[PATCH v26 08/13] libstdc++: Optimize std::add_lvalue_reference compilation performance

[Jonathan Wakely]

On 11/05/24 02:01 -0700, Ken Matsui wrote:
>This patch optimizes the compilation performance of
>std::add_lvalue_reference by dispatching to the new
>__add_lvalue_reference built-in trait.
>
>libstdc++-v3/ChangeLog:
>
>	* include/std/type_traits (add_lvalue_reference): Use
>	__add_lvalue_reference built-in trait.
>	(__add_lvalue_reference_helper): Likewise.
>
>Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>
>---
> libstdc++-v3/include/std/type_traits | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
>
>diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
>index 5b74e44d0a6..effa3fbcb75 100644
>--- a/libstdc++-v3/include/std/type_traits
>+++ b/libstdc++-v3/include/std/type_traits
>@@ -1157,6 +1157,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>     };
>
>   /// @cond undocumented
>+#if _GLIBCXX_USE_BUILTIN_TRAIT(__add_lvalue_reference)
>+  template<typename _Tp>
>+    struct __add_lvalue_reference_helper
>+    { using type = __add_lvalue_reference(_Tp); };
>+#else
>   template<typename _Tp, typename = void>
>     struct __add_lvalue_reference_helper
>     { using type = _Tp; };
>@@ -1164,6 +1169,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>   template<typename _Tp>
>     struct __add_lvalue_reference_helper<_Tp, __void_t<_Tp&>>
>     { using type = _Tp&; };
>+#endif
>
>   template<typename _Tp>
>     using __add_lval_ref_t = typename __add_lvalue_reference_helper<_Tp>::type;

This still requires instantiating a class template, which means that
all uses of __add_lval_ref_t (e.g. in is_copy_constructible) still
instantiate __add_lvalue_reference_helper.

Could we do this instead, and then no change to
std::add_lvalue_reference is needed?

   /// @cond undocumented
#if _GLIBCXX_USE_BUILTIN_TRAIT(__add_lvalue_reference)
   template<typename _Tp>
     using __add_lval_ref_t = __add_lvalue_reference(_Tp);
#else
   template<typename _Tp, typename = void>
     struct __add_lvalue_reference_helper
     { using type = _Tp; };

   template<typename _Tp>
     struct __add_lvalue_reference_helper<_Tp, __void_t<_Tp&>>
     { using type = _Tp&; };

   template<typename _Tp>
     using __add_lval_ref_t = typename __add_lvalue_reference_helper<_Tp>::type;
#endif
   /// @endcond

We never use __add_lval_ref_t in a context that requires it to be
mangled, so I think this is OK.

>@@ -1731,9 +1737,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> #endif
>
>   /// add_lvalue_reference
>+#if _GLIBCXX_USE_BUILTIN_TRAIT(__add_lvalue_reference)
>+  template<typename _Tp>
>+    struct add_lvalue_reference
>+    { using type = __add_lvalue_reference(_Tp); };
>+#else
>   template<typename _Tp>
>     struct add_lvalue_reference
>     { using type = __add_lval_ref_t<_Tp>; };
>+#endif
>
>   /// add_rvalue_reference
>   template<typename _Tp>
>-- 
>2.44.0
>