今天我在java中遇到了一个相当惊人的行为，or比and慢!
我什至做了一个测试用例，您可以在下面看到它。现在我想知道为什么会这样？
我做错了什么还是只发生在我的电脑上？
我看不出有任何理由 or 应该比 and 慢，特别是有这个显着差异。我想用其他一些语言来测试这个现象，你对这个有什么一般性的想法吗？
public class TestClass {
public static void main(String[] args) {
long[] or = new long[10];
long[] and = new long[10];
long lStartTime, lEndTime, difference = 0;
for (int idx = 0; idx < 10; idx++) {
lStartTime = System.nanoTime();
for (int i= 0; i < 1000000000; i++) {
int j = i | i+1 ;
}
lEndTime = System.nanoTime();
difference = lEndTime - lStartTime;
System.out.println("Elapsed milliseconds: " + difference/1000000);
or[idx] = difference;
lStartTime = System.nanoTime();
for (int i= 0; i < 1000000000; i++) {
int j = i & i+1 ;
}
lEndTime = System.nanoTime();
difference = lEndTime - lStartTime;
System.out.println("Elapsed milliseconds: " + difference/1000000);
and[idx] = difference;
System.out.println("------------------------------------" );
}
long tmp = 0;
for (long l : or) {
tmp += l;
}
tmp /= 10;
System.out.println("Elapsed milliseconds for or: " + tmp/1000000);
tmp = 0;
for (long l : and) {
tmp += l;
}
tmp /= 10;
System.out.println("Elapsed milliseconds for and: " + tmp/1000000);
}
}
结果:
Elapsed milliseconds: 1600 Elapsed milliseconds: 1332 ------------------------------------ Elapsed milliseconds: 1609 Elapsed milliseconds: 1335 ------------------------------------ Elapsed milliseconds: 1609 Elapsed milliseconds: 1335 ------------------------------------ Elapsed milliseconds: 1542 Elapsed milliseconds: 1314 ------------------------------------ Elapsed milliseconds: 1705 Elapsed milliseconds: 1324 ------------------------------------ Elapsed milliseconds: 1559 Elapsed milliseconds: 1315 ------------------------------------ Elapsed milliseconds: 1526 Elapsed milliseconds: 1314 ------------------------------------ Elapsed milliseconds: 1568 Elapsed milliseconds: 1340 ------------------------------------ Elapsed milliseconds: 1551 Elapsed milliseconds: 1318 ------------------------------------ Elapsed milliseconds: 1574 Elapsed milliseconds: 1321 ------------------------------------ Elapsed milliseconds for or: 1584 Elapsed milliseconds for and: 1325
最佳答案
Bit-Wise OR 并不比 Bit-Wise AND 慢!
在代码中的时间测量片段之间交换，你会得到相反的结果:
for (int idx = 0; idx < 10; idx++) {
lStartTime = System.nanoTime();
for (int i= 0; i < 1000000000; i++) {
int j = i & i+1 ;
}
lEndTime = System.nanoTime();
difference = lEndTime - lStartTime;
System.out.println("Elapsed milliseconds: " + difference/1000000);
and[idx] = difference;
lStartTime = System.nanoTime();
for (int i= 0; i < 1000000000; i++) {
int j = i | i+1 ;
}
lEndTime = System.nanoTime();
difference = lEndTime - lStartTime;
System.out.println("Elapsed milliseconds: " + difference/1000000);
or[idx] = difference;
System.out.println("------------------------------------");
}
我倾向于猜测 JVM 在第二个片段中应用了某种运行时优化。
关于java - 为什么 "or"在 Java 中比 "and"慢？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/22575304/