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| Author | SHA1 | Date | |
|---|---|---|---|
 Niels Lohmann
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@@ -1,21 +1,21 @@
|
||||
<svg class="nj-fig nj-charts nj-charts--compile" viewBox="0 0 470 516" role="img" aria-labelledby="nj-charts-ct nj-charts-ct-d" xmlns="http://www.w3.org/2000/svg">
|
||||
<svg class="nj-fig nj-charts nj-charts--compile" viewBox="0 0 470 620" role="img" aria-labelledby="nj-charts-ct nj-charts-ct-d" xmlns="http://www.w3.org/2000/svg">
|
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<title id="nj-charts-ct">Change in compile time against the default configuration at -O0, Apple clang and G​CC 16</title>
|
||||
<desc id="nj-charts-ct-d">Grouped horizontal bars, negative is faster. json_fwd.hpp instead of json.hpp on a header-only translation unit: Apple clang -62 %, G​CC 16 -82 %; extern template on a parsing translation unit: Apple clang -7 %, G​CC 16 -21 %; JSON_NO_AUTOMATIC_UDLS on a model translation unit: Apple clang -11 %, G​CC 16 -11 %; JSON_NO_AUTOMATIC_UDLS on a parsing translation unit: Apple clang -1 %, G​CC 16 -2 %; JSON_HAS_FILESYSTEM=0 + JSON_NO_IO on a model translation unit: Apple clang -8 %, G​CC 16 -11 %; JSON_HAS_FILESYSTEM=0 on a model translation unit: Apple clang -5 %, G​CC 16 -9 %; JSON_NO_IO on a model translation unit: Apple clang -2 %, G​CC 16 -2 %; JSON_USE_GLOBAL_UDLS=0 on a model translation unit: Apple clang 0 %, G​CC 16 -1 %.</desc>
|
||||
<desc id="nj-charts-ct-d">Grouped horizontal bars, negative is faster. json_fwd.hpp instead of json.hpp on a header-only translation unit: Apple clang -62 %, G​CC 16 -82 %; extern template on a parsing translation unit: Apple clang -7 %, G​CC 16 -21 %; JSON_NO_AUTOMATIC_UDLS on a model translation unit: Apple clang -11 %, G​CC 16 -11 %; JSON_NO_AUTOMATIC_UDLS on a parsing translation unit: Apple clang -1 %, G​CC 16 -2 %; JSON_HAS_FILESYSTEM=0 + JSON_NO_IO on a model translation unit: Apple clang -8 %, G​CC 16 -11 %; JSON_HAS_FILESYSTEM=0 on a model translation unit: Apple clang -5 %, G​CC 16 -9 %; JSON_HAS_STD_FORMAT=0 on a model translation unit: Apple clang -1 %, G​CC 16 -4 %; JSON_HAS_RANGES=0 on a model translation unit: Apple clang -4 %, G​CC 16 -1 %; JSON_NO_IO on a model translation unit: Apple clang -2 %, G​CC 16 -2 %; JSON_USE_GLOBAL_UDLS=0 on a model translation unit: Apple clang 0 %, G​CC 16 -1 %.</desc>
|
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<rect class="nj-charts-clang" x="8" y="10" width="12" height="12" rx="3"/>
|
||||
<text class="nj-charts-leg" x="26" y="20.5">Apple clang</text>
|
||||
<rect class="nj-charts-gcc" x="126" y="10" width="12" height="12" rx="3"/>
|
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<text class="nj-charts-leg" x="144" y="20.5">G​CC 16</text>
|
||||
<line class="nj-charts-grid" x1="47.4" y1="58" x2="47.4" y2="472"/>
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||||
<text class="nj-charts-tick" x="47.4" y="488">−80 %</text>
|
||||
<line class="nj-charts-grid" x1="126.2" y1="58" x2="126.2" y2="472"/>
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||||
<text class="nj-charts-tick" x="126.2" y="488">−60 %</text>
|
||||
<line class="nj-charts-grid" x1="205" y1="58" x2="205" y2="472"/>
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||||
<text class="nj-charts-tick" x="205" y="488">−40 %</text>
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||||
<line class="nj-charts-grid" x1="283.8" y1="58" x2="283.8" y2="472"/>
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||||
<text class="nj-charts-tick" x="283.8" y="488">−20 %</text>
|
||||
<text class="nj-charts-tick" x="362.6" y="488">0 %</text>
|
||||
<text class="nj-charts-tick nj-charts-tick--l" x="8" y="506">← faster</text>
|
||||
<text class="nj-charts-tick nj-charts-tick--r" x="462" y="506">slower →</text>
|
||||
<line class="nj-charts-grid" x1="47.4" y1="58" x2="47.4" y2="576"/>
|
||||
<text class="nj-charts-tick" x="47.4" y="592">−80 %</text>
