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

Fri Nov 16 05:06:15 PST 2018

> On Nov 16, 2018, at 00:18, JF Bastien via libcxx-dev <libcxx-dev at lists.llvm.org> wrote:
> 
> 
> 
>> On Nov 15, 2018, at 5:41 PM, Kristina Brooks via libcxx-dev <libcxx-dev at lists.llvm.org <mailto:libcxx-dev at lists.llvm.org>> wrote:
>> 
>> 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.
> 
> According to: https://itanium-cxx-abi.github.io/cxx-abi/ <https://itanium-cxx-abi.github.io/cxx-abi/>
> 
> The primary discussion forum for the ABI is the GitHub repository <http://github.com/itanium-cxx-abi/cxx-abi/>. Please open a new issue for any new topic you want to discuss. Previously, the ABI was discussed on a mailing list, cxx-abi-dev, whose archives <http://sourcerytools.com/pipermail/cxx-abi-dev/> are still accessible.
> 
> CC’ing John since he’ll know more about this, seeing as how he’s the most active contributor to the repository :-)
> 
> This issue from Richard seems like a good starting point: https://github.com/itanium-cxx-abi/cxx-abi/issues/42 <https://github.com/itanium-cxx-abi/cxx-abi/issues/42>
> 
> 
>> 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 <mailto: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 <mailto: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?
> 
> What I had in mind was distinguishing `std::__N` as you say, and by construction that would distinguish between implementations because libc++ and libstdc++ use different namespaces.

But that would also prevent libstdc++ from ever using that mangling scheme (not that I care about libstdc++, but you know).

Louis

> 
> 
>>> 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> <mailto: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 <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>
>>>>>>  <mailto: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>
>>>>>>      <mailto: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 <http://lists.llvm.org/cgi-bin/mailman/listinfo/libcxx-dev>>
>>>>>> 
>>>>>>      -- Marshall
>>>>>> 
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>>>>>> 
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