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Re: Correct line/column numbers in byte compiler messages

From: Alan Mackenzie
Subject: Re: Correct line/column numbers in byte compiler messages
Date: Sat, 21 Mar 2020 20:19:54 +0000
User-agent: Mutt/1.10.1 (2018-07-13)

Hello, Andrea.

On Sat, Mar 21, 2020 at 18:37:13 +0000, Andrea Corallo wrote:
> Have to apologize this is probably the quarantine effect ....

As of today, we're under quarantine, too.  :-(

> .... but I couldn't resist testing this:

> #+BEGIN_SRC lisp
> ;; -*- lexical-binding: t; -*-
> (require 'cl-lib)
> (defvar elb-list (cl-loop for i from 0 to 1500000
>                           if (cl-oddp i)
>                           collect 'a
>                           else
>                           collect 'b))

> (defun elb-eq ()
>   (let ((n 0))
>     (dolist (l elb-list n)
>       (when (eq 'b l)
>         (cl-incf n)))))

> (defun elb-eq-entry ()
>   (dotimes (_ 1000)
>     (elb-eq)))

> Results:

> b619777dd6 (baseline) 50.09s
> accurate-warning-pos  51.28s

> This is about 2% perf penalty.

On my Ryzen, I'm seeing a 50% penalty.  :-(  (Admittedly that's
comparing the year old branch to current master.  I suppose I should
build the correct comparable revision and try again.)  This suggests
that the branch prediction logic isn't present (or isn't active) on the

> Interestingly with the __builtin_expect trick applied exec time gets
> back to 50.65s.

How do you do this?  I couldn't make much sense of the documentation of
__builtin_expect.  :-(

> We could probably find a benchmark that better highlights the difference
> (this is potentially dominated by cache misses while pointer chasing the
> list) but is it worth?

Could I ask you to do the following timing.

Evaluate the following (e.g. in *scratch*):

(defmacro time-it (&rest forms)
  "Time the running of a sequence of forms using `float-time'.
Call like this: \"M-: (time-it (foo ...) (bar ...) ...)\"."
  `(let ((start (float-time)))
    (- (float-time) start)))

(defun time-scroll (&optional arg)
  (interactive "P")
  (message "%s"
            (condition-case nil
                (while t
                  (if arg (scroll-down) (scroll-up))
                  (sit-for 0))
              (error nil)))))

, visit .../emacs/src/xdisp.c, and do M-: (time-scroll).  This scrolls
through the buffer and prints a timing in the minibuffer.  (N.B. to run
this again, type something at BOB and undo it, thus marking the
fontification as stale.)

I'm seeing 19.4s vs. 22.2s, which is around 15% difference.  :-(

> Regards

>   Andrea

> --
> address@hidden

Alan Mackenzie (Nuremberg, Germany).

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