sizeOf() calculation from ClassLayout seems to be wrong

[Enis Söztutar]

Hi,

I was testing ClassLayout with small byte[]'s, and noticed something with a
byte[] size of 9:

[B object internals:

 OFFSET  SIZE  TYPE DESCRIPTION                    VALUE

      0     4       (object header)                01 00 00 00 (00000001
00000000 00000000 00000000) (1)

      4     4       (object header)                00 00 00 00 (00000000
00000000 00000000 00000000) (0)

      8     4       (object header)                f5 00 00 f8 (11110101
00000000 00000000 11111000) (-134217483)

     12     4       (object header)                11 00 00 00 (00010001
00000000 00000000 00000000) (17)

     16    17  byte [B.<elements>                  N/A

     33     7       (loss due to the next object alignment)

Instance size: 40 bytes

Space losses: 0 bytes internal + 7 bytes external = 7 bytes total

We seem to be calculating an extra 8 bytes in the byte[] layout after the
header. The instance size is calculated as 40, however,
VM.current().sizeOf() returns 32.


This little UT also fails with the latest source code base:

public class ArraySizeTest {

  @Test

  public void testByteArraySize() {

    System.err.println(VM.current().details()); // debug

    for (int i = 0; i < 100; i++) {

      byte[] b = new byte[i];

      long vmSize = VM.current().sizeOf(i);

      long clSize = ClassLayout.parseInstance(b).instanceSize();

      Assert.assertEquals("sizeOf() from VM and ClassLayout does not match,
VM.sizeOf()="

        + vmSize + ", ClassLayout.instanceSize()=" + clSize, vmSize, clSize
);

    }

  }

}

I was not able to open a JIRA issue for this, figured I can report here.

Thanks,

Enis