Scan strings with SSE2, SSSE3, or NEON (#5853)
* Scan strings with SSE2, SSSE3, or NEON The string fast path found the end of a run of plain characters 8 bytes at a time (SWAR) and validated non-ASCII text one sequence at a time, calling validate_one_utf8() out of line for each. The vector code of json_view's scanner, in the new shared header detail/input/simd_scan.hpp, now does both 16 bytes at a time: SSE2 finds the end of a run on x86-64, and the UTF-8 check of Keiser and Lemire runs with NEON on AArch64 and with SSSE3 on x86-64. SSSE3 is not part of x86-64, so the check is compiled for it with a function attribute and used when the CPU has it (cached in a constant-initialized atomic), else the scalar check; code compiled with -mssse3 skips the CPU check. The first word of a run is still checked with SWAR (the first four with NEON, where a vector mask costs more), as most keys and short values end there, and the scalar helpers are always inlined. On anything else, the scalar code is unchanged. The new macro JSON_NO_SIMD selects the portable code everywhere, including the SSE2/NEON digit formatting of the floating-point writer. The lexer tests now also run with JSON_NO_SIMD and, on x86-64, with -mssse3, and a new test places every byte that ends a run at each offset across several 16-byte blocks, after ASCII and after CJK text. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Keep the string scan from slowing down short strings and dump() on Arm Three benchmarks were 7-9% slower than develop on Apple M1 (Apple Clang, -O3 -flto): Accept/jeopardy, Dump/citm_catalog, and Dump/jeopardy. - dump(): the serializer passes a string without its closing quote, so plain_run_end() ran out of input and called the out-of-line NEON loop for every string, with fewer than 16 bytes left. The loop is now only called for 16 bytes or more. - accept(): no string of jeopardy.json reaches the vector UTF-8 check, yet compiling it into the lexer's scan function made it 5% slower: the function runs once per token and now saved more registers and kept a stack slot. lexer_string_bulk_run() checks the first four words inline and continues out of line, and the serializer keeps string_bulk_run() inline, which is faster there. - The byte that ends a run is read from the scanned buffer instead of through get(), which Apple Clang merged with the other get() calls that load the read position again right after it was stored (3%). Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Define string_bulk_run_from() only where lexer_string_bulk_run() calls it With NEON and simdutf in C++17, lexer_string_bulk_run() takes the string_bulk_run() path, so the out-of-line helper was unused. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Do not dispatch the SSSE3 string scan with GCC's C++20 modules GCC does not keep the target("ssse3") attribute on functions it reads back from a module interface, so the module build (ci_module_cpp20, gcc:latest) rejected the intrinsics in vector_utf8_run() with "needs isa option -mssse3". With __cpp_modules, GCC now uses the portable UTF-8 check like other compilers without the target attribute; the SSE2 scan stays, and -mssse3 or more still compiles the SSSE3 check directly. Clang is not affected. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Include <cpuid.h> only once when json.hpp and json_view.hpp are both used The SSSE3 dispatch of the string scan and the one of json_view each include <cpuid.h>. Before GCC 11 the header has no include guard, so a translation unit that includes both headers fails with GCC 10: "redefinition of 'unsigned int __get_cpuid_max(unsigned int, unsigned int*)'". Guard both includes with NLOHMANN_JSON_CPUID_INCLUDED. Signed-off-by: Niels Lohmann <mail@nlohmann.me> * Add an ABI tag for JSON_NO_SIMD JSON_NO_SIMD selects between two definitions of the internal scanning and formatting functions. Like JSON_VIEW_NO_SIMD (_vns), it now appends _ns to the inline namespace, so translation units built with and without it can be linked into one program without ODR violations. Signed-off-by: Niels Lohmann <mail@nlohmann.me> --------- Signed-off-by: Niels Lohmann <mail@nlohmann.me>
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@@ -38,6 +38,7 @@ header. See also the [macro overview page](../../features/macros.md).
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- [**JSON_HAS_THREE_WAY_COMPARISON**](json_has_three_way_comparison.md) - control 3-way comparison support
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- [**JSON_NO_AUTOMATIC_UDLS**](json_no_automatic_udls.md) - do not include the user-defined string literals (UDLs) automatically
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- [**JSON_NO_IO**](json_no_io.md) - switch off functions relying on certain C++ I/O headers
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- [**JSON_NO_SIMD**](json_no_simd.md) - use only portable code instead of vector instructions (SSE2, SSSE3, NEON)
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- [**JSON_NO_THREAD_LOCAL**](json_no_thread_local.md) - switch off the use of `thread_local` storage
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- [**JSON_SKIP_UNSUPPORTED_COMPILER_CHECK**](json_skip_unsupported_compiler_check.md) - do not warn about unsupported compilers
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- [**JSON_USE_GLOBAL_UDLS**](json_use_global_udls.md) - place user-defined string literals (UDLs) into the global namespace
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@@ -0,0 +1,83 @@
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# JSON_NO_SIMD
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```cpp
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#define JSON_NO_SIMD
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```
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When defined, the library uses only portable C++ code where it would otherwise use vector instructions: when scanning
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strings during parsing, and when writing floating-point numbers during serialization.
