How to use ARM Assembly code in an Android project?

Question

I'm not particularly experienced with Assembly and ARM, but I was able to write a few routines in it and I'd like to see how they run on an ARM-equipped Android device (Nexus S). What is the procedure for including an Assembly code file into an Android project? Can I only call it from native code, or from Java also?

Answer 1

You can call assembly from Android using the Java Native Interface and the Android NDK.

Cedric mentions using the asm keyword, while I prefer to include assembly source code. I have posted a tutorial to do this at my site: http://www.eggwall.com/2011/09/android-arm-assembly-calling-assembly.html

You can download the source code for my example and see how it works. Once you see a functioning example, it is easy to modify it to your needs.

Answer 2

Minimal example with inline and separate source file

Some care has to be taken to not compile the raw assembly under the wrong arch. Here we use:

• #ifdefs on C files
• ifeqs on Android.mk

This example on GitHub. Tested on Ubuntu 16.04, Android NDK 12, Sony Xperia Z3 D6643 (ARMv7) with Android 5.1.1.

jni/main.c

#include <stdio.h>

#include <jni.h>

#ifdef __arm__
int asm_main(void);
#endif

jstring Java_com_cirosantilli_android_1cheat_ndk_1asm_Main_jniMethod(
        JNIEnv* env, jobject thiz) {
    enum Constexpr { N = 256 };
    char s[N];
    size_t cur = 0;

    int x = 0;
#ifdef __arm__
    cur += snprintf(s + cur, N - cur, "arm ");
    /* Inline test. Increment x by 1. */
    asm (
        "add %0, #1"
        : "=r" (x)
        : "0" (x)
    );
    /* Separate source test. Increment x by 1. */
    x += asm_main();
#endif
    if (x == 2)
        cur += snprintf(s + cur, N - cur, "%s", "0");
    else
        cur += snprintf(s + cur, N - cur, "%s", "1");

    return (*env)->NewStringUTF(env, s);
}

jni/main_asm.S

.text
/* Function that just returns 1. */
.global asm_main
asm_main:
    mov r0, #1
    bx lr

jni/Android.mk

LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := main
LOCAL_SRC_FILES := main.c
# http://stackoverflow.com/questions/12614417/android-ndk-how-to-get-compiler-architecture-in-android-mk-dynamically
ifneq (,$(filter $(TARGET_ARCH_ABI),armeabi armeabi-v7a))
    LOCAL_SRC_FILES := $(LOCAL_SRC_FILES) main_asm.S
endif
include $(BUILD_SHARED_LIBRARY)

com/cirosantilli/android_cheat/ndk_asm/Main.java

package com.cirosantilli.android_cheat.ndk_asm;

import android.app.Activity;
import android.widget.TextView;
import android.os.Bundle;

public class Main extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        TextView tv = new TextView(this);
        tv.setText(jniMethod());
        setContentView(tv);
    }
    public native String jniMethod();
    static {
        System.loadLibrary("main");
    }
}