Optimizing fontTools.ttLib.TTFont.getGlyphID

I have a project with many thousands of glyphs, which translates to thousands of calls to fontTools.ttLib.TTFont.getGlyphID when calling fontTools.ttLib.TTFont.save. I was able to make it faster (17.66 s to 4.236 s of cumulative time) by adding the following to the beginning of the function.

		try:
			return self._reverseGlyphOrderDict[glyphName]
		except (AttributeError, KeyError):
			pass

The function is slow because it checks that _reverseGlyphOrderDict and glyphOrder are up to date and consistent. My version skips those checks. Is that safe? I think it is, for my use case, because I have finished creating glyphs by the time the function is first called.

Would it be possible to add something like this optimization to fontTools? If getGlyphID is called because save was called, it seems like it could assume that _reverseGlyphOrderDict and glyphOrder are frozen for the duration of save.

[anthrotype]

i’m a bit surprised that calling getGlyphID is three times slower than directly looking up self._reverseGlyphOrderDict. For the most common case of a glyphName which is already contained in self._reverseGlyphOrderDict, and the latter being in sync with self.glyphOrder, the extra work is basically:

1. one hasattr check (line 566),
2. one call to getGlyphOrder, which in turn simply returns self.glyphOrder (line 568),
3. one dict contains check (line 570)
4. one dict getitem call (line 602),
5. and one string comparison (line 603).

I wonder why we care about checking the consistency between glyphOrder and _reverseGlyphOrderDict every single time getGlyphID is used. Would it be enough to just have setGlyphOrder method be the only supported way of modifying the self.glyphOrder, and have that method invalidate (i.e. remove the attribute) the self._reverseGlyphOrderDict cache? Then the getGlyphID method can simply assume that whenever a _reverseGlyphOrderDict attribute is present, it must be in sync with the glyphOrder, and do try get glyphName from it, except rebuilding the reverse order when missing.

If a client messes directly with self.glyphOrder or with self._reversedGlyphOrderDict it’s not fontTools’ fault.

[behdad]

@dscorbett Can you test https://github.com/fonttools/fonttools/pull/2398 ?