Re: java memory allocation with gcj

[isuru herath <isuru81 at yahoo dot com>]

Hi Andrew,

I am really sorry for what happened. I pressed the wrong button. Please see 
the original mail below.

Thanks for the reply. I tried in x $eax gdb after the new being executed an 
it was giving me the following. 
0x804a0e0 <_ZN13test_java_new6class$E>: 0xb7a4afe8

When converted to decimal it was 134521056 which seems close to that of C. 
Could you please tell me what the other value represents here.

Seems like the way I was doing is wrong. But I couldn't find another way.

This is my Java class.

class test_java_new
{
        public static void main(String[]args)
        {
                int isuru[]=new int[2];
                isuru[0] = 12345;
        }
}

Then I compiled this with gcj -S. Then I open the test_java_new.s file.
There I add following lines after the 
call    _Jv_NewPrimArray line.

The lines I am adding are
        pushl   %ecx            #;for start location
        movl    %eax, %ecx      #;copy value in eax to ecx
        push    %eax            #;the memory tag
        movl    $5, %eax        #;set eax to HEAP tag
        xchg    %bx, %bx        #;magic instruction
        popl    %eax
        popl    %ecx

Why I am doing this is, when the xchg %bx, %bx is executed my simulator (Simics) triggers an event and my hardware module can get the control of the system. So that I can pass information from the user program to my hardware module. But when I check the eax register at my hardware module after xchg    %bx, %bx , I got that small number which is 166816. But when I add the same assembly code soon after the call malloc in a gcc generated assembly of a C program, my hardware module gets the correct address. Therefore I was thinking it should be the same here.

Some other thing I noticed is that when I invoke x $eax after the 
isuru[0] = 12345; I got 0X28bc0 which is quite similar to 166848. I don't know this helps you to help me to clarify my problem.

I really appreciate your help.

thanks and regards,
Isuru


--- On Thu, 11/5/09, Andrew Haley <aph@redhat.com> wrote:

> From: Andrew Haley <aph@redhat.com>
> Subject: Re: java memory allocation with gcj
> To: "isuru herath" <isuru81@yahoo.com>
> Date: Thursday, November 5, 2009, 4:09 AM
> isuru herath wrote:
> > Hi Andrew,
> > 
> > Thanks for the reply. So I was thinking soon after the
> call to _Jv_NewPrimArray the eax register should have the
> starting location of the the memory allocated by the mmap.
> But the value at the eax register was 166816.
> 
> So, if you're in gdb, try
> 
>   x $eax
> 
> 166816 doesn't seem unreasonable.  It can be a valid
> address.
> 
> > When I tried this with C, the value at the eax
> register after the malloc call was 155648008. I also put a
> printf(&name) in the C code and it prints the same
> address.
> > Then I tried store a value in the Java array and then
> tried to read the memory address 166816 and it wasnot giving
> me the value that I stored in my program. 
> 
> I think I need to see a test case.  Please, I need the
> code, and the steps
> you took.
> 
> > Even the memory addresses given from C and java are
> different in sizes as well.
> > 
> > Am I missing something here. I just need to know how
> to find the address of the block allocated by the "new" when
> running as native binary.
> > 
> > any help/advice on this regard is greatly
> appreciated.
> 
> Hard to say.  I know you must be doing something
> wrong, but I don't
> know what.  If you can send me a test case that
> illustrates what
> you're doing, I'll have a look.
> 
> Andrew.
>