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PR libstdc++/80285 optimize std::make_shared for -fno-rtti

Message ID 20170516131644.GR5109@redhat.com
State New
Headers show

Commit Message

Jonathan Wakely May 16, 2017, 1:16 p.m. UTC 
On 11/05/17 15:19 +0100, Jonathan Wakely wrote:
>std::make_shared didn't support embedding the object within the
>shared_ptr bookkeeping structure when -fno-rtti was used. This was
>because the trick I use for getting the address of the embedded
>object relied on typeid, so without RTTI it just creating the object
>separately on the heap.
>
>This removes the alternative code path for -fno-rtti and simply forms
>a fake std::type_info& reference using reinterpret_cast. This allows
>us to embed the object and get its address without typeid, so we avoid
>the second allocation even when -fno-rtti is used.
>
>	PR libstdc++/80285
>	* include/bits/shared_ptr_base.h (_Sp_make_shared_tag::_S_ti): Define
>	function to get unique fake std::type_info reference.
>	(_Sp_counted_ptr_inplace::_M_get_deleter) [!__cpp_rtti]: Compare to
>	_S_ti() fake reference.
>	(__shared_ptr(_Sp_make_shared_tag, const Alloc&, Args&&...)): Share
>	single implementation with or without RTTI enable.
>	[!__cpp_rtti]: Pass fake reference to _M_get_deleter.
>	* testsuite/20_util/shared_ptr/creation/alloc.cc: Change expected
>	allocation and deallocation counts.
>	* testsuite/20_util/shared_ptr/creation/single_allocation.cc: New.
>	* testsuite/20_util/shared_ptr/creation/single_allocation_no_rtti.cc:
>	New.
>
>Tested powerpc64le-linux, committed to trunk.

This fix is needed for gnu-versioned-namespace builds, because
std::type_info should not be declared inside the versioned namespace.

Tested x86_64-linux, comitted to trunk.
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Patch

commit e14f89bd9e529b3343c0b8c5fce3377394f6cbc7
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue May 16 14:13:40 2017 +0100

    Fix forward declaration of std::type_info for versioned-namespace
    
    	PR libstdc++/80285
    	* include/bits/shared_ptr_base.h [!__cpp_rtti] (type_info): Declare
    	outside versioned namespace.

diff --git a/libstdc++-v3/include/bits/shared_ptr_base.h b/libstdc++-v3/include/bits/shared_ptr_base.h
index 6918579..b4a5edf 100644
--- a/libstdc++-v3/include/bits/shared_ptr_base.h
+++ b/libstdc++-v3/include/bits/shared_ptr_base.h
@@ -59,6 +59,10 @@ 
 
 namespace std _GLIBCXX_VISIBILITY(default)
 {
+#if !__cpp_rtti
+  class type_info;
+#endif
+
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
 #if _GLIBCXX_USE_DEPRECATED
@@ -68,10 +72,6 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #pragma GCC diagnostic pop
 #endif
 
-#if !__cpp_rtti
-  class type_info;
-#endif
-
  /**
    *  @brief  Exception possibly thrown by @c shared_ptr.
    *  @ingroup exceptions