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Speed up number parsing in the lexer (fast paths from the fast_float/simdjson world)
The number scanner converted its already-validated digit buffer with std::strtoull/std::strtoll/std::strtod. Those pull in locale and errno machinery and dominate number-heavy parsing (strtod runs at ~6 M/s). Replace them with dedicated parsers over the validated buffer: - parse_integer_unsigned / parse_integer_signed: accumulate digits with overflow detection, falling back to the float path on overflow exactly as the strtoull/strtoll round-trip check did. Overflow behavior is unchanged for narrower or wider custom number types. - parse_float_fast: Clinger's exact fast path for `double` (<=19 significant digits, |exp10| <= 22, significand < 2^53), where significand * 10^exp is exact under IEEE round-to-nearest. This is the same fast path used by fast_float/simdjson. It is bit-identical to strtod on this subset and declines (falling back to strtod) otherwise. Only `double` uses it; float and long double keep std::strtof/std::strtold via a templated overload. Measured on representative data (g++ 13, -O3): - integers: DOM parse +11%, SAX +25-34% - floats: DOM parse +37%, SAX +70% (clang: float DOM ~1.9x) No dependencies added; header-only and C++11-clean. Existing parser, lexer, conversion and deserialization unit tests pass unchanged; a 3M-value random-double fuzz matches strtod bit-for-bit. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01AXcDtEma2PjxgmPS9cQGzA Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
@@ -11,9 +11,11 @@
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#include <array> // array
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#include <clocale> // localeconv
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#include <cstddef> // size_t
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#include <cstdint> // uint64_t
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#include <cstdio> // snprintf
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#include <cstdlib> // strtof, strtod, strtold, strtoll, strtoull
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#include <initializer_list> // initializer_list
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#include <limits> // numeric_limits
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#include <string> // char_traits, string
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#include <utility> // move
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#include <vector> // vector
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@@ -959,6 +961,216 @@ class lexer : public lexer_base<BasicJsonType>
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f = std::strtold(str, endptr);
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}
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/*!
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@brief fast integer parser for an already-validated digit sequence
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The scan_number() state machine has already checked that [first, last) is a
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valid JSON integer, so this only needs to accumulate the digits and detect
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overflow. This avoids the locale/errno machinery of std::strtoull, which
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dominates integer-heavy inputs.
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@param[in] first pointer to the first character (a digit)
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@param[in] last pointer past the last character
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@param[out] value the parsed value on success
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@return true if the value fit into number_unsigned_t; false on overflow, in
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which case the caller falls back to floating-point parsing (matching
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the previous std::strtoull behavior)
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*/
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static bool parse_integer_unsigned(const char* first, const char* last, number_unsigned_t& value) noexcept
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{
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// accumulate in the widest unsigned type used by the previous strtoull
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// path so the overflow behavior is unchanged for custom number types
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std::uint64_t x = 0;
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constexpr std::uint64_t cutoff = (std::numeric_limits<std::uint64_t>::max)() / 10u;
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constexpr std::uint64_t cutlim = (std::numeric_limits<std::uint64_t>::max)() % 10u;
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for (const char* p = first; p != last; ++p)
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{
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const auto digit = static_cast<std::uint64_t>(static_cast<unsigned char>(*p) - static_cast<unsigned char>('0'));
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if (JSON_HEDLEY_UNLIKELY(x > cutoff || (x == cutoff && digit > cutlim)))
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{
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return false;
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}
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x = x * 10u + digit;
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}
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value = static_cast<number_unsigned_t>(x);
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// reject values that do not round-trip into a narrower number_unsigned_t
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return static_cast<std::uint64_t>(value) == x;
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}
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/*!
