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

Tue Sep 15 15:18:23 PDT 2020

> What is ultimately the behavior for "unspecified" symbols, when no
export-list is provided by the user?

If symbols have `unspecified` visibility, and no export list is provided,
symbols will not get exported.

> Are you saying that on the existing AIX compilers/linkers, the
export-list cannot be used to make a symbol "hidden" (non-exported), if it
had been explicitly annotated visibility("default") in the source (assuming
visibility declarations weren't being ignored)? That only "unspecified"
symbols can be modified by the export-list?

The export list is able to override the visibility settings in the object
file in some situations, but not in all situations. The scenario you
described above could be overridden by export list.
There is a comprehensive list for all scenarios here:
https://www.ibm.com/developerworks/aix/library/au-aix-symbol-visibility-part2/index.html
in
Table 1.

Again, this is more for getting the default right on AIX. GCC and xlclang
on AIX have the consistent default behavior as what we described for
-mignore-xcoff-visibility.

On Tue, Sep 15, 2020 at 5:20 PM James Y Knight <jyknight at google.com> wrote:

> On Tue, Sep 15, 2020 at 3:29 PM Jason Liu via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> > 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?
>>
>> I guess I understand the point of confusion, so I will try to clarify it
>> a little bit.
>> It goes back to Clang does not have "unspecified" visibility mapping yet.
>> On AIX, the baseline visibility is not "default" or export. The baseline
>> visibility is "unspecified" when no attribute is marked on the symbol.
>>
>
> What is ultimately the behavior for "unspecified" symbols, when no
> export-list is provided by the user?
>
> And -mignore-xcoff-visibility means let all the symbols have "unspecified"
>> visibility, whether or not any visibility attribute is specified on the
>> symbol. So that user could use export list in the link step to control the
>> visibility on AIX, which gives a more fine-grained control.
>>
>
> On GNU/ELF, the export-list functionality (called version-script there)
> allows you to remove symbols from the list to be exported, but not add
> them. So, visibility("default") marks it as potentially-exported, but the
> version-script can override and make the symbol hidden if it wants to.
>
> Are you saying that on the existing AIX compilers/linkers, the export-list
> cannot be used to make a symbol "hidden" (non-exported), if it had been
> explicitly annotated visibility("default") in the source (assuming
> visibility declarations weren't being ignored)? That only "unspecified"
> symbols can be modified by the export-list?
>
>
>
>> As it is right now, the llc on AIX actually treats
>> `GlobalValue::DefaultVisibility` as "unspecified" as it is the baseline
>> visibility mode on AIX. So it is a bit messy there, and we will need to
>> clean that up when we add "unspecified" visibility attribute to clang for
>> AIX. But I don't think it should prevent us from making the default
>> behavior(unspecified visibility for all symbols) right on AIX for now.
>> Hope that would clean up the confusion.
>>
>>
>> On Tue, Sep 15, 2020 at 2:35 PM Fāng-ruì Sòng via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>>
>>> On Tue, Sep 15, 2020 at 11:21 AM James Y Knight via cfe-dev
>>> <cfe-dev at lists.llvm.org> wrote:
>>> >
>>> > 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?
>>>
>>> The same feeling here... Aren't most
>>> __attribute__((visibility("hidden"))) guarded by macros? AIX can turn
>>> off the macros...
>>> _______________________________________________
>>> cfe-dev mailing list
>>> cfe-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>
>> _______________________________________________
>> cfe-dev mailing list
>> cfe-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20200915/f9355d4d/attachment-0001.html>