Speed up compilation of std::visit() by hard-coding the most common cases

Author: higher-performance

libcxx/include/variant

@@ -1323,6 +1323,10 @@ private:
   friend struct __variant_detail::__visitation::__variant;
 };
 
+template <class... _Types>
+variant<_Types...> __upcast_to_variant(const volatile variant<_Types...>*);
+void __upcast_to_variant(...);
+
 template <size_t _Ip, class... _Types>
 _LIBCPP_HIDE_FROM_ABI constexpr bool __holds_alternative(const variant<_Types...>& __v) noexcept {
   return __v.index() == _Ip;
@@ -1576,22 +1580,100 @@ _LIBCPP_HIDE_FROM_ABI constexpr void __throw_if_valueless(_Vs&&... __vs) {
   }
 }
 
+#    define _LIBCPP_VARIANT_DISPATCH_COUNT 11 // Speed up compilation for the common cases
+
 template < class _Visitor, class... _Vs, typename>
 _LIBCPP_HIDE_FROM_ABI constexpr decltype(auto) visit(_Visitor&& __visitor, _Vs&&... __vs) {
-  using __variant_detail::__visitation::__variant;
-  std::__throw_if_valueless(std::forward<_Vs>(__vs)...);
-  return __variant::__visit_value(std::forward<_Visitor>(__visitor), std::forward<_Vs>(__vs)...);
+  using __variant_type = decltype(std::__upcast_to_variant(std::declval<remove_reference_t<_Vs>*>()...));
+  constexpr size_t __variant_size =
+      conditional_t<is_void_v<__variant_type>,
+                    std::integral_constant<size_t, variant_npos>,
+                    variant_size<__variant_type>>::value;
+  if constexpr (__variant_size <= _LIBCPP_VARIANT_DISPATCH_COUNT) {
+    using __variant_detail::__access::__variant;
+#    define _LIBCPP_VARIANT_DISPATCH_INDEX(_I)                                                                         \
+    case _I:                                                                                                           \
+      if constexpr (_I < __variant_size) {                                                                             \
+        return std::__invoke(                                                                                          \
+            std::forward<_Visitor>(__visitor), __variant::__get_alt<_I>(std::forward<_Vs>(__vs)...).__value);          \
+      }                                                                                                                \
+      [[__fallthrough__]]
+    switch ((..., __vs.__variant_type::index())) {
+      enum : size_t { _I0 = __LINE__ + 1 };           // New line required
+      _LIBCPP_VARIANT_DISPATCH_INDEX(__LINE__ - _I0); // New line required
+      _LIBCPP_VARIANT_DISPATCH_INDEX(__LINE__ - _I0); // New line required
+      _LIBCPP_VARIANT_DISPATCH_INDEX(__LINE__ - _I0); // New line required
+      _LIBCPP_VARIANT_DISPATCH_INDEX(__LINE__ - _I0); // New line required
+      _LIBCPP_VARIANT_DISPATCH_INDEX(__LINE__ - _I0); // New line required
+      _LIBCPP_VARIANT_DISPATCH_INDEX(__LINE__ - _I0); // New line required
+      _LIBCPP_VARIANT_DISPATCH_INDEX(__LINE__ - _I0); // New line required
+      _LIBCPP_VARIANT_DISPATCH_INDEX(__LINE__ - _I0); // New line required
+      _LIBCPP_VARIANT_DISPATCH_INDEX(__LINE__ - _I0); // New line required
+      _LIBCPP_VARIANT_DISPATCH_INDEX(__LINE__ - _I0); // New line required
+      _LIBCPP_VARIANT_DISPATCH_INDEX(__LINE__ - _I0); // New line required
+      static_assert(__LINE__ - _I0 == _LIBCPP_VARIANT_DISPATCH_COUNT, "index count mismatch");
+    default:
+      std::__throw_bad_variant_access();
+    }
+#    undef _LIBCPP_VARIANT_DISPATCH_INDEX
+  } else {
+    using __variant_detail::__visitation::__variant;
+    std::__throw_if_valueless(std::forward<_Vs>(__vs)...);
+    return __variant::__visit_value(std::forward<_Visitor>(__visitor), std::forward<_Vs>(__vs)...);
+  }
 }
 
