Avoid unnecessary AttributedString copies on iOS

[dheerajkoppu]

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

Benchmark code

#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);
  }
}

Changelog:

[IOS] [CHANGED] - Pass AttributedString by const reference in RCTTextLayoutManager and add a move constructor for AttributedStringBox.

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.