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SWAR: test for special bytes in the reduced form

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The usual test for a zero byte, (x - ones) & ~x & high, needs `& ~x` only
to keep bytes from 0x80 out. The tests for quotes, backslashes and control
characters in string_scan.hpp and the serializer also flag every byte from
0x80 with `v & high`, so the `& ~x` terms can be dropped. Clang finds this
reduction itself; GCC does not and emits 24 instead of 16 instructions.

The masks are the same for 1.3 billion random words and for every pair of
adjacent byte values. With GCC 15 on x86-64: accept() -2 to -4 %,
sax_parse() -2 %, dump() -3 %. Clang is unchanged.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann committed 2026-11-06 12:17:57 +01:00
1 parent 788b12213b
commit 2f817b9174
3 files changed
+16 -28
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include/nlohmann/detail/input/string_scan.hpp
+6 -10
View File
@@ -58,11 +58,10 @@ inline std::uint64_t swar_string_special(std::uint64_t v) noexcept
constexpr std::uint64_t high = 0x8080808080808080ull;
const std::uint64_t q = v ^ 0x2222222222222222ull; // '"' (0x22)
const std::uint64_t b = v ^ 0x5C5C5C5C5C5C5C5Cull; // '\\' (0x5C)
const std::uint64_t has_quote = (q - ones) & ~q & high;
const std::uint64_t has_backslash = (b - ones) & ~b & high;
const std::uint64_t has_control = (v - 0x2020202020202020ull) & ~v & high; // < 0x20
const std::uint64_t has_non_ascii = v & high; // >= 0x80
return has_quote | has_backslash | has_control | has_non_ascii;
// The usual test for a zero byte is (x - ones) & ~x & high. The `& ~x`
// only removes bytes from 0x80, and `v & high` flags those anyway, so it
// can be dropped. Clang finds this reduction itself; GCC does not.
return ((q - ones) | (b - ones) | (v - 0x2020202020202020ull) | v) & high;
}
// return the index of the first is_string_special() byte in [data, data+n), or
@@ -143,11 +142,8 @@ inline std::size_t find_ascii_copyable_run(const unsigned char* data, std::size_
const std::uint64_t q = v ^ 0x2222222222222222ull; // '"' (0x22)
const std::uint64_t b = v ^ 0x5C5C5C5C5C5C5C5Cull; // '\\' (0x5C)
const std::uint64_t d = v ^ 0x7F7F7F7F7F7F7F7Full; // DEL (0x7F)
const std::uint64_t stop = ((q - ones) & ~q & high) // == '"'
| ((b - ones) & ~b & high) // == '\\'
| ((d - ones) & ~d & high) // == 0x7F
| ((v - 0x2020202020202020ull) & ~v & high) // < 0x20
| (v & high); // >= 0x80
// (see swar_string_special(): the bytes from 0x80 are stops anyway)
const std::uint64_t stop = ((q - ones) | (b - ones) | (d - ones) | (v - 0x2020202020202020ull) | v) & high;
if (stop != 0)
{
// the lowest flagged byte is the first one to stop at (see
include/nlohmann/detail/output/serializer.hpp
+2 -4
View File
@@ -724,10 +724,8 @@ class serializer
constexpr std::uint64_t ones = 0x0101010101010101ull;
const std::uint64_t q = v ^ 0x2222222222222222ull; // '"'
const std::uint64_t b = v ^ 0x5C5C5C5C5C5C5C5Cull; // '\\'
return (((q - ones) & ~q) | ((b - ones) & ~b) // == '"', == '\\'
| ((v - 0x2020202020202020ull) & ~v) // < 0x20
| v | (v + ones)) // >= 0x7F
& 0x8080808080808080ull;
// (see swar_string_special(): the bytes from 0x80 are flagged anyway)
return ((q - ones) | (b - ones) | (v - 0x2020202020202020ull) | v | (v + ones)) & 0x8080808080808080ull;
}
template<typename WordType>
single_include/nlohmann/json.hpp
Generated
+8 -14
View File
@@ -12537,11 +12537,10 @@ inline std::uint64_t swar_string_special(std::uint64_t v) noexcept
constexpr std::uint64_t high = 0x8080808080808080ull;
const std::uint64_t q = v ^ 0x2222222222222222ull; // '"' (0x22)
const std::uint64_t b = v ^ 0x5C5C5C5C5C5C5C5Cull; // '\\' (0x5C)
const std::uint64_t has_quote = (q - ones) & ~q & high;
const std::uint64_t has_backslash = (b - ones) & ~b & high;
const std::uint64_t has_control = (v - 0x2020202020202020ull) & ~v & high; // < 0x20
const std::uint64_t has_non_ascii = v & high; // >= 0x80
return has_quote | has_backslash | has_control | has_non_ascii;
// The usual test for a zero byte is (x - ones) & ~x & high. The `& ~x`
// only removes bytes from 0x80, and `v & high` flags those anyway, so it
// can be dropped. Clang finds this reduction itself; GCC does not.
return ((q - ones) | (b - ones) | (v - 0x2020202020202020ull) | v) & high;
}
// return the index of the first is_string_special() byte in [data, data+n), or
@@ -12622,11 +12621,8 @@ inline std::size_t find_ascii_copyable_run(const unsigned char* data, std::size_
const std::uint64_t q = v ^ 0x2222222222222222ull; // '"' (0x22)
const std::uint64_t b = v ^ 0x5C5C5C5C5C5C5C5Cull; // '\\' (0x5C)
const std::uint64_t d = v ^ 0x7F7F7F7F7F7F7F7Full; // DEL (0x7F)
const std::uint64_t stop = ((q - ones) & ~q & high) // == '"'
| ((b - ones) & ~b & high) // == '\\'
| ((d - ones) & ~d & high) // == 0x7F
| ((v - 0x2020202020202020ull) & ~v & high) // < 0x20
| (v & high); // >= 0x80
// (see swar_string_special(): the bytes from 0x80 are stops anyway)
const std::uint64_t stop = ((q - ones) | (b - ones) | (d - ones) | (v - 0x2020202020202020ull) | v) & high;
if (stop != 0)
{
// the lowest flagged byte is the first one to stop at (see
@@ -37015,10 +37011,8 @@ class serializer
constexpr std::uint64_t ones = 0x0101010101010101ull;
const std::uint64_t q = v ^ 0x2222222222222222ull; // '"'
const std::uint64_t b = v ^ 0x5C5C5C5C5C5C5C5Cull; // '\\'
return (((q - ones) & ~q) | ((b - ones) & ~b) // == '"', == '\\'
| ((v - 0x2020202020202020ull) & ~v) // < 0x20
| v | (v + ones)) // >= 0x7F
& 0x8080808080808080ull;
// (see swar_string_special(): the bytes from 0x80 are flagged anyway)
return ((q - ones) | (b - ones) | (v - 0x2020202020202020ull) | v | (v + ones)) & 0x8080808080808080ull;
}
template<typename WordType>