There are many cases where C2 eliminates the array pre-zeroing followed by arraycopy into the array. However, there are cases where this surprisingly does not work.

For example, see this benchmark:
 http://cr.openjdk.java.net/~shade/8146828/ArrayZeroingBench.java
 http://cr.openjdk.java.net/~shade/8146828/benchmarks.jar

The most puzzling example is this:

    @Benchmark
    public Foo[] arraycopy_srcLength() {
        Object[] src = this.src;
        Foo[] dst = new Foo[size];
        System.arraycopy(src, 0, dst, 0, src.length);
        return dst;
    }

    @Benchmark
    public Foo[] arraycopy_dstLength() {
        Object[] src = this.src;
        Foo[] dst = new Foo[size];
        System.arraycopy(src, 0, dst, 0, dst.length);
        return dst;
    }

In srcLength case we successfully eliminate the pre-zeroing on destination array, while in dstLength we don't!

See "rep stos" in dstLength case, and no zeroing in srcLength case:
 http://cr.openjdk.java.net/~shade/8146828/jdk9b99.perfasm
 http://cr.openjdk.java.net/~shade/8146828/jdk9b99-size10.perfasm