STINNER Victor added the comment:
"_Py_hashtable_hash_ptr is quite crude in that it doesn't even try to
compensate for pointer alignment, so there will automatically be many
collisions. Only one hash bucket every 8 or 16 will be used, at best."
I chose to use _Py_HashPointer() to drop (shift) lower bits. Do you mean that
it's not enough? Python memory allocator uses an alignement on 8 bytes (2**3).
Py_uhash_t
_Py_hashtable_hash_ptr(const void *key)
{
return (Py_uhash_t)_Py_HashPointer((void *)key);
}
Py_hash_t
_Py_HashPointer(void *p)
{
Py_hash_t x;
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 = (Py_hash_t)y;
if (x == -1)
x = -2;
return x;
}
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<http://bugs.python.org/issue21556>
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