package combine;

import java.util.Random;
/**
 * csapp優化程式效能從不同角度
 * @author Administrator
 *
 */
public class Combine {
	static double combine1(Data data) {//初始版本
		data.sum[0]=1;
		for (int i = 0; i < data.a.length; i++) {
			double val;
			val = data.getElement(data, i);
			data.sum[0]=data.sum[0]+val;
		}
		return 0;
	}
	static double combine2(Data data) {//用l替代data.a.length
		data.sum[1]=1;
		int l = data.a.length;
		for (int i = 0; i < l; i++) {
			double val;
			val = data.getElement(data, i);
			data.sum[1]=data.sum[1]+val;
		}
		return 0;
	}
	static double combine3(Data data) {//用double a[]儲存data.a
		data.sum[2]=1;
		int l = data.a.length;
		double a[] = data.a;
		for (int i = 0; i < l; i++) {
			data.sum[2]=data.sum[2]+a[i];
		}
		return 0;
	}
	static double combine4(Data data) {//data.sum[0]替換成s
		data.sum[3]=1;
		int l = data.a.length;
		double a[] = data.a;
		double s = 1;
		for (int i = 0; i < l; i++) {
			s+=a[i];
		}
		data.sum[3] = s;
		return 0;
	}
	static double combine5(Data data) {//迴圈展開
		int l = data.a.length;
		int limit = l-1;
		double a[] = data.a;
		double s = 1;
		int i = 0;
		for (; i < limit; i+=2) {
			s=(s+a[i])+a[i+1];
		}
		for (; i < l; i++) {
			s=s+a[i];
		}
		data.sum[4] = s;
		return 0;
	}
	static double combine6(Data data) {//多個累計變數
		int l = data.a.length;
		int limit = l-1;
		double a[] = data.a;
		double s1 = 1;
		double s2 = 0;
		int i = 0;
		for (; i < limit; i+=2) {
			s1 = s1 + a[i];
			s2 = s2 + a[i+1];
		}
		for (; i < l; i++) {
			s1=s1+a[i];
		}
		data.sum[5] = s1+s2;
		return 0;
	}
	static double combine7(Data data) {//重新結合變換
		double d[]=data.a;
		int l = d.length;
		int limit = l-1;
		double s=1;
		int i = 0;
		for (; i < limit; i+=2) {
			s=s+(d[i]+d[i+1]);//d[i]+d[i+1]和s累加可並行
		}
		for (; i < l; i++) {
			s+=d[i];
		}
		data.sum[6]=s;
		return s;
	}
	public static void main(String[] args) {
		int n = 300000000;
		Data data = new Data(n);
		CalTime.start();
		combine1(data);
		long t1 = CalTime.end();
		CalTime.start();
		combine2(data);
		long t2 = CalTime.end();
		CalTime.start();
		combine3(data);
		long t3 = CalTime.end();
		CalTime.start();
		combine4(data);
		long t4 = CalTime.end();
		CalTime.start();
		combine5(data);
		long t5 = CalTime.end();
		CalTime.start();
		combine6(data);
		long t6 = CalTime.end();
		CalTime.start();
		combine7(data);
		long t7 = CalTime.end();
		for (int i = 0; i < data.sum.length; i++) {
			System.out.println(data.sum[i]);
		}
		System.out.println(t1+" "+t2+" "+t3+" "+t4+" "+t5+" "+t6+" "+t7);
	}

}
 

儲存隨機數的Data類：

package combine;

import java.util.Random;

public class Data {
	
	double a[];
	double sum[];
	
	public Data(int n) {
		Random random = new Random(System.currentTimeMillis());
		a = new double[n];
		 sum = new double[7];
		 for (int i = 0; i < n; i++) {
				a[i] = random.nextInt(n);
			}
	}
	double getElement(Data data,int index){
		if(index<0||index>=data.a.length){
			return 0;
		}
		return data.a[index];
	}
	
}
 

package combine;

public class CalTime {
	private static long timeStart;
	public static void start() {
		timeStart = System.currentTimeMillis();
	}
	public static long end(){
		return System.currentTimeMillis()-timeStart;
	}
}

290 288 284 277 276 198 194
289 284 282 275 277 195 192

289 284 282 281 277 194 191

可以看到加速比在n=300000000下才在1.51這樣。。。。

書上用c可以從combine1的10CPE(每元素週期)下降到combine7的0.8CPE，

不懂哪裡不對。。