Re: Analysis of redisplay performance on Windows

[Chong Yidong <cyd@stupidchicken.com>]

Jason Rumney <jasonr@gnu.org> writes:

> That is why I suspect there may be a problem with the setting of
> row->contains_overlapping_glyphs_p on Windows. We avoid calling those
> functions if that variable is not set. It may also be much more
> expensive to look up the glyph index points in Windows, that is the case
> for obtaining text extents, so it wouldn't surprise me.
>
> Another factor is that we do this all one character at a time, and each
> time on Windows we select the font into the current GC, then reselect
> the previously set font. This operation is also expensive, so it would
> help to have functions in the font backend interface for selecting and
> deselecting fonts. I already tried using prepare_face and done_face for
> this, but the calls overlap - ie we prepare a second face without
> calling done_face for the first.

Could you verify that setting the overlaps argument unconditionally to 1
in draw_glyphs indeed removes the performance problem?  If so, we can
work around this.