[cfe-dev] different gcc/clang behavior for Internal visibility

[Reid Kleckner via cfe-dev]

IIRC, internal visibility is a guarantee that a function is not escaped and
called indirectly from another DSO. This allows the compiler to avoid
setting up a TOC or PIC base register in the prologue for architectures
that have one. This was notably expensive for x86_32 because of the
call/pop code sequence required to materialize EIP. There are other RISC-y
architectures (PPC? 64? not sure) with PIC base registers that could
benefit from internal visibility support, but I think most of them are
considered legacy architectures at this point, so it's not top priority.

On Mon, May 10, 2021 at 12:52 PM Fangrui Song via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> On 2021-05-10, Y Song via cfe-dev wrote:
> >Hi,
> >
> >The bpf linker project tries to explore to use INTERNAL visibility as in
> >   https://lore.kernel.org/bpf/20210507054119.270888-1-andrii@kernel.org/
> >
> >But we found clang actually changed user "internal" visibility to
> "hidden".
> >For example, I have the following example,
> >
> >$ cat t.c
> >int __attribute__((visibility("internal"))) foo() { return 0; }
> >$ clang -c t.c && llvm-readelf -s t.o | grep foo
> >     3: 0000000000000000     8 FUNC    GLOBAL HIDDEN      2 foo
> >$ gcc -c t.c && llvm-readelf -s t.o | grep foo
> >     8: 0000000000000000    11 FUNC    GLOBAL INTERNAL    1 foo
> >$
> >
> >Looks like this is caused by clang Attr.td,
> >
> >diff --git a/clang/include/clang/Basic/Attr.td
> >b/clang/include/clang/Basic/Attr.td
> >index 5e04f32187cd..4559a1bcfe42 100644
> >--- a/clang/include/clang/Basic/Attr.td
> >+++ b/clang/include/clang/Basic/Attr.td
> >@@ -2776,7 +2776,7 @@ def Visibility : InheritableAttr {
> >   let Spellings = [GCC<"visibility">];
> >   let Args = [EnumArgument<"Visibility", "VisibilityType",
> >                            ["default", "hidden", "internal",
> "protected"],
> >-                           ["Default", "Hidden", "Hidden",
> "Protected"]>];
> >+                           ["Default", "Hidden", "Internal",
> "Protected"]>];
> >   let MeaningfulToClassTemplateDefinition = 1;
> >   let Documentation = [Undocumented];
> > }
> >@@ -2786,7 +2786,7 @@ def TypeVisibility : InheritableAttr {
> >   let Spellings = [Clang<"type_visibility">];
> >   let Args = [EnumArgument<"Visibility", "VisibilityType",
> >                            ["default", "hidden", "internal",
> "protected"],
> >-                           ["Default", "Hidden", "Hidden",
> "Protected"]>];
> >+                           ["Default", "Hidden", "Internal",
> "Protected"]>];
> > //  let Subjects = [Tag, ObjCInterface, Namespace];
> >   let Documentation = [Undocumented];
> > }
> >
> >One of early commits,
> >
> >commit 570024a8d9b4a4aa4a35f077a0a65003dc7b71fe
> >Author: Eli Friedman <eli.friedman at gmail.com>
> >Date:   Thu Aug 5 06:57:20 2010 +0000
> >
> >    Implement #pragma GCC visibility.
> >
> >    llvm-svn: 110315
> >
> >I see
> >
> >+    else if (VisType->isStr("internal"))
> >+      type = VisibilityAttr::HiddenVisibility; // FIXME
> >
> >Do we have any plan to support Internal visibility?
>
> I don't think there is value supporting STV_INTERNAL.
>
> STV_INTERNAL as we see today in the ELF specification was requested by SGI.
>
> > The meaning of this visibility attribute may be defined by processor
> > supplements to further constrain hidden symbols. A processor
> > supplement's definition should be such that generic tools can safely
> > treat internal symbols as hidden.  An internal symbol contained in a
> > relocatable object must be either removed or converted to STB_LOCAL
> > binding by the link-editor when the relocatable object is included in an
> > executable file or shared object.
>
> I recall from reading somewhere that it is used by its propritery
> compiler for some LTO like optimizations. STV_INTERNAL is identical to
> STV_HIDDEN in GNU/Solaris (and likely HP-UX).
>
> For the Linux kernel, my suggestion is to just use
> __attribute__((visibility("hidden"))).
> Then there will be no confusion that "internal" translates to STV_HIDDEN.
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