[lib2to3] Make grammar pickling faster (#6491)

* Now uses pickle protocol 4

* Doesn't wrap the grammar's `__dict__` in ordered dictionaries anymore as
dictionaries in Python 3.6+ are ordered by default

This still produces deterministic pickles (that hash the same with MD5).
Tested with different PYTHONHASHSEED values.

Author: ambv

Lib/lib2to3/pgen2/grammar.py

@@ -86,21 +86,9 @@ def __init__(self):
         self.start = 256
 
     def dump(self, filename):
-        """Dump the grammar tables to a pickle file.
-
-        dump() recursively changes all dict to OrderedDict, so the pickled file
-        is not exactly the same as what was passed in to dump(). load() uses the
-        pickled file to create the tables, but  only changes OrderedDict to dict
-        at the top level; it does not recursively change OrderedDict to dict.
-        So, the loaded tables are different from the original tables that were
-        passed to load() in that some of the OrderedDict (from the pickled file)
-        are not changed back to dict. For parsing, this has no effect on
-        performance because OrderedDict uses dict's __getitem__ with nothing in
-        between.
-        """
+        """Dump the grammar tables to a pickle file."""
         with open(filename, "wb") as f:
-            d = _make_deterministic(self.__dict__)
-            pickle.dump(d, f, 2)
+            pickle.dump(self.__dict__, f, pickle.HIGHEST_PROTOCOL)
 
     def load(self, filename):
         """Load the grammar tables from a pickle file."""
@@ -141,17 +129,6 @@ def report(self):
         print("start", self.start)
 
 
-def _make_deterministic(top):
-    if isinstance(top, dict):
-        return collections.OrderedDict(
-            sorted(((k, _make_deterministic(v)) for k, v in top.items())))
-    if isinstance(top, list):
-        return [_make_deterministic(e) for e in top]
-    if isinstance(top, tuple):
-        return tuple(_make_deterministic(e) for e in top)
-    return top
-
-
 # Map from operator to number (since tokenize doesn't do this)
 
 opmap_raw = """

Misc/NEWS.d/next/Library/2018-04-16-16-21-09.bpo-23403.rxR1Q_.rst

@@ -0,0 +1 @@
+lib2to3 now uses pickle protocol 4 for pre-computed grammars.