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Slow fontification in C mode buffers

From: Eli Zaretskii
Subject: Slow fontification in C mode buffers
Date: Sat, 03 Dec 2011 15:19:47 +0200

Did anyone else notice that scrolling the first time through a very
large comment in C mode became extremely slow lately?  For example,
visit xdisp.c in "emacs -Q", type M-<, then hit C-v several times in
quick succession.  Then watch in disbelief how long it takes for Emacs
to scroll by these several screenfuls.  In my case (6.5 year old
hardware), I even see the "hourglass cursor" kick in, and the CPU
meter shows 100% utilization of one execution unit for about 5

If you then type M-< and again hit C-v several times, scrolling will
be at its usual speed.  Scrolling outside of large comments is also
reasonably fast, even for the first time.

Therefore, my prime suspect is font-lock, which is triggered by the
need to display a portion of the buffer that was not fontified yet.
This suspicion is corroborated by the fact that jit-stealth font-lock
of xdisp.c causes extremely high peaks of CPU usage, and Emacs
response time becomes very sluggish.

Is there something in C Mode's arrangements for fontification,
specifically related to comments, that was changed lately and that can
explain this slowdown?

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