Re: Enhance std::hash for pointers

[Richard Biener <richard.guenther@gmail.com>]

On Wed, May 6, 2015 at 10:10 PM, François Dumont <frs.dumont@gmail.com> wrote:
> Hi
>
>     Following Marc Glisse comment #4
> on:https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65641 I would like to
> propose this enhancement to the hash functor for pointers. It simply gets
> rid of the irrelevant bits on pointers hash code based on memory alignment
> of the pointed type. The only drawback I can think of is that the type needs
> to be complete at std::hash instantiation time but is it really an issue ?
>
>     IMO it is quite obvious that the resulting hash code will be better but

If you use a real hashing function that's not true.  That is, something
else than GCCs pointer_hash (void *p) { return (uintptr_t)p >>3; }.

Richard.

> if anyone has a good method to prove it I can try to implement it. The test
> I have added in quality.cc is very basic and just reflect enhancement
> following Marc's comment.
>
> 2015-05-05  François Dumont <fdumont@gcc.gnu.org>
>
>     * include/bits/functional_hash.h
>     (std::__detail::_Lowest_power_of_two<size_t>): New.
>     (std::hash<_Tp*>::operator()): Use latter.
>     * testsuite/20_util/hash/quality.cc (pointer_quality_test): New.
>
> Tested under Linux x86_64.
>
> François
>