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

[James Y Knight via cfe-dev]

On Fri, Sep 11, 2020 at 3:55 PM Hubert Tong <
hubert.reinterpretcast at gmail.com> wrote:

> On Fri, Sep 11, 2020 at 2:58 PM James Y Knight <jyknight at google.com>
> wrote:
>
>> 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.
>>
> It makes sense for Clang on AIX if it improves the consistency of
> behaviour with other compilers on that platform. GCC on AIX does not
> support encoding visibility into XCOFF and the default behaviour of the IBM
> xlclang/xlclang++ compiler on AIX is to not encode visibility into XCOFF.
> That there will be future complementary changes does not seem to be a
> practical reason for leaving Clang gratuitously different on AIX from other
> AIX compilers.
>
>>
If clang did *not* implement this feature, symbols marked
visibility("hidden") or visibility("protected") will actually be marked
hidden/protected in the output on AIX. If clang *does* implement this
feature, such symbols will instead be marked as exported
default-visibility. There is no change for symbols marked
visibility("default").

So, the request here is that Clang on AIX should ignore the attempt to hide
symbols, resulting in symbols being exported, even though the code thought
it was hiding them? That's the desired behavior change?

I remain unclear as to how that is a useful or important feature. Are you
worried about compatibility with code which incorrectly specified
visibility("hidden"), when it actually did need to export the symbol?
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