Speed up varscalar with caching (#2798)

* Speed up varscalar with caching

* Don't use cached_property

* Make model pool a class attribute

* Don't catch things on the values side

* Remove unused import

Lib/fontTools/feaLib/builder.py

@@ -770,6 +770,7 @@ class Builder(object):
                 gdef.remap_device_varidxes(varidx_map)
                 if 'GPOS' in self.font:
                     self.font['GPOS'].table.remap_device_varidxes(varidx_map)
+            VariableScalar.clear_cache()
         if any(
             (
                 gdef.GlyphClassDef,

Lib/fontTools/feaLib/variableScalar.py

@@ -8,6 +8,15 @@ def Location(loc):
 class VariableScalar:
     """A scalar with different values at different points in the designspace."""
 
+    # We will often use exactly the same locations (i.e. the font's
+    # masters) for a large number of variable scalars. Instead of
+    # creating a model for each, let's share the models.
+    model_pool = {}
+
+    @classmethod
+    def clear_cache(cls):
+        cls.model_pool = {}
+
     def __init__(self, location_value={}):
         self.values = {}
         self.axes = {}
@@ -84,7 +93,12 @@ class VariableScalar:
     @property
     def model(self):
         locations = [dict(self._normalized_location(k)) for k in self.values.keys()]
-        return VariationModel(locations)
+        key = tuple([tuple(x.items()) for x in locations])
+        if key in self.model_pool:
+            return self.model_pool[key]
+        m = VariationModel(locations)
+        self.model_pool[key] = m
+        return m
 
     def get_deltas_and_supports(self):
         values = list(self.values.values())