mirror of
https://github.com/nlohmann/json.git
synced 2026-10-06 14:40:32 +00:00
Niels Lohmann
606c1e710b
The vector UTF-8 check of json_view needed SSSE3 at compile time (JSON_VIEW_USE_SSSE3 with -mssse3), so default x86-64 builds validated non-ASCII text one sequence at a time. The check is now compiled for SSSE3 with a function attribute (GCC 4.9 and later, Clang; MSVC compiles the intrinsics anyway) and used where CPUID reports SSSE3. The answer is kept in an atomic that is initialized at compile time, so neither a guard nor a global constructor is needed. The definitions do not depend on compiler flags, so there is no ODR issue. JSON_VIEW_USE_SSSE3 now only skips the CPU check. On x86-64 Linux (Haswell), twitter.json parses 23% faster with Clang 18 and 25-34% faster with GCC 13, now ahead of yyjson. Also always inline read_eight_bytes() and parse_eight_digits(): GCC called both in the number loops of the lexer and of json_view (52 call sites), which cost about 10% on citm_catalog.json at -O2. Document that reusing a document with read() avoids the page faults of a fresh node index (about 40% of a 55 MB parse on x86-64 Linux). Signed-off-by: Niels Lohmann <mail@nlohmann.me>
220 lines
7.7 KiB
C++
220 lines
7.7 KiB
C++
// __ _____ _____ _____
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// __| | __| | | | JSON for Modern C++
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// | | |__ | | | | | | version 3.12.0
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// |_____|_____|_____|_|___| https://github.com/nlohmann/json
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//
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// SPDX-FileCopyrightText: 2013-2026 Niels Lohmann <https://nlohmann.me>
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// SPDX-FileCopyrightText: 2020 YaoYuan <https://github.com/ibireme/yyjson>
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// SPDX-License-Identifier: MIT
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#pragma once
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#include <array> // array
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#include <cstddef> // size_t
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#include <cstdint> // uint8_t, uint16_t, uint64_t
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#include <cstring> // memcpy
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#include <nlohmann/json.hpp>
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#include <nlohmann/detail/view/macro_scope.hpp>
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#include <nlohmann/detail/view/simd.hpp>
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// Scanning primitives of the view's parser. The unrolled checks at fixed
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// offsets follow yyjson (https://github.com/ibireme/yyjson, MIT license): the
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// loads do not depend on each other, so the CPU can run ahead. Words are read
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// with read_eight_bytes(), so nothing here depends on the byte order.
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NLOHMANN_JSON_NAMESPACE_BEGIN
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namespace detail
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{
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namespace view
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{
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/// 1 for bytes that may appear verbatim in a string: 0x20..0x7F except '"' and '\\'
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inline const std::uint8_t* string_plain() noexcept
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{
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static const std::array<std::uint8_t, 256> table =
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{
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{
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0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0x00..0x1F
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1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, // 0x20..0x3F ('"')
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1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 1, 1, 1, // 0x40..0x5F ('\\')
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1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, // 0x60..0x7F
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0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0x80..0x9F
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0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0xA0..0xBF
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0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0xC0..0xDF
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0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0xE0..0xFF
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}
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};
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return table.data();
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}
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NLOHMANN_VIEW_ALWAYS_INLINE bool is_digit(unsigned char c) noexcept
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{
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return static_cast<unsigned char>(c - '0') <= 9;
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}
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/// two bytes as they are in memory (only compared with byte-symmetric patterns)
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NLOHMANN_VIEW_ALWAYS_INLINE std::uint16_t load16(const unsigned char* p) noexcept
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{
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std::uint16_t w = 0;
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std::memcpy(&w, p, 2);
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return w;
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}
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/// Advance over plain string bytes and well-formed UTF-8. Stops at a quote,
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/// a backslash, a control character, ill-formed UTF-8, or the end. The first
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/// bytes are checked one by one, so that the position advances by constants
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/// in predicted branches: 16 for keys, whose lengths repeat from record to
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/// record, and 8 for string values (Value) where a vector loop follows, as
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/// their lengths vary more. Longer runs continue 16 bytes at a time with NEON
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/// or SSE2, else eight bytes at a time.
