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bug#15841: Display bugs with cache-long-lines non-nil

From: Michael Heerdegen
Subject: bug#15841: Display bugs with cache-long-lines non-nil
Date: Mon, 11 Nov 2013 04:39:56 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3.50 (gnu/linux)

Eli Zaretskii <address@hidden> writes:

> > Anyway, I'll tell you when I have a recipe.

Some elaborations, feel free to ignore.

The culprit was my own code: it placed myriads of invisible overlays
with no properties into the buffer.  Under these extreme circumstances,
`line-number-at-pos' indeed gets extremely slow at the end of my 10000
lines buffer: one invocation needs over a second.  I saw that with elp
as well as with profiler.  Setting `cache-long-scans' to nil (or
removing the overlays) cures this.

Although this is a corner case, I wonder why overlays slow down
`line-number-at-pos' so much for `cache-long-scans' non-nil - is that
expected?  Or can the profiler times I saw span redisplay times?

Because, when I use this:

(defmacro my-measure-time (expr)
  "Eval EXPR, display how much time it took."
  (with-gensyms (time)
    `(let ((,time (current-time)))
       (message "%s secs"
          (float-time (time-subtract (current-time) ,time))))))

and evaluate (my-measure-time (line-number-at-pos)) manually with M-:
(in the same situation), it shows a very tiny value.  But I'm sure that
from code, `line-number-at-pos' really lasts over a second.  Strange, I
don't understand it.



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