|
||||
<line class="nj-charts-grid" x1="126.2" y1="58" x2="126.2" y2="576"/>
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<text class="nj-charts-tick" x="126.2" y="592">−60 %</text>
|
||||
<line class="nj-charts-grid" x1="205" y1="58" x2="205" y2="576"/>
|
||||
<text class="nj-charts-tick" x="205" y="592">−40 %</text>
|
||||
<line class="nj-charts-grid" x1="283.8" y1="58" x2="283.8" y2="576"/>
|
||||
<text class="nj-charts-tick" x="283.8" y="592">−20 %</text>
|
||||
<text class="nj-charts-tick" x="362.6" y="592">0 %</text>
|
||||
<text class="nj-charts-tick nj-charts-tick--l" x="8" y="610">← faster</text>
|
||||
<text class="nj-charts-tick nj-charts-tick--r" x="462" y="610">slower →</text>
|
||||
<text class="nj-charts-rowlabel" x="8" y="62"><tspan class="nj-charts-code">json_fwd.hpp instead of json.hpp</tspan><tspan class="nj-charts-sub"> · header-only TU</tspan></text>
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<path class="nj-charts-clang" d="M362.6 70 H122.32 Q118.32 70 118.32 74 V78 Q118.32 82 122.32 82 H362.6 Z"><title>json_fwd.hpp instead of json.hpp, header-only TU (Apple clang): −62 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="80.5">−62 %</text>
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||||
@@ -46,15 +46,25 @@
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<text class="nj-charts-val" x="367.6" y="340.5">−5 %</text>
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<path class="nj-charts-gcc" d="M362.6 344 H331.14 Q327.14 344 327.14 348 V352 Q327.14 356 331.14 356 H362.6 Z"><title>JSON_HAS_FILESYSTEM=0, model TU (G​CC 16): −9 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="354.5">−9 %</text>
|
||||
<text class="nj-charts-rowlabel" x="8" y="374"><tspan class="nj-charts-code">JSON_NO_IO</tspan><tspan class="nj-charts-sub"> · model TU*</tspan></text>
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<path class="nj-charts-clang" d="M362.6 382 H358.72 Q354.72 382 354.72 386 V390 Q354.72 394 358.72 394 H362.6 Z"><title>JSON_NO_IO, model TU (Apple clang): −2 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="392.5">−2 %</text>
|
||||
<path class="nj-charts-gcc" d="M362.6 396 H358.72 Q354.72 396 354.72 400 V404 Q354.72 408 358.72 408 H362.6 Z"><title>JSON_NO_IO, model TU (G​CC 16): −2 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="406.5">−2 %</text>
|
||||
<text class="nj-charts-rowlabel" x="8" y="426"><tspan class="nj-charts-code">JSON_USE_GLOBAL_UDLS=0</tspan><tspan class="nj-charts-sub"> · model TU*</tspan></text>
|
||||
<path class="nj-charts-clang" d="M362.6 434 h0.5 v12 h-0.5 Z"><title>JSON_USE_GLOBAL_UDLS=0, model TU (Apple clang): 0 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="444.5">0 %</text>
|
||||
<path class="nj-charts-gcc" d="M362.6 448 H362.6 Q358.66 448 358.66 451.94 V456.06 Q358.66 460 362.6 460 H362.6 Z"><title>JSON_USE_GLOBAL_UDLS=0, model TU (G​CC 16): −1 %</title></path>
|
||||
<text class="nj-charts-rowlabel" x="8" y="374"><tspan class="nj-charts-code">JSON_HAS_STD_FORMAT=0</tspan><tspan class="nj-charts-sub"> · model TU</tspan></text>
|
||||
<path class="nj-charts-clang" d="M362.6 382 H362.6 Q358.66 382 358.66 385.94 V390.06 Q358.66 394 362.6 394 H362.6 Z"><title>JSON_HAS_STD_FORMAT=0, model TU (Apple clang): −1 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="392.5">−1 %</text>
|
||||
<path class="nj-charts-gcc" d="M362.6 396 H350.84 Q346.84 396 346.84 400 V404 Q346.84 408 350.84 408 H362.6 Z"><title>JSON_HAS_STD_FORMAT=0, model TU (G​CC 16): −4 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="406.5">−4 %</text>
|
||||
<text class="nj-charts-rowlabel" x="8" y="426"><tspan class="nj-charts-code">JSON_HAS_RANGES=0</tspan><tspan class="nj-charts-sub"> · model TU</tspan></text>
|
||||
<path class="nj-charts-clang" d="M362.6 434 H350.84 Q346.84 434 346.84 438 V442 Q346.84 446 350.84 446 H362.6 Z"><title>JSON_HAS_RANGES=0, model TU (Apple clang): −4 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="444.5">−4 %</text>