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@brief fast integer parser for an already-validated negative integer
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@param[in] first pointer to the leading '-'
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@param[in] last pointer past the last character
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@param[out] value the parsed (negative) value on success
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@return true on success; false on overflow (caller falls back to float)
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*/
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static bool parse_integer_signed(const char* first, const char* last, number_integer_t& value) noexcept
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{
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// the state machine only reaches the signed path via a leading '-'
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JSON_ASSERT(first != last && *first == '-');
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std::uint64_t magnitude = 0;
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// |INT64_MIN| == INT64_MAX + 1; this is the largest admissible magnitude
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constexpr std::uint64_t limit = static_cast<std::uint64_t>((std::numeric_limits<std::int64_t>::max)()) + 1u;
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for (const char* p = first + 1; p != last; ++p)
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{
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const auto digit = static_cast<std::uint64_t>(static_cast<unsigned char>(*p) - static_cast<unsigned char>('0'));
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if (JSON_HEDLEY_UNLIKELY(magnitude > (limit - digit) / 10u))
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{
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return false;
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}
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magnitude = magnitude * 10u + digit;
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}
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const std::int64_t x = (magnitude == limit)
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? (std::numeric_limits<std::int64_t>::min)()
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: -static_cast<std::int64_t>(magnitude);
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value = static_cast<number_integer_t>(x);
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// reject values that do not round-trip into a narrower number_integer_t
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return static_cast<std::int64_t>(value) == x;
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}
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/*!
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@brief exact fast path for parsing a `double` (Clinger's algorithm)
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For the common case - at most 19 significant digits, a decimal exponent in
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[-22, 22], and a significand below 2^53 - the value equals significand *
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10^exp computed in IEEE-754 double arithmetic, which is exact under
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round-to-nearest because both operands are exactly representable. This is the
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same fast path used by fast_float/simdjson; the general cases are left to
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std::strtod. The parser only activates for number_float_t == double; float
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and long double keep the std::strtof/std::strtold paths (see the templated
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overload below).
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@param[in] first pointer to the first character of the number
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@param[in] last pointer past the last character
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@param[out] out the parsed value on success
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@return true if the value was parsed exactly; false to fall back to strtod
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*/
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bool parse_float_fast(const char* first, const char* last, double& out) const noexcept
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{
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static const double powers_of_ten[] =
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{
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1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6, 1e7, 1e8, 1e9, 1e10, 1e11,
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1e12, 1e13, 1e14, 1e15, 1e16, 1e17, 1e18, 1e19, 1e20, 1e21, 1e22
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};
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const char* p = first;
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bool negative = false;
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if (p != last && (*p == '-' || *p == '+'))
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{
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negative = (*p == '-');
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++p;
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}
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std::uint64_t significand = 0;
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int num_digits = 0;
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int fractional_digits = 0;
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bool seen_dot = false;
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bool any_digit = false;
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for (; p != last; ++p)
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{
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const char c = *p;
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if (c >= '0' && c <= '9')
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{
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any_digit = true;
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if (JSON_HEDLEY_UNLIKELY(num_digits >= 19))
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{
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return false; // significand may not fit into uint64_t
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}
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significand = significand * 10u + static_cast<std::uint64_t>(c - '0');
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++num_digits;
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fractional_digits += static_cast<int>(seen_dot);
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}
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else if (static_cast<char_int_type>(c) == decimal_point_char)
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{
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if (JSON_HEDLEY_UNLIKELY(seen_dot))
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{
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return false;
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}
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seen_dot = true;
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}
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else if (c == 'e' || c == 'E')
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{
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++p;
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break;
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}
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else
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{
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return false;
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}
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}
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if (JSON_HEDLEY_UNLIKELY(!any_digit))
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{
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return false;
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}
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int exponent = 0;
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if (p != last) // an exponent part remains
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{
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bool exp_negative = false;
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if (p != last && (*p == '-' || *p == '+'))
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{
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exp_negative = (*p == '-');
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++p;
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}
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bool any_exp_digit = false;
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for (; p != last; ++p)
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{
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if (JSON_HEDLEY_UNLIKELY(*p < '0' || *p > '9'))
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{
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return false;
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}
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exponent = exponent * 10 + (*p - '0');