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template<bool Value = false>
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NLOHMANN_VIEW_ALWAYS_INLINE const unsigned char* scan_string_run(const unsigned char* p, const unsigned char* e) noexcept
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{
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const std::uint8_t* plain = string_plain();
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for (;;)
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{
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if (e - p >= 16)
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{
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#define NLOHMANN_VIEW_STEP(i) if (NLOHMANN_VIEW_LIKELY(plain[p[i]] != 0)) {} else { p += (i); goto stop; }
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NLOHMANN_VIEW_STEP(0) NLOHMANN_VIEW_STEP(1) NLOHMANN_VIEW_STEP(2) NLOHMANN_VIEW_STEP(3)
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NLOHMANN_VIEW_STEP(4) NLOHMANN_VIEW_STEP(5) NLOHMANN_VIEW_STEP(6) NLOHMANN_VIEW_STEP(7)
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if (!Value || !NLOHMANN_VIEW_VECTOR)
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{
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NLOHMANN_VIEW_STEP(8) NLOHMANN_VIEW_STEP(9) NLOHMANN_VIEW_STEP(10) NLOHMANN_VIEW_STEP(11)
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NLOHMANN_VIEW_STEP(12) NLOHMANN_VIEW_STEP(13) NLOHMANN_VIEW_STEP(14) NLOHMANN_VIEW_STEP(15)
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p += 8;
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}
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#undef NLOHMANN_VIEW_STEP
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p += 8;
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#if NLOHMANN_VIEW_VECTOR
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p = vector_plain_run(p, e);
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if (p != e && plain[*p] == 0)
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{
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goto stop;
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}
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#else
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while (e - p >= 8)
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{
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const std::uint64_t special = swar_string_special(read_eight_bytes(p));
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if (special != 0)
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{
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p += count_trailing_zeros(special) / 8;
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goto stop;
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}
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p += 8;
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}
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#endif
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continue;
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}
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while (p != e && plain[*p] != 0)
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{
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++p;
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}
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if (p == e)
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{
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return p;
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}
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stop:
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if (*p < 0x80)
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{
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return p; // quote, backslash, or control character
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}
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#if NLOHMANN_VIEW_VECTOR_UTF8
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#if NLOHMANN_VIEW_SSSE3_DISPATCH
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if (NLOHMANN_VIEW_LIKELY(cpu_has_ssse3()))
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#endif
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{
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// non-ASCII: the vector check, out of line
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return scan_string_vector(p, e, plain);
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}
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#endif
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#if !NLOHMANN_VIEW_VECTOR_UTF8 || NLOHMANN_VIEW_SSSE3_DISPATCH
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// non-ASCII: a run of well-formed sequences (the library's check, so
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// that exactly what json::parse accepts is accepted)
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do
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{
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const std::size_t n = validate_one_utf8(p, static_cast<std::size_t>(e - p));
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if (n == 0)
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{
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return p;
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}
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p += n;
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}
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while (p != e && *p >= 0x80);
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#endif
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}
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}
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/// advance over ASCII digits
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NLOHMANN_VIEW_ALWAYS_INLINE const unsigned char* skip_digits(const unsigned char* p, const unsigned char* e) noexcept
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{
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while (e - p >= 16)
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{
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#define NLOHMANN_VIEW_STEP(i) if (NLOHMANN_VIEW_LIKELY(is_digit(p[i]))) {} else { return p + (i); }
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NLOHMANN_VIEW_REPEAT16(NLOHMANN_VIEW_STEP)
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#undef NLOHMANN_VIEW_STEP
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p += 16;
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}
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while (p != e && is_digit(*p))
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{
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++p;
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}
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return p;
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}
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/// powers of ten up to 10^19 as integers
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inline std::uint64_t int_pow10(unsigned k) noexcept
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{
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static const std::array<std::uint64_t, 20> table =
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{
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{
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1u, 10u, 100u, 1000u, 10000u, 100000u, 1000000u, 10000000u, 100000000u, 1000000000u,
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10000000000u, 100000000000u, 1000000000000u, 10000000000000u, 100000000000000u, 1000000000000000u,
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10000000000000000u, 100000000000000000u, 1000000000000000000u, 10000000000000000000u
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}
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};
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return table[k];
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}
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/// value of 0 < k < 8 digits at p in one step if [p, p + 8) lies below
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/// limit, else one digit at a time (whole blocks of eight digits are read by
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/// parse_upto19() directly)
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NLOHMANN_VIEW_ALWAYS_INLINE std::uint64_t parse_upto8(const unsigned char* p, unsigned k, const unsigned char* limit) noexcept
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{
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if (NLOHMANN_VIEW_LIKELY(limit - p >= 8))
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{
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// move the k digits to the top and pad the vacated low bytes with '0'
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const unsigned shift = 8 * (8 - k);
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return parse_eight_digits((read_eight_bytes(p) << shift) | (0x3030303030303030u >> (8 * k)));
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}
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std::uint64_t v = 0;
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for (unsigned i = 0; i < k; ++i)
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{
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v = (v * 10) + static_cast<std::uint64_t>(p[i] - '0');
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}
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return v;
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}
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/// value of k <= 19 digits at p
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NLOHMANN_VIEW_ALWAYS_INLINE std::uint64_t parse_upto19(const unsigned char* p, unsigned k, const unsigned char* limit) noexcept
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{
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std::uint64_t w = 0;
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while (k >= 8)
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{
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// (eight digits of the token: they lie below limit)
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w = (w * 100000000u) + parse_eight_digits(read_eight_bytes(p));
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p += 8;
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k -= 8;
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}
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if (k != 0)
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{
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w = (w * int_pow10(k)) + parse_upto8(p, k, limit);
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}
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return w;
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}
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} // namespace view
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} // namespace detail
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NLOHMANN_JSON_NAMESPACE_END
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