java - 为什么 JIT 在绑定(bind)检查消除方面做得如此糟糕？

Question

我正在测试 HotSpot JIT 数组绑定(bind)检查消除功能。下面是相同堆排序实现的两个版本，一个使用普通数组索引，另一个 sun.misc.Unsafe API，没有绑定(bind)检查:

public class HeapSort {
    // copied from http://en.wikibooks.org/wiki/Algorithm_Implementation/Sorting/Heapsort#C
    static int heapSortSimple(int[] arr) {
        int t;
        int n = arr.length, parent = n / 2, index, childI;
        while (true) {
            if (parent > 0) {
                t = arr[--parent]; // 1, i. e. first indexing
            } else {
                if (--n == 0) break;
                t = arr[n]; // 2
                arr[n] = arr[0]; // 3, 4
            }
            index = parent;
            childI = (index << 1) + 1;
            while (childI < n) {
                int childV = arr[childI]; // 5
                int right;
                if (childI + 1 < n && (right = arr[childI + 1]) > childV) { // 6
                    childI++;
                    childV = right;
                }
                if (childV > t) {
                    arr[index] = childV; // 7
                    index = childI;
                    childI = (index << 1) + 1;
                } else {
                    break;
                }
            }
            arr[index] = t; // 8
        }
        return arr[arr.length - 1];
    }

    static int heapSortUnsafe(int[] arr) {
        int t;
        long n = arr.length * INT_SCALE, parent = (arr.length / 2) * INT_SCALE, index, childI;
        while (true) {
            if (parent > 0) {
                t = U.getInt(arr, INT_BASE + (parent -= INT_SCALE));
            } else {
                if ((n -= INT_SCALE) == 0) break;
                t = U.getInt(arr, INT_BASE + n);
                U.putInt(arr, INT_BASE + n, U.getInt(arr, INT_BASE));
            }
            index = parent;
            childI = (index << 1) + INT_SCALE;
            while (childI < n) {
                int childV = U.getInt(arr, INT_BASE + childI);
                int right;
                if (childI + INT_SCALE < n &&
                        (right = U.getInt(arr, INT_BASE + (childI + INT_SCALE))) > childV) {
                    childI += INT_SCALE;
                    childV = right;
                }
                if (childV > t) {
                    U.putInt(arr, INT_BASE + index, childV);
                    index = childI;
                    childI = (index << 1) + INT_SCALE;
                } else {
                    break;
                }
            }
            U.putInt(arr, INT_BASE + index, t);
        }
        return arr[arr.length - 1];
    }

    @OutputTimeUnit(TimeUnit.MICROSECONDS)
    @BenchmarkMode(Mode.AverageTime)
    @Warmup(iterations = 5, time = 1)
    @Measurement(iterations = 10, time = 1)
    @State(Scope.Thread)
    @Threads(1)
    @Fork(1)
    public static class Benchmarks {
        static final int N = 1024;
        int[] a = new int[N];

        @Setup(Level.Invocation)
        public void fill() {
            Random r = ThreadLocalRandom.current();
            for (int i = 0; i < N; i++) {
                a[i] = r.nextInt();
            }
        }

        @GenerateMicroBenchmark
        public static int simple(Benchmarks st) {
            int[] arr = st.a;
            return heapSortSimple(arr);
        }

        @GenerateMicroBenchmark
        public static int unsafe(Benchmarks st) {
            int[] arr = st.a;
            return heapSortUnsafe(arr);
        }
    }

    public static void main(String[] args) {
        Benchmarks bs = new Benchmarks();

        // verify simple sort
        bs.fill();
        int[] a1 = bs.a;
        int[] a2 = a1.clone();
        Arrays.sort(a2);
        heapSortSimple(a1);
        if (!Arrays.equals(a2, a1))
            throw new AssertionError();

        // let JIT to generate optimized assembly
        for (int i = 0; i < 10000; i++) {
            bs.fill();
            heapSortSimple(bs.a);
        }

        // verify unsafe sort
        bs.fill();
        a1 = bs.a;
        a2 = a1.clone();
        Arrays.sort(a2);
        heapSortUnsafe(a1);
        if (!Arrays.equals(a2, a1))
            throw new AssertionError();

        for (int i = 0; i < 10000; i++) {
            bs.fill();
            heapSortUnsafe(bs.a);
        }
    }

    static final Unsafe U;
    static final long INT_BASE;
    static final long INT_SCALE = 4;

    static {
        try {
            Field f = Unsafe.class.getDeclaredField("theUnsafe");
            f.setAccessible(true);
            U = (Unsafe) f.get(null);
        } catch (Exception e) {
            throw new IllegalStateException(e);
        }
        INT_BASE = U.arrayBaseOffset(int[].class);
    }
}

不安全版本在 Intel SB 和 AMD K10 CPU 上始终快 13%。

我查看了生成的程序集:

• 为所有索引操作(1-8)消除了下限检查
• 仅对操作 5 取消上限检查，合并 2 和 3 的检查
• 是的，对于操作 4 (arr[0])，在每次迭代中都会检查arr.length != 0<

很明显，所有绑定(bind)检查分支都是完美预测的，这就是为什么使用简单索引的堆排序比不安全的排序慢 13%。

我认为 JIT 至少为操作 1、2 和 3 优化了边界检查，其中索引从数组长度以下的某个值稳定地递减到零。

问题在标题中:为什么 HotSpot JIT 在这种情况下在消除绑定(bind)检查方面做得如此糟糕？

Answer 1

我不认为所有的胶乳都受到限制。

传递零长度数组会导致 IOOB。尝试在循环之前让 if (n==0) return。