3058d527ce
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>
1.6 KiB
JSON_VIEW_NO_SIMD
#define JSON_VIEW_NO_SIMD
When defined, the parser of basic_json_document (<nlohmann/json_view.hpp>)
uses only portable C++ to scan strings. By default, it scans long runs of string bytes 16 at a time with NEON on
AArch64 (with GCC and Clang) and SSE2 on x86-64, which are part of the baseline instruction sets of these
architectures, and validates non-ASCII text with NEON (or SSSE3, see
JSON_VIEW_USE_SSSE3).
The same input is accepted or rejected either way, with the same values, and errors are reported the same way; only the speed differs. The macro exists for platforms whose compilers lack the intrinsics headers, and to test the portable code.
!!! warning "Define consistently"
The macro selects between two definitions of the same inline functions. It must therefore be defined identically for
**every** translation unit that includes `<nlohmann/json_view.hpp>`; prefer a compile definition on the target.
Default definition
By default, #!cpp JSON_VIEW_NO_SIMD is not defined, and the vector code is used where available.
#undef JSON_VIEW_NO_SIMD
Examples
??? example
The code below uses the portable string scanning of the view.
```cpp
#define JSON_VIEW_NO_SIMD
#include <nlohmann/json_view.hpp>
...
```
See also
- JSON_VIEW_USE_SSSE3 - validate non-ASCII strings with SSSE3 on x86-64
- json_view - the zero-copy view
Version history
- Added in version 3.13.0.