[Python-Dev] Hashing proposal: change only string-only dicts
"Martin v. Löwis"
martin at v.loewis.de
Wed Jan 18 18:55:31 CET 2012
Am 18.01.2012 17:01, schrieb PJ Eby:
> On Tue, Jan 17, 2012 at 7:58 PM, "Martin v. Löwis" <martin at v.loewis.de
> <mailto:martin at v.loewis.de>> wrote:
> Am 17.01.2012 22:26, schrieb Antoine Pitrou:
> > Only 2 bits are used in ob_sstate, meaning 30 are left. These 30 bits
> > could cache a "hash perturbation" computed from the string and the
> > random bits:
> > - hash() would use ob_shash
> > - dict_lookup() would use ((ob_shash * 1000003) ^ (ob_sstate & ~3))
> > This way, you cache almost all computations, adding only a computation
> > and a couple logical ops when looking up a string in a dict.
> That's a good idea. For Unicode, it might be best to add another slot
> into the object, even though this increases the object size.
> Wouldn't that break the ABI in 2.x?
I was thinking about adding the field at the end, so I thought it
shouldn't. However, if somebody inherits from PyUnicodeObject, it still
might - so my new proposal is to add the extra hash into the str block,
either at str[-1], or after the terminating 0. This would cause an
average increase of four bytes of the storage (0 bytes in 50% of the
cases, 8 bytes because of padding in the other 50%).
What do you think?
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