dirs: speed up by storing number of direct children per dir

The Python version of the dirs type stores only the number of direct
children associated with each directory. That means that while adding
a directory, it only has to walk backwards until it runs into a
directory that is already in its map. The C version walks all the way
to the top-most directory. By copying the Python version's clever
trick to the C code, we can speed it up quite a bit.

On the Firefox repo, perfdirs now runs in 0.031390, from 0.056518
before the undoing Sid's optimization in the previous change, and
0.061835 before previous his optimization. More practically, it speeds
up 'hg status nonexistent' on the Firefox repo from 0.176s to 0.155s.

It's unclear why the C version did not have the same cleverness
implemented from the start, especially given that they were both
written by the same person (Bryan O'Sullivan) very close in time:

  856960173630 (scmutil: add a dirs class, 2013-04-10)
  02ee846b246a (scmutil: rewrite dirs in C, use if available, 2013-04-10)

mercurial/dirs.c

@@ -1,293 +1,295 b''
 /*
  dirs.c - dynamic directory diddling for dirstates
 
  Copyright 2013 Facebook
 
  This software may be used and distributed according to the terms of
  the GNU General Public License, incorporated herein by reference.
 */
 
 #define PY_SSIZE_T_CLEAN
 #include <Python.h>
 #include "util.h"
 
 /*
  * This is a multiset of directory names, built from the files that
  * appear in a dirstate or manifest.
  *
  * A few implementation notes:
  *
  * We modify Python integers for refcounting, but those integers are
  * never visible to Python code.
  *
  * We mutate strings in-place, but leave them immutable once they can
  * be seen by Python code.
  */
 typedef struct {
 	PyObject_HEAD
 	PyObject *dict;
 } dirsObject;
 
 static inline Py_ssize_t _finddir(PyObject *path, Py_ssize_t pos)
 {
 	const char *s = PyString_AS_STRING(path);
 
 	while (pos != -1) {
 		if (s[pos] == '/')
 			break;
 		pos -= 1;
 	}
 
 	return pos;
 }
 
 static int _addpath(PyObject *dirs, PyObject *path)
 {
 	const char *cpath = PyString_AS_STRING(path);
 	Py_ssize_t pos = PyString_GET_SIZE(path);
 	PyObject *key = NULL;
 	int ret = -1;
 
 	while ((pos = _finddir(path, pos - 1)) != -1) {
 		PyObject *val;
 
 		/* It's likely that every prefix already has an entry
 		   in our dict. Try to avoid allocating and
 		   deallocating a string for each prefix we check. */
 		if (key != NULL)
 			((PyStringObject *)key)->ob_shash = -1;
 		else {
 			/* Force Python to not reuse a small shared string. */
 			key = PyString_FromStringAndSize(cpath,
 							 pos < 2 ? 2 : pos);
 			if (key == NULL)
 				goto bail;
 		}
 		PyString_GET_SIZE(key) = pos;
 		PyString_AS_STRING(key)[pos] = '\0';
 
 		val = PyDict_GetItem(dirs, key);
 		if (val != NULL) {
 			PyInt_AS_LONG(val) += 1;
-			continue;
+			break;
 		}
 
 		/* Force Python to not reuse a small shared int. */
 		val = PyInt_FromLong(0x1eadbeef);
 
 		if (val == NULL)
 			goto bail;
 
 		PyInt_AS_LONG(val) = 1;
 		ret = PyDict_SetItem(dirs, key, val);
 		Py_DECREF(val);
 		if (ret == -1)
 			goto bail;
 		Py_CLEAR(key);
 	}
 	ret = 0;
 
 bail:
 	Py_XDECREF(key);
 
 	return ret;
 }
 
 static int _delpath(PyObject *dirs, PyObject *path)
 {
 	Py_ssize_t pos = PyString_GET_SIZE(path);
 	PyObject *key = NULL;
 	int ret = -1;
 
 	while ((pos = _finddir(path, pos - 1)) != -1) {
 		PyObject *val;
 
 		key = PyString_FromStringAndSize(PyString_AS_STRING(path), pos);
 
 		if (key == NULL)
 			goto bail;
 
 		val = PyDict_GetItem(dirs, key);
 		if (val == NULL) {
 			PyErr_SetString(PyExc_ValueError,
 					"expected a value, found none");
 			goto bail;
 		}
 
