Re: [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 <kmat...@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