|
||||
<path class="nj-charts-gcc" d="M362.6 448 H362.6 Q358.66 448 358.66 451.94 V456.06 Q358.66 460 362.6 460 H362.6 Z"><title>JSON_HAS_RANGES=0, model TU (G​CC 16): −1 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="458.5">−1 %</text>
|
||||
<line class="nj-charts-zero" x1="362.6" y1="58" x2="362.6" y2="472"/>
|
||||
<text class="nj-charts-rowlabel" x="8" y="478"><tspan class="nj-charts-code">JSON_NO_IO</tspan><tspan class="nj-charts-sub"> · model TU*</tspan></text>
|
||||
<path class="nj-charts-clang" d="M362.6 486 H358.72 Q354.72 486 354.72 490 V494 Q354.72 498 358.72 498 H362.6 Z"><title>JSON_NO_IO, model TU (Apple clang): −2 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="496.5">−2 %</text>
|
||||
<path class="nj-charts-gcc" d="M362.6 500 H358.72 Q354.72 500 354.72 504 V508 Q354.72 512 358.72 512 H362.6 Z"><title>JSON_NO_IO, model TU (G​CC 16): −2 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="510.5">−2 %</text>
|
||||
<text class="nj-charts-rowlabel" x="8" y="530"><tspan class="nj-charts-code">JSON_USE_GLOBAL_UDLS=0</tspan><tspan class="nj-charts-sub"> · model TU*</tspan></text>
|
||||
<path class="nj-charts-clang" d="M362.6 538 h0.5 v12 h-0.5 Z"><title>JSON_USE_GLOBAL_UDLS=0, model TU (Apple clang): 0 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="548.5">0 %</text>
|
||||
<path class="nj-charts-gcc" d="M362.6 552 H362.6 Q358.66 552 358.66 555.94 V560.06 Q358.66 564 362.6 564 H362.6 Z"><title>JSON_USE_GLOBAL_UDLS=0, model TU (G​CC 16): −1 %</title></path>
|
||||
<text class="nj-charts-val" x="367.6" y="562.5">−1 %</text>
|
||||
<line class="nj-charts-zero" x1="362.6" y1="58" x2="362.6" y2="576"/>
|
||||
</svg>
|
||||
|
Before Width: | Height: | Size: 7.5 KiB After Width: | Height: | Size: 9.0 KiB |
@@ -11,6 +11,11 @@ that cost, ordered by how much they typically save.
|
||||
16.2.0 on macOS (Apple M1 Max). All configurations were run interleaved, in alternating order. They show the order
|
||||
of magnitude to expect; measure your own code before and after a change.
|
||||
|
||||
The two sections on `std::format` and ranges apply to C++20 only and were measured with `-std=c++20`, so their
|
||||
baselines are slower than the others. Their differences are mostly within the run-to-run noise of a few percent:
|
||||
two complete runs of the same configuration differed by up to 10% in the median. The tables show the later run (one
|
||||
outlier cell was repeated).
|
||||
|
||||
The translation units are listed below. They replace the ones used for earlier versions of this page, whose
|
||||
sources were not published, so the numbers differ from earlier versions.
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||||
|
||||
@@ -66,7 +71,8 @@ that cost, ordered by how much they typically save.
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||||
--8<-- "images/compile_times.svg"
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</div>
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||||
<p class="nj-charts-caption" markdown>Change in compile time at `-O0` against the default configuration of the same
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||||
translation unit, taken from the tables below (medians of nine runs, see above). * computed from the milliseconds in
|
||||
translation unit, taken from the tables below (medians of nine runs, see above; the `std::format` and ranges rows are
|
||||
measured with `-std=c++20`). * computed from the milliseconds in
|
||||
[Options without effect on compile times](#options-without-effect-on-compile-times).</p>
|
||||
|
||||
## Include `json_fwd.hpp` in headers
|
||||
@@ -214,6 +220,88 @@ from and to `std::string`; whether that fallback exists depends on the platform'
|
||||
(`std::wstring` on Windows), so check code that converts paths. `JSON_NO_IO` additionally removes all stream
|
||||
functions (see [Options without effect](#options-without-effect-on-compile-times)).