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The macro is an escape hatch, for example for toolchains that cannot compile the vector code, for sanitizers or
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analyzers that do not understand it, or to compare the speed of both variants. It does not change any result: values,
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exceptions, and error positions are the same either way. Only the speed differs.
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## Default definition
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By default, `#!cpp JSON_NO_SIMD` is not defined, and vector instructions are used where the target has them.
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```cpp
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#undef JSON_NO_SIMD
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```
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## Notes
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!!! note "Which instructions are used"
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- **x86-64:** SSE2, which every x86-64 CPU has, finds the end of runs of plain string characters 16 bytes at a time
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and writes the digits of floating-point numbers. The UTF-8 check of non-ASCII text needs SSSE3, which is not part
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of x86-64: it is compiled for SSSE3 with a function attribute and used when the CPU has SSSE3 (every x86-64 CPU
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since about 2011), else the portable check. Code compiled for SSSE3 anyway (e.g., with `-mssse3` or `-march=native`)
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skips this check of the CPU. 32-bit x86 uses SSE2 for strings when the compiler targets it.
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- **AArch64:** NEON, which every 64-bit Arm CPU has, checks UTF-8 text and long runs of plain string characters 16
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bytes at a time, and writes the digits of floating-point numbers. With MSVC, only the floating-point digits use
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NEON.
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- **Other targets:** portable code only.
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Strings are only scanned this way for contiguous input and for the characters a `#!cpp std::istream` holds in its
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stream buffer (see [`JSON_DISABLE_STREAM_BULK_SCAN`](json_disable_stream_bulk_scan.md)); other inputs are read one
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character at a time.
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!!! note "Interaction with `JSON_USE_SIMDUTF`"
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With [`JSON_USE_SIMDUTF`](json_use_simdutf.md), runs of string characters are first validated by simdutf; the code
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described here is used for what simdutf does not accept, so that errors are reported at the same position.
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`JSON_NO_SIMD` does not affect simdutf.
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!!! note "ABI compatibility"
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The macro selects between two definitions of the library's internal scanning and formatting functions. Whether it
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is defined is encoded in the [namespace](../../features/namespace.md) (tag `_ns`), resulting in distinct symbol
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names. Translation units compiled with and without it can therefore be linked into the same program without One
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Definition Rule (ODR) violations, but they cannot exchange instances of library types.
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## Examples
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??? example "Example: switching off vector instructions"
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The program parses and serializes the same values with and without the macro:
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```cpp
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#define JSON_NO_SIMD
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#include <iostream>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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const json j = json::parse(R"({"name": "日本語 text", "pi": 3.141592653589793})");
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std::cout << j.dump() << '\n'; // {"name":"日本語 text","pi":3.141592653589793}
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}
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```
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## See also
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- [**parse**](../basic_json/parse.md) - deserialize from a compatible input
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- [**dump**](../basic_json/dump.md) - serialization
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- [**JSON_USE_SIMDUTF**](json_use_simdutf.md) - use the simdutf library to accelerate UTF-8 validation
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- [**JSON_DISABLE_STREAM_BULK_SCAN**](json_disable_stream_bulk_scan.md) - read a `std::istream` one character at a time
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- [Parsing: input sources and performance](../../features/parsing/index.md#input-sources-and-performance)
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## Version history
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- Added in version 3.13.0.
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@@ -6,8 +6,9 @@
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When defined, the parser validates the UTF-8 content of JSON strings that come from a **contiguous byte input**
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(`std::string`, `std::vector<char>`/`<std::uint8_t>`, string literals, `const char*` ranges, …) using the
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[simdutf](https://github.com/simdutf/simdutf) library instead of the built-in scalar validator. On text with many
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non-ASCII characters (e.g. CJK or emoji) this can validate several times faster.
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[simdutf](https://github.com/simdutf/simdutf) library instead of the built-in validator (which uses SSSE3 or NEON where
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available, see [`JSON_NO_SIMD`](json_no_simd.md)). On text with many non-ASCII characters (e.g. CJK or emoji) this can
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validate faster, depending on the CPU.
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This is an **opt-in external dependency**. The library itself remains header-only and its behavior is unchanged: the
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same input is accepted or rejected either way, and every parse error is reported at the same position with the same
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@@ -39,6 +39,6 @@ the library.
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## Version history
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- Added in version 3.11.0. Changed inline namespace name in version 3.11.2. Added the ABI tag `_dp` in
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version 3.12.0, and the ABI tags `_bics`, `_psp`, `_snul`, `_sbu8`, `_ekmo`, `_dsbs`, `_dufio`, `_vns`, and
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`_vssse3` in version 3.13.0; see
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version 3.12.0, and the ABI tags `_bics`, `_psp`, `_snul`, `_sbu8`, `_ekmo`, `_dsbs`, `_dufio`, `_ns`, `_vns`,
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and `_vssse3` in version 3.13.0; see
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[`nlohmann` Namespace](../../features/namespace.md#structure).
