[cfe-dev] add new option -fnovisibility for clang

[James Y Knight via cfe-dev]

On Thu, Sep 10, 2020 at 10:30 PM Hubert Tong <
hubert.reinterpretcast at gmail.com> wrote:

> On Thu, Sep 10, 2020 at 9:07 PM James Y Knight <jyknight at google.com>
> wrote:
>
>>
>>
>> On Thu, Sep 10, 2020, 6:57 PM Hubert Tong <
>> hubert.reinterpretcast at gmail.com> wrote:
>>
>>> On Thu, Sep 10, 2020 at 6:30 PM James Y Knight via cfe-dev <
>>> cfe-dev at lists.llvm.org> wrote:
>>>
>>>> But *why* does AIX want to ignore visibility restrictions encoded via
>>>> attribute? Especially by default?
>>>>
>>> One reason is that AIX performs more early/less lazy symbol resolution
>>> (even with runtime linking) than, say, Linux. So, if a project goes with an
>>> export-by-default model (via attribute in some scope), they can cause
>>> link-time errors from symbol references to undefined symbols from code that
>>> would otherwise have been discarded as unreferenced by the linker. It seems
>>> such code is not uncommon (and has the questionable effect of exporting
>>> template instantiations based on "client provided" types that are not part
>>> of the "library"/"utility" code in question).
>>>
>>
>> But, the default visibility is "default" -- which is the most export-y
>> visibility setting there is. So, without specifying visibility options on
>> the command line, the only thing you can do with visibility attributes in
>> code is to *remove *exports, not add additional ones.
>>
>> What am I missing?
>>
> That AIX is not an ELF platform. AIX has a default visibility for XCOFF
> called "unspecified". This is because the ELF-based visibility options do
> not quite match the base case on AIX. That the most export-y visibility is
> named "default" at the source level is an ELF-ism.
>

But Clang doesn't have an "unspecified" visibility, only default,
protected, and hidden. And this proposal doesn't propose to add one,
either. So I don't see how this command-line option makes sense in Clang,
right now.

Perhaps it would make sense if proposed as a component of a larger change
to visibility handling in clang.
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