[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 391883.
Quuxplusone edited the summary of this revision.
Quuxplusone added a comment.

  However, there's a problem on both GCC and Clang: they can't mangle
  `__is_same(T,U)` if it appears anywhere that affects mangling. That's
  a hard error. And it turns out that GCC puts dependent return types
  into the mangling more aggressively than Clang, so we need to avoid
  using raw `_IsSame` (or type aliases such as `__all` that expand into
  `_IsSame`) in one return type, for GCC's benefit.


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
@@ -550,7 +550,7 @@
 
 // is_same
 
-#if __has_keyword(__is_same)
+#if __has_builtin(__is_same)
 
 template <class _Tp, class _Up>
 struct _LIBCPP_TEMPLATE_VIS is_same : _BoolConstant<__is_same(_Tp, _Up)> { };
@@ -560,6 +560,16 @@
 inline constexpr bool is_same_v = __is_same(_Tp, _Up);
 #endif
 
+// _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 on supported
+// compilers _IsSame<A,B> and _IsSame<C,D> may be the same type (namely, false_type).
+
+template <class _Tp, class _Up>
+using _IsSame = _BoolConstant<__is_same(_Tp, _Up)>;
+
+template <class _Tp, class _Up>
+using _IsNotSame = _BoolConstant<!__is_same(_Tp, _Up)>;
+
 #else
 
 template <class _Tp, class _Up> struct _LIBCPP_TEMPLATE_VIS is_same           : public false_type {};
@@ -570,26 +580,13 @@
 inline constexpr bool is_same_v = is_same<_Tp, _Up>::value;
 #endif
 
-#endif // __is_same
-
 template <class _Tp, class _Up>
-using _IsSame = _BoolConstant<
-#ifdef __clang__
-    __is_same(_Tp, _Up)
-#else
-    is_same<_Tp, _Up>::value
-#endif
->;
+using _IsSame = is_same<_Tp, _Up>;
 
 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>::value>;
 
+#endif // __has_builtin(__is_same)
 
 template <class _Tp>
 using __test_for_primary_template = __enable_if_t<
@@ -602,8 +599,6 @@
 
 struct __two {char __lx[2];};
 
-// helper class:
-
 // is_const
 
 #if __has_keyword(__is_const)
Index: libcxx/include/tuple
===================================================================
--- libcxx/include/tuple
+++ libcxx/include/tuple
@@ -1116,13 +1116,12 @@
 tuple(allocator_arg_t, _Alloc, tuple<_Tp...>) -> tuple<_Tp...>;
 #endif
 
+template <class ..._Ts>
+struct __all_are_swappable : __all<__is_swappable<_Ts>::value...> {};
+
 template <class ..._Tp>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
-typename enable_if
-<
-    __all<__is_swappable<_Tp>::value...>::value,
-    void
->::type
+typename enable_if<__all_are_swappable<_Tp...>::value>::type
 swap(tuple<_Tp...>& __t, tuple<_Tp...>& __u)
                  _NOEXCEPT_(__all<__is_nothrow_swappable<_Tp>::value...>::value)
     {__t.swap(__u);}


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