If __index_type is a smaller type than size_t, then the result of size_t(__index_type(-1)) is not equal to size_t(-1), but to an incorrect value such as size_t(255) or size_t(65535). The old implementation of variant<T...>::index() uses (size_t(__index_type(_M_index + 1)) - 1) which is always correct, but generates suboptimal code for many common cases.
When the __index_type is size_t or valueless variants are not possible we can just return the value directly. When the number of alternatives is sufficiently small the result of converting the _M_index value to the corresponding signed type will be either non-negative or -1. In those cases converting to the signed type and then to size_t will either produce the correct positive value or will sign extend -1 to (size_t)-1 as desired. For the remaining case we keep the existing arithmetic operations to ensure the correct result. PR libstdc++/91788 (partial) * include/std/variant (variant::index()): Improve codegen for cases where conversion to size_t already works correctly. Tested x86_64-linux, committed to trunk.
commit 6ef8d9750c46e3d781c4d8cf169f66a13e0d0827 Author: Jonathan Wakely <jwak...@redhat.com> Date: Mon Sep 23 16:07:54 2019 +0100 PR libstdc++/91788 improve codegen for std::variant<T...>::index() If __index_type is a smaller type than size_t, then the result of size_t(__index_type(-1)) is not equal to size_t(-1), but to an incorrect value such as size_t(255) or size_t(65535). The old implementation of variant<T...>::index() uses (size_t(__index_type(_M_index + 1)) - 1) which is always correct, but generates suboptimal code for many common cases. When the __index_type is size_t or valueless variants are not possible we can just return the value directly. When the number of alternatives is sufficiently small the result of converting the _M_index value to the corresponding signed type will be either non-negative or -1. In those cases converting to the signed type and then to size_t will either produce the correct positive value or will sign extend -1 to (size_t)-1 as desired. For the remaining case we keep the existing arithmetic operations to ensure the correct result. PR libstdc++/91788 (partial) * include/std/variant (variant::index()): Improve codegen for cases where conversion to size_t already works correctly. diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant index d93ea86ea29..c0043243ec2 100644 --- a/libstdc++-v3/include/std/variant +++ b/libstdc++-v3/include/std/variant @@ -1518,7 +1518,17 @@ namespace __variant { return !this->_M_valid(); } constexpr size_t index() const noexcept - { return size_t(typename _Base::__index_type(this->_M_index + 1)) - 1; } + { + using __index_type = typename _Base::__index_type; + if constexpr (is_same_v<__index_type, size_t>) + return this->_M_index; + else if constexpr (__detail::__variant::__never_valueless<_Types...>()) + return this->_M_index; + else if constexpr (sizeof...(_Types) <= __index_type(-1) / 2) + return make_signed_t<__index_type>(this->_M_index); + else + return size_t(__index_type(this->_M_index + 1)) - 1; + } void swap(variant& __rhs)