Re: Enhance std::hash for pointers

[Christopher Jefferson <chris at bubblescope dot net>]

My concern with accepting this patch is that many of libstdc++'s hash
functions are awful from a mixing point of view -- you would get
exactly the same problem from users who have integers which are always
a multiple of a power of 2 (which is in practice not uncommon). This
would give exactly the same problem.

Rather than try to "fix" one hash function like this, we should just
accept our hash functions might have low quality lower order bits.



On 8 May 2015 at 21:18, FranÃois Dumont <frs.dumont@gmail.com> wrote:
> On 08/05/2015 10:02, Richard Biener wrote:
>>
>> On Wed, May 6, 2015 at 10:10 PM, FranÃois 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; }.
>
>
> Sorry, I don't understand your remark. Do you mean that if someone is not
> using std::hash to hash its pointers he won't benefit from the enhancement ?
>
> It is a good point however to see that gcc is using something similar
> internally.
>
> FranÃois
>