六、在Android中调用汇编代码

AndroidARM

C可以调用汇编代码，而Android则通过JNI调用C代码。
所以，可以通过写汇编代码和C，用ndk-build编译成so，让Android调用。

1、建立hello-jni项目

直接使用 \android-ndk-r10d\samples\hello-jni 项目。
将jni目录删除，用Android Studio初始化hello-jni。
代码修改为：

/*
 * Copyright (C) 2009 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.example.hellojni;
import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;
public class HelloJni extends Activity {
    /* this is used to load the 'hello-jni' library on application
     * startup. The library has already been unpacked into
     * /data/data/com.example.hellojni/lib/libhello-jni.so at
     * installation time by the package manager.
     */
    static {
        System.loadLibrary("hello-jni");
    }
    /**
     * Called when the activity is first created.
     */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        /* Create a TextView and set its content.
         * the text is retrieved by calling a native
         * function.
         */
        TextView tv = new TextView(this);
        tv.setText(stringFromJNI() + factorialJNI(13));
        setContentView(tv);
    }
    /* A native method that is implemented by the
     * 'hello-jni' native library, which is packaged
     * with this application.
     */
    public native String stringFromJNI();
    /* This is another native method declaration that is *not*
     * implemented by 'hello-jni'. This is simply to show that
     * you can declare as many native methods in your Java code
     * as you want, their implementation is searched in the
     * currently loaded native libraries only the first time
     * you call them.
     *
     * Trying to call this function will result in a
     * java.lang.UnsatisfiedLinkError exception !
     */
    public native String unimplementedStringFromJNI();
    /**
     * Multiply the number by 10.
     *
     * @param input, the number to be multiplied
     * @return the multiple of the number
     */
    public native int factorialJNI(int input);
}

2、编译 so

• jni/multiple.s
  汇编代码，作用是将输入的数字乘以10，并返回结果。

    @ This file is jni/multiple.s
    .text
    .align  2
    .global armFunction
    .type   armFunction, %function
armFunction:
    @ Multiply by 10. Input value and return value in r0
    stmfd   sp!, {fp,ip,lr}
    mov r3, r0, asl #3  @ r3=r0<<3=r0*8
    add r0, r3, r0, asl #1  @ r0=r3+r0<<1=r0*8+r0*2=r0*10
    ldmfd   sp!, {fp,ip,lr}
    bx  lr
    .size   armFunction, .-armFunction

• jni/hello.c
  根据\android-ndk-r10d\samples\hello-jni\jni\hello.c修改，65-71 行是调用汇编函数的代码。

/*
 * Copyright (C) 2009 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 */
#include <string.h>
#include <jni.h>
/* This is a trivial JNI example where we use a native method
 * to return a new VM String. See the corresponding Java source
 * file located at:
 *
 *   apps/samples/hello-jni/project/src/com/example/hellojni/HelloJni.java
 */
jstring
Java_com_example_hellojni_HelloJni_stringFromJNI( JNIEnv* env,
                                                  jobject thiz )
{
#if defined(__arm__)
  #if defined(__ARM_ARCH_7A__)
    #if defined(__ARM_NEON__)
      #if defined(__ARM_PCS_VFP)
        #define ABI "armeabi-v7a/NEON (hard-float)"
      #else
        #define ABI "armeabi-v7a/NEON"
      #endif
    #else
      #if defined(__ARM_PCS_VFP)
        #define ABI "armeabi-v7a (hard-float)"
      #else
        #define ABI "armeabi-v7a"
      #endif
    #endif
  #else
   #define ABI "armeabi"
  #endif
#elif defined(__i386__)
   #define ABI "x86"
#elif defined(__x86_64__)
   #define ABI "x86_64"
#elif defined(__mips64)  /* mips64el-* toolchain defines __mips__ too */
   #define ABI "mips64"
#elif defined(__mips__)
   #define ABI "mips"
#elif defined(__aarch64__)
   #define ABI "arm64-v8a"
#else
   #define ABI "unknown"
#endif
    return (*env)->NewStringUTF(env, "Hello from JNI !  Compiled with ABI " ABI ".");
}
/* This stub calls the function. It helps to have a stub like this to
 * save yourself the hassle of defining the function call in
 * Assembly. */
jint Java_com_example_hellojni_HelloJni_factorialJNI(
    JNIEnv* env, jobject object, jint input) {
    /* Try calling some local code */
    return armFunction(input);
}

• jni/Android.mk
  根据\android-ndk-r10d\samples\hello-jni\jni\Android.mk修改

# Copyright (C) 2009 The Android Open Source Project
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#      http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
# I want ARM, not thumb.
LOCAL_ARM_MODE := arm
# Name of the local module
LOCAL_MODULE    := hello-jni
# The files that make up the source code
LOCAL_SRC_FILES := hello-jni.c multiple.s
include $(BUILD_SHARED_LIBRARY)

Android.mk
hello-jni.c
multiple.s
准备好这3个文件后，就可以使用ndk-build来编译so了。
其中libs就是包含so的目录，将这个目录，拷贝给Android项目使用即可。

$ ndk-build
[armeabi] Compile arm    : hello-jni <= multiple.s
[armeabi] SharedLibrary  : libhello-jni.so
[armeabi] Install        : libhello-jni.so => libs/armeabi/libhello-jni.so

代码打包地址：http://yunpan.cn/ccaxuNLVH5c9R 访问密码 cff9

参考：
http://www.eggwall.com/2011/09/android-arm-assembly-calling-assembly.html