[PATCH] Simplify _Hashtable::_M_merge_multi

[François Dumont]

For the record, this small change have been committed as part of another 
larger one:

https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=bcc8dea6a45b46febfa76df6f5e3e5b13f3b4a58


On 19/02/2025 13:49, Jonathan Wakely wrote:
> On Mon, 17 Feb 2025 at 11:59, François Dumont <frs.dumont@gmail.com> wrote:
>>
>> On 16/02/2025 23:14, Jonathan Wakely wrote:
>>> On Sun, 16 Feb 2025 at 21:15, François Dumont <frs.dumont@gmail.com> wrote:
>>>> Hi
>>>>
>>>> A minor simplification.
>>>>
>>>> libstdc++: Simplify _Hashtable::_M_merge_multi
>>>>
>>>> When merging two hashtable instances of the same type we do not need
>>>> to go through _M_src_hash_code that also check for identical Hash functor
>>>> type.
>>> But that check is very cheap, do we really gain much here?
>> No runtime gain no, just limit instantiations.
>>
>>> If we're getting rid of _M_src_hash_code in this overload of
>>> _M_merge_multi, should we get rid of it in the other overload too,
>>> since that's the only other place that uses _M_src_hash_code?
>> Seems complicated, so maybe we can avoid is_same_v and is_empty_v with
>> this smaller patch ?
> is_same_v and is_empty_v are extremely cheap, they are variable
> templates that use a compiler intrinsic.
>
>>       libstdc++: [_Hashtable] Avoid is_same_v and is_empty_v instantiations
>>
>>       There is no need to check for same Hash functor type and this type
>> being stateless
>>       if there is no hash code cached to reuse.
>>
>>       libstdc++-v3/ChangeLog:
>>
>>               * include/bits/hashtable.h (_Hashtable<>::_M_src_hash_code):
>>               Add __hash_cached::value check.
>>
>> Let me know if interested.
> OK for trunk
>