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Niels Lohmann 3058d527ce Scan json_view strings with SIMD and index large objects

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>

2026-10-11 11:30:55 +02:00

8.6 KiB

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Macros

Some aspects of the library can be configured by defining preprocessor macros before including the json.hpp header. See also the macro overview page.

Runtime assertions

JSON_ASSERT(x) - control behavior of runtime assertions

Exceptions

JSON_CATCH_USER(exception)
JSON_THROW_USER(exception)
JSON_TRY_USER - control exceptions
JSON_DIAGNOSTICS - control extended diagnostics
JSON_DIAGNOSTIC_POSITIONS - access positions of elements
JSON_NOEXCEPTION - switch off exceptions

Parsing

JSON_PRECISE_STREAM_POSITION - opt in to leaving an input stream positioned right after a parsed number
JSON_STRICT_BINARY_UTF8 - opt in to checking strings for valid UTF-8 in the CBOR, UBJSON, BJData, and BSON writers
JSON_STRICT_NUL_HANDLING - opt in to rejecting a NUL byte in the input instead of treating it as end of input

Language support

JSON_HAS_CPP_11
JSON_HAS_CPP_14
JSON_HAS_CPP_17
JSON_HAS_CPP_20 - set supported C++ standard
JSON_HAS_FILESYSTEM
JSON_HAS_EXPERIMENTAL_FILESYSTEM - control std::filesystem support
JSON_HAS_RANGE_VIEW_CONVERSION - control construction from std::ranges views
JSON_HAS_RANGES - control std::ranges support
JSON_HAS_STATIC_RTTI - control RTTI (run time type information) support
JSON_HAS_STD_FORMAT - control std::format/std::formatter support
JSON_HAS_THREE_WAY_COMPARISON - control 3-way comparison support
JSON_NO_AUTOMATIC_UDLS - do not include the user-defined string literals (UDLs) automatically
JSON_NO_IO - switch off functions relying on certain C++ I/O headers
JSON_NO_THREAD_LOCAL - switch off the use of thread_local storage
JSON_SKIP_UNSUPPORTED_COMPILER_CHECK - do not warn about unsupported compilers
JSON_USE_GLOBAL_UDLS - place user-defined string literals (UDLs) into the global namespace
JSON_USE_SIMDUTF - use the simdutf library to accelerate UTF-8 validation
JSON_VIEW_NO_SIMD - use only portable code in the parser of json_view.hpp
JSON_VIEW_USE_SSSE3 - validate non-ASCII strings with SSSE3 in the parser of json_view.hpp

Library version

JSON_SKIP_LIBRARY_VERSION_CHECK - skip library version check
NLOHMANN_JSON_VERSION_MAJOR
NLOHMANN_JSON_VERSION_MINOR
NLOHMANN_JSON_VERSION_PATCH - library version information

Library namespace

NLOHMANN_JSON_NAMESPACE - full name of the nlohmann namespace
NLOHMANN_JSON_NAMESPACE_BEGIN
NLOHMANN_JSON_NAMESPACE_END - open and close the library namespace
NLOHMANN_JSON_NAMESPACE_NO_VERSION - disable the version component of the inline namespace

Type conversions

JSON_BRACE_INIT_COPY_SEMANTICS - opt in to copy/move semantics for single-element brace initialization
JSON_DISABLE_ENUM_SERIALIZATION - switch off default serialization/deserialization functions for enums
JSON_DISABLE_TUPLE_REFERENCE_CONVERSION - switch off conversion from a one-element tuple of a JSON reference
JSON_USE_IMPLICIT_CONVERSIONS - control implicit conversions
JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS - opt in to storing maps with enum keys as objects

Deprecated functions

JSON_DELETE_DEPRECATED_FUNCTIONS - opt in to deleting the deprecated functions ahead of their removal in version 4.0.0

Comparison behavior

JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON - control comparison of discarded values

Serialization/deserialization macros

Enums

NLOHMANN_JSON_SERIALIZE_ENUM - serialize/deserialize an enum
NLOHMANN_JSON_SERIALIZE_ENUM_STRICT - serialize/deserialize an enum with exceptions

Classes and structs

NLOHMANN_DEFINE_TYPE_INTRUSIVE - serialize/deserialize a non-derived class with private members
NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT - serialize/deserialize a non-derived class with private members; uses default values
NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE - serialize a non-derived class with private members
NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE - serialize/deserialize a non-derived class
NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT - serialize/deserialize a non-derived class; uses default values
NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE - serialize a non-derived class
NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE - serialize/deserialize a derived class with private members
NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT - serialize/deserialize a derived class with private members; uses default values
NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE - serialize a derived class with private members
NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE - serialize/deserialize a derived class
NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT - serialize/deserialize a derived class; uses default values
NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE - serialize a derived class
NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_NAMES - serialize/deserialize a non-derived class with private members; uses custom names
NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT_WITH_NAMES - serialize/deserialize a non-derived class with private members; uses default values; uses custom names
NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES - serialize a non-derived class with private members; uses custom names
NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_NAMES - serialize/deserialize a non-derived class; uses custom names
NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT_WITH_NAMES - serialize/deserialize a non-derived class; uses default values; uses custom names
NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES - serialize a non-derived class; uses custom names
NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_NAMES - serialize/deserialize a derived class with private members; uses custom names
NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT_WITH_NAMES - serialize/deserialize a derived class with private members; uses default values; uses custom names
NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES - serialize a derived class with private members; uses custom names
NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_NAMES - serialize/deserialize a derived class; uses custom names
NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT_WITH_NAMES - serialize/deserialize a derived class; uses default values; uses custom names
NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES - serialize a derived class; uses custom names

8.6 KiB Raw Blame History