Emit C++ TraceSection markers through ATrace on Android

[tomekzaw]

Summary:

TraceSection (ReactCommon/cxxreact/TraceSection.h) is the C++ tracing primitive used throughout the renderer: ShadowTree::commit, UIManager::createNode, the Yoga measure callbacks, FabricMountingManager::executeMount and others. On Android it only does something when built with WITH_PERFETTO or WITH_FBSYSTRACE, and the OSS build sets neither, so it compiles to DummyTraceSection.

Current behavior. Perfetto and systrace captures of OSS Android apps show only the Java/Kotlin sections. The C++ half of the renderer (commit, layout, text measurement, building mount instructions) is invisible.

With this change. A new Android branch calls the NDK's ATrace_beginSection and ATrace_endSection:

• It includes <android/trace.h> and calls the NDK functions directly. They exist from API 23, and React Native's minSdk is 24. libreactnative already links libandroid, and libhermestooling gets it through libreactnative, so no build changes are needed.
• Each section checks ATrace_isEnabled() first, so when nobody is tracing it costs one call.
• WITH_PERFETTO and WITH_FBSYSTRACE keep precedence.
• Section arguments (key/value pairs) are ignored for now.

Why. Without this, profiling React Native on Android with standard tools can't show where commit and layout time goes. In a capture of RNTester mounting a 100-post feed, this branch shows 7084 UIManager::createNode sections and 1632 Yoga measure-callback sections, among others; on main none of them appear.

Changelog:

[ANDROID] [ADDED] - C++ TraceSection markers now appear in Perfetto and systrace captures

Test Plan:

• RNTester assembleDebug and assembleRelease (-PreactNativeArchitectures=arm64-v8a) and yarn format-check-cpp pass.
• Recorded Perfetto traces (atrace_apps: "com.facebook.react.uiapp") of the same RNTester scenario on main and on this branch. The C++ sections appear only on this branch.
• Rebuilt after switching to direct calls: libreactnative.so imports the three ATrace_* functions from libandroid.so, and a Perfetto capture of an RNTester release launch on a physical device shows the C++ sections (UIManager::createNode, ShadowTree::commit, FabricMountingManager::executeMount, ...).
• Overhead with tracing off (measured on the first revision, which looked the functions up with dlsym; direct calls do the same work minus the null checks). Emulator (Pixel 10 Pro AVD, API 37), release builds, current main vs. this branch, 6 runs, medians:

scenario	total CPU, main	total CPU, this PR	change
mount 100-post feed	128.8 ms	130.0 ms	+1%
mount 1000 labels	117.7 ms	117.1 ms	−1%

Update phases were within emulator noise, with mixed signs.

🤖 Generated with Claude Code

[meta-codesync]

@Abbondanzo has imported this pull request. If you are a Meta employee, you can view this in D125730780.

[Abbondanzo]

<android/trace.h> should have these

[tomekzaw]

Thanks! Applied all three suggestions in 5c5202e.

A note on linking: libreactnative already links libandroid, and libhermestooling picks it up through libreactnative, so calling ATrace directly needs no CMake changes. RNTester debug and release builds pass, and the C++ sections still show up in a Perfetto capture.