A solid improvement.

BTW, if I just stick benchmark-progn around icomplete-completions like

diff --git a/lisp/icomplete.el b/lisp/icomplete.el
index 08b4ef2030..b9fe3e1836 100644
--- a/lisp/icomplete.el
+++ b/lisp/icomplete.el
@@ -678,12 +678,13 @@ icomplete-exhibit
                   ;; seems to trigger it fairly often!
                   (while-no-input-ignore-events '(selection-request))
                   (text (while-no-input
-                         (icomplete-completions
-                          field-string
-                          (icomplete--completion-table)
-                          (icomplete--completion-predicate)
-                          (if (window-minibuffer-p)
-                              (eq minibuffer--require-match t)))))
+                         (benchmark-progn
+                           (icomplete-completions
+                            field-string
+                            (icomplete--completion-table)
+                            (icomplete--completion-predicate)
+                            (if (window-minibuffer-p)
+                                (eq minibuffer--require-match t))))))
                   (buffer-undo-list t)
                   deactivate-mark)
              ;; Do nothing if while-no-input was aborted.

...it reports

Elapsed time: 0.329006s (0.246073s in 10 GCs)

vs

Elapsed time: 0.169200s (0.113762s in 5 GCs)

I suppose the 40-70ms difference is due to delay in typing.