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

["Fāng-ruì Sòng via libstdc++" <libstdc++ at gcc dot gnu dot org>]

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

Attachment: patch.txt
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