void function declared attribute const

[Jonathan Wakely]

On 02/08/17 15:41 -0600, Martin Sebor wrote:
>Hi Honza,
>
>While testing improvements to GCC attribute handling I came
>across the warning below:
>
>In file included from 
>/ssd/src/gcc/81544/libstdc++-v3/src/c++98/mt_allocator.cc:31:0:
>/ssd/build/gcc-81544/x86_64-pc-linux-gnu/libstdc++-v3/include/ext/mt_allocator.h:359:43: 
>warning: ‘const’ attribute on function returning ‘void’ [-Wattributes]
>       _M_destroy_thread_key(void*) throw ();
>                                          ^
>
>Git log shows you added the attribute in r146330 and there was
>a lively debate preceding the change on the libstdc++ mailing
>list:
>  https://gcc.gnu.org/ml/libstdc++/2009-04/msg00053.html
>
>A patch including this function was posted here:
>  https://gcc.gnu.org/ml/libstdc++/2009-04/msg00111.html
>
>I've re-read much of the discussion but couldn't find my answer
>there so I'm wondering if you could help me understand the
>rationale for it.  (Btw., I understand what the attribute does,
>I'm just not sure why it's helpful or even used in this instance.
>
>The function is declared like this in the header:
>
>      // XXX GLIBCXX_ABI Deprecated
>      _GLIBCXX_CONST void
>      _M_destroy_thread_key(void*) throw ();
>
>and defined like so in mt_allocator.cc:
>
>  void
>  __pool<true>::_M_destroy_thread_key(void*) throw () { }
>
>So the definition trivially meets the requirements on a const
>function, and calls to it will be eliminated (with optimization).
>
>My question is: since the function is deprecated and its
>definition only provided for ABI compatibility, should calls
>to it ever actually be compiled with future revisions of GCC?
>
>I mean, since it's an implementation-only function, there should
>be no code outside of libstdc++ that makes direct calls to it.
>Code that did make calls to it (e.g., as a result of macro or
>inline expansion) was presumably compiled into object files by
>older versions of GCC (before the deprecation and before the
>addition of the attribute) and won't benefit from the const
>attribute now.  And since the function is deprecated, no code
>newly compiled with GCC should make calls to it, either directly
>or otherwise.  (Would using attribute deprecated on the function
>make sense?)
>
>The reason for my question is to understand if the warning is
>justified (it's based on the documentation of attribute const:
>"It does not make sense for a const function to return void.")
>
>If it does make sense to declare a function const that returns
>void then I'll remove the warning and update the manual and
>mention this use case.  Otherwise, if you confirm that the
>function shouldn't be called in new code I'll submit a patch
>to remove the const attribute.

I think you're right that it won't be called from new code.