Niels Lohmann
4939f8d667
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>
2.1 KiB
JSON_VIEW_USE_SSSE3
#define JSON_VIEW_USE_SSSE3
When defined on x86-64, the parser of basic_json_document
(<nlohmann/json_view.hpp>) validates non-ASCII text in strings with SSSE3 without asking the CPU first.
By default, the parser checks once at run time whether the CPU has SSSE3 (all x86-64 CPUs since about 2011 have it)
and then validates non-ASCII text 16 bytes at a time, using the "lookup4" algorithm of
simdjson; on CPUs without SSSE3, it validates one UTF-8 sequence at a time.
The vector check is compiled for SSSE3 with a function attribute (GCC 4.9 and later, Clang), so this needs no compiler
option. With MSVC, the check uses __cpuid. On AArch64, the vector check uses NEON and is always on.
Define the macro only together with a compiler option that enables SSSE3 (e.g. -mssse3, or -march= with a CPU that
has it), and only for programs that run on such CPUs. It saves the check of the CPU, which costs little. The same
input is accepted or rejected either way; only the speed of non-ASCII text differs.
!!! note "ABI compatibility"
The macro selects between two definitions of the same inline functions. Whether it is defined is encoded in the
[namespace](../../features/namespace.md) (tag `_vssse3`, on every architecture), resulting in distinct symbol
names. Translation units compiled with and without it can therefore be linked into the same program without One
Definition Rule (ODR) violations, but they cannot exchange instances of library types.
Default definition
By default, #!cpp JSON_VIEW_USE_SSSE3 is not defined.
#undef JSON_VIEW_USE_SSSE3
Examples
??? example
With CMake, for a program that only runs on CPUs with SSSE3:
```cmake
target_compile_definitions(your_target PRIVATE JSON_VIEW_USE_SSSE3)
target_compile_options(your_target PRIVATE -mssse3)
```
See also
- JSON_VIEW_NO_SIMD - use only portable code in the view's parser
- JSON_USE_SIMDUTF - validate UTF-8 with simdutf in
basic_json's parser
Version history
- Added in version 3.13.0.