[Python-checkins] r69582 - in python/trunk: Misc/NEWS Objects/object.c
antoine.pitrou
python-checkins at python.org
Fri Feb 13 14:52:33 CET 2009
Author: antoine.pitrou
Date: Fri Feb 13 14:52:33 2009
New Revision: 69582
Log:
Issue #5186: Reduce hash collisions for objects with no __hash__ method by
rotating the object pointer by 4 bits to the right.
Modified:
python/trunk/Misc/NEWS
python/trunk/Objects/object.c
Modified: python/trunk/Misc/NEWS
==============================================================================
--- python/trunk/Misc/NEWS (original)
+++ python/trunk/Misc/NEWS Fri Feb 13 14:52:33 2009
@@ -12,6 +12,9 @@
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.
Modified: python/trunk/Objects/object.c
==============================================================================
--- python/trunk/Objects/object.c (original)
+++ python/trunk/Objects/object.c Fri Feb 13 14:52:33 2009
@@ -1072,23 +1072,15 @@
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
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