[libcxx-dev] Option to disable inline namespacing completely?

[Kristina Brooks via libcxx-dev]

Well that was in my original suggestion, but I don't know who is in charge of IA64 mangling, I
don't think Clang developers can push a change like this forward without some RFC to whatever
committee maintains the IA64 ABI or am I wrong? I suppose libc++ ABI could be considered separate
enough in which case we just need to come up with a compatible scheme for "short" manglings that
is also guaranteed not to clash with the standard IA64 ABI.

Since you and Eric are mostly in charge of libcxxabi, could you propose a draft for a compatible
short mangling that would allow doing this as a breaking change (for unstable ABI versions, even
2+ since it's not stable yet unless Fuchsia has settled on it)? That way it's possible to add that
into the Clang mangler. I'm still not sure if this requires a blessing from whoever is in charge
of IA64 ABI, I think that would be a good idea to mention the draft to them at least.

Thanks.

On 16/11/2018 01:26, Louis Dionne wrote:
> 
> 
>> On Nov 15, 2018, at 20:06, JF Bastien via libcxx-dev <libcxx-dev at lists.llvm.org> wrote:
>>
>>
>>
>>> On Nov 15, 2018, at 3:59 PM, Kristina Brooks via libcxx-dev <libcxx-dev at lists.llvm.org> wrote:
>>>
>>> Yes exactly, seems like an obvious win on a mono-ABI system, if preserving debug data
>>> this saves a lot, not to mention symbol table strings. Also, I'm purely speculating here
>>> but this would also speed up compilation and linking time by an insignificant margin, but
>>> nevertheless, it's an improvement.
>>>
>>> If the ABI ever needs to change, on those kinds of systems, it's easy enough with an OS
>>> update.
>>
>> Or, we could update the Itanium ABI so that the current mangling still works and is compatible with a new mangling which is shorter for libc++ (yet remains purposefully incompatible with libstdc++’s mangling) 🙂
>>
> 
> Is it necessary/desirable to distinguish between implementations of the standard library at the Itanium ABI level? Wouldn't it be possible to define a compressed scheme for `std::__N` in full generality, and handle the ability to differentiate between implementations differently?
> 
> Louis
> 
>>
>>> Thanks.
>>>
>>> On 14/11/2018 17:06, Eric Fiselier wrote:
>>>>
>>>>
>>>> On Wed, Nov 14, 2018 at 12:04 PM Eric Fiselier <eric at efcs.ca <mailto:eric at efcs.ca>> wrote:
>>>>
>>>>   The itanium specification provides special compressed mangled names for std::string when it's not in an inline
>>>>   namespace [1]
>>>>
>>>>   Specifically, `Ss` vs `St3__112basic_string`
>>>>
>>>>
>>>> Correction... `Ss` vs `NSt3__112basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEE`
>>>>
>>>>
>>>>
>>>>   [1] https://itanium-cxx-abi.github.io/cxx-abi/abi.html#mangling-compression
>>>>
>>>>   On Wed, Nov 14, 2018 at 10:03 AM Marshall Clow via libcxx-dev <libcxx-dev at lists.llvm.org
>>>>   <mailto:libcxx-dev at lists.llvm.org>> wrote:
>>>>
>>>>       On Sun, Nov 11, 2018 at 9:49 PM Kristina Brooks via libcxx-dev <libcxx-dev at lists.llvm.org
>>>>       <mailto:libcxx-dev at lists.llvm.org>> wrote:
>>>>
>>>>           Hi,
>>>>
>>>>           I was wondering, would it be possible to add an option to disable inline namespacing completely (as a CMake
>>>>           configuration flag) in libc++ for the sake of being able to use shorthand mangling and without having to
>>>>           resort to
>>>>           handling it on IA64 mangler level. This has many use cases for example distributions of anything that
>>>>           includes libc++ as
>>>>           one and only libc++ and does not allow non-vendor software to be installed. On an embedded system, assuming
>>>>           debug info
>>>>           is generated, and given how common some of the debug data takes a very significant amount of space given the
>>>>           complex
>>>>           definition of something like `std::__2::basic_string<...>` versus the short form (`std::string` having a
>>>>           shorthand
>>>>           mangling is a godsend since it's 95% shorter, not an accurate figure but basically definitely above 90%).
>>>>
>>>>
>>>>       I'm confused here.
>>>>       Why are you comparing `std::string` to `std::__2::basic_string<......" ?
>>>>
>>>>       `std::__2::string` is quite short.<http://lists.llvm.org/cgi-bin/mailman/listinfo/libcxx-dev>
>>>>
>>>>       -- Marshall
>>>>
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>>>>
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