ndk-samples/orderfile/README.md

Order file demo

Order files are text files containing symbols representing functions names. Linkers (lld) uses order files to layout functions in a specific order. These binaries with ordered symbols will reduce page faults and improve a program's launch time due to the efficient loading of symbols during a program’s cold-start.

Files

• app/src/main/cpp/orderfile.cpp: The source code for the orderfile library that is used by the Kotlin app.
• app/src/main/cpp/CMakeLists.txt: The CMakeLists either sets the orderfile library as generating profiles or loading the orderfile.
• app/src/main/java/MainActivity.kt: The Kotlin app source code.

Profile Steps

1. For simplicity, we have setup the CMakeLists.txt and you just need make sure set(GENERATE_PROFILES ON) is not commented. You need to pass any optimization flag except -O0. The mapping file is not generated and the profile instrumentation does not work without an optimization flag.
2. Run the app on Android Studio. You can either run it on a physical or virtual device. You will see "Hello World" on the screen.
3. To pull the data from the device, you'll need to move it from an app-writable directory to a shell readable directory for adb pull. We also need to transfer the output into hexadecimal format.

adb shell "run-as com.example.orderfiledemo sh -c 'cat /data/user/0/com.example.orderfiledemo/cache/demo.output.order' | cat > /data/local/tmp/demo.output.order"
adb pull /data/local/tmp/demo.output.order .

# Convert to hexdeciaml format on Linux, Mac, or ChromeOS
hexdump -C demo.output.order > demo.prof

# Convert to hexdecimal format on Windows
certutil -f -encodeHex demo.output.order demo.prof

4. Once you get both mapping file and profile file, you can use this script to create the order file:

python3 create_orderfile.py --profile-file demo.prof --mapping-file mapping.txt --output app/src/main/cpp/demo.orderfile

Load Steps

1. For load, you need to uncomment set(USE_PROFILE "${CMAKE_SOURCE_DIR}/demo.orderfile") and make sure set(GENERATE_PROFILES ON) is commented.

2. If you want to validate the shared library's layout is different, you need to find liborderfiledemo.so and run nm

nm -n liborderfiledemo.so

Difference between Java and Kotlin App

The main difference between a Java app and a Kotlin app is the syntax. You can easily change this Kotlin example into a Java example.

• Load Library

# Kotlin
companion object {
    init {
        System.loadLibrary("orderfiledemo")
    }
}

# Java
static {
    System.loadLibrary("orderfiledemo");
}

• Recognize an external method

# Kotlin
external fun runWorkload(tempDir: String)

# Java
private native void runWorkload(String tempDir);

• Get the cache directory

# Kotlin
runWorkload(applicationContext.cacheDir.toString())

# Java
runWorkload(getcacheDir().toString())