Class layout question

[Steve Ellcey <sje at cup dot hp dot com>]

I have been looking at bringing up the gcj compiler on IA64 HP-UX.  It
currently runs on IA64 Linux and with some local changes I can compile
(but not run) java programs on HP-UX.  Looking at the differences
between HP-UX and Linux I see they are generating the exact same code
sequence but different class layouts.

Obviously either the code should be different, or the class layout should
be the same.  I was wondering if someone could point me to where the
difference in the class layout might be coming from.  For the java program
below:

public class x
{
  public static void main(String[] args)
  {
  }
}

I look at the assembly language file and on HP-UX I see:

        .type   _ZN1x6class$E#, @object
        .size   _ZN1x6class$E#, 280
_ZN1x6class$E:
        data8   _ZTVN4java4lang5ClassE#+16
        data8   0      <** This does not get output in Linux
        data8   401000
        data8   _Utf5#
        data2   1
        .skip   6
        data8   _ZN4java4lang6Object6class$E#
        data4   0
        .skip   4
        data8   0
        data8   0
        data8   _MT_x#
	(more follows)

In the Linux version the 'data8 0' is not put out and _ZN1x6class$E is 8
bytes shorter than on HP-UX.  Note, this is LP64 mode on both HP-UX and
Linux so that is not an issue, though maybe the compiler thinks HP-UX is
in ILP32 mode and is doing some padding, I don't know.

Anyway, I was looking for some information on class layouts and what my
extra number might be, where it is generated, and what macros/functions
might be controlling it's output.

Steve Ellcey
sje@cup.hp.com