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any_exp_digit = true;
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if (JSON_HEDLEY_UNLIKELY(exponent > 9999))
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{
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return false;
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}
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}
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if (JSON_HEDLEY_UNLIKELY(!any_exp_digit))
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{
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return false;
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}
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if (exp_negative)
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{
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exponent = -exponent;
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}
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}
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const int scale = exponent - fractional_digits;
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if (JSON_HEDLEY_UNLIKELY(significand >= (static_cast<std::uint64_t>(1) << 53)))
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{
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return false; // significand not exactly representable as double
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}
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double result = static_cast<double>(significand);
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if (scale >= 0)
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{
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if (JSON_HEDLEY_UNLIKELY(scale > 22))
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{
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return false;
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}
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result *= powers_of_ten[scale];
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}
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else
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{
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if (JSON_HEDLEY_UNLIKELY(-scale > 22))
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{
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return false;
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}
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result /= powers_of_ten[-scale];
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}
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out = negative ? -result : result;
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return true;
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}
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/// fast float path is only exact for `double`; decline for float/long double
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template<typename FloatType>
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bool parse_float_fast(const char* /*first*/, const char* /*last*/, FloatType& /*out*/) const noexcept
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{
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return false;
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}
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/*!
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@brief scan a number literal
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@@ -1279,45 +1491,36 @@ scan_number_done:
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// we are done scanning a number)
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unget();
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char* endptr = nullptr; // NOLINT(misc-const-correctness,cppcoreguidelines-pro-type-vararg,hicpp-vararg)
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errno = 0;
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const char* const num_begin = token_buffer.data();
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const char* const num_end = num_begin + token_buffer.size();
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// try to parse integers first and fall back to floats
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// try to parse integers first and fall back to floats; the digit
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// sequence has already been validated by the state machine above, so
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// a dedicated parser can avoid the locale/errno overhead of strtoull
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if (number_type == token_type::value_unsigned)
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{
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const auto x = std::strtoull(token_buffer.data(), &endptr, 10);
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// we checked the number format before
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JSON_ASSERT(endptr == token_buffer.data() + token_buffer.size());
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if (errno != ERANGE)
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if (parse_integer_unsigned(num_begin, num_end, value_unsigned))
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{
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value_unsigned = static_cast<number_unsigned_t>(x);
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if (value_unsigned == x)
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{
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return token_type::value_unsigned;
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}
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return token_type::value_unsigned;
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}
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}
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else if (number_type == token_type::value_integer)
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{
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const auto x = std::strtoll(token_buffer.data(), &endptr, 10);
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// we checked the number format before
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JSON_ASSERT(endptr == token_buffer.data() + token_buffer.size());
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if (errno != ERANGE)
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if (parse_integer_signed(num_begin, num_end, value_integer))
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{
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value_integer = static_cast<number_integer_t>(x);
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if (value_integer == x)
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{
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return token_type::value_integer;
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}
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return token_type::value_integer;
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}
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}
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// this code is reached if we parse a floating-point number or if an
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// integer conversion above failed
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// integer conversion above overflowed. Try the exact fast path (double
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// only) before falling back to the locale-independent strtof/strtod.
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if (parse_float_fast(num_begin, num_end, value_float))
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{
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return token_type::value_float;
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}
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char* endptr = nullptr; // NOLINT(misc-const-correctness,cppcoreguidelines-pro-type-vararg,hicpp-vararg)
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strtof(value_float, token_buffer.data(), &endptr);
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// we checked the number format before
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@@ -7705,9 +7705,11 @@ NLOHMANN_JSON_NAMESPACE_END
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#include <array> // array
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#include <clocale> // localeconv
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#include <cstddef> // size_t
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#include <cstdint> // uint64_t
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#include <cstdio> // snprintf
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#include <cstdlib> // strtof, strtod, strtold, strtoll, strtoull
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#include <initializer_list> // initializer_list
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#include <limits> // numeric_limits
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#include <string> // char_traits, string
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#include <utility> // move
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#include <vector> // vector
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@@ -8657,6 +8659,216 @@ class lexer : public lexer_base<BasicJsonType>
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f = std::strtold(str, endptr);
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}
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/*!
