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.
This commit is contained in:
Raymond Hettinger 2007-12-18 21:24:09 +00:00
parent 3c887b2802
commit fd7ed407d7
6 changed files with 26 additions and 6 deletions

View file

@ -549,6 +549,23 @@ dictresize(PyDictObject *mp, Py_ssize_t minused)
return 0;
}
/* Create a new dictionary pre-sized to hold an estimated number of elements.
Underestimates are okay because the dictionary will resize as necessary.
Overestimates just mean the dictionary will be more sparse than usual.
*/
PyObject *
_PyDict_NewPresized(Py_ssize_t minused)
{
PyObject *op = PyDict_New();
if (minused>5 && op != NULL && dictresize((PyDictObject *)op, minused) == -1) {
Py_DECREF(op);
return NULL;
}
return op;
}
/* Note that, for historical reasons, PyDict_GetItem() suppresses all errors
* that may occur (originally dicts supported only string keys, and exceptions
* weren't possible). So, while the original intent was that a NULL return