 #    if _LIBCPP_STD_VER >= 20
 template < class _Rp, class _Visitor, class... _Vs, typename>
 _LIBCPP_HIDE_FROM_ABI constexpr _Rp visit(_Visitor&& __visitor, _Vs&&... __vs) {
-  using __variant_detail::__visitation::__variant;
-  std::__throw_if_valueless(std::forward<_Vs>(__vs)...);
-  return __variant::__visit_value<_Rp>(std::forward<_Visitor>(__visitor), std::forward<_Vs>(__vs)...);
+  using __variant_type = decltype(std::__upcast_to_variant(std::declval<remove_reference_t<_Vs>*>()...));
+  constexpr size_t __variant_size =
+      conditional_t<is_void_v<__variant_type>,
+                    std::integral_constant<size_t, variant_npos>,
+                    variant_size<__variant_type>>::value;
+  if constexpr (__variant_size <= _LIBCPP_VARIANT_DISPATCH_COUNT) {
+    using __variant_detail::__access::__variant;
+#      define _LIBCPP_VARIANT_DISPATCH_INDEX(_I)                                                                       \
+      case _I:                                                                                                         \
+        if constexpr (_I < __variant_size) {                                                                           \
+          if constexpr (std::is_void_v<_Rp>) {                                                                         \
+            std::__invoke(                                                                                             \
+                std::forward<_Visitor>(__visitor), __variant::__get_alt<_I>(std::forward<_Vs>(__vs)...).__value);      \
+            break;                                                                                                     \
+          } else {                                                                                                     \
+            return std::__invoke(                                                                                      \
+                std::forward<_Visitor>(__visitor), __variant::__get_alt<_I>(std::forward<_Vs>(__vs)...).__value);      \
+          }                                                                                                            \
+        }                                                                                                              \
+        [[__fallthrough__]]
+    switch ((..., __vs.__variant_type::index())) {
+      enum : size_t { _I0 = __LINE__ + 1 };           // New line required
+      _LIBCPP_VARIANT_DISPATCH_INDEX(__LINE__ - _I0); // New line required
+      _LIBCPP_VARIANT_DISPATCH_INDEX(__LINE__ - _I0); // New line required
+      _LIBCPP_VARIANT_DISPATCH_INDEX(__LINE__ - _I0); // New line required
+      _LIBCPP_VARIANT_DISPATCH_INDEX(__LINE__ - _I0); // New line required
+      _LIBCPP_VARIANT_DISPATCH_INDEX(__LINE__ - _I0); // New line required
+      _LIBCPP_VARIANT_DISPATCH_INDEX(__LINE__ - _I0); // New line required
+      _LIBCPP_VARIANT_DISPATCH_INDEX(__LINE__ - _I0); // New line required
+      _LIBCPP_VARIANT_DISPATCH_INDEX(__LINE__ - _I0); // New line required
+      _LIBCPP_VARIANT_DISPATCH_INDEX(__LINE__ - _I0); // New line required
+      _LIBCPP_VARIANT_DISPATCH_INDEX(__LINE__ - _I0); // New line required
+      _LIBCPP_VARIANT_DISPATCH_INDEX(__LINE__ - _I0); // New line required
+      static_assert(__LINE__ - _I0 == _LIBCPP_VARIANT_DISPATCH_COUNT, "index count mismatch");
+    default:
+      std::__throw_bad_variant_access();
+    }
+#      undef _LIBCPP_VARIANT_DISPATCH_INDEX
+  } else {
+    using __variant_detail::__visitation::__variant;
+    std::__throw_if_valueless(std::forward<_Vs>(__vs)...);
+    return __variant::__visit_value<_Rp>(std::forward<_Visitor>(__visitor), std::forward<_Vs>(__vs)...);
+  }
 }
 #    endif
 
+#    undef _LIBCPP_VARIANT_DISPATCH_COUNT
+
 template <class... _Types>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 auto
 swap(variant<_Types...>& __lhs, variant<_Types...>& __rhs) noexcept(noexcept(__lhs.swap(__rhs)))