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.
diff --git a/Python/ceval.c b/Python/ceval.c
index 1af998d..b6501b3 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -1997,7 +1997,7 @@
break;
case BUILD_MAP:
- x = PyDict_New();
+ x = _PyDict_NewPresized((Py_ssize_t)oparg);
PUSH(x);
if (x != NULL) continue;
break;