https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87431
--- Comment #19 from Jonathan Wakely <redi at gcc dot gnu.org> --- Author: redi Date: Fri Apr 5 16:56:09 2019 New Revision: 270170 URL: https://gcc.gnu.org/viewcvs?rev=270170&root=gcc&view=rev Log: PR libstdc++/87431 re-adjust never-valueless optimizations Avoid creating arbitrarily large objects on the stack when emplacing trivially copyable objects into a variant. Currently we provide the strong exception-safety guarantee for all trivially copyable types, by constructing a second variant and then doing a non-throwing move assignment from the temporary. This patch restricts that behaviour to trivially copyable types that are no larger than 256 bytes. For larger types the object will be emplaced directly into the variant, and if its initialization throws then the variant becomes valueless. Also implement Antony Polukhin's suggestion to whitelist specific types that are not trivially copyable but can be efficiently move-assigned. Emplacing those types will never cause a variant to become valueless. The whitelisted types are: std::shared_ptr, std::weak_ptr, std::unique_ptr, std::function, and std::any. Additionally, std::basic_string, std::vector, and __gnu_debug::vector are whitelisted if their allocator traits give them a non-throwing move assignment operator. Specifically, this means std::string is whitelisted, but std::pmr::string is not. As part of this patch, additional if-constexpr branches are added for the cases where the initialization is known to be non-throwing (so the overhead of the try-catch block can be avoided) and where a scalar is being produced by a potentially-throwing conversion operator (so that the overhead of constructing and move-assigning a variant is avoided). These changes should have no semantic effect, just better codegen. PR libstdc++/87431 (again) * include/bits/basic_string.h (__variant::_Never_valueless_alt): Define partial specialization for basic_string. * include/bits/shared_ptr.h (_Never_valueless_alt): Likewise for shared_ptr and weak_ptr. * include/bits/std_function.h (_Never_valueless_alt): Likewise for function. * include/bits/stl_vector.h (_Never_valueless_alt): Likewise for vector. * include/bits/unique_ptr.h (_Never_valueless_alt): Likewise for unique_ptr. * include/debug/vector (_Never_valueless_alt): Likewise for debug vector. * include/std/any (_Never_valueless_alt): Define explicit specialization for any. * include/std/variant (_Never_valueless_alt): Define primary template. (__never_valueless): Use _Never_valueless_alt instead of is_trivially_copyable. (variant::emplace<N>(Args&&...)): Add special case for non-throwing initializations to avoid try-catch overhead. Add special case for scalars produced by potentially-throwing conversions. Use _Never_valueless_alt instead of is_trivially_copyable for the remaining strong exception-safety cases. (variant::emplace<N>(initializer_list<U>, Args&&...)): Likewise. * testsuite/20_util/variant/87431.cc: Run both test functions. * testsuite/20_util/variant/exception_safety.cc: New test. * testsuite/20_util/variant/run.cc: Use pmr::string instead of string, so the variant becomes valueless. Added: trunk/libstdc++-v3/testsuite/20_util/variant/exception_safety.cc Modified: trunk/libstdc++-v3/ChangeLog trunk/libstdc++-v3/include/bits/basic_string.h trunk/libstdc++-v3/include/bits/shared_ptr.h trunk/libstdc++-v3/include/bits/std_function.h trunk/libstdc++-v3/include/bits/stl_vector.h trunk/libstdc++-v3/include/bits/unique_ptr.h trunk/libstdc++-v3/include/debug/vector trunk/libstdc++-v3/include/std/any trunk/libstdc++-v3/include/std/variant trunk/libstdc++-v3/testsuite/20_util/variant/87431.cc trunk/libstdc++-v3/testsuite/20_util/variant/run.cc
