Why is Arrays.copyOf 2 times faster than System.arraycopy for small arrays?

Question

I was recently playing with some benchmarks and found very interesting results that I can't explain right now. Here is the benchmark:

@BenchmarkMode(Mode.Throughput)
@Fork(1)
@State(Scope.Thread)
@Warmup(iterations = 10, time = 1, batchSize = 1000)
@Measurement(iterations = 10, time = 1, batchSize = 1000)
public class ArrayCopy {

    @Param({"1","5","10","100", "1000"})
    private int size;
    private int[] ar;

    @Setup
    public void setup() {
        ar = new int[size];
        for (int i = 0; i < size; i++) {
            ar[i] = i;
        }
    }

    @Benchmark
    public int[] SystemArrayCopy() {
        final int length = size;
        int[] result = new int[length];
        System.arraycopy(ar, 0, result, 0, length);
        return result;
    }

    @Benchmark
    public int[] javaArrayCopy() {
        final int length = size;
        int[] result = new int[length];
        for (int i = 0; i < length; i++) {
            result[i] = ar[i];
        }
        return result;
    }

    @Benchmark
    public int[] arraysCopyOf() {
        final int length = size;
        return Arrays.copyOf(ar, length);
    }

}

Result:

Benchmark                  (size)   Mode  Cnt       Score      Error  Units
ArrayCopy.SystemArrayCopy       1  thrpt   10   52533.503 ± 2938.553  ops/s
ArrayCopy.SystemArrayCopy       5  thrpt   10   52518.875 ± 4973.229  ops/s
ArrayCopy.SystemArrayCopy      10  thrpt   10   53527.400 ± 4291.669  ops/s
ArrayCopy.SystemArrayCopy     100  thrpt   10   18948.334 ±  929.156  ops/s
ArrayCopy.SystemArrayCopy    1000  thrpt   10    2782.739 ±  184.484  ops/s
ArrayCopy.arraysCopyOf          1  thrpt   10  111665.763 ± 8928.007  ops/s
ArrayCopy.arraysCopyOf          5  thrpt   10   97358.978 ± 5457.597  ops/s
ArrayCopy.arraysCopyOf         10  thrpt   10   93523.975 ± 9282.989  ops/s
ArrayCopy.arraysCopyOf        100  thrpt   10   19716.960 ±  728.051  ops/s
ArrayCopy.arraysCopyOf       1000  thrpt   10    1897.061 ±  242.788  ops/s
ArrayCopy.javaArrayCopy         1  thrpt   10   58053.872 ± 4955.749  ops/s
ArrayCopy.javaArrayCopy         5  thrpt   10   49708.647 ± 3579.826  ops/s
ArrayCopy.javaArrayCopy        10  thrpt   10   48111.857 ± 4603.024  ops/s
ArrayCopy.javaArrayCopy       100  thrpt   10   18768.866 ±  445.238  ops/s
ArrayCopy.javaArrayCopy      1000  thrpt   10    2462.207 ±  126.549  ops/s

So there are two strange things here:

• Arrays.copyOf is 2 times faster than System.arraycopy for small arrays (1,5,10 size). However, on a large array of size 1000 Arrays.copyOf becomes almost 2 times slower. I know that both methods are intrinsics, so I would expect the same performance. Where does this difference come from?
• Manual copy for a 1-element array is faster than System.arraycopy. It's not clear to me why. Does anybody know?

VM version: JDK 1.8.0_131, VM 25.131-b11

Answer 1

Your System.arraycopy benchmark is not semantically equivalent to Arrays.copyOf.

It will be if you replace

    System.arraycopy(ar, 0, result, 0, length);

with

    System.arraycopy(ar, 0, result, 0, Math.min(ar.length, length));

With this change, the performance of both benchmarks will also become similar.

Why is the first variant slower then?

1. Without knowing how length relates to ar.length JVM needs to perform additional bounds check and be prepared to throw IndexOutOfBoundsException when length > ar.length.

2. This also breaks the optimization to eliminate redundant zeroing. You know, every allocated array must be initialized with zeros. However, JIT can avoid zeroing if it sees that the array is filled right after creation. But -prof perfasm clearly shows that the original System.arraycopy benchmark spends a significant amount of time clearing the allocated array:

    0,84%    0x000000000365d35f: shr    $0x3,%rcx
    0,06%    0x000000000365d363: add    $0xfffffffffffffffe,%rcx
    0,69%    0x000000000365d367: xor    %rax,%rax
             0x000000000365d36a: shl    $0x3,%rcx
   21,02%    0x000000000365d36e: rep rex.W stos %al,%es:(%rdi)  ;*newarray
   

Manual copy appeared faster for small arrays, because unlike System.arraycopy it does not perform any runtime calls to VM functions.