Merged revisions 69582-69583 via svnmerge from

svn+ssh://pythondev@svn.python.org/python/trunk

........
  r69582 | antoine.pitrou | 2009-02-13 14:52:33 +0100 (ven., 13 févr. 2009) | 4 lines

  Issue #5186: Reduce hash collisions for objects with no __hash__ method by
  rotating the object pointer by 4 bits to the right.
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  r69583 | antoine.pitrou | 2009-02-13 14:57:40 +0100 (ven., 13 févr. 2009) | 3 lines

  Fix compiler warning (gcc)
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diff --git a/Misc/NEWS b/Misc/NEWS
index 33a1c9831fc9edd807517318f9a9db0dd0a6e134..3aa3b49c71151a0bf4cc2798e1c3f018b26c3f50 100644 (file)
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -12,6 +12,9 @@ What's New in Python 3.1 alpha 0
 Core and Builtins
 -----------------
 
+- Issue #5186: Reduce hash collisions for objects with no __hash__ method by
+  rotating the object pointer by 4 bits to the right.
+
 - Issue #4575: Fix Py_IS_INFINITY macro to work correctly on x87 FPUs:
   it now forces its argument to double before testing for infinity.
 

diff --git a/Objects/object.c b/Objects/object.c
index ee18bc3c22e02f132c2dc4666d1576675c1335b0..85837977ee2972567295c08d4b7b4c3b0800a4fd 100644 (file)
--- a/Objects/object.c
+++ b/Objects/object.c
@@ -699,23 +699,15 @@ _Py_HashDouble(double v)
 long
 _Py_HashPointer(void *p)
 {
-#if SIZEOF_LONG >= SIZEOF_VOID_P
-       return (long)p;
-#else
-       /* convert to a Python long and hash that */
-       PyObject* longobj;
        long x;
-
-       if ((longobj = PyLong_FromVoidPtr(p)) == NULL) {
-               x = -1;
-               goto finally;
-       }
-       x = PyObject_Hash(longobj);
-
-finally:
-       Py_XDECREF(longobj);
+       size_t y = (size_t)p;
+       /* bottom 3 or 4 bits are likely to be 0; rotate y by 4 to avoid
+          excessive hash collisions for dicts and sets */
+       y = (y >> 4) | (y << (8 * SIZEOF_VOID_P - 4));
+       x = (long)y;
+       if (x == -1)
+               x = -2;
        return x;
-#endif
 }
 
 long