Give meaning to the oparg for BUILD_MAP: estimated size of the dictionary.

Allows dictionaries to be pre-sized (upto 255 elements) saving time lost
to re-sizes with their attendant mallocs and re-insertions.

Has zero effect on small dictionaries (5 elements or fewer), a slight
benefit for dicts upto 22 elements (because they had to resize once
anyway), and more benefit for dicts upto 255 elements (saving multiple
resizes during the build-up and reducing the number of collisions on
the first insertions).  Beyond 255 elements, there is no addional benefit.

1 files changed, 1 insertions, 0 deletions

diff --git a/Include/dictobject.h b/Include/dictobject.h
index fec62958fe..c5378cf2dd 100644
--- a/Include/dictobject.h
+++ b/Include/dictobject.h
@@ -110,6 +110,7 @@ PyAPI_FUNC(Py_ssize_t) PyDict_Size(PyObject *mp);
 PyAPI_FUNC(PyObject *) PyDict_Copy(PyObject *mp);
 PyAPI_FUNC(int) PyDict_Contains(PyObject *mp, PyObject *key);
 PyAPI_FUNC(int) _PyDict_Contains(PyObject *mp, PyObject *key, long hash);
+PyAPI_FUNC(PyObject *) _PyDict_NewPresized(Py_ssize_t minused);
 
 /* PyDict_Update(mp, other) is equivalent to PyDict_Merge(mp, other, 1). */
 PyAPI_FUNC(int) PyDict_Update(PyObject *mp, PyObject *other);