|
||||
|
||||
## Switch off `std::format` support
|
||||
|
||||
With C++20 and a standard library that provides `<format>`, the library includes it to provide a
|
||||
[`std::formatter<basic_json>`](../api/basic_json/std_formatter.md) specialization (see
|
||||
[`JSON_HAS_STD_FORMAT`](../api/macros/json_has_std_format.md)). If you do not format JSON values with `std::format`,
|
||||
switch the detection off for the whole project:
|
||||
|
||||
```cmake
|
||||
target_compile_definitions(my_target PRIVATE JSON_HAS_STD_FORMAT=0)
|
||||
```
|
||||
|
||||
| Compiler | Translation unit | Default (`-O0` / `-O2`) | `std::format` off (`-O0` / `-O2`) | Change |
|
||||
|-------------|------------------|------------------------:|----------------------------------:|-----------:|
|
||||
| Apple clang | model | 1283 ms / 1544 ms | 1269 ms / 1539 ms | −1% / 0% |
|
||||
| GCC 16 | model | 2026 ms / 2482 ms | 1944 ms / 2416 ms | −4% / −3% |
|
||||
| Apple clang | parsing | 1350 ms / 2278 ms | 1347 ms / 2300 ms | 0% / +1% |
|
||||
| GCC 16 | parsing | 2715 ms / 4351 ms | 2644 ms / 4287 ms | −3% / −1% |
|
||||
|
||||
<p class="nj-table-caption" markdown>Compile time with `JSON_HAS_STD_FORMAT=0` against the default configuration, both with `-std=c++20`, at `-O0` and `-O2`.</p>
|
||||
|
||||
The saving is small, and only GCC shows it: libstdc++'s `<format>` is an additional header that nothing else in the
|
||||
translation unit includes, so the macro removes it (and the headers it pulls in). With libc++ (Apple clang), other
|
||||
standard headers include `<format>` anyway, so the macro only removes the library's own formatter, and the difference is
|
||||
within the noise.
|
||||
|
||||
The trade-off: the `std::formatter<basic_json>` specialization is removed, so `#!cpp std::format("{}", j)`,
|
||||
and `#!cpp std::format_to` with a JSON value no longer compile (in a test with both compilers on
|
||||
macOS, the error was that `std::formatter` must be specialized for the type). Use [`dump()`](../api/basic_json/dump.md),
|
||||
or the stream operators, instead.
|
||||
|
||||
## Switch off ranges support
|
||||
|
||||
With C++20 and a standard library that provides `<ranges>`, the library includes it to declare the
|
||||
[`items()`](../api/basic_json/items.md) proxy a *borrowed range*, and uses the `std::ranges` concepts and algorithms for
|
||||
construction from range views and for iterator input (see [`JSON_HAS_RANGES`](../api/macros/json_has_ranges.md)). If you
|
||||
do not need these, switch the detection off for the whole project:
|
||||
|
||||
```cmake
|
||||
target_compile_definitions(my_target PRIVATE JSON_HAS_RANGES=0)
|
||||
```
|
||||
|
||||
| Compiler | Translation unit | Default (`-O0` / `-O2`) | Ranges off (`-O0` / `-O2`) | Change |
|
||||
|-------------|------------------|------------------------:|---------------------------:|-----------:|
|
||||
| Apple clang | model | 1283 ms / 1544 ms | 1237 ms / 1495 ms | −4% / −3% |
|
||||
| GCC 16 | model | 2026 ms / 2482 ms | 2004 ms / 2464 ms | −1% / −1% |
|
||||
| Apple clang | parsing | 1350 ms / 2278 ms | 1311 ms / 2284 ms | −3% / 0% |
|
||||
| GCC 16 | parsing | 2715 ms / 4351 ms | 2705 ms / 4427 ms | 0% / +2% |
|
||||
|
||||
<p class="nj-table-caption" markdown>Compile time with `JSON_HAS_RANGES=0` against the default configuration, both with `-std=c++20`, at `-O0` and `-O2`.</p>
|
||||
|
||||
The saving is small as well, and with GCC it is not distinguishable from the noise: its `<format>` includes `<span>`,
|
||||
so with `std::format` support left on, part of the ranges machinery is still parsed.
