[Dwarf-Discuss] dwarf and local variables's offset

[Jan Kratochvil]

Hi,

DW_AT_frame_base specifies some virtual "frame base" address. Technically it
can be arbitrary address, it is used just by the DWARF expressions as
a reference point.

DWARF would work the same if you had:
>     Offset   Begin    End      Expression
>     00000000 080483c4 080483ec (DW_OP_breg5: 108)
>     00000000 <End of list>
+
> >> ?<2><8d>: Abbrev Number: 6 (DW_TAG_variable)
> > [...]
> >> ? ? <98> ? DW_AT_location ? ?: 2 byte block: 91 ?? ? ? ?(DW_OP_fbreg: -116)

As some constant can be added to the frame base and later subtracted for the
value locations addresses.

DWARF3 says:
# The frame base for a procedure is typically an address fixed relative to the
# first unit of storage allocated for the procedure?s stack frame.

So the address is right above the frame pointer and return address on the
stack.  For i386 -fno-omit-frame-pointer code that corresponds to +8 = +4 +4
as there is both the return address and the stored caller's %ebp value.

This is a typical i386 -fno-omit-frame-pointer function prologue:
--- Here frame base is %esp + 4 as the return address is already stored there
    by the caller's `call' instruction.
 XXXXXXX:       55                      push   %ebp
--- Here frame base is %esp + 8 as also the %ebp reg. got stored to the stack.
 XXXXXXX:       89 e5                   mov    %esp,%ebp
--- Here frame base is %ebp + 8 as since this point compiler may temporarily
    push something on the stack (%esp) while %ebp should remain constant
    through the function.

(.debug_loc you provided in your mail looks as imperfect, it could better
describe the first 3 bytes of the function containing the prologue.)


> 2) Why DW_OP_breg5  and  (%ebp )  are not same????

Frame base is therefore unrelated to the register %ebp.  Also for
-fomit-frame-pointer - which is even default on x86_64 - there is nothing like
%ebp to choose.  Only %esp (%rsp) has a valid value and the location list
(.debug_loc) has to track the actual value as %esp/%rsp is being changed
through the function.


> ie) Why it is 8 only???
> If I compile my program again, it can be some other value???

GCC follows what the DWARF spec. says so it should behave deterministically.


Regards,
Jan


On Mon, 04 May 2009 11:04:52 +0200, ranjith kumar wrote:
> Hi,
>         In the contes of  the .debug_loc section:
> _____________________________________________
>     Offset   Begin    End      Expression
>     00000000 080483c4 080483ec (DW_OP_breg5: 8)
>     00000000 <End of list>
> ___________________________________________
> 
> 1)  What does "DW_OP_breg5: 8" mean???
> ie) Why it is 8 only???
> If I compile my program again, it can be some other value???
> Or that value is constant for the given  machine? If so it represents
> what information about the machine?
> 
> 2) Why DW_OP_breg5  and  (%ebp )  are not same????
> 
>     I could not figure it out it in dwarf3.pdf.
> Thanks in advace.