Enhance std::hash for pointers

François Dumont frs.dumont@gmail.com
Mon May 11 21:06:00 GMT 2015

     My proposal should be consider out of any context. We don't know 
what std::hash is used for in user code, this is why I am proposing this 
patch even if for the moment it doesn't make any difference considering 
only our usage of it.

     Your remark would make more sens if we were talking about changing 
std::unordered_xxx containers number of buckets policy to a power of 2


On 09/05/2015 00:10, Christopher Jefferson wrote:
> 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, François Dumont<frs.dumont@gmail.com>  wrote:
>> On 08/05/2015 10:02, Richard Biener wrote:
>>> 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; }.
>> 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.
>> François

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