[cfe-dev] [libc++] Guarantee that TUs built with different versions of libc++ can be linked together

[Louis Dionne via cfe-dev]

> On Aug 14, 2018, at 13:56, John McCall <rjmccall at apple.com> wrote:
> 
>> On Aug 14, 2018, at 1:19 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com <mailto:dexonsmith at apple.com>> wrote:
>> I support this change too.
>> 
>> At least in the short term, I think vendors should still have the capability to provide this guarantee without user opt-in by changing the default macro settings (switching it to a user opt-out).
> 
> I agree with both of these points.

I implemented these in the latest revision of https://reviews.llvm.org/D50652 <https://reviews.llvm.org/D50652>.

With https://reviews.llvm.org/D50652 <https://reviews.llvm.org/D50652>, by default, internal_linkage is NOT used and it is NOT safe to link TUs compiled with different versions of libc++. When _LIBCPP_ABI_HIDDEN_USE_INTERNAL_LINKAGE is defined, internal_linkage is used and it IS safe again to link TUs compiled with different versions of libc++.

To make that the default behavior, libc++ can be configured with `cmake <…> -DLIBCXX_ABI_HIDDEN_USE_INTERNAL_LINKAGE=ON` and _LIBCPP_ABI_HIDDEN_USE_INTERNAL_LINKAGE will be defined by default.

Bikeshedding on the name of that option is welcome on the review page — but I’d like to keep the form _LIBCPP_ABI_XXX to be consistent with other ABI-related macros.

Louis

> 
> John.
> 
>> 
>>> On Aug. 14, 2018, at 10:11, Reid Kleckner <rnk at google.com <mailto:rnk at google.com>> wrote:
>>> 
>>> I 100% support not providing this guarantee by default. It always seemed a bit too heroic to me for libc++ to try to provide this support.
>>> 
>>> I think most people with standard library version skew typically only have skew between separate DSOs.
>>> 
>>> On Tue, Aug 14, 2018 at 6:49 AM Louis Dionne via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
>>> Hi,
>>> 
>>> libc++ currently allows TUs that were built using different versions of libc++ to be linked together. Roughly speaking, this is achieved by making sure that anything not ABI-stable is local to each TU. This also allows distributing a static archive without caring too much about the version of libc++ it was compiled with, at least in principle. Supporting this use case creates code bloat for all users of libc++, since private functions are duplicated in each TU.
>>> 
>>> I posit that few clients actually care about this, yet everybody is paying for it in terms of executable size. Hence, I would like to change the default guarantee offered by libc++ from "this use case is supported" to "this use case is not supported". The use case would still be supported by defining a macro for those that need it.
>>> 
>>> Does anybody have objections to this change?
>>> 
>>> Louis
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>> 
> 

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