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Niels Lohmann 3ff37ead75 Scan json_view strings with SIMD and index large objects (#5628 )

* 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>

* Fix GCC module build and MSVC C4127 in the json_view SIMD code

GCC ignores the target attribute in modules, so the SSSE3 dispatch is
disabled for the module interface (the check stays portable, SSE2 is kept).
Make the 8-vs-16 byte unrolling condition in scan_string_run a
preprocessor/template split to avoid a constant condition (C4127).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Remove an accidentally committed GCC module cache

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Bound the probe length of the large-object hash index

The key hash is not seeded, so keys chosen to collide made building the table quadratic (20,000 colliding keys took 470 ms to parse). A key may now sit at most 64 slots from its home slot; if a key would sit further away, the table is dropped and the object is searched linearly. Lookups stop after the same distance.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Release the spare capacity of the hash index in shrink_to_fit

shrink_to_fit() now trims the tables of large objects like the node array and the decoded strings, and the list of large objects is released as soon as the tables are built.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Use the portable string scan on non-x86 targets that define __SSE2__

WebAssembly with -msse2 defines __SSE2__, but has no <cpuid.h> or x86 intrinsics headers.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Document the hash index number in node::extra

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Keep the last of duplicate keys in the object hash index

Lookups in objects with 128 members or more now return the last member of
a repeated key, like the linear search of smaller objects and like
materialize() and basic_json::parse(). build_object_index let the first
occurrence win, so the same text gave different results depending on the
size of the object.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Keep the first of duplicate keys in the object hash index again

Lookups in objects with 128 members or more return the first member of a
repeated key again, like the linear search of smaller objects. This undoes
the code change of a69542046; its test now expects the first member from
lookups (with and without a table) and the last value from materialize()
and basic_json::parse().

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Fix clang-tidy 22 findings in the json_view headers

Silence bugprone-casting-through-void for the SSE loads (a reinterpret_cast
would trip -Wcast-align=strict), use auto for a cast initialiser, add
parentheses to a mixed expression, and name bugprone-std-namespace-modification
in the NOLINTs of the tuple_size/tuple_element specialisations.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Fix clang-tidy 22 findings in unit-json_view.cpp and unit-json_view_builder.cpp

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Avoid GCC useless-cast and strict-overflow warnings in unit-json_view.cpp

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Index strings with size_t in the colliding-keys test (MSVC C4244 on Win32)

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Make json_view.hpp pass include-what-you-use

ci_single_binaries runs IWYU with --error on every header. For json_view.hpp
it suggested adding <array> (std::array is used), the headers that json.hpp
already provides (abi_config, abi_macros, input_adapters, json_pointer,
cpp_future, string_concat, value_t, json_fwd), and <version> for
std::nullptr_t, and removing <cstddef>.

- include <array>
- keep <cstddef> (nullptr_t, size_t; IWYU attributes them to <version> and <cstring>)
- tell IWYU not to suggest the headers that json.hpp provides: the amalgamated
  json_view.hpp only includes json.hpp, so including them here would duplicate
  their definitions

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Move the dump, comparison, and large-object tests to unit-json_view_dump.cpp

The dump and comparison tests moved to their own file in the dump pull
request; the hash index tests of this branch join them there.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Regenerate the amalgamated headers

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Follow the renames from the review of the view builder

The review of #5620 renamed document_data's tape/tape_size/tape_cap to
nodes/node_count/node_capacity and made the decode arena a
std::vector<char>; adapt the code of this change to it.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Rename slots in the object index for Qt compatibility

Qt defines slots as a macro, so a Qt header included before
json_view.hpp broke the declarations of the local variable (see #5865).
The test variable is renamed as well.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Address the review of the json_view object index and SIMD scan

From gregmarr's review of #5628:

- Docs and comments say that the hash index is one of the object's keys.
- object_index.hpp: conditional_static_cast for the hash, "slot" for the
  table position, no flag for a degenerate table, and a static_assert
  that the smallest indexed object is larger than the probe window.
- scan.hpp unrolls with NLOHMANN_VIEW_REPEAT_FIRST_8/SECOND_8.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Regenerate the amalgamated headers

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Encode JSON_VIEW_NO_SIMD and JSON_VIEW_USE_SSSE3 in the ABI namespace

Both macros select between definitions of the same inline functions, so
translation units that disagree on them violated the ODR. Like the other
ABI-affecting macros, they now add a tag to the inline namespace (_vns and
_vssse3), and the docs replace the "Define consistently" warning with the
usual ABI compatibility note. From gregmarr's review of #5628.

