Re: Analysis of redisplay performance on Windows

[Jason Rumney <jasonr@gnu.org>]

Chong Yidong wrote:
> Jason Rumney <jasonr@gnu.org> writes:

> What you wrote seems to imply that left_overwritten and
> right_overwritten aren't really the problem; they have a performance
> impact simply because they cause more glyphs to be drawn (i.e., for
> redrawing overwritten glyphs).  But the ultimate problem is that drawing
> glyphs is a much slower operation, compared to GNU/Linux.  The question
> is, what's the reason for this slowness, and can we fix it?

One difference between Emacs 22 and 23 is that we compute glyph indexes
properly in Emacs 23, while on 22 we use unicode code points. Since we
call font->encode_char once per character rather than for a whole run,
the overhead of selecting fonts into the GC is multiplied. As I
suggested in my earlier email, new functions in the font backend
interface to select a font for working with and releasing it when done,
would help, as we could then skip doing this in functions like
encode_char and text_extents.