Use the cache in get_base_units and get_dimensionality. Added benchmarks.

bench_base.yaml

@@ -21,6 +21,20 @@
     - name: (item)
       stmt: q = ureg['meter']
 
+- name: base units
+  setup: |
+         import pint
+         ureg = pint.UnitRegistry()
+  stmts:
+    - name: meter
+      stmt: ureg.get_base_units('meter')
+    - name: yard
+      stmt: ureg.get_base_units('yard')
+    - name: meter / second
+      stmt: ureg.get_base_units('meter / second')
+    - name: yard / minute
+      stmt: ureg.get_base_units('yard / minute')
+
 - name: build cache
   setup: |
          import pint

pint/unit.py

@@ -790,6 +790,9 @@ class UnitRegistry(object):
         if isinstance(input_units, string_types):
             input_units = ParserHelper.from_string(input_units)
 
+        if input_units in self._base_units_cache:
+            return self._dimensionality_cache[input_units]
+
         for key, value in input_units.items():
             if _is_dim(key):
                 reg = self._dimensions[key]
@@ -828,6 +831,9 @@ class UnitRegistry(object):
         if isinstance(input_units, string_types):
             input_units = ParserHelper.from_string(input_units)
 
+        if input_units in self._base_units_cache:
+            return self._base_units_cache[input_units]
+
         factor = 1.
         units = UnitsContainer()
         for key, value in input_units.items():