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Avoid unnecessary AttributedString copies on iOS (#59018)
Summary:
Measuring `<Text>` on iOS makes a bunch of unnecessary copies of `AttributedString`.
There are a few places where this happens:
- `ParagraphShadowNode` copies a local string into `AttributedStringBox` even though the local is about to be destroyed.
- `TextLayoutManager::measure` uses `auto` instead of a reference, which makes another copy.
- `ensurePlaceholderIfEmpty_DO_NOT_USE` copies the string even when it's not empty.
- The `RCTTextLayoutManager` methods take the string by value, which also copies it.
This PR avoids those extra copies by using `std::move`, passing references where possible, and only creating a placeholder when the string is actually empty. Nothing changes in how text is rendered.
With these changes, a cache hit goes from **5 copies to 2**, and a cache miss goes from **11 to 6**. Draw, hit-testing, and frame updates go from **2 copies to 0**. The remaining copies mostly come from cache keys, which I left alone.
It's a pretty small cleanup, but it removes some unnecessary work.
I also ran a quick benchmark on an M4 Pro using clang `-O2`, taking the median of 7 runs:
| Fragments | Deep copy | Box creation (before → after) | measure() entry (before → after) |
|---|---|---|---|
| 1 | 41 ns | 87 → 52 ns | 77 → 1 ns |
| 3 | 76 ns | 160 → 89 ns | 150 → 1 ns |
| 10 | 211 ns | 436 → 227 ns | 427 → 1 ns |
<details>
<summary>Benchmark code</summary>
```cpp
#include <react/renderer/attributedstring/AttributedString.h>
#include <react/renderer/attributedstring/AttributedStringBox.h>
#include <react/renderer/attributedstring/PlaceholderAttributedString.h>
#include <algorithm>
#include <chrono>
#include <cstdio>
#include <memory>
#include <optional>
#include <vector>
using namespace facebook::react;
static AttributedString makeString(int fragmentCount) {
AttributedString attributedString;
auto owner = std::make_shared<int>(0);
for (int i = 0; i < fragmentCount; i++) {
AttributedString::Fragment fragment;
fragment.string = "This is a typical row label in a list";
fragment.textAttributes.fontSize = 16;
fragment.parentShadowView.tag = i + 1;
// Aliasing shared_ptrs: real refcount traffic without constructing Props/EventEmitter/State.
fragment.parentShadowView.props = std::shared_ptr<const Props>(owner, nullptr);
fragment.parentShadowView.eventEmitter = std::shared_ptr<EventEmitter>(owner, nullptr);
fragment.parentShadowView.state = std::shared_ptr<const State>(owner, nullptr);
attributedString.appendFragment(std::move(fragment));
}
return attributedString;
}
template <typename F>
static double nsPerIteration(F&& body, int iterations) {
std::vector<double> samples;
for (int run = 0; run < 7; run++) {
auto start = std::chrono::steady_clock::now();
for (int i = 0; i < iterations; i++) body();
auto end = std::chrono::steady_clock::now();
samples.push_back(std::chrono::duration<double, std::nano>(end - start).count() / iterations);
}
std::sort(samples.begin(), samples.end());
return samples[3];
}
#define ESCAPE(x) asm volatile("" : : "r"(&(x)) : "memory")
int main() {
const int iterations = 200000;
std::printf("| Fragments | Deep copy | Box from lvalue (before) | Box from rvalue (after) | measure() entry before | measure() entry after |\n");
std::printf("|---|---|---|---|---|---|\n");
for (int n : {1, 3, 10}) {
const auto source = makeString(n);
AttributedStringBox box{source};
double copy = nsPerIteration([&] { AttributedString c = source; ESCAPE(c); }, iterations);
// The three ParagraphShadowNode sites: a dying local handed to the box.
double boxBefore = nsPerIteration([&] { auto local = source; AttributedStringBox b{local}; ESCAPE(b); }, iterations);
double boxAfter = nsPerIteration([&] { auto local = source; AttributedStringBox b{std::move(local)}; ESCAPE(b); }, iterations);
// The iOS TextLayoutManager::measure entry, before and after.
double entryBefore = nsPerIteration([&] {
auto original = box.getValue();
auto attributedString = ensurePlaceholderIfEmpty_DO_NOT_USE(original);
ESCAPE(attributedString);
}, iterations);
double entryAfter = nsPerIteration([&] {
const auto& original = box.getValue();
auto placeholder = original.isEmpty() ? std::optional<AttributedString>{ensurePlaceholderIfEmpty_DO_NOT_USE(original)} : std::nullopt;
const auto& attributedString = placeholder ? *placeholder : original;
ESCAPE(attributedString);
}, iterations);
std::printf("| %d | %.0f ns | %.0f ns | %.0f ns | %.0f ns | %.0f ns |\n", n, copy, boxBefore, boxAfter, entryBefore, entryAfter);
}
}
```
</details>
## Changelog:
[IOS] [CHANGED] - Pass `AttributedString` by const reference in `RCTTextLayoutManager` and add a move constructor for `AttributedStringBox`.
Pull Request resolved: #59018
Test Plan:
- Added `testValueMoveConstructor` to `AttributedStringBoxTest`.
- `yarn fantom Text`: all 223 tests pass.
- RNTester builds and runs on the iPhone 18 Pro simulator, and all 172 `RNTesterUnitTests` pass.
- `yarn format-check-cpp` passes.
- Updated API snapshots.
- Haven't benchmarked this on a physical device yet.
Reviewed By: cipolleschi
Differential Revision: D124965503
Pulled By: javache
fbshipit-source-id: e7e0add9aeb1dc90768ad852b0446f9804bcc83c
Copy file name to clipboardExpand all lines: packages/react-native/ReactCommon/react/renderer/textlayoutmanager/platform/ios/react/renderer/textlayoutmanager/RCTTextLayoutManager.h
+8-8Lines changed: 8 additions & 8 deletions
Original file line number
Diff line number
Diff line change
@@ -29,7 +29,7 @@ using RCTTextLayoutFragmentEnumerationBlock =
Copy file name to clipboardExpand all lines: packages/react-native/ReactCommon/react/renderer/textlayoutmanager/platform/ios/react/renderer/textlayoutmanager/RCTTextLayoutManager.mm
Copy file name to clipboardExpand all lines: packages/react-native/ReactCommon/react/renderer/textlayoutmanager/platform/ios/react/renderer/textlayoutmanager/TextLayoutManager.mm
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