Subject: Re: bug#51766: 29.0.50; Return value of buffer-chars-modified-tick changes when buffer text is not yet changed before inserting a character for non-latin input methods

From: Ihor Radchenko <[email-masked]>
Date: Fri, 12 Nov 2021 21:39:54 +0800

Eli Zaretskii <[email-masked]> writes:

>> quail changes the buffer after org-element--after-change-function call,
>> but before org-element--before-change-function. So, all Org can see is
>> that something has been changed in buffer, but there is no way to tell
>> what it was. Org cannot distinguish between harmless buffer edits by
>> quail (they do not change buffer text) and other kinds of "silent"
>> changes.
>
> OK, but why does this invalidate what Org does?  All it means, AFAIU,
> is that in some cases Org will do unnecessary processing.  Those cases
> are probably not too frequent.

Normally, buffer changes under inhibit-modification-hooks are not
frequent indeed. But not with quail + non-latin input methods. Every
single self-insert-command triggers such change.

> IOW, why invalidating the cache unnecessarily is such a big deal?

It is critical for Org to know which part of buffer was changed (i.e.
beg, end, length that are normally passed as arguments of
after-change-functions). org-element-cache can contain >100k elements
for especially large buffers. Manually checking which elements are
changed without knowing the changed region is inefficient. Clearing the
cache is not too much of a big deal, but causes slowdown when user runs
a query on Org buffers (i.e. in agenda or sparse trees) - the buffer has
to be re-parsed. When every edit triggers cache invalidation (that's
what happens when user uses non-latin input method), the slowdown is
pretty much guaranteed. Moreover, too frequent cache resets increase the
load on Emacs' garbage collector (cache size is typically a multiple of
buffer size). Overloading garbage collector leads to overall Emacs
slowdown.

>> The hooks are called, but after quail already triggered
>> buffer-chars-modified-tick increase. If quail called
>> before-change-functions before buffer-chars-modified-tick increases, it
>> would be useful for my scenario. Though I am not sure how feasible it
>> is. Just an idea.
>
> Would it help if Org looked at both buffer-modified-tick and
> buffer-chars-modified-tick?

When buffer-chars-modified-tick is changed, buffer-modified-tick is also
changed. AFAIU, buffer-chars-modified-tick registers a subset of buffer
modifications that actually change buffer text. buffer-modified-tick
also registers text property changes. So, I do not see how
buffer-modified-tick can help.

Best,
Ihor