If you run a simple benchmark like this:

    @Benchmark
    public int stream_unsafe_char() {
        int s = 0;
        for (int c = 0; c < size; c++) {
            s += U.getChar(charArr, CHAR_ARR_OFFSET + CHAR_ARR_SCALE * c);
        }
        return s;
    }

...then you will notice that generated code has a stray "movslq":

 0x00007ffad8741180: movslq %r8d,%r9  <--- unnecessary sign extension
 0x00007ffad8741183: movzwl 0x10(%r10,%r9,2)
 0x00007ffad8741189: add    %ecx,%edx         
 0x00007ffad874118b: inc    %r8d               
 0x00007ffad874118e: cmp    %r11d,%r8d
 0x00007ffad8741191: jl     0x00007ffad8741180  

Indeed, the baseline test that does the plain Java access:

    @Benchmark
    public int plain() {
        int s = 0;
        for (int c = 0; c < size; c++) {
            s += charArr[c];
        }
        return s;
    }

...does without sign extension:

 0x00007fc3d0f84ea0: movzwl 0x10(%r11,%r8,2),%ecx
 0x00007fc3d0f84ea6: add    %ecx,%edx
 0x00007fc3d0f84ea8: inc    %r8d
 0x00007fc3d0f84eab: cmp    %r10d,%r8d
 0x00007fc3d0f84eae: jl     0x00007fc3d0f84ea0  

This conversion costs some cycles on my 1x4x2 4 GHz Haswell, running with Linux x86_64, 8u40 EA:

Benchmark            (size)  Mode  Cnt    Score   Error  Units
UnsafeMovslq.plain     1000  avgt    5  343.719 �� 0.694  ns/op
UnsafeMovslq.unsafe    1000  avgt    5  362.350 �� 5.302  ns/op