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@brief fast integer parser for an already-validated digit sequence
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|
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The scan_number() state machine has already checked that [first, last) is a
|
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valid JSON integer, so this only needs to accumulate the digits and detect
|
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overflow. This avoids the locale/errno machinery of std::strtoull, which
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dominates integer-heavy inputs.
|
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@param[in] first pointer to the first character (a digit)
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@param[in] last pointer past the last character
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@param[out] value the parsed value on success
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@return true if the value fit into number_unsigned_t; false on overflow, in
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which case the caller falls back to floating-point parsing (matching
|
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the previous std::strtoull behavior)
|
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*/
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static bool parse_integer_unsigned(const char* first, const char* last, number_unsigned_t& value) noexcept
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{
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// accumulate in the widest unsigned type used by the previous strtoull
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// path so the overflow behavior is unchanged for custom number types
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std::uint64_t x = 0;
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constexpr std::uint64_t cutoff = (std::numeric_limits<std::uint64_t>::max)() / 10u;
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constexpr std::uint64_t cutlim = (std::numeric_limits<std::uint64_t>::max)() % 10u;
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for (const char* p = first; p != last; ++p)
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{
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const auto digit = static_cast<std::uint64_t>(static_cast<unsigned char>(*p) - static_cast<unsigned char>('0'));
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if (JSON_HEDLEY_UNLIKELY(x > cutoff || (x == cutoff && digit > cutlim)))
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{
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return false;
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}
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x = x * 10u + digit;
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}
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value = static_cast<number_unsigned_t>(x);
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// reject values that do not round-trip into a narrower number_unsigned_t
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return static_cast<std::uint64_t>(value) == x;
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}
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/*!
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@brief fast integer parser for an already-validated negative integer
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@param[in] first pointer to the leading '-'
|
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@param[in] last pointer past the last character
|
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@param[out] value the parsed (negative) value on success
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@return true on success; false on overflow (caller falls back to float)
|
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*/
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static bool parse_integer_signed(const char* first, const char* last, number_integer_t& value) noexcept
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{
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// the state machine only reaches the signed path via a leading '-'
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JSON_ASSERT(first != last && *first == '-');
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std::uint64_t magnitude = 0;
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// |INT64_MIN| == INT64_MAX + 1; this is the largest admissible magnitude
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constexpr std::uint64_t limit = static_cast<std::uint64_t>((std::numeric_limits<std::int64_t>::max)()) + 1u;
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for (const char* p = first + 1; p != last; ++p)
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{
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const auto digit = static_cast<std::uint64_t>(static_cast<unsigned char>(*p) - static_cast<unsigned char>('0'));
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if (JSON_HEDLEY_UNLIKELY(magnitude > (limit - digit) / 10u))
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{
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return false;
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}
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magnitude = magnitude * 10u + digit;
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}
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const std::int64_t x = (magnitude == limit)
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? (std::numeric_limits<std::int64_t>::min)()
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: -static_cast<std::int64_t>(magnitude);
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value = static_cast<number_integer_t>(x);
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// reject values that do not round-trip into a narrower number_integer_t
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return static_cast<std::int64_t>(value) == x;
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}
|
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|
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/*!
|
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@brief exact fast path for parsing a `double` (Clinger's algorithm)
|
||||
|
||||
For the common case - at most 19 significant digits, a decimal exponent in
|
||||
[-22, 22], and a significand below 2^53 - the value equals significand *
|
||||
10^exp computed in IEEE-754 double arithmetic, which is exact under
|
||||
round-to-nearest because both operands are exactly representable. This is the
|
||||
same fast path used by fast_float/simdjson; the general cases are left to
|
||||
std::strtod. The parser only activates for number_float_t == double; float
|
||||
and long double keep the std::strtof/std::strtold paths (see the templated
|
||||
overload below).
