[patch] Remove the linker_private and linker_private_weak linkages

[Rafael Espíndola]

ping

On 7 March 2014 15:18, Rafael Espíndola <rafael.espindola at gmail.com> wrote:
> Hi,
>
> These linkages were introduced some time ago, but it was never very
> clear what exactly their semantics were or what they should be used
> for. Some investigation found these uses:
>
> * utf-16 strings in clang.
> * non-unnamed_addr strings produced by the sanitizers.
>
> It turns out they were just working around a more fundamental problem.
> For some sections a MachO linker needs a symbol in order to split the
> section into atoms, and llvm had no idea that was the case. I fixed
> that in r201700 and it is now safe to use the private linkage. When
> the object ends up in a section that requires symbols, llvm will use a
> 'l' prefix instead of a 'L' prefix and things just work.
>
> With that, these linkages were already dead, but there was a potential
> future user in the objc metadata information. I am still looking at
> CGObjcMac.cpp, but at this point I am convinced that linker_private
> and linker_private_weak are not what they need.
>
> The objc uses are currently split in
>
> * Regular symbols (no '\01' prefix). LLVM already directly provides
> whatever semantics they need.
> * Uses of a private name (start with "\01L" or "\01l") and private
> linkage. We can drop the "\01L" and "\01l" prefixes as soon as llvm
> agrees with clang on L being ok or not for a given section. I have two
> patches in code review for this.
> * Uses of private name and weak linkage.
>
> The last case is the one that one could think would fit one of these
> linkages. That is not the case. The semantics are
>
> * the linker will merge these symbol by *name*.
> * the linker will hide them in the final DSO.
>
> Given that the merging is done by name, any of the private (or
> internal) linkages would be a bad match. They allow llvm to rename the
> symbols, and that is really not what we want. From the llvm point of
> view, these objects should really be (linkonce|weak)(_odr)?.
>
> For now, just keeping the "\01l" prefix is probably the best for these
> symbols. If we one day want to have a more direct support in llvm,
> IMHO what we should add is not a linkage, it is just a hidden_symbol
> attribute. It would be applicable to multiple linkages. For example,
> on weak it would produce the current behavior we have for objc
> metadata. On internal, it would be equivalent to private (and we
> should then remove private).
>
> Cheers,
> Rafael