From: INADA Naoki <songofacandy@gmail.com>
Date: Mon, 21 Nov 2016 15:57:02 +0000 (+0900)
Subject: Issue #28731: Optimize _PyDict_NewPresized() to create correct size dict
X-Git-Tag: v3.7.0a1~1918
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=92c50eee5273e75ef186ce1ffb750d7288a80767;p=python

Issue #28731: Optimize _PyDict_NewPresized() to create correct size dict

Improve speed of dict literal with constant keys up to 30%.
---

diff --git a/Misc/NEWS b/Misc/NEWS
index ddaae95bec..6916011c44 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,9 @@ What's New in Python 3.7.0 alpha 1
 Core and Builtins
 -----------------
 
+- Issue #28731: Optimize _PyDict_NewPresized() to create correct size dict.
+  Improve speed of dict literal with constant keys up to 30%.
+
 - Issue #28532: Show sys.version when -V option is supplied twice.
 
 - Issue #28746: Fix the set_inheritable() file descriptor method on platforms
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index 320dff6dfc..2a01f70b71 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -389,7 +389,7 @@ dk_set_index(PyDictKeysObject *keys, Py_ssize_t i, Py_ssize_t ix)
  * This can be used to reserve enough size to insert n entries without
  * resizing.
  */
-#define ESTIMATE_SIZE(n)  (((n)*3) >> 1)
+#define ESTIMATE_SIZE(n)  (((n)*3+1) >> 1)
 
 /* Alternative fraction that is otherwise close enough to 2n/3 to make
  * little difference. 8 * 2/3 == 8 * 5/8 == 5. 16 * 2/3 == 16 * 5/8 == 10.
@@ -1361,12 +1361,26 @@ make_keys_shared(PyObject *op)
 PyObject *
 _PyDict_NewPresized(Py_ssize_t minused)
 {
+    const Py_ssize_t max_presize = 128 * 1024;
     Py_ssize_t newsize;
     PyDictKeysObject *new_keys;
-    for (newsize = PyDict_MINSIZE;
-         newsize <= minused && newsize > 0;
-         newsize <<= 1)
-        ;
+
+    /* There are no strict guarantee that returned dict can contain minused
+     * items without resize.  So we create medium size dict instead of very
+     * large dict or MemoryError.
+     */
+    if (minused > USABLE_FRACTION(max_presize)) {
+        newsize = max_presize;
+    }
+    else {
+        Py_ssize_t minsize = ESTIMATE_SIZE(minused);
+        newsize = PyDict_MINSIZE;
+        while (newsize < minsize) {
+            newsize <<= 1;
+        }
+    }
+    assert(IS_POWER_OF_2(newsize));
+
     new_keys = new_keys_object(newsize);
     if (new_keys == NULL)
         return NULL;