RTTI names for classes in anonymous namespaces

RTTI names for classes in anonymous namespaces

[Varun Kumar E]

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 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(*).
      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

Re: RTTI names for classes in anonymous namespaces

[Jonathan Wakely]

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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.


      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
>

Re: RTTI names for classes in anonymous namespaces

[Jonathan Wakely]

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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
>>
>

Re: RTTI names for classes in anonymous namespaces.

[Jonathan Wakely]

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On Wed, 23 Aug 2023, 09:19 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(*).

      Could you please explain the reasoning behind this?


It means that type_info equality can be determined by just comparing the
const char* pointers, instead of using strcmp to match equal names.

The pointer value will be unique, so will only compare equal to itself.


As Itanium ABI
> c++ has no mention of this.
>

The Itanium ABI doesn't dictate the form of those strings.

When you call std::type_info::name() the '*' is not in the result. It's
just an implementation detail.

RTTI names for classes in anonymous namespaces.

[Varun Kumar E]

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 hello all,

      The RTTI names generated by gcc for classes in anonymous namespaces
begin with the prefix asterisk(*).
      Could you please explain the reasoning behind this? As Itanium ABI
c++ has no mention of this.

regards,
Varun