mirror of
https://github.com/nlohmann/json.git
synced 2026-10-06 22:47:13 +00:00
Niels Lohmann
6ee5e89803
Speed up json_view's parser with SIMD scanning and a hash table for large objects. Long runs of string bytes are scanned 16 bytes at a time with NEON (AArch64, GCC and Clang) and SSE2 (x86-64), both baseline instruction sets. Keys keep 16 table checks before the vector loop, because their lengths repeat from record to record; string values get 8, because their lengths vary more. Non-ASCII text is validated 16 bytes at a time with simdjson's "lookup4" check (Keiser and Lemire, 2021), with NEON on AArch64 and, on x86-64, with SSSE3. SSSE3 is not part of baseline x86-64, so the check is compiled for SSSE3 with a function attribute and used only where CPUID reports it, which all x86-64 CPUs since about 2011 do; the answer is cached in a statically initialized atomic, so there is no guard of a local static and no global constructor. The same input is accepted either way. JSON_VIEW_NO_SIMD selects the portable code. On x86-64, string runs are now checked vector-first: one SSE2 compare from the first byte finds the end of most keys and short values, instead of a branch per byte for the first 8-16 bytes. AArch64 keeps the byte-wise steps, where a NEON mask costs more and the branches predict well. Entering an object or array no longer stalls: open() stores the parent's frame field by field instead of building it on the stack and reading it back with wider loads, which waited for the narrower stores to retire. Objects with 128 members or more get an open-addressing hash table built when the object closes, so operator[], at(), find(), contains(), count(), value(), and JSON pointers take constant time on average in such objects; of duplicate keys, the first is kept, as for the linear search. The idea comes from Boost.JSON. simdjson is credited in simd.hpp's SPDX block, the README, and license.md. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
229 lines
8.0 KiB
C++
229 lines
8.0 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. With SSE2, the run is checked 16 bytes
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/// at a time from its first byte instead: on x86-64, one compare that finds
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/// the end of most keys and short values is faster than a branch per byte (on
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/// AArch64, where a NEON mask costs more and branches predict well, slower).
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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 NLOHMANN_VIEW_SSE2
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p = vector_plain_run(p, e);
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#else
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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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#endif
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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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#if !NLOHMANN_VIEW_SSE2
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stop:
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#endif
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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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