|
||||
Both macros together save more than either alone:
|
||||
|
||||
```cmake
|
||||
target_compile_definitions(my_target PRIVATE JSON_HAS_STD_FORMAT=0 JSON_HAS_RANGES=0)
|
||||
```
|
||||
|
||||
| Compiler | Translation unit | Default (`-O0` / `-O2`) | Both off (`-O0` / `-O2`) | Change |
|
||||
|-------------|------------------|------------------------:|-------------------------:|-----------:|
|
||||
| Apple clang | model | 1283 ms / 1544 ms | 1223 ms / 1504 ms | −5% / −3% |
|
||||
| GCC 16 | model | 2026 ms / 2482 ms | 1914 ms / 2378 ms | −6% / −4% |
|
||||
| Apple clang | parsing | 1350 ms / 2278 ms | 1311 ms / 2271 ms | −3% / 0% |
|
||||
| GCC 16 | parsing | 2715 ms / 4351 ms | 2635 ms / 4260 ms | −3% / −2% |
|
||||
|
||||
<p class="nj-table-caption" markdown>Compile time with `JSON_HAS_STD_FORMAT=0` and `JSON_HAS_RANGES=0` against the default configuration, both with `-std=c++20`, at `-O0` and `-O2`.</p>
|
||||
|
||||
The trade-offs of `JSON_HAS_RANGES=0` are larger than those of the other options on this page:
|
||||
|
||||
- `items()` is no longer a borrowed range. On a temporary, `#!cpp std::ranges::begin(make_json().items())` no longer
|
||||
compiles, and `std::ranges` algorithms such as `#!cpp std::ranges::find_if(make_json().items(), pred)` return
|
||||
`std::ranges::dangling` instead of an iterator, so code that uses the result no longer compiles. Calling them on
|
||||
`#!cpp j.items()` for a named `j` is not affected.
|
||||
- [`JSON_HAS_RANGE_VIEW_CONVERSION`](../api/macros/json_has_range_view_conversion.md) becomes `0`, so a JSON array can no
|
||||
longer be constructed from a `std::ranges::view` such as the result of `std::views::filter`. The assignment
|
||||
`#!cpp json j = v | std::views::filter(pred);` no longer compiles.
|
||||
- Parsing from an iterator pair loses the C++20 fast path for contiguous iterators that are not pointers (for
|
||||
example those of `std::vector`) and falls back to byte-at-a-time scanning. The result is the same; parsing may be
|
||||
slower (not measured here).
|
||||
|
||||
The first two were checked with both compilers on macOS; the third is read from the library's source.
|
||||
|
||||
## Use C++20 modules
|
||||
|
||||
With a toolchain that supports named modules, `import nlohmann.json;` compiles the library once into a module and
|
||||
@@ -343,6 +431,8 @@ and −2% / 0% (GCC). To get a saving, combine it with
|
||||
|
||||
- [`JSON_HAS_FILESYSTEM`](../api/macros/json_has_filesystem.md) - control the conversions from and to
|
||||
`std::filesystem::path`
|
||||
- [`JSON_HAS_STD_FORMAT`](../api/macros/json_has_std_format.md) - control the `std::formatter` specialization
|
||||
- [`JSON_HAS_RANGES`](../api/macros/json_has_ranges.md) - control the use of `std::ranges`
|
||||
- [`JSON_NO_AUTOMATIC_UDLS`](../api/macros/json_no_automatic_udls.md) - do not include the user-defined string
|
||||
literals automatically
|
||||
- [Modules](../features/modules.md) - C++20 module support
|
||||
|
||||
@@ -73,6 +73,8 @@ COMPILE = [
|
||||
("JSON_NO_AUTOMATIC_UDLS", "parsing TU", -1, -2, False),
|
||||
("JSON_HAS_FILESYSTEM=0 + JSON_NO_IO", "model TU", -8, -11, False),
|
||||
("JSON_HAS_FILESYSTEM=0", "model TU", -5, -9, False),
|
||||
("JSON_HAS_STD_FORMAT=0", "model TU", -1, -4, False),
|
||||
("JSON_HAS_RANGES=0", "model TU", -4, -1, False),
|
||||
# computed from the ms in "Options without effect on compile times": 1034/1051, 1756/1801, 1055/1051, 1783/1801
|
||||
("JSON_NO_IO", "model TU", round((1034 / 1051 - 1) * 100), round((1756 / 1801 - 1) * 100), True),
|
||||
("JSON_USE_GLOBAL_UDLS=0", "model TU", round((1055 / 1051 - 1) * 100), round((1783 / 1801 - 1) * 100), True),
|
||||
|
||||
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Block a user