nlohmann_json.natvis covers twice as many tags now, so it lists four times
as many namespaces (14.7 MB instead of 3.4 MB).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Regenerate the amalgamated headers

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Do not include type_traits.hpp directly in the object index

json.hpp, which object_index.hpp already includes, provides it; included
directly, the json_view amalgamation (which only treats nlohmann/json.hpp
as external) copied the json internals into json_view.hpp a second time,
so the fuzzers, which include both single headers, failed with
redefinitions (make_void, integer_sequence, priority_tag, ...).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Compare keys with key_equals() in the object hash index

As the linear lookups do since the second review round of #5622.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Regenerate the amalgamated headers

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

2026-10-21 10:21:28 +02:00

6.3 KiB

Raw Blame History

`nlohmann` Namespace

The 3.11.0 release introduced an inline namespace to allow different parts of a codebase to safely use different versions of the JSON library as long as they never exchange instances of library types.

Structure

The complete default namespace name is derived as follows:

The root namespace is always nlohmann.
The inline namespace starts with json_abi and is followed by several optional ABI tags according to the value of these ABI-affecting macros, in order:
- JSON_DIAGNOSTICS defined non-zero appends _diag.
- JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON defined non-zero appends _ldvcmp.
- JSON_DIAGNOSTIC_POSITIONS defined non-zero appends _dp.
- JSON_BRACE_INIT_COPY_SEMANTICS defined non-zero appends _bics.
- JSON_PRECISE_STREAM_POSITION defined non-zero appends _psp.
- JSON_STRICT_NUL_HANDLING defined non-zero appends _snul.
- JSON_STRICT_BINARY_UTF8 defined non-zero appends _sbu8.
- JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS defined non-zero appends _ekmo.
- JSON_DISABLE_STREAM_BULK_SCAN defined non-zero appends _dsbs.
- JSON_DISABLE_UNLOCKED_FILE_IO defined non-zero appends _dufio.
- JSON_VIEW_NO_SIMD defined appends _vns.
- JSON_VIEW_USE_SSSE3 defined appends _vssse3.
The inline namespace ends with the suffix _v followed by the 3 components of the version number separated by underscores. To omit the version component, see Disabling the version component below.

For example, the namespace name for version 3.11.2 with JSON_DIAGNOSTICS defined to 1 is:

nlohmann::json_abi_diag_v3_11_2

Purpose

Several incompatibilities have been observed. Amongst the most common ones is linking code compiled with different definitions of JSON_DIAGNOSTICS. This is illustrated in the diagram below.

graph
    json["<strong>nlohmann_json (v3.10.5)</strong><br>JSON_DIAGNOSTICS=0"]
    json_diag["<strong>nlohmann_json (v3.10.5)</strong><br>JSON_DIAGNOSTICS=1"]
    library["<strong>some library</strong>"]
    app["<strong>application</strong>"]
    
    library --> json
    app --> json_diag
    app --> library

In releases prior to 3.11.0, mixing any version of the JSON library with different JSON_DIAGNOSTICS settings would result in a crashing application. If some_library never passes instances of JSON library types to the application, this scenario became safe in version 3.11.0 and above due to the inline namespace yielding distinct symbol names.

Limitations

Neither the compiler nor the linker will issue as much as a warning when translation units – intended to be linked together and that include different versions and/or configurations of the JSON library – exchange and use library types.

There is an exception when forward declarations are used (i.e., when including json_fwd.hpp) in which case the linker may complain about undefined references.

Disabling the version component

Different versions are not necessarily ABI-incompatible, but the project does not actively track changes in the ABI and recommends that all parts of a codebase exchanging library types be built with the same version. Users can, at their own risk, disable the version component of the inline namespace, allowing different versions – but not configurations – to be used in cases where the linker would otherwise output undefined reference errors.

To do so, define NLOHMANN_JSON_NAMESPACE_NO_VERSION to 1.

This applies to version 3.11.2 and above only; versions 3.11.0 and 3.11.1 can apply the technique described in the next section to emulate the effect of the NLOHMANN_JSON_NAMESPACE_NO_VERSION macro.

!!! danger "Use at your own risk"

Disabling the namespace version component and mixing ABI-incompatible versions will result in crashes or incorrect
behavior. You have been warned!

Disabling the inline namespace completely

When interoperability with code using a pre-3.11.0 version of the library is required, users can, at their own risk restore the old namespace layout by redefining NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END as follows:

#define NLOHMANN_JSON_NAMESPACE_BEGIN  namespace nlohmann {
#define NLOHMANN_JSON_NAMESPACE_END    }