bug#64391: buffer narrowing slowdown regression in emacs 29

[Mattias Engdegård]

1 juli 2023 kl. 06.59 skrev Andrew Cohen <acohen@ust.hk>:

> Just for fun I simply commented it out (since the test code doesn't use
> labeled restrictions) and ran the test code, and the problem disappears:

The attached patch combines narrow-to-region and internal--label-restriction 
into a single function, internal--narrow-to-region. (We could also add the 
label as an optional argument to narrow-to-region.)

As a result, the up-front consing and marker allocation disappear entirely for 
normal (unlabelled) narrowing, as does the expensive buffer-local 
`outermost-restriction` variable.

Labelled narrowing is almost as expensive as before but is, we hope, less 
common. Maybe we can work separately on reducing its cost.

(And to those reading the patch: yes, some work on doc strings is needed. 
Suggestions welcome.)

narrow-to-region-with-label.diff
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