https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96419
Bug ID: 96419 Summary: Constant propoagation works on global variable, but not in a function Product: gcc Version: 10.1.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: milasudril at gmail dot com Target Milestone: --- I tried to implement a map-like data structure, with compile-time keys: #include <algorithm> #include <array> #include <type_traits> namespace fixed_flatmap_detail { template<class T, size_t N, class Compare> constexpr auto sort(std::array<T, N> const& x, Compare const& compare) { auto tmp = x; std::ranges::sort(tmp, compare); return tmp; } } template<class Keys, class Value, class Compare = std::less<decltype(Keys::items[0])>> class FixedFlatmap { public: using key_type = std::remove_reference_t<decltype(Keys::items[0])>; using value_type = Value; static constexpr auto size() { return std::size(Keys::items); } static constexpr auto const keys() { return s_keys; } constexpr auto const values() const { return m_vals; } constexpr auto values() { return m_vals; } constexpr auto find(key_type const& key) const { auto i = std::ranges::lower_bound(s_keys, key, Compare{}); if(i != std::end(s_keys) && !Compare{}(key, *i)) [[likely]] { return std::begin(m_vals) + (i - std::begin(s_keys)); } return static_cast<value_type const*>(nullptr); } constexpr auto find(key_type const& key) { return const_cast<value_type*>(std::as_const(*this).find(key)); } private: static constexpr auto s_keys = fixed_flatmap_detail::sort(Keys::items, Compare{}); std::array<value_type, size()> m_vals; }; struct KeyStruct { static constexpr std::array<std::string_view, 3> items{"Foo", "Bar", "Kaka"}; }; FixedFlatmap<KeyStruct, int> my_vals{}; auto this_value_is_computed_at_compile_time = my_vals.find("Kaka"); int* test_lookup(FixedFlatmap<KeyStruct, int>& vals) { return vals.find("Foo"); // == static_cast<int*>(static_cast<byte*>(&vals) + sizeof(int)) } Interestingly gcc succeeds to compute `find` on a global variable, but fails as soon as the same structure is allocated in a function. I am not an expert in compilers, but realize that it could be trickier to compute it on a non-global object (base address is not known at compile-time). However, the binary search does not even use *this. Thus, `std::lower_bound` and outcome validation should be possible to compute. Godbolt: https://gcc.godbolt.org/z/c7E3P9