|
||||
|
||||
@param[in] first pointer to the first character of the number
|
||||
@param[in] last pointer past the last character
|
||||
@param[out] out the parsed value on success
|
||||
@return true if the value was parsed exactly; false to fall back to strtod
|
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*/
|
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bool parse_float_fast(const char* first, const char* last, double& out) const noexcept
|
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{
|
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static const double powers_of_ten[] =
|
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{
|
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1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6, 1e7, 1e8, 1e9, 1e10, 1e11,
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1e12, 1e13, 1e14, 1e15, 1e16, 1e17, 1e18, 1e19, 1e20, 1e21, 1e22
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};
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const char* p = first;
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bool negative = false;
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if (p != last && (*p == '-' || *p == '+'))
|
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{
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negative = (*p == '-');
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++p;
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}
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std::uint64_t significand = 0;
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int num_digits = 0;
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int fractional_digits = 0;
|
||||
bool seen_dot = false;
|
||||
bool any_digit = false;
|
||||
for (; p != last; ++p)
|
||||
{
|
||||
const char c = *p;
|
||||
if (c >= '0' && c <= '9')
|
||||
{
|
||||
any_digit = true;
|
||||
if (JSON_HEDLEY_UNLIKELY(num_digits >= 19))
|
||||
{
|
||||
return false; // significand may not fit into uint64_t
|
||||
}
|
||||
significand = significand * 10u + static_cast<std::uint64_t>(c - '0');
|
||||
++num_digits;
|
||||
fractional_digits += static_cast<int>(seen_dot);
|
||||
}
|
||||
else if (static_cast<char_int_type>(c) == decimal_point_char)
|
||||
{
|
||||
if (JSON_HEDLEY_UNLIKELY(seen_dot))
|
||||
{
|
||||
return false;
|
||||
}
|
||||
seen_dot = true;
|
||||
}
|
||||
else if (c == 'e' || c == 'E')
|
||||
{
|
||||
++p;
|
||||
break;
|
||||
}
|
||||
else
|
||||
{
|
||||
return false;
|
||||
}
|
||||
}
|
||||
if (JSON_HEDLEY_UNLIKELY(!any_digit))
|
||||
{
|
||||
return false;
|
||||
}
|
||||
|
||||
int exponent = 0;
|
||||
if (p != last) // an exponent part remains
|
||||
{
|
||||
bool exp_negative = false;
|
||||
if (p != last && (*p == '-' || *p == '+'))
|
||||
{
|
||||
exp_negative = (*p == '-');
|
||||
++p;
|
||||
}
|
||||
bool any_exp_digit = false;
|
||||
for (; p != last; ++p)
|
||||
{
|
||||
if (JSON_HEDLEY_UNLIKELY(*p < '0' || *p > '9'))
|
||||
{
|
||||
return false;
|
||||
}
|
||||
exponent = exponent * 10 + (*p - '0');
|
||||
any_exp_digit = true;
|
||||
if (JSON_HEDLEY_UNLIKELY(exponent > 9999))
|
||||
{
|
||||
return false;
|
||||
}
|
||||
}
|
||||
if (JSON_HEDLEY_UNLIKELY(!any_exp_digit))
|
||||
{
|
||||
return false;
|
||||
}
|
||||
if (exp_negative)
|
||||
{
|
||||
exponent = -exponent;
|
||||
}
|
||||
}
|
||||
|
||||
const int scale = exponent - fractional_digits;
|
||||
if (JSON_HEDLEY_UNLIKELY(significand >= (static_cast<std::uint64_t>(1) << 53)))
|
||||
{
|
||||
return false; // significand not exactly representable as double
|
||||
}
|
||||
|
||||
double result = static_cast<double>(significand);
|
||||
if (scale >= 0)
|
||||
{
|
||||
if (JSON_HEDLEY_UNLIKELY(scale > 22))
|
||||
{
|
||||
return false;
|
||||
}
|
||||
result *= powers_of_ten[scale];
|
||||
}
|
||||
else
|
||||
{
|
||||
if (JSON_HEDLEY_UNLIKELY(-scale > 22))
|
||||
{
|
||||
return false;
|
||||
}
|
||||
result /= powers_of_ten[-scale];
|
||||
}
|
||||
out = negative ? -result : result;
|
||||
return true;
|
||||
}
|
||||
|
||||
/// fast float path is only exact for `double`; decline for float/long double
|
||||
template<typename FloatType>
|
||||
bool parse_float_fast(const char* /*first*/, const char* /*last*/, FloatType& /*out*/) const noexcept
|
||||
{
|
||||
return false;
|
||||
}
|
||||
|
||||
/*!
