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Replace snprintf with a branch-free writer for \uXXXX escapes

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dump_escaped called std::snprintf(..., "\u%04x", ...) once per escaped
code point in the string serialization hot path. snprintf re-parses
the format string and pulls in locale/printf machinery on every call,
which is far heavier than the fixed 6-/12-byte output warrants. This
is hot for any string containing control characters, and for all
non-ASCII text when ensure_ascii is set.

Replace it with write_u_escape, a small helper that writes the escape
directly into string_buffer via a nibble-to-hex lookup table, mirroring
the existing hand-rolled dump_integer fast path in the same file.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-07-04 22:42:55 +02:00
parent c034480c22
commit 4cad4e08a4
3 changed files with 86 additions and 18 deletions
Download Patch File Download Diff File Expand all files Collapse all files
include/nlohmann/detail/output/serializer.hpp
+30 -9
View File
@@ -465,18 +465,12 @@ class serializer
{
if (codepoint <= 0xFFFF)
{
// NOLINTNEXTLINE(cppcoreguidelines-pro-type-vararg,hicpp-vararg)
static_cast<void>((std::snprintf)(string_buffer.data() + bytes, 7, "\\u%04x",
static_cast<std::uint16_t>(codepoint)));
bytes += 6;
bytes = write_u_escape(bytes, static_cast<std::uint16_t>(codepoint));
}
else
{
// NOLINTNEXTLINE(cppcoreguidelines-pro-type-vararg,hicpp-vararg)
static_cast<void>((std::snprintf)(string_buffer.data() + bytes, 13, "\\u%04x\\u%04x",
static_cast<std::uint16_t>(0xD7C0u + (codepoint >> 10u)),
static_cast<std::uint16_t>(0xDC00u + (codepoint & 0x3FFu))));
bytes += 12;
bytes = write_u_escape(bytes, static_cast<std::uint16_t>(0xD7C0u + (codepoint >> 10u)));
bytes = write_u_escape(bytes, static_cast<std::uint16_t>(0xDC00u + (codepoint & 0x3FFu)));
}
}
else
@@ -683,6 +677,33 @@ class serializer
return result;
}
/*!
* @brief write a lowercase "\uXXXX" escape sequence into @a string_buffer
*
* Branch-free replacement for `snprintf(buf, 7, "\\u%04x", codeunit)` in the
* string escaping hot path. It writes exactly six characters ('\\', 'u' and
* four hex digits) at position @a pos of @a string_buffer via a nibble
* lookup table, avoiding the format-string parsing and locale machinery of
* `snprintf`.
*
* @param[in] pos position in @a string_buffer to write at; there must
* be at least 6 bytes of headroom
* @param[in] codeunit 16-bit value to encode
* @return position just after the written escape sequence (@a pos + 6)
*/
std::size_t write_u_escape(std::size_t pos, std::uint16_t codeunit) noexcept
{
JSON_ASSERT(string_buffer.size() - pos >= 6);
static constexpr const char nibble_to_hex[] = "0123456789abcdef";
string_buffer[pos + 0] = '\\';
string_buffer[pos + 1] = 'u';
string_buffer[pos + 2] = nibble_to_hex[(codeunit >> 12u) & 0x0Fu];
string_buffer[pos + 3] = nibble_to_hex[(codeunit >> 8u) & 0x0Fu];
string_buffer[pos + 4] = nibble_to_hex[(codeunit >> 4u) & 0x0Fu];
string_buffer[pos + 5] = nibble_to_hex[codeunit & 0x0Fu];
return pos + 6;
}
// templates to avoid warnings about useless casts
template <typename NumberType, enable_if_t<std::is_signed<NumberType>::value, int> = 0>
bool is_negative_number(NumberType x)
single_include/nlohmann/json.hpp
+30 -9
View File
@@ -19752,18 +19752,12 @@ class serializer
{
if (codepoint <= 0xFFFF)
{
// NOLINTNEXTLINE(cppcoreguidelines-pro-type-vararg,hicpp-vararg)
static_cast<void>((std::snprintf)(string_buffer.data() + bytes, 7, "\\u%04x",
static_cast<std::uint16_t>(codepoint)));
bytes += 6;
bytes = write_u_escape(bytes, static_cast<std::uint16_t>(codepoint));
}
else
{
// NOLINTNEXTLINE(cppcoreguidelines-pro-type-vararg,hicpp-vararg)
static_cast<void>((std::snprintf)(string_buffer.data() + bytes, 13, "\\u%04x\\u%04x",
static_cast<std::uint16_t>(0xD7C0u + (codepoint >> 10u)),
static_cast<std::uint16_t>(0xDC00u + (codepoint & 0x3FFu))));
bytes += 12;
bytes = write_u_escape(bytes, static_cast<std::uint16_t>(0xD7C0u + (codepoint >> 10u)));
bytes = write_u_escape(bytes, static_cast<std::uint16_t>(0xDC00u + (codepoint & 0x3FFu)));
}
}
else
@@ -19970,6 +19964,33 @@ class serializer
return result;
}
/*!
