GNU bug report logs - #77924
31.0.50; [Feature branch] Change marker implementation

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Message #284 received at 77924 <at> debbugs.gnu.org (full text, mbox):

From: Gerd Möllmann <gerd.moellmann <at> gmail.com>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: Eli Zaretskii <eliz <at> gnu.org>, stefankangas <at> gmail.com, 77924 <at> debbugs.gnu.org
Subject: Re: bug#77924: 31.0.50; [Feature branch] Change marker implementation
Date: Sat, 26 Apr 2025 21:25:23 +0200

[Message part 1 (text/plain, inline)]

Gerd Möllmann <gerd.moellmann <at> gmail.com> writes:

> Stefan Monnier <monnier <at> iro.umontreal.ca> writes:
>
>>> * Results
>>>
>>>   | test          | non-gc (s) | gc (s) | gcs | total (s) | err |
>>>   |---------------+------------+--------+-----+-----------+-----|
>>>   | font-lock     |       0.52 |   0.32 |  23 |      0.83 |  0% |
>>>   | smie          |       1.18 |   0.46 |  33 |      1.64 |  0% |
>>>   | smie-nonascii |       2.10 |   0.49 |  35 |      2.59 |  0% |
>>>   |---------------+------------+--------+-----+-----------+-----|
>>>   | total         |       3.79 |   1.27 |  92 |      5.07 |  0% |
>>>
>>> Very nice! Can you tell the story a bit? For entertainment? :-).
>>
>> I didn't have time to get to the bottom of it, but here's what happened:
>>
>> - rebuilt with `--enable-profiling`.
>> - ran the test then gprof.
>> - told me >50% of the time was spent in `buf_charpos_to_bytepos`.
>> - tweaked the source code to prevent inlining of the other functions
>>   used in that code, to get more info from the profiler.
>> - the new profile showed that a lot more time was spent in
>>   `bytepos_backward_to_charpos` than in `bytepos_forward_to_charpos`.
>> - I looked at the possible source of asymmetry and figured it might
>>   be because of that corner case where we (needlessly) scan
>>   backward from `next` when charpos is right at `prev`.
>> - tried the patch which confirmed my suspicion.
>> - called it a day.
>
> Thanks!
>
> And ouch, I hope that thing I tried there doesn't count as a premature
> optimization (looks like one). That would betoo embarrassing :-).

To fix this, I would propose

[0001-Don-t-optimize-prematurely-in-text_index_charpos_to_.patch (text/x-patch, attachment)]

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