https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91788
--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> --- Author: redi Date: Mon Sep 23 15:54:16 2019 New Revision: 276056 URL: https://gcc.gnu.org/viewcvs?rev=276056&root=gcc&view=rev Log: 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. Modified: trunk/libstdc++-v3/ChangeLog trunk/libstdc++-v3/include/std/variant
