Re: [committed] libstdc++: Optimize std::any_cast by replacing indirect call

[Jonathan Wakely <jwakely@redhat.com>]

Apparently my mailer decided to sent this email as From: Tim, rather
than me. Sorry for any confusion.  The patch is from Tim, but the
email to the lists was sent by me (jwakely). Hopefully this one will
have the right From: header on it!


On 04/06/21 18:02 +0100, Tim Adye wrote:
>This significantly improves the performance of std::any_cast, by
>avoiding an indirect call to the _S_manage function through a function
>pointer. Before we make that indirect call we've already established
>that the contained value has the expected type, which means we also know
>the manager type, and so can call one of its members directly.
>
>We also know the precise type in the any::emplace functions, because
>we've just constructed that type, so we can use the new member there
>too. That doesn't seem to affect performance, but we might as well use
>the new _S_access function anyway.
>
>Signed-off-by: Tim Adye <Tim.Adye@cern.ch>
>Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
>
>libstdc++-v3/ChangeLog:
>
>	* include/std/any (any::_Manager::_S_access): New static
>	function to access the contained value.
>	(any::emplace, __any_caster): Use _S_access member of the
>	manager type.
>
>Tested powerpc64le-linux. Committed to trunk.
>
>This patch was contributed by Tim Adye and accepted in line with the
>new policy announced in https://gcc.gnu.org/pipermail/gcc/2021-June/236182.html
>
>Thanks, Tim!
>

>commit f6bb145c0bff19767931d37733be11c8acc6fa00
>Author: Tim Adye <Tim.Adye@cern.ch>
>Date:   Fri Jun 4 15:59:38 2021
>
>    libstdc++: Optimize std::any_cast by replacing indirect call
>
>    This significantly improves the performance of std::any_cast, by
>    avoiding an indirect call to the _S_manage function through a function
>    pointer. Before we make that indirect call we've already established
>    that the contained value has the expected type, which means we also know
>    the manager type, and so can call one of its members directly.
>
>    We also know the precise type in the any::emplace functions, because
>    we've just constructed that type, so we can use the new member there
>    too. That doesn't seem to affect performance, but we might as well use
>    the new _S_access function anyway.
>
>    Signed-off-by: Tim Adye <Tim.Adye@cern.ch>
>    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
>
>    libstdc++-v3/ChangeLog:
>
>            * include/std/any (any::_Manager::_S_access): New static
>            function to access the contained value.
>            (any::emplace, __any_caster): Use _S_access member of the
>            manager type.
>
>diff --git a/libstdc++-v3/include/std/any b/libstdc++-v3/include/std/any
>index 391e43339a0..21120a9146f 100644
>--- a/libstdc++-v3/include/std/any
>+++ b/libstdc++-v3/include/std/any
>@@ -263,9 +263,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>       {
> 	using _VTp = decay_t<_Tp>;
> 	__do_emplace<_VTp>(std::forward<_Args>(__args)...);
>-	any::_Arg __arg;
>-	this->_M_manager(any::_Op_access, this, &__arg);
>-	return *static_cast<_VTp*>(__arg._M_obj);
>+	return *any::_Manager<_VTp>::_S_access(_M_storage);
>       }
>
>     /// Emplace with an object created from @p __il and @p __args as
>@@ -276,9 +274,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>       {
> 	using _VTp = decay_t<_Tp>;
> 	__do_emplace<_VTp, _Up>(__il, std::forward<_Args>(__args)...);
>-	any::_Arg __arg;
>-	this->_M_manager(any::_Op_access, this, &__arg);
>-	return *static_cast<_VTp*>(__arg._M_obj);
>+	return *any::_Manager<_VTp>::_S_access(_M_storage);
>       }
>
>     // modifiers
>@@ -384,6 +380,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> 	    void* __addr = &__storage._M_buffer;
> 	    ::new (__addr) _Tp(std::forward<_Args>(__args)...);
> 	  }
>+
>+	static _Tp*
>+	_S_access(const _Storage& __storage)
>+	{
>+	  // The contained object is in __storage._M_buffer
>+	  const void* __addr = &__storage._M_buffer;
>+	  return static_cast<_Tp*>(const_cast<void*>(__addr));
>+	}
>       };
>
>     // Manage external contained object.
>@@ -405,6 +409,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> 	  {
> 	    __storage._M_ptr = new _Tp(std::forward<_Args>(__args)...);
> 	  }
>+	static _Tp*
>+	_S_access(const _Storage& __storage)
>+	{
>+	  // The contained object is in *__storage._M_ptr
>+	  return static_cast<_Tp*>(__storage._M_ptr);
>+	}
>       };
>   };
>
>@@ -511,9 +521,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> #endif
> 	  )
> 	{
>-	  any::_Arg __arg;
>-	  __any->_M_manager(any::_Op_access, __any, &__arg);
>-	  return __arg._M_obj;
>+	  return any::_Manager<_Up>::_S_access(__any->_M_storage);
> 	}
>       return nullptr;
>     }