26-Jul-2013 01:25, Walter Bright пишет:
On 7/25/2013 1:00 PM, bearophile wrote:
Walter Bright:
It's not the hashing that's slow. It's the lookup that is.
By "different hashing scheme" I meant different strategies in
resolving hash
collisions, likes double hashing, internal hashing, cuckoo hashing,
and so on
and on. Maybe one of such alternative strategies is more fit for the
needs of
dmd compilation. (I think that currently the Python dicts are using a
hashing
strategy different from the built-in dictionaries of D. The Python
style of
hashing was implemented in D some months ago, but I don't remember
what happened
to that project later).
Hash collisions are not the problem - I sized the hash bucket array to
make it fairly sparse. Neither is the hash algorithm.
The slowness was in the frackin' "convert the hash to an index in the
bucket", which is a modulus operation.
Then it's past due to finally stop the madness of modulo prime table and
use a power of 2 size. In essence what modulo prime does is simply
enhancing the quality of your hash function w.r.t. collisions (it helps
to distribute values more evenly).
Any of new decent hashes are good enough to work with plain slice the
lower bits approach.
Also, computing the hash is done exactly once, in the lexer.
All the more reason to use good hash function and kill the modulo prime.
Thereafter,
all identifiers are known only by their handles, which are (not
coincidentally) the pointer to the identifier, and by its very nature is
unique.
--
Dmitry Olshansky
