libstdc++: Remove try/catch overhead in std::variant::emplace

The __variant_construct_by_index helper function sets the new index
before constructing the new object. This means that if the construction
throws then the exception needs to be caught, so the index can be reset
to variant_npos, and then the exception rethrown. This means callers are
responsible for restoring the variant's invariants and they need the
overhead of a catch handler and a rethrow.

If we don't set the index until after construction completes then the
invariant is never broken, and callers can ignore the exception and let
it propagate. The callers all call _M_reset() first, which sets index to
variant_npos as required while the variant is valueless.

We need to be slightly careful here, because changing the order of
operations in __variant_construct_by_index and removing the try-block
from variant::emplace<I> changes an implicit ABI contract between those
two functions. If the linker were to create an executable containing an
instantiation of the old __variant_construct_by_index and an
instantiation of the new variant::emplace<I> code then we would have a
combination that breaks the invariant and doesn't have the exception
handling to restore it. To avoid this problem, we can rename the
__variant_construct_by_index function so that the new emplace<I> code
calls a new symbol, and is unaffected by the behaviour of the old
symbol.

libstdc++-v3/ChangeLog:

	* include/std/variant (__detail::__variant::__get_storage):
	Remove unused function.
	(__variant_construct_by_index): Set index after construction is
	complete. Rename to ...
	(__detail::__variant::__construct_by_index): ... this.
	(variant): Use new name for __variant_construct_by_index friend
	declaration. Remove __get_storage friend declaration.
	(variant::emplace): Use new name and remove try-blocks.

(cherry picked from commit e27771e5dcd8cf2cb757db6177a3485acd28b88f)

diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index f101c4267447c3c48f99a506e35ce6e34a90e30e..285c5be6521544d7bc20720cb869a4eb4bffbed7 100644 (file)
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -1089,19 +1089,21 @@ namespace __variant
        >;
     }
 
-} // namespace __variant
-} // namespace __detail
-
   template<size_t _Np, typename _Variant, typename... _Args>
-    void __variant_construct_by_index(_Variant& __v, _Args&&... __args)
+    inline void
+    __construct_by_index(_Variant& __v, _Args&&... __args)
     {
-      __v._M_index = _Np;
       auto&& __storage = __detail::__variant::__get<_Np>(__v);
       ::new ((void*)std::addressof(__storage))
         remove_reference_t<decltype(__storage)>
          (std::forward<_Args>(__args)...);
+      // Construction didn't throw, so can set the new index now:
+      __v._M_index = _Np;
     }
 
+} // namespace __variant
+} // namespace __detail
+
   template<typename _Tp, typename... _Types>
     constexpr bool
     holds_alternative(const variant<_Types...>& __v) noexcept
@@ -1337,8 +1339,9 @@ namespace __variant
       template <typename... _UTypes, typename _Tp>
        friend decltype(auto) __variant_cast(_Tp&&);
       template<size_t _Np, typename _Variant, typename... _Args>
-       friend void __variant_construct_by_index(_Variant& __v,
-                                                _Args&&... __args);
+       friend void
+       __detail::__variant::__construct_by_index(_Variant& __v,
+                                                 _Args&&... __args);
 
       static_assert(sizeof...(_Types) > 0,
                    "variant must have at least one alternative");
@@ -1502,12 +1505,13 @@ namespace __variant
          static_assert(_Np < sizeof...(_Types),
                        "The index must be in [0, number of alternatives)");
          using type = variant_alternative_t<_Np, variant>;
+         namespace __variant = std::__detail::__variant;
          // Provide the strong exception-safety guarantee when possible,
          // to avoid becoming valueless.
          if constexpr (is_nothrow_constructible_v<type, _Args...>)
            {
              this->_M_reset();
-             __variant_construct_by_index<_Np>(*this,
+             __variant::__construct_by_index<_Np>(*this,
                  std::forward<_Args>(__args)...);
            }
          else if constexpr (is_scalar_v<type>)
@@ -1516,9 +1520,9 @@ namespace __variant
              const type __tmp(std::forward<_Args>(__args)...);
              // But these steps won't throw:
              this->_M_reset();
-             __variant_construct_by_index<_Np>(*this, __tmp);
+             __variant::__construct_by_index<_Np>(*this, __tmp);
            }
-         else if constexpr (__detail::__variant::_Never_valueless_alt<type>()
+         else if constexpr (__variant::_Never_valueless_alt<type>()
              && _Traits::_S_move_assign)
            {
              // This construction might throw:
@@ -1532,17 +1536,8 @@ namespace __variant
              // This case only provides the basic exception-safety guarantee,
              // i.e. the variant can become valueless.
              this->_M_reset();
-             __try
-               {
-                 __variant_construct_by_index<_Np>(*this,
-                   std::forward<_Args>(__args)...);
-               }
-             __catch (...)
-               {
-                 using __index_type = decltype(this->_M_index);
-                 this->_M_index = static_cast<__index_type>(variant_npos);
-                 __throw_exception_again;
-               }
+             __variant::__construct_by_index<_Np>(*this,
+               std::forward<_Args>(__args)...);
            }
          return std::get<_Np>(*this);
        }
@@ -1556,6 +1551,7 @@ namespace __variant
          static_assert(_Np < sizeof...(_Types),
                        "The index must be in [0, number of alternatives)");
          using type = variant_alternative_t<_Np, variant>;
+         namespace __variant = std::__detail::__variant;
          // Provide the strong exception-safety guarantee when possible,
          // to avoid becoming valueless.
          if constexpr (is_nothrow_constructible_v<type,
@@ -1563,10 +1559,10 @@ namespace __variant
                                                   _Args...>)
            {
              this->_M_reset();
-             __variant_construct_by_index<_Np>(*this, __il,
+             __variant::__construct_by_index<_Np>(*this, __il,
                  std::forward<_Args>(__args)...);
            }
-         else if constexpr (__detail::__variant::_Never_valueless_alt<type>()
+         else if constexpr (__variant::_Never_valueless_alt<type>()
              && _Traits::_S_move_assign)
            {
              // This construction might throw:
@@ -1580,17 +1576,8 @@ namespace __variant
              // This case only provides the basic exception-safety guarantee,
              // i.e. the variant can become valueless.
              this->_M_reset();
-             __try
-               {
-                 __variant_construct_by_index<_Np>(*this, __il,
-                   std::forward<_Args>(__args)...);
-               }
-             __catch (...)
-               {
-                 using __index_type = decltype(this->_M_index);
-                 this->_M_index = static_cast<__index_type>(variant_npos);
-                 __throw_exception_again;
-               }
+             __variant::__construct_by_index<_Np>(*this, __il,
+               std::forward<_Args>(__args)...);
            }
          return std::get<_Np>(*this);
        }