问题 一些运行/ JIT的故障后Java循环变慢了？

看起来Java是将值添加到变量的方式 r。 我做了一些改动，添加了方法 run2()：

public class TestPerformanceOfStaticVsDynamicCalls {
    private static final long RUNS = 1_000_000_000L;
    public static void main(String[] args) {
        System.out.println("Test run 1 =================================");
        new TestPerformanceOfStaticVsDynamicCalls().run();
        System.out.println("Test run 2 =================================");
        new TestPerformanceOfStaticVsDynamicCalls().run2();
    }
    private void run2() {
        long r = 0;
        long start, end;
        for (int loop = 0; loop < 10; loop++) {
            // Benchmark
            long stat = 0;
            start = System.currentTimeMillis();
            for (long i = 0; i < RUNS; i++) {
                stat += addStatic(1, i);
            }
            end = System.currentTimeMillis();
            System.out.println("Static: " + (end - start) + " ms");
            long dyna = 0;
            start = System.currentTimeMillis();
            for (long i = 0; i < RUNS; i++) {
                dyna += addDynamic(1, i);
            }
            end = System.currentTimeMillis();
            System.out.println("Dynamic: " + (end - start) + " ms");
            // If you really want to have values in "r" then...
            r += stat + dyna;
            // Do something with r to keep compiler happy
            System.out.println(r);
        }
    }
    private void run() {
        long r = 0;
        long start, end;
        for (int loop = 0; loop < 10; loop++) {
            // Benchmark
            start = System.currentTimeMillis();
            for (long i = 0; i < RUNS; i++) {
                r += addStatic(1, i);
            }
            end = System.currentTimeMillis();
            System.out.println("Static: " + (end - start) + " ms");
            start = System.currentTimeMillis();
            for (long i = 0; i < RUNS; i++) {
                r += addDynamic(1, i);
            }
            end = System.currentTimeMillis();
            System.out.println("Dynamic: " + (end - start) + " ms");
            // If you really want to have values in "r" then...
            // Do something with r to keep compiler happy
            System.out.println(r);
        }
    }
    private long addDynamic(long a, long b) {
        return a + b;
    }
    private static long addStatic(long a, long b) {
        return a + b;
    }
}

结果是：

Test run 1 =================================
Static: 582 ms
Dynamic: 579 ms
1000000001000000000
Static: 2065 ms
Dynamic: 2352 ms
2000000002000000000
Static: 2084 ms
Dynamic: 2345 ms
3000000003000000000
Static: 2095 ms
Dynamic: 2347 ms
4000000004000000000
Static: 2102 ms
Dynamic: 2338 ms
5000000005000000000
Static: 2073 ms
Dynamic: 2345 ms
6000000006000000000
Static: 2074 ms
Dynamic: 2341 ms
7000000007000000000
Static: 2102 ms
Dynamic: 2355 ms
8000000008000000000
Static: 2062 ms
Dynamic: 2354 ms
9000000009000000000
Static: 2057 ms
Dynamic: 2350 ms
-8446744063709551616
Test run 2 =================================
Static: 584 ms
Dynamic: 582 ms
1000000001000000000
Static: 587 ms
Dynamic: 577 ms
2000000002000000000
Static: 577 ms
Dynamic: 579 ms
3000000003000000000
Static: 577 ms
Dynamic: 577 ms
4000000004000000000
Static: 578 ms
Dynamic: 579 ms
5000000005000000000
Static: 578 ms
Dynamic: 580 ms
6000000006000000000
Static: 577 ms
Dynamic: 579 ms
7000000007000000000
Static: 578 ms
Dynamic: 577 ms
8000000008000000000
Static: 580 ms
Dynamic: 578 ms
9000000009000000000
Static: 576 ms
Dynamic: 579 ms
-8446744063709551616

至于为什么直接添加到 r， 我没有任何线索。也许有人可以提供有关访问原因的更多见解 r 在 - 的里面 loop block 使事情变得更慢。

只需一个注意事项。如果我使用，我只能观察到这种奇怪的行为 long 对于 r 和 i秒。如果我将它们转换为 int 然后我得到这些时间：

Static: 352 ms
Dynamic: 353 ms
Static: 348 ms
Dynamic: 349 ms
Static: 349 ms
Dynamic: 348 ms
Static: 349 ms
Dynamic: 344 ms

所以有可能 结论 是避免 long 在那些情况下。至少在Linux / Amd64 Java7中，性能很重要。