Java是否通过两个幂的乘法优化除法？ [英] Does Java optimize division by powers of two to bitshifting?

问题描述

Java编译器或的JIT编译器是否以2的常数乘以bithifting来优化除法或乘法？

Does the Java compiler or the JIT compiler optimize divisions or multiplications by a constant power of two down to bitshifting?

是以下两个优化为相同的语句？

For example, are the following two statements optimized to be the same?

int median = start + (end - start) >>> 1;
int median = start + (end - start) / 2;

_{（基本上这个问题但是对于Java）}

_{(basically this question but for Java)}

推荐答案

不，Java编译器不这样做，因为它不能确定 ）将会。为什么这么重要？负整数上的位移产生与普通除法不同的结果。在这里您可以看到演示：这个简单的测试：

No, the Java compiler doesn't do that, because it can't be sure on what the sign of (end - start) will be. Why does this matter? Bit shifts on negative integers yield a different result than an ordinary division. Here you can see a demo: this simple test:

System.out.println((-10) >> 1);  // prints -5
System.out.println((-11) >> 1);  // prints -6
System.out.println((-11) / 2);   // prints -5

还要注意，我使用>> 而不是>>> 。 >>> 是无符号位移，而>>

Also note that I used >> instead of >>>. A >>> is an unsigned bitshift, while >> is signed.

System.out.println((-10) >>> 1); // prints 2147483643

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@ Mystical：基准，表明编译器/ JVM不进行优化： https://ideone.com/aKDShA

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