Remove the linker_private and linker_private_weak linkages.

These linkages were introduced some time ago, but it was never very
clear what exactly their semantics were or what they should be used
for. Some investigation found these uses:

* utf-16 strings in clang.
* non-unnamed_addr strings produced by the sanitizers.

It turns out they were just working around a more fundamental problem.
For some sections a MachO linker needs a symbol in order to split the
section into atoms, and llvm had no idea that was the case. I fixed
that in r201700 and it is now safe to use the private linkage. When
the object ends up in a section that requires symbols, llvm will use a
'l' prefix instead of a 'L' prefix and things just work.

With that, these linkages were already dead, but there was a potential
future user in the objc metadata information. I am still looking at
CGObjcMac.cpp, but at this point I am convinced that linker_private
and linker_private_weak are not what they need.

The objc uses are currently split in

* Regular symbols (no '\01' prefix). LLVM already directly provides
whatever semantics they need.
* Uses of a private name (start with "\01L" or "\01l") and private
linkage. We can drop the "\01L" and "\01l" prefixes as soon as llvm
agrees with clang on L being ok or not for a given section. I have two
patches in code review for this.
* Uses of private name and weak linkage.

The last case is the one that one could think would fit one of these
linkages. That is not the case. The semantics are

* the linker will merge these symbol by *name*.
* the linker will hide them in the final DSO.

Given that the merging is done by name, any of the private (or
internal) linkages would be a bad match. They allow llvm to rename the
symbols, and that is really not what we want. From the llvm point of
view, these objects should really be (linkonce|weak)(_odr)?.

For now, just keeping the "\01l" prefix is probably the best for these
symbols. If we one day want to have a more direct support in llvm,
IMHO what we should add is not a linkage, it is just a hidden_symbol
attribute. It would be applicable to multiple linkages. For example,
on weak it would produce the current behavior we have for objc
metadata. On internal, it would be equivalent to private (and we
should then remove private).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203866 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 1 insertions, 12 deletions

diff --git a/docs/LangRef.rst b/docs/LangRef.rst
index 0a46e8375e..c959f56ef1 100644
--- a/docs/LangRef.rst
+++ b/docs/LangRef.rst
@@ -197,16 +197,6 @@ linkage:
     private to be renamed as necessary to avoid collisions. Because the
     symbol is private to the module, all references can be updated. This
     doesn't show up in any symbol table in the object file.
-``linker_private``
-    Similar to ``private``, but the symbol is passed through the
-    assembler and evaluated by the linker. Unlike normal strong symbols,
-    they are removed by the linker from the final linked image
-    (executable or dynamic library).
-``linker_private_weak``
-    Similar to "``linker_private``", but the symbol is weak. Note that
-    ``linker_private_weak`` symbols are subject to coalescing by the
-    linker. The symbols are removed by the linker from the final linked
-    image (executable or dynamic library).
 ``internal``
     Similar to private, but the value shows as a local symbol
     (``STB_LOCAL`` in the case of ELF) in the object file. This
@@ -681,8 +671,7 @@ Syntax::
 
     @<Name> = [Visibility] [DLLStorageClass] alias [Linkage] <AliaseeTy> @<Aliasee>
 
-The linkage must be one of ``private``, ``linker_private``,
-``linker_private_weak``, ``internal``, ``linkonce``, ``weak``,
+The linkage must be one of ``private``, ``internal``, ``linkonce``, ``weak``,
 ``linkonce_odr``, ``weak_odr``, ``external``. Note that some system linkers
 might not correctly handle dropping a weak symbol that is aliased by a non-weak
 alias.