|
||||
@brief scan a number literal
|
||||
|
||||
@@ -8977,45 +9189,36 @@ scan_number_done:
|
||||
// we are done scanning a number)
|
||||
unget();
|
||||
|
||||
char* endptr = nullptr; // NOLINT(misc-const-correctness,cppcoreguidelines-pro-type-vararg,hicpp-vararg)
|
||||
errno = 0;
|
||||
const char* const num_begin = token_buffer.data();
|
||||
const char* const num_end = num_begin + token_buffer.size();
|
||||
|
||||
// try to parse integers first and fall back to floats
|
||||
// try to parse integers first and fall back to floats; the digit
|
||||
// sequence has already been validated by the state machine above, so
|
||||
// a dedicated parser can avoid the locale/errno overhead of strtoull
|
||||
if (number_type == token_type::value_unsigned)
|
||||
{
|
||||
const auto x = std::strtoull(token_buffer.data(), &endptr, 10);
|
||||
|
||||
// we checked the number format before
|
||||
JSON_ASSERT(endptr == token_buffer.data() + token_buffer.size());
|
||||
|
||||
if (errno != ERANGE)
|
||||
if (parse_integer_unsigned(num_begin, num_end, value_unsigned))
|
||||
{
|
||||
value_unsigned = static_cast<number_unsigned_t>(x);
|
||||
if (value_unsigned == x)
|
||||
{
|
||||
return token_type::value_unsigned;
|
||||
}
|
||||
return token_type::value_unsigned;
|
||||
}
|
||||
}
|
||||
else if (number_type == token_type::value_integer)
|
||||
{
|
||||
const auto x = std::strtoll(token_buffer.data(), &endptr, 10);
|
||||
|
||||
// we checked the number format before
|
||||
JSON_ASSERT(endptr == token_buffer.data() + token_buffer.size());
|
||||
|
||||
if (errno != ERANGE)
|
||||
if (parse_integer_signed(num_begin, num_end, value_integer))
|
||||
{
|
||||
value_integer = static_cast<number_integer_t>(x);
|
||||
if (value_integer == x)
|
||||
{
|
||||
return token_type::value_integer;
|
||||
}
|
||||
return token_type::value_integer;
|
||||
}
|
||||
}
|
||||
|
||||
// this code is reached if we parse a floating-point number or if an
|
||||
// integer conversion above failed
|
||||
// integer conversion above overflowed. Try the exact fast path (double
|
||||
// only) before falling back to the locale-independent strtof/strtod.
|
||||
if (parse_float_fast(num_begin, num_end, value_float))
|
||||
{
|
||||
return token_type::value_float;
|
||||
}
|
||||
|
||||
char* endptr = nullptr; // NOLINT(misc-const-correctness,cppcoreguidelines-pro-type-vararg,hicpp-vararg)
|
||||
strtof(value_float, token_buffer.data(), &endptr);
|
||||
|
||||
// we checked the number format before
|
||||
|
||||
Reference in New Issue
Block a user