Re: [PATCH][Hashtable 5/6] Remove H1/H2 template parameters

["François Dumont" <frs.dumont@gmail.com>]

Deeply sorry, I indeed didn't sent the patch I wanted to commit. It was 
in 3 commits on my side and it looks like I had miss the last one 
regarding the changes for _ExtractKey.

But moreover I had change the ebo helper index wrongly, I missed the abi 
change here, thanks for fixing it.


On 26/08/20 6:45 pm, Jonathan Wakely wrote:
> On 26/08/20 16:58 +0100, Jonathan Wakely wrote:
>> On 26/08/20 16:55 +0100, Jonathan Wakely wrote:
>>> On 26/08/20 16:30 +0100, Jonathan Wakely wrote:
>>>> I'm seeing new FAILures with this:
>>>>
>>>> FAIL: 20_util/function_objects/searchers.cc (test for excess errors)
>>>> UNRESOLVED: 20_util/function_objects/searchers.cc compilation 
>>>> failed to produce executable
>>>> FAIL: experimental/functional/searchers.cc (test for excess errors)
>>>> UNRESOLVED: experimental/functional/searchers.cc compilation failed 
>>>> to produce executable
>>>>
>>>> It looks like what you committed is not what you sent for review. The
>>>> patch sent for review has:
>>>>
>>>> /// Specialization: hash function and range-hashing function, no
>>>> /// caching of hash codes.
>>>> /// Provides typedef and accessor required by C++ 11.
>>>> template<typename _Key, typename _Value, typename _ExtractKey,
>>>> -          typename _H1, typename _H2>
>>>> -    struct _Hash_code_base<_Key, _Value, _ExtractKey, _H1, _H2,
>>>> -                          _Default_ranged_hash, false>
>>>> +          typename _Hash, typename _RangeHash, typename _Unused>
>>>> +    struct _Hash_code_base<_Key, _Value, _ExtractKey, _Hash, 
>>>> _RangeHash,
>>>> +                          _Unused, false>
>>>>   : private _Hashtable_ebo_helper<0, _ExtractKey>,
>>>> -      private _Hashtable_ebo_helper<1, _H1>,
>>>> -      private _Hashtable_ebo_helper<2, _H2>
>>>> +      private _Hashtable_ebo_helper<1, _Hash>
>>>>   {
>>>>
>>>>
>>>> But what you committed has:
>>>>
>>>> /// Specialization: hash function and range-hashing function, no
>>>> /// caching of hash codes.
>>>> /// Provides typedef and accessor required by C++ 11.
>>>> template<typename _Key, typename _Value, typename _ExtractKey,
>>>> -          typename _H1, typename _H2>
>>>> -    struct _Hash_code_base<_Key, _Value, _ExtractKey, _H1, _H2,
>>>> -                          _Default_ranged_hash, false>
>>>> -    : private _Hashtable_ebo_helper<0, _ExtractKey>,
>>>> -      private _Hashtable_ebo_helper<1, _H1>,
>>>> -      private _Hashtable_ebo_helper<2, _H2>
>>>> +          typename _Hash, typename _RangeHash, typename _Unused>
>>>> +    struct _Hash_code_base<_Key, _Value, _ExtractKey, _Hash, 
>>>> _RangeHash,
>>>> +                          _Unused, false>
>>>> +    : private _Hashtable_ebo_helper<0, _Hash>
>>>>   {
>>>>
>>>>
>>>> Note that you've changed the type of the base class from:
>>>>
>>>> +      private _Hashtable_ebo_helper<1, _Hash>
>>>>
>>>> to
>>>>
>>>> +      private _Hashtable_ebo_helper<0, _Hash>
>>>>
>>>> This causes an ambiguity:
>>>>
>>>> /home/jwakely/src/gcc/build/x86_64-pc-linux-gnu/libstdc++-v3/include/bits/hashtable_policy.h:1706: 
>>>> error: 'std::__detail::_Hashtable_ebo_helper<0, test03()::<unnamed 
>>>> struct>, true>' is an ambiguous base of 
>>>> 'std::__detail::_Hashtable_base<char, std::pair<const char, long 
>>>> int>, std::__detail::_Select1st, test03()::<unnamed struct>, 
>>>> test03()::<unnamed struct>, std::__detail::_Mod_range_hashing, 
>>>> std::__detail::_Default_ranged_hash, 
>>>> std::__detail::_Hashtable_traits<true, false, true> >'
>>>>
>>>> However, what I don't understand is why we are storing that _Hash type
>>>> more than once as a base class. That seems wrong (but not something we
>>>> can change without ABI impact).
>>>
>>> Ah, we're not storing it more than once.
>>>
>>> The problem is:
>>>
>>> template<typename _Key, typename _Value,
>>>        typename _ExtractKey, typename _Equal,
>>>        typename _H1, typename _H2, typename _Hash, typename _Traits>
>>> struct _Hashtable_base
>>> : public _Hash_code_base<_Key, _Value, _ExtractKey, _H1, _H2, _Hash,
>>>                _Traits::__hash_cached::value>,
>>>   private _Hashtable_ebo_helper<0, _Equal>
>>>
>>> This has a base of _Hashtable_ebo_helper<0, _Equal> so it used to
>>> have these bases:
>>>
>>> _Hashtable_ebo_helper<0, _ExtractKey>
>>> _Hashtable_ebo_helper<1, _Hash>
>>> _Hashtable_ebo_helper<2, _RangeHash>
>>> _Hashtable_ebo_helper<0, _Equal>
>>>
>>> but after your change it has these bases:
>>>
>>> _Hashtable_ebo_helper<0, _Hash>
>>> _Hashtable_ebo_helper<0, _Equal>
>>>
>>> In the case
>>> where _Equal and _Hash are the same type (which is what I was testing
>>> in the test that fail, because I'm sneaky) that means:
>>>
>>> _Hashtable_ebo_helper<0, T>
>>> _Hashtable_ebo_helper<0, T>
>>>
>>> which is obviously ambiguous.
>>>
>>> I think the _hash_code_base should still use the index 1 for its base
>>> class, i.e. _Hashtable_ebo_helper<1, _Hash>. That way we have these:
>>>
>>> _Hashtable_ebo_helper<1, _Hash>
>>> _Hashtable_ebo_helper<0, _Equal>
>>>
>>> which works even if they're the same types.
>>
>> Here's a minimal test:
>>
>> #include <unordered_map>
>>
>> struct Evil : std::hash<int>, std::equal_to<int>
>> {
>> };
>>
>> int main()
>> {
>>  std::unordered_map<int, int, Evil, Evil> h;
>>  h.key_eq();
>> }
>>
>> This fails on current trunk.
>
> Fixed by the attached patch.
>
> Tested powerpc64le-linux, committed to trunk.
>
>