Rei Thiessen wrote:
"Tobias Schwinger" <tschwinger@isonews2.com> wrote in message news:f9elpo$86r$1@sea.gmane.org...
Interesting. Note, that this optimization does not pay off in general: Worse distribution (no avalanche effect because bits are cut off) ==> more probing ==> more data cache misses.
If the hash function is poor, then modulo with a prime number will certainly help; however, I don't think containers should be responsible for improving the effectiveness of hash functions. If I already have a fine source of hash values, then applying further (expensive!) operations are of diminishing value.
A prime modulus certainly improves the distribution for the (bit-twiddling-based) default hash function in Boost (I've tried it). Whether or not it pays off in a concrete case depends on so many factors, that it's petty pointless to speculate or argue.
Please, no more type template parameters! Add another type member to the traits of that container and keep the primary interface simple, instead.
(If "traits" here refers to value traits) I don't think bucket pointer, bucket size, and the modulo options belong in the value traits, > since they pertain more to the container than to the templated types. I do support adding a "bucket trait" class, and perhaps folding the SizeType parameter into the bucket trait class as well. (I'm not sure whether converting the currently internal implementation types for the buckets into a trait-based system has been discussed already)
The interface is slightly awkward because of too many template arguments, already. It's fine to split up the configuration into different pieces but then let's arbitrate the ordering, applying some kind of policy mixing (e.g. a mechanism to provide named arguments as provided by Boost.Parameter). container<T, offset_ptr_storage<T>, fixed_bucket_size<256> > being the same as container<T, fixed_bucket_size<256>, offset_ptr_storage<T> > (once could probably get rid of that 'T' argument to 'offset_ptr_storage'). I'd also like to see a 'T' parameter for straightforwardness template< class T, class Policy1 = /unspecified/ , ... > class container; so one could just say container<T> given 'T' provides appropriate hooks.
The modulo option parameter can perhaps be folded into an expanded concept of a hash function class.
Hmm... Maybe it's better to stick with the standard concepts for the hash function.
If 'dynamic_bucket_size_policy' is configured in the container's traits, the ctor and the 'set_bucket_size' member function accept an integer. If 'constant_bucket_size_policy' is used an attempt to use an integer will yield a compile error. EBCO can be exploited for a zero-overhead implementation.
That'll work.
One note about my code: There's actually no need to implement special optimization for power of too, since the expression 'x % I', where I is a non-type, integral template argument equal to a power of two will be translated to an AND instruction by every decent compiler (sorry, I was in a bit of a hurry when I wrote it). Regards, Tobias