问题描述
Java 编译器或 JIT 编译器是否将除法或乘法优化为 2 的恒定幂以进行位移?
Does the Java compiler or the JIT compiler optimize divisions or multiplications by a constant power of two down to bitshifting?
例如，以下两个语句是否优化为相同?
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，因为它无法确定 (end - start) 的标志是什么 将.为什么这很重要?负整数的位移产生与普通除法不同的结果.在这里你可以看到一个演示: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
另外请注意，我使用 >> 而不是 >>>.>>> 是无符号位移，而 >> 是有符号的.
Also note that I used >> instead of >>>. A >>> is an unsigned bitshift, while >> is signed.
System.out.println((-10) >>> 1); // prints 2147483643
@Mystical:我写了一个基准测试，表明编译器/JVM 没有进行优化:https://ideone.com/aKDShA
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