[llvm-dev] Internal visibility (original) (raw)

Reid Kleckner via llvm-dev llvm-dev at lists.llvm.org
Thu Feb 7 16:08:16 PST 2019

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

LLVM Developers mailing list llvm-dev at lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190207/81b9bebd/attachment.html>

More information about the llvm-dev mailing list