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@@ -174,6 +174,13 @@ security reasons (e.g., Intel Software Guard Extensions (SGX)).
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See [full documentation of `JSON_NO_IO`](../api/macros/json_no_io.md).
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## `JSON_NO_SIMD`
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When defined, the library uses only portable code instead of vector instructions (SSE2, SSSE3, NEON) for scanning
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strings and writing floating-point numbers. Results are the same either way. It is not defined by default.
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See [full documentation of `JSON_NO_SIMD`](../api/macros/json_no_simd.md).
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## `JSON_NO_THREAD_LOCAL`
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When defined, the library does not use `#!cpp thread_local` storage. Copying a value and comparing two values then
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@@ -27,6 +27,7 @@ The complete default namespace name is derived as follows:
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`_dsbs`.
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- [`JSON_DISABLE_UNLOCKED_FILE_IO`](../api/macros/json_disable_unlocked_file_io.md) defined non-zero appends
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`_dufio`.
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- [`JSON_NO_SIMD`](../api/macros/json_no_simd.md) defined appends `_ns`.
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- [`JSON_VIEW_NO_SIMD`](../api/macros/json_view_no_simd.md) defined appends `_vns`.
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- [`JSON_VIEW_USE_SSSE3`](../api/macros/json_view_use_ssse3.md) defined appends `_vssse3`.
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- The inline namespace ends with the suffix `_v` followed by the 3 components of the version number separated by
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@@ -114,6 +115,7 @@ follows:
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([`JSON_STRICT_BINARY_UTF8`](../api/macros/json_strict_binary_utf8.md)), `_ekmo`
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([`JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS`](../api/macros/json_use_objects_for_enum_keyed_maps.md)), `_dsbs`
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([`JSON_DISABLE_STREAM_BULK_SCAN`](../api/macros/json_disable_stream_bulk_scan.md)), `_dufio`
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([`JSON_DISABLE_UNLOCKED_FILE_IO`](../api/macros/json_disable_unlocked_file_io.md)), `_vns`
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([`JSON_DISABLE_UNLOCKED_FILE_IO`](../api/macros/json_disable_unlocked_file_io.md)), `_ns`
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([`JSON_NO_SIMD`](../api/macros/json_no_simd.md)), `_vns`
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([`JSON_VIEW_NO_SIMD`](../api/macros/json_view_no_simd.md)), and `_vssse3`
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([`JSON_VIEW_USE_SSSE3`](../api/macros/json_view_use_ssse3.md)) in version 3.13.0.
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@@ -79,6 +79,10 @@ follow the usual `std::streambuf` contract, and
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[`JSON_DISABLE_UNLOCKED_FILE_IO`](../../api/macros/json_disable_unlocked_file_io.md) for programs in which another
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thread must use the same `#!cpp FILE` object while it is parsed.
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Where the target has them, strings are scanned with vector instructions: SSE2 on x86-64 (and SSSE3 for UTF-8 text, when
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the CPU has it) and NEON on AArch64. [`JSON_NO_SIMD`](../../api/macros/json_no_simd.md) selects portable code instead,
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again without changing any result.
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## SAX vs. DOM parsing
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The library offers two parsing models:
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@@ -64,7 +64,7 @@ below are relative to `include/nlohmann`.
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|-----------|----------|
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| Value type enumeration | [`detail/value_t.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/value_t.hpp) |
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| Input adapters | [`detail/input/input_adapters.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/input/input_adapters.hpp) |
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| Lexer | [`detail/input/lexer.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/input/lexer.hpp), [`detail/input/number_parse.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/input/number_parse.hpp), [`detail/input/string_scan.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/input/string_scan.hpp) |
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| Lexer | [`detail/input/lexer.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/input/lexer.hpp), [`detail/input/number_parse.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/input/number_parse.hpp), [`detail/input/string_scan.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/input/string_scan.hpp), [`detail/input/simd_scan.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/input/simd_scan.hpp) |
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| Parser | [`detail/input/parser.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/input/parser.hpp) |
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| SAX interface and DOM builders | [`detail/input/json_sax.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/input/json_sax.hpp) |
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| Binary format readers | [`detail/input/binary_reader.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/input/binary_reader.hpp) |
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@@ -383,6 +383,7 @@ nav:
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- 'JSON_NO_AUTOMATIC_UDLS': api/macros/json_no_automatic_udls.md
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- 'JSON_NOEXCEPTION': api/macros/json_noexception.md
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- 'JSON_NO_IO': api/macros/json_no_io.md
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- 'JSON_NO_SIMD': api/macros/json_no_simd.md
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- 'JSON_NO_THREAD_LOCAL': api/macros/json_no_thread_local.md
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- 'JSON_PRECISE_STREAM_POSITION': api/macros/json_precise_stream_position.md
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- 'JSON_SKIP_LIBRARY_VERSION_CHECK': api/macros/json_skip_library_version_check.md
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