* @brief write a lowercase "\uXXXX" escape sequence into @a string_buffer
*
* Branch-free replacement for `snprintf(buf, 7, "\\u%04x", codeunit)` in the
* string escaping hot path. It writes exactly six characters ('\\', 'u' and
* four hex digits) at position @a pos of @a string_buffer via a nibble
* lookup table, avoiding the format-string parsing and locale machinery of
* `snprintf`.
*
* @param[in] pos position in @a string_buffer to write at; there must
* be at least 6 bytes of headroom
* @param[in] codeunit 16-bit value to encode
* @return position just after the written escape sequence (@a pos + 6)
*/
std::size_t write_u_escape(std::size_t pos, std::uint16_t codeunit) noexcept
{
JSON_ASSERT(string_buffer.size() - pos >= 6);
static constexpr const char nibble_to_hex[] = "0123456789abcdef";
string_buffer[pos + 0] = '\\';
string_buffer[pos + 1] = 'u';
string_buffer[pos + 2] = nibble_to_hex[(codeunit >> 12u) & 0x0Fu];
string_buffer[pos + 3] = nibble_to_hex[(codeunit >> 8u) & 0x0Fu];
string_buffer[pos + 4] = nibble_to_hex[(codeunit >> 4u) & 0x0Fu];
string_buffer[pos + 5] = nibble_to_hex[codeunit & 0x0Fu];
return pos + 6;
}
// templates to avoid warnings about useless casts
template <typename NumberType, enable_if_t<std::is_signed<NumberType>::value, int> = 0>
bool is_negative_number(NumberType x)
tests/src/unit-convenience.cpp
+26
View File
@@ -168,6 +168,32 @@ TEST_CASE("convenience functions")
CHECK_THROWS_WITH_AS(check_escaped("\xC2"), "[json.exception.type_error.316] incomplete UTF-8 string; last byte: 0xC2", json::type_error&);
}
SECTION("string escape with ensure_ascii")
{
// control characters are escaped regardless of ensure_ascii
check_escaped("\x01", "\\u0001", true);
check_escaped("\x1f", "\\u001f", true);
// non-ASCII code points in the Basic Multilingual Plane are emitted as
// a single lowercase \uXXXX escape (exercises every nibble position)
check_escaped("\xC2\x80", "\\u0080", true); // U+0080
check_escaped("\xC3\xBF", "\\u00ff", true); // U+00FF (ÿ)
check_escaped("\xDF\xBF", "\\u07ff", true); // U+07FF
check_escaped("\xE4\xBD\xA0", "\\u4f60", true); // U+4F60 (你)
check_escaped("\xEA\xAF\x8D", "\\uabcd", true); // U+ABCD
check_escaped("\xEF\xBF\xBD", "\\ufffd", true); // U+FFFD (replacement char, all-f nibbles)
// code points outside the BMP are emitted as a UTF-16 surrogate pair
// of two lowercase \uXXXX escapes
check_escaped("\xF0\x90\x80\x80", "\\ud800\\udc00", true); // U+10000 (lowest astral)
check_escaped("\xF0\x9F\x98\x80", "\\ud83d\\ude00", true); // U+1F600 (😀)
check_escaped("\xF4\x8F\xBF\xBF", "\\udbff\\udfff", true); // U+10FFFF (highest code point)
// with ensure_ascii disabled, non-ASCII input is passed through verbatim
check_escaped("\xE4\xBD\xA0", "\xE4\xBD\xA0", false);
check_escaped("\xF0\x9F\x98\x80", "\xF0\x9F\x98\x80", false);
}
SECTION("string concat")
{
using nlohmann::detail::concat;