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


On 19/07/18 16:58 -0700, Fāng-ruì Sòng via libstdc++ wrote:
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)


Thanks, I've tested this and committed it to trunk.

I think we want this on gcc-8-branch too, but it's too late for the
8.2 release now.


--
宋方睿

From 6da8cec298766ce043d9c6dcda7b87142228dafb Mon Sep 17 00:00:00 2001
From: Fangrui Song <maskray@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



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