[PATCH] Explicitly mark _S_ti() as default visibility to work around clang -fvisibility-inlines-hidden bug

[Fāng-ruì Sòng via gcc-patches]

clang (including trunk and many older versions) incorrectly marks static local 
variables (__tag) hidden when -fvisibility-inlines-hidden is used.

% cat b.cc
#include <memory>
std::shared_ptr<int> foo(int x) {
 return std::make_shared<int>(x);
}
% g++-8 -fvisibility-inlines-hidden -fno-rtti -c b.cc
% readelf -s b.o | grep _S_ti
  163: 0000000000000000     1 OBJECT  UNIQUE DEFAULT   67 
_ZZNSt19_Sp_make_shared_tag5_S_tiEvE5__tag
  164: 0000000000000000     8 FUNC    WEAK   HIDDEN    68 
_ZNSt19_Sp_make_shared_tag5_S_tiEv
% ~/Dev/llvm/static-release/bin/clang++ -fvisibility-inlines-hidden -fno-rtti 
-c b.cc
% readelf -s b.o | grep _S_ti
  129: 0000000000000000    16 FUNC    WEAK   HIDDEN    34 
_ZNSt19_Sp_make_shared_tag5_S_tiEv
  155: 0000000000000000     1 OBJECT  WEAK   HIDDEN   202 
_ZZNSt19_Sp_make_shared_tag5_S_tiEvE5__tag

This can lead to multiple instances of __tag when shares objects are used.
The function
virtual void* std::_Sp_counted_ptr_inplace::_M_get_deleter(const 
std::type_info& __ti) noexcept
may return nullptr and causes std::make_shared<T>() to return nullptr 
(-fvisibility-inlines-hidden -fno-rtti).

After applying this patch (tagging _S_ti() with default visibility to override 
-fvisibility-inlines-hidden)

% readelf -s b.o | grep _S_ti
  129: 0000000000000000    16 FUNC    WEAK   DEFAULT   34 
_ZNSt19_Sp_make_shared_tag5_S_tiEv
  155: 0000000000000000     1 OBJECT  WEAK   DEFAULT  202 
_ZZNSt19_Sp_make_shared_tag5_S_tiEvE5__tag


This issue caused 10+ check-all tests of a -DUSE_SHARED_LLVM=On build of llvm 
(compiled with clang trunk) to SIGSEGV (because std::make_shared returned 
nullptr) and this patch fixes it.


       * include/bits/shared_ptr_base.h (_S_ti): Use
       _GLIBCXX_VISIBILITY(default)


--
宋方睿

>From 6da8cec298766ce043d9c6dcda7b87142228dafb Mon Sep 17 00:00:00 2001
From: Fangrui Song <mask...@google.com>
Date: Thu, 19 Jul 2018 16:40:26 -0700
Subject: [PATCH 1/1] Explicitly mark _S_ti() as default visibility to work
 around clang -fvisibility-inlines-hidden bug

clang (including trunk and many older versions) incorrectly marks static
local variables (__tag) hidden when -fvisibility-inlines-hidden is used.
This can lead to multiple instances of __tag when shares objects are used.

        * include/bits/shared_ptr_base.h (_S_ti): Use
        _GLIBCXX_VISIBILITY(default)
---
 libstdc++-v3/include/bits/shared_ptr_base.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/shared_ptr_base.h 
b/libstdc++-v3/include/bits/shared_ptr_base.h
index f3994da158f..870aeb9bfda 100644
--- a/libstdc++-v3/include/bits/shared_ptr_base.h
+++ b/libstdc++-v3/include/bits/shared_ptr_base.h
@@ -508,7 +508,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       friend class _Sp_counted_ptr_inplace;
 
     static const type_info&
-    _S_ti() noexcept
+    _S_ti() noexcept _GLIBCXX_VISIBILITY(default)
     {
       alignas(type_info) static constexpr _Sp_make_shared_tag __tag;
       return reinterpret_cast<const type_info&>(__tag);
-- 
2.18.0