re PR libstdc++/89825 (Jump table for variant visitation could be shortened for never...

PR libstdc++/89825

Fix based on a suggestion by Antony Polukhin.
* include/std/variant (_Extra_visit_slot_needed): New.
(_Multi_array): Use it.
(_S_apply_all_alts): Likewise.

From-SVN: r269947

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 622426076743d5f2758f936c5f736b473bd34f11..2e595b3eca8b4c5153780f8991699bff7c47a201 100644 (file)
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,13 @@
+2019-03-26  Ville Voutilainen  <ville.voutilainen@gmail.com>
+
+       PR libstdc++/89825
+       Fix based on a suggestion by Antony Polukhin.
+       * include/std/variant (__never_valueless): New.
+       (_M_valid): Use it.
+       (_Extra_visit_slot_needed): New.
+       (_Multi_array): Use it.
+       (_S_apply_all_alts): Likewise.
+
 2019-03-26  Ville Voutilainen  <ville.voutilainen@gmail.com>
 
        PR libstdc++/89824

diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index 36314638e633f92bbd8f5d1a85de9d711a44d470..3932a8a0d1476bc073d8ee45fdb80c881cd66673 100644 (file)
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -323,6 +323,12 @@ namespace __variant
       _Variadic_union<_Rest...> _M_rest;
     };
 
+  template <typename... _Types>
+  constexpr bool __never_valueless()
+  {
+    return (is_trivially_copyable_v<_Types> && ...);
+  }
+
   // Defines index and the dtor, possibly trivial.
   template<bool __trivially_destructible, typename... _Types>
     struct _Variant_storage;
@@ -408,7 +414,7 @@ namespace __variant
       constexpr bool
       _M_valid() const noexcept
       {
-       if constexpr ((is_trivially_copyable_v<_Types> && ...))
+       if constexpr (__never_valueless<_Types...>())
          return true;
        return this->_M_index != __index_type(variant_npos);
       }
@@ -747,6 +753,20 @@ namespace __variant
     void* __get_storage(_Variant&& __v)
     { return __v._M_storage(); }
 
+  template <typename _Maybe_variant_cookie, typename _Variant>
+  struct _Extra_visit_slot_needed
+  {
+    template <typename> struct _Variant_never_valueless;
+
+    template <typename... _Types>
+    struct _Variant_never_valueless<variant<_Types...>>
+      : bool_constant<__never_valueless<_Types...>()> {};
+
+    static constexpr bool value =
+      is_same_v<_Maybe_variant_cookie, __variant_cookie>
+      && !_Variant_never_valueless<__remove_cvref_t<_Variant>>::value;
+  };
+
   // Used for storing multi-dimensional vtable.
   template<typename _Tp, size_t... _Dimensions>
     struct _Multi_array
@@ -764,8 +784,11 @@ namespace __variant
           size_t __first, size_t... __rest>
     struct _Multi_array<_Ret(*)(_Visitor, _Variants...), __first, __rest...>
     {
+      static constexpr size_t __index =
+       sizeof...(_Variants) - sizeof...(__rest) - 1;
+      using _Variant = typename _Nth_type<__index, _Variants...>::type;
       static constexpr int __do_cookie =
-       is_same_v<_Ret, __variant_cookie> ? 1 : 0;
+       _Extra_visit_slot_needed<_Ret, _Variant>::value ? 1 : 0;
       using _Tp = _Ret(*)(_Visitor, _Variants...);
       template<typename... _Args>
        constexpr const _Tp&
@@ -832,7 +855,7 @@ namespace __variant
        _S_apply_all_alts(_Array_type& __vtable,
                          std::index_sequence<__var_indices...>)
        {
-         if constexpr (is_same_v<_Result_type, __variant_cookie>)
+         if constexpr (_Extra_visit_slot_needed<_Result_type, _Next>::value)
            (_S_apply_single_alt<true, __var_indices>(
              __vtable._M_arr[__var_indices + 1],
              &(__vtable._M_arr[0])), ...);