Re: [v3] Fix management of non empty hash functor

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

On 10 January 2013 21:02, François Dumont wrote:
> Hi
>
>     Here is an other version of this patch. Indeed there were no need to
> expose many stuff public. Inheriting from _Hash_code_base is fine, it is not
> final and it deals with EBO itself. I only kept usage of
> _Hashtable_ebo_helper when embedding H2 functor. As it is an extension we
> could have impose it not to be final but it doesn't cost a lot to deal with
> it. Finally I only needed a single friend declaration to get access to the
> H2 part of _Hash_code_base.

OK.

>     I didn't touch the default cache policy for the moment except reducing
> constraints on the hash functor. I prefer to submit an other patch to change
> when we cache or not depending on the hash functor expected performance.

OK.  The reduced constraints are good.  Does this actually affect
performance?  In my tests it doesn't, so I assume we still need to
change the caching decision to notice any performance improvements?

(Do the performance benchmarks actually tell us anything useful?
When I run them I get such varying results it doesn't seem to be reliable.)

>     I also took the time to replace some typedef expressions with using
> ones. I really know what is the rule about using one or the other but I
> remembered that Benjamin spent quite some time changing typedef in using so
> I prefer to stick to this approach in this file, even if there are still
> some typedef left.

OK, that doesn't make any difference so isn't important which is used.


>     Tested under linux x86_64 normal and debug modes.
>
> 2013-01-10  François Dumont  <fdumont@gcc.gnu.org>
>
>
>     * include/bits/hashtable_policy.h (_Local_iterator_base): Use
>     _Hashtable_ebo_helper to embed necessary functors into the
>     local_iterator when necessary. Pass information about functors

Repeating "necessary" seems unnecessary here :)

>     involved in hash code by copy.
>     * include/bits/hashtable.h (__cache_default): Do not cache for
>     builtin integral types unless the hash functor is not noexcept
>     qualified or is not default constructible. Adapt static assertions
>     and local iteraror instantiations.

^^ "iteraror"

+      // When hash codes are not cached local iterator inherits from
+      // __hash_code_base above to compute node bucket index so it has to be
+      // default constructible.
+      static_assert(__if_hash_not_cached<
+		      is_default_constructible<__hash_code_base>>::value,
+		    "Cache the hash code or make functors involved in hash code"
+		    " and bucket index computation default constructibles");

"constructible" not "constructibles"

This is OK for trunk, but not 4.7