[HarfBuzz] Some optimizations

[Behdad Esfahbod]

Hi,

For Boston Summit this year I sat down optimizing harfbuzz buffer a bit
to avoid unnecessary copy/swap'ing when glyphs do not change.  The idea
is to use the same string for in and out, essentially working in-place,
until a change that doesn't fit in-place is needed at which point we
lazily create a separate out buffer and copy over.

Unfortunately I didn't start this in a git-svn pango checkout and ended
up piling changes on top of each other.

Around the final round it was about 1700 lines worth of diff, but to
find why a particular Invalid_SubTable error was being hit I needed to
see where the error is originating from, so I renamed remaining FT_Err
instances to HB_Err and made the entire code call _hb_err() when
returning an original error, so you can set a breakpoint on it.  It
helped debugging the bug in a minute, but also means that the patch is
over 7000 lines now :(.  Sorry Simon :(.

Anyway, it's here:

  http://bugzilla.gnome.org/show_bug.cgi?id=485559

Quick summary:

2007-10-10  Behdad Esfahbod  <behdad at gnome.org>

        Bug 485559 – Boston Summit HarfBuzz optimizations

        * pango/opentype/*: HarfBuzz hacking to:

          - Rename last remaining FT_Err stuff to HB_Err.

          - Fix a couple invalid table paths to be permissive so
            fonts work better.  Particularly GDEF table for Nafees
            Nastaliq is loaded and works great now.

          - Optimize harfbuzz buffer to not copy/swap for simple
          one-to-one and "copy" GSUB operations.

        * pango/pango-ot*: Update to FT_Err to HB_Err renaming.


I think the easiest way to apply it to harfbuzz upstream is to rip out
the FT_Err and _hb_err changes out, apply it, and redo them...  I'll
give it a try if it's too much pain for Simon to do (which I totally
understand if it is), but other than that, it can use a pair of eyes.
Specially the harfbuzz-buffer changes.

Tomorrow I'll make pango cache a single harfbuzz buffer instead of
recreating them all the time.

A very warm feeling of doing this all is that I feel much more
comfortable with the GSUB code now.  GPOS still has a bit of mystery to
discover.  All in all quite good just in time for picking up the rewrite
again, which I'm starting next week.

Regards,

-- 
behdad
http://behdad.org/

"Those who would give up Essential Liberty to purchase a little
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