[libcxx-commits] [PATCH] D115100: [libc++] Enable the optimized _IsSame on GCC as well as Clang.

[Arthur O'Dwyer via Phabricator via libcxx-commits]

Quuxplusone updated this revision to Diff 394067.
Quuxplusone edited the summary of this revision.
Quuxplusone added a comment.

Updated with @ldionne's suggestion. Let's see if buildkite is happy with this; if so, then I'm ready to land it and forget it. :)


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D115100/new/

https://reviews.llvm.org/D115100

Files:
  libcxx/include/__tuple
  libcxx/include/type_traits


Index: libcxx/include/type_traits
===================================================================
--- libcxx/include/type_traits
+++ libcxx/include/type_traits
@@ -550,8 +550,6 @@
 
 // is_same
 
-#if __has_keyword(__is_same)
-
 template <class _Tp, class _Up>
 struct _LIBCPP_TEMPLATE_VIS is_same : _BoolConstant<__is_same(_Tp, _Up)> { };
 
@@ -560,36 +558,17 @@
 inline constexpr bool is_same_v = __is_same(_Tp, _Up);
 #endif
 
-#else
-
-template <class _Tp, class _Up> struct _LIBCPP_TEMPLATE_VIS is_same           : public false_type {};
-template <class _Tp>            struct _LIBCPP_TEMPLATE_VIS is_same<_Tp, _Tp> : public true_type {};
+// _IsSame<T,U> has the same effect as is_same<T,U> but instantiates fewer types:
+// is_same<A,B> and is_same<C,D> are guaranteed to be different types, but
+// _IsSame<A,B> and _IsSame<C,D> are the same type (namely, false_type).
+// If we ever need to support a compiler without the __is_same builtin,
+// we can make this an alias template for `is_same<T,U>::type`.
 
-#if _LIBCPP_STD_VER > 14
 template <class _Tp, class _Up>
-inline constexpr bool is_same_v = is_same<_Tp, _Up>::value;
-#endif
-
-#endif // __is_same
+using _IsSame = _BoolConstant<__is_same(_Tp, _Up)>;
 
 template <class _Tp, class _Up>
-using _IsSame = _BoolConstant<
-#ifdef __clang__
-    __is_same(_Tp, _Up)
-#else
-    is_same<_Tp, _Up>::value
-#endif
->;
-
-template <class _Tp, class _Up>
-using _IsNotSame = _BoolConstant<
-#ifdef __clang__
-    !__is_same(_Tp, _Up)
-#else
-    !is_same<_Tp, _Up>::value
-#endif
->;
-
+using _IsNotSame = _BoolConstant<!__is_same(_Tp, _Up)>;
 
 template <class _Tp>
 using __test_for_primary_template = __enable_if_t<
@@ -600,9 +579,9 @@
     __test_for_primary_template, _Tp
   >;
 
-struct __two {char __lx[2];};
+// helper class
 
-// helper class:
+struct __two {char __lx[2];};
 
 // is_const
 
Index: libcxx/include/__tuple
===================================================================
--- libcxx/include/__tuple
+++ libcxx/include/__tuple
@@ -387,7 +387,7 @@
 struct __all_dummy;
 
 template <bool ..._Pred>
-using __all = _IsSame<__all_dummy<_Pred...>, __all_dummy<((void)_Pred, true)...>>;
+struct __all : _IsSame<__all_dummy<_Pred...>, __all_dummy<((void)_Pred, true)...>> {};
 
 struct __tuple_sfinae_base {
   template <template <class, class...> class _Trait,


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