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

[Louis Dionne via libcxx-dev]

> 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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