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Niels Lohmann
c634c24877
Add detail helpers that call the standard library function when the library provides it (checked with the __cpp_lib_* feature-test macros, so no new ABI-tag macros) and a C++11 implementation otherwise: - detail::clamp (std::clamp): by value, so that static constexpr members need no out-of-line definition before C++17; used in json_view's dump() buffer sizing, where step() can now be a static constexpr member. - detail::bit_cast (std::bit_cast): replaces dtoa_impl::reinterpret_bits and the memcpy bit copies in the number parser, to_chars, the json_view serializer, and binary_writer::to_char_type. - detail::byteswap (std::byteswap): one implementation instead of the binary writer's own byte_swap() overloads and the bswap chain in to_chars' store_msb_first(). - count_leading_zeros/count_trailing_zeros: two 32-bit _BitScanReverse/ _BitScanForward calls for MSVC x86/ARM32 instead of the loop, and std::countl_zero/countr_zero for compilers without builtins. - conjunction, disjunction, negation, bool_constant, and uncvref_t map to their std counterparts in C++17/C++20. Builtins and intrinsics stay ahead of the <bit> functions, as MSVC's std::countl_zero checks for LZCNT at run time unless AVX2 is enabled. The generated code is unchanged with Clang (C++11/20/23, arm64 and x86-64) and GCC 16 for parsing, dumping, and the binary formats. The binary reader keeps its own byte_swap() and the writer its memcpy calls: the smaller code from detail::byteswap/std::bit_cast there made GCC 16 inline get_number()/write_number() differently, which slowed reading by 10-18 % and writing by about 1 %. Signed-off-by: Niels Lohmann <mail@nlohmann.me>