-		if (--PyInt_AS_LONG(val) <= 0 &&
-		    PyDict_DelItem(dirs, key) == -1)
+		if (--PyInt_AS_LONG(val) <= 0) {
+			if (PyDict_DelItem(dirs, key) == -1)
 			goto bail;
+		} else
+			break;
 		Py_CLEAR(key);
 	}
 	ret = 0;
 
 bail:
 	Py_XDECREF(key);
 
 	return ret;
 }
 
 static int dirs_fromdict(PyObject *dirs, PyObject *source, char skipchar)
 {
 	PyObject *key, *value;
 	Py_ssize_t pos = 0;
 
 	while (PyDict_Next(source, &pos, &key, &value)) {
 		if (!PyString_Check(key)) {
 			PyErr_SetString(PyExc_TypeError, "expected string key");
 			return -1;
 		}
 		if (skipchar) {
 			if (!dirstate_tuple_check(value)) {
 				PyErr_SetString(PyExc_TypeError,
 						"expected a dirstate tuple");
 				return -1;
 			}
 			if (((dirstateTupleObject *)value)->state == skipchar)
 				continue;
 		}
 
 		if (_addpath(dirs, key) == -1)
 			return -1;
 	}
 
 	return 0;
 }
 
 static int dirs_fromiter(PyObject *dirs, PyObject *source)
 {
 	PyObject *iter, *item = NULL;
 	int ret;
 
 	iter = PyObject_GetIter(source);
 	if (iter == NULL)
 		return -1;
 
 	while ((item = PyIter_Next(iter)) != NULL) {
 		if (!PyString_Check(item)) {
 			PyErr_SetString(PyExc_TypeError, "expected string");
 			break;
 		}
 
 		if (_addpath(dirs, item) == -1)
 			break;
 		Py_CLEAR(item);
 	}
 
 	ret = PyErr_Occurred() ? -1 : 0;
 	Py_DECREF(iter);
 	Py_XDECREF(item);
 	return ret;
 }
 
 /*
  * Calculate a refcounted set of directory names for the files in a
  * dirstate.
  */
 static int dirs_init(dirsObject *self, PyObject *args)
 {
 	PyObject *dirs = NULL, *source = NULL;
 	char skipchar = 0;
 	int ret = -1;
 
 	self->dict = NULL;
 
 	if (!PyArg_ParseTuple(args, "|Oc:__init__", &source, &skipchar))
 		return -1;
 
 	dirs = PyDict_New();
 
 	if (dirs == NULL)
 		return -1;
 
 	if (source == NULL)
 		ret = 0;
 	else if (PyDict_Check(source))
 		ret = dirs_fromdict(dirs, source, skipchar);
 	else if (skipchar)
 		PyErr_SetString(PyExc_ValueError,
 				"skip character is only supported "
 				"with a dict source");
 	else
 		ret = dirs_fromiter(dirs, source);
 
 	if (ret == -1)
 		Py_XDECREF(dirs);
 	else
 		self->dict = dirs;
 
 	return ret;
 }
 
 PyObject *dirs_addpath(dirsObject *self, PyObject *args)
 {
 	PyObject *path;
 
 	if (!PyArg_ParseTuple(args, "O!:addpath", &PyString_Type, &path))
 		return NULL;
 
 	if (_addpath(self->dict, path) == -1)
 		return NULL;
 
 	Py_RETURN_NONE;
 }
 
 static PyObject *dirs_delpath(dirsObject *self, PyObject *args)
 {
 	PyObject *path;
 
 	if (!PyArg_ParseTuple(args, "O!:delpath", &PyString_Type, &path))
 		return NULL;
 
 	if (_delpath(self->dict, path) == -1)
 		return NULL;
 
 	Py_RETURN_NONE;
 }
 
 static int dirs_contains(dirsObject *self, PyObject *value)
 {
 	return PyString_Check(value) ? PyDict_Contains(self->dict, value) : 0;
 }
 
 static void dirs_dealloc(dirsObject *self)
 {
 	Py_XDECREF(self->dict);
 	PyObject_Del(self);
 }
 
 static PyObject *dirs_iter(dirsObject *self)
 {
 	return PyObject_GetIter(self->dict);
 }
 
 static PySequenceMethods dirs_sequence_methods;
 
 static PyMethodDef dirs_methods[] = {
 	{"addpath", (PyCFunction)dirs_addpath, METH_VARARGS, "add a path"},
 	{"delpath", (PyCFunction)dirs_delpath, METH_VARARGS, "remove a path"},
 	{NULL} /* Sentinel */
 };
 
 static PyTypeObject dirsType = { PyObject_HEAD_INIT(NULL) };
 
 void dirs_module_init(PyObject *mod)
 {
 	dirs_sequence_methods.sq_contains = (objobjproc)dirs_contains;
 	dirsType.tp_name = "parsers.dirs";
 	dirsType.tp_new = PyType_GenericNew;
 	dirsType.tp_basicsize = sizeof(dirsObject);
 	dirsType.tp_dealloc = (destructor)dirs_dealloc;
 	dirsType.tp_as_sequence = &dirs_sequence_methods;
 	dirsType.tp_flags = Py_TPFLAGS_DEFAULT;
 	dirsType.tp_doc = "dirs";
 	dirsType.tp_iter = (getiterfunc)dirs_iter;
 	dirsType.tp_methods = dirs_methods;
 	dirsType.tp_init = (initproc)dirs_init;
 
 	if (PyType_Ready(&dirsType) < 0)
 		return;
 	Py_INCREF(&dirsType);
 
 	PyModule_AddObject(mod, "dirs", (PyObject *)&dirsType);
 }