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

[Jonathan Wakely <jwakely at redhat dot com>]

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