[llvm-dev] Internal visibility

[Reid Kleckner via llvm-dev]

I don't think there's any technical reason not to add support for internal
visibility. In 2015, I added the alias in clang:

$ git show 7737bd9f9fb5d9cfd5717f36791036fa08d57b95
commit 7737bd9f9fb5d9cfd5717f36791036fa08d57b95
Author: Reid Kleckner <rnk at google.com>
Date:   Wed Oct 21 22:01:02 2015 +0000

    [Driver] Alias -fvisibility=internal to -fvisibility=hidden

    The ELF symbol visibilities are:
    - internal: Not visibile across DSOs, cannot pass address across DSOs
    - hidden: Not visibile across DSOs, can be called indirectly
    - default: Usually visible across DSOs, possibly interposable
    - protected: Visible across DSOs, not interposable

    LLVM only supports the latter 3 visibilities. Internal visibility is in
    theory useful, as it allows you to assume that the caller is maintaining
    a PIC register for you in %ebx, or in some other pre-arranged location.
    As far as LLVM is concerned, this isn't worth the trouble. Using hidden
    visibility is always correct, so we can just do that.

    Resolves PR9183.

    llvm-svn: 250954

While I did say "As far as LLVM is concerned, this isn't worth the
trouble.", but that was mostly just a reflection of the fact that I filed
this bug in 2011 and it never got any attention: https://llvm.org/pr9183.

Given what I know about how PIC code works for PPC, it seems like it might
be worth someone's time to add this visibility mode.

On Thu, Feb 7, 2019 at 2:54 PM Rafik Zurob via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello
>
> In GCC on Linux on Power, -fvisibility=internal enables internal
> visibility.
>
> $ cat t.c
> int foo(int i) { return i + 1; }
> $ gcc t.c -c -fvisibility=internal
> $ readelf -s t.o | grep foo
>      8: 0000000000000000    60 FUNC    GLOBAL INTERNAL    1 foo
> $
>
> But in Clang/LLVM, GlobalValue::VisibilityTypes does not have internal
> visibility:
>
>   /// An enumeration for the kinds of visibility of global values.
>   enum VisibilityTypes {
>     DefaultVisibility = 0,  ///< The GV is visible
>     HiddenVisibility,       ///< The GV is hidden
>     ProtectedVisibility     ///< The GV is protected
>   };
>
> and using -fvisibility=internal in Clang uses the
> close-but-not-quite-the-same hidden visibility:
>
> $ clang t.c -c -fvisibility=internal
> $ readelf -s t.o | grep foo
>      2: 0000000000000000    36 FUNC    GLOBAL HIDDEN     2 foo
> $
>
> Given that -fvisibility=internal is accepted, I assume this behaviour is
> intentional.  Can anyone help me understand why that is, or point me to
> where I can find out more?
>
> Thanks
>
> Rafik
>
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