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Date: Fri, 08 May 2015 08:02:00 -0000
Message-ID: <CAFiYyc2asQNBV2_1ezunOnVeVDpDzF5nBE4jyD+2t6PvsPJGYA@mail.gmail.com>
Subject: Re: Enhance std::hash for pointers
From: Richard Biener <richard.guenther@gmail.com>
To: =?UTF-8?Q?Fran=C3=A7ois_Dumont?= <frs.dumont@gmail.com>
Cc: "libstdc++@gcc.gnu.org" <libstdc++@gcc.gnu.org>, gcc-patches <gcc-patches@gcc.gnu.org>
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On Wed, May 6, 2015 at 10:10 PM, Fran=C3=A7ois Dumont <frs.dumont@gmail.com=
> wrote:
> Hi
>
>     Following Marc Glisse comment #4
> on:https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D65641 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 ne=
eds
> 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 b=
ut

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 te=
st
> I have added in quality.cc is very basic and just reflect enhancement
> following Marc's comment.
>
> 2015-05-05  Fran=C3=A7ois 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.
>
> Fran=C3=A7ois
>