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.

--- a/Python/ceval.c	Tue Dec 18 20:10:42 2007 +0000
+++ b/Python/ceval.c	Tue Dec 18 21:24:09 2007 +0000
@@ -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;