灵感来自这个问题：
Why is it faster to process a sorted array than an unsorted array?
Why is it faster to process a sorted array than an unsorted array?
我写了自己的分支预测实验：
public class BranchPrediction {
public static void main(final String[] args) {
long start;
long sum = 0;
/* No branch */
start = System.nanoTime();
sum = 0;
for (long i = 0; i < 10000000000L; ++i)
sum += i;
System.out.println(System.nanoTime() - start);
System.out.println(sum);
/* With branch */
start = System.nanoTime();
sum = 0;
for (long i = 0; i < 10000000000L; ++i)
if (i >= 0)
sum += i;
System.out.println(System.nanoTime() - start);
System.out.println(sum);
/* No branch (again) */
start = System.nanoTime();
sum = 0;
for (long i = 0; i < 10000000000L; ++i)
sum += i;
System.out.println(System.nanoTime() - start);
System.out.println(sum);
/* With branch (again) */
start = System.nanoTime();
sum = 0;
for (long i = 0; i < 10000000000L; ++i)
if (i >= 0)
sum += i;
System.out.println(System.nanoTime() - start);
System.out.println(sum);
}
}
结果让我感到困惑：根据程序输出,带分支的循环比没有分支循环的循环可靠得快.
输出示例：
7949691477 -5340232226128654848 6947699555 -5340232226128654848 7920972795 -5340232226128654848 7055459799 -5340232226128654848
为什么会这样？
编辑：
>反汇编类显示Java编译器没有优化(遗漏)任何东西(https://gist.github.com/HouzuoGuo/5692424)
> Why is it faster to process a sorted array than an unsorted array?作者使用的Java基准技术与我的相同.
>该机器是Intel核心i7,运行Linux 3.2 64位和Oracle JVM 1.7 64位
>当我超过循环迭代的次数时,with-branch循环比非分支循环运行多SECONDS.
