cs app深入理解计算机系统：第五章 优化程序性能 几个优化的java实现

package combine;import java.util.Random;/*** csapp优化程序性能从不同角度* @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.lengthdata.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[]存储data.adata.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]替换成sdata.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) {//循环展开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) {//多个累计变量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) {//重新结合变换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累加可并行}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);}}

存储随机数的Data类：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;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];}}计算时间类：package combine;public class CalTime {private static long timeStart;public static void start() {timeStart = System.currentTimeMillis();}public static long end(){return System.currentTimeMillis()-timeStart;}}多次运行得到运行时间：t1~t7:290 288 284 277 276 198 194289 284 282 275 277 195 192289 284 282 281 277 194 191可以看到加速比在n=300000000下才在1.51这样。。。。书上用c可以从combine1的10CPE(每元素周期)下降到combine7的0.8CPE，不懂哪里不对。。