[cfe-dev] libc++ is not using always_inline anymore!

[Louis Dionne via cfe-dev]

> On Nov 11, 2018, at 00:08, Louis Dionne via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> 
> 
> On Nov 10, 2018, at 23:14, Mehdi AMINI <joker.eph at gmail.com <mailto:joker.eph at gmail.com>> wrote:
> 
>> 
>> 
>> Le mar. 30 oct. 2018 à 15:57, Louis Dionne via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> a écrit :
>> 
>>> On Oct 30, 2018, at 18:00, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>>> 
>>> Awesome!
>>> 
>>> What are the new semantics? That this ABI stability guarantee is provided by hiding the functions in each user so they can't be deduplicated with anotehr user's copy? (what about other copies that are from the same build? I guess even those won't get coalesced/collapsed together? Would that be useful to support?)
>> 
>> There are currently two modes (in LLVM trunk, and that is the plan for LLVM 8 too):
>> 
>> 1. (the default) All TUs linked together inside the same final linked image need to have use the same libc++ version. Inline functions are ODR-merged across TUs like they normally are. In this mode, we don't use any funny attribute to control linkage (neither always_inline nor internal_linkage).
>> 
>> 2. (can be opted-in) TUs can be linked together even if they use different headers of libc++. This is achieved by using internal_linkage on implementation detail functions of libc++. Those functions are local to a TU and they are NOT ODR-merge across TUs. This results in more code duplication than option (1).
>> 
>>> 
>>> I assume this doesn't change the defaults, but does it make it any easier for users who don't need the ABI stability guarantee? (or was it already easy/no change here?)
>> 
>> It actually does change the default. However, it depends of what ABI guarantee you're talking about.
>> 
>> 1. The ABI stability of the shared objects is always (and has always been, and will most likely always) be guaranteed. The only way to change that is to explicitly use the _LIBCPP_ABI_UNSTABLE macro, which says "use all the ABI breaking features". This obviously only works if you're also linking against a library that was built to provide that ABI. This ability to use the unstable ABI has been provided for a long time, it wasn't the default, and it still isn't the default -- my change is completely orthogonal to that.
>> 
>> 2. The "ABI stability" of static archives is a different matter. The question here is whether you can link programs against static archives built with different versions of libc++. The answer used to be YES by default, not it is NO by default. If you want to retain that ability, you need to use the `_LIBCPP_HIDE_FROM_ABI_PER_TU` macro. And also please give us a heads up so we know someone is using it.
>> 
>> In general I'm worried of "undefined behavior" that isn't caught by a tool, ideally at build time otherwise at runtime. I would really encourage to not introduce any default behavior where you can't provide an easy detection mechanism to the user.
> 
> Can you please expand on what you mean here? Are you referring to the potential for ODR violations if someone links TUs built against different versions of the libc++ headers? If so, that situation exists for every single C++ library out in the wild.

More specifically, what I mean here is that anyone relying on this guarantee is walking an incredibly thin line, and so I think it is reasonable for such users to explicitly opt into the guarantee.

Louis

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