Re: [Bug libstdc++/54075] [4.7.1] unordered_map insert still slower than 4.6.2

[François Dumont <frs dot dumont at gmail dot com>]

On 11/13/2012 11:53 PM, Paolo Carlini wrote:
> Regarding performance, I have done a small evolution of the 54075.cc 
> test proposed last time. It is now checking performance with and 
> without cache of hash code. Result is:
> > 54075.cc                     std::unordered_set 300000 Foo insertions 
> > without cache       9r    9u    0s 13765616mem    0pf
> > 54075.cc                     std::unordered_set 300000 Foo insertions 
> > with cache      14r   13u    0s 18562064mem    0pf
> > 54075.cc                     std::tr1::unordered_set 300000 Foo 
> > insertions without cache       9r    8u    1s 13765616mem    0pf
> > 54075.cc                     std::tr1::unordered_set 300000 Foo 
> > insertions with cache      14r   13u    0s 18561952mem    0pf
> >
> >     So the difference of performance in this case only seems to come 
> > from caching the hash code or not. In reported use case default 
> > behavior of std::unordered_set is to cache hash codes and 
> > std::tr1::unordered_set not to cache it. We should perhaps review 
> > default behavior regarding caching the hash code. Perhaps cache it if 
> > the hash functor can throw and not cache it otherwise, not easy to 
> > find out what's best to do.
> Ah good. I think we finally have nailed the core performance issue. 
> And, as it turns out, I'm a bit confused about the logic we have in 
> place now for the defaults: can you please summarize what we are doing 
> and which are the trade offs (leaving out the technicalities having to 
> do with the final types)?
We do not cache if the following conditions are all met:
- key type is an integral
- hash functor is empty and not final
- hash functor doesn't throw

As you can see we cache in most of the cases.

> I think the most interesting are three:
>
>     1- std::hash<int>
>     2- std::hash<std::string>
>     3- user_defined_hash<xxx> which cannot throw
>
> In the first we should normally not cache; in the second, from a 
> performance point of view (from the exception safety point of view we 
> could do both, because std::hash<std::string> doesn't throw anyway) it 
> would be better to cache; the third case is rather tricky, because, 
> like the case of std::string, from the exception safety point of view 
> we could do both, thus it's purely a performance issue. Do I 
> understand correctly that currently we handle 2- and 3- above in the 
> same way, thus we cache?
yes, because types are not integral.
> It seems to me that whereas that kind of default makes a lot of sense 
> for std::string, doesn't necessarily make sense for everything else, 
> and it seems to me that such kind of default makes a suboptimal use of 
> the knowledge we have via __is_noexcept_hash that the functor doesn't 
> throw. That seems instead a sort of user-hint to not cache! Given the 
> unfortunate situation that the user has no way to explicitly pick a 
> behavior when instantiating the container, we can imagine that he can 
> anyway provide a strong if indirect hint by decorating or not with 
> noexcept the call operator. We could even document that as part of our 
> implementation defined behavior. How does it sound? Do we have a way 
> to figure out what other implementations are doing? Outside std::hash, 
> it should be pretty easy to instantiate with a special functor which 
> internally keeps a counter... if we have evidence that the other best 
> implementations don't cache for 3- we should definitely do the same.
>
> To summarize my intuitions are (again, leaving out the final 
> technicalities)
>
>     a- std::hash specializations for scalar types -> no cache
>     b- std::hash specialization for std::string (or maybe everything 
> else, for simplicity) -> cache
>     c- user defined functor -> cache or not basing on __is_noexcept_hash
    I don't understand why we would make a distinction between 
std::hash specialization and user defined functor, especially because 
hash specialization can throw. I like the idea of caching based on 
noexcept as its the only way users can tweak this behavior. Of course it 
will mean that we will need to check for std::string explicitly.

François