Re: RTTI names for classes in anonymous namespaces

[Jonathan Wakely <jwakely.gcc@gmail.com>]

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On Wed, 23 Aug 2023, 09:54 Jonathan Wakely, <jwakely.gcc@gmail.com> wrote:

>
>
> On Wed, 23 Aug 2023, 09:41 Varun Kumar E via Libstdc++, <
> libstdc++@gcc.gnu.org> wrote:
>
>>  hello all,
>>
>>       The RTTI names generated by gcc for classes in anonymous namespaces
>> begin with the prefix asterisk(*).
>>       Once the __name pointer is different why do we not return
>> immediately? Instead we perform a string comparison if the name does not
>> begin with asterisk(*).
>>
>
> This would have been more more useful as a reply to your previous email on
> the subject (which I already replied to) rather than a separate thread.
>
> When using shared objects there is no guarantee that duplicate RTTI data
> gets combined, so we can have two or more copies of the data for a single
> type. Using strcmp to compare the names works correctly in this case.
>
> For types in anonymous namespaces (or types with no linkage) the type
> cannot exist in two different shared objects, so we know that the RTTI data
> is unique. That means a pointer comparison is ok.
>


And as I said in the other thread, the asterisk is present when the pointer
comparison is ok.

If either name begins with * then it's a type that only exists in one
object file, and so its RTTI data is unique.



>
>       Could you please explain the reasoning behind this.
>>
>>       code snippet below:
>>
>>    type_info::operator==(const type_info& __arg) const _GLIBCXX_NOEXCEPT
>>   {
>>       if (std::__is_constant_evaluated())
>>          return this == &__arg;
>>
>>       if (__name == __arg.__name)
>>          return true;
>>
>>       #if !__GXX_TYPEINFO_EQUALITY_INLINE
>>          // ABI requires comparisons to be non-inline.
>>          return __equal(__arg);
>>      #elif !__GXX_MERGED_TYPEINFO_NAMES
>>          // Need to do string comparison.
>>          return __name[0] != '*' && __builtin_strcmp (__name, __arg.name
>> ())
>> == 0;
>>       #else
>>          return false;
>>       #endif
>>      }
>>
>>      Code link: typeinfo - gcc-mirror/gcc - Sourcegraph
>> <
>> https://sourcegraph.com/github.com/gcc-mirror/gcc/-/blob/libstdc++-v3/libsupc++/typeinfo?L205
>> >
>>
>>
>> regards,
>> Varun
>>
>