Re: C++ debugging progress

[Jason Merrill <jason at redhat dot com>]

>>>>> "Jason" == Jason Merrill <jason@redhat.com> writes:

> Yep.  Since Base is a virtual base of Left, the DW_AT_data_member_location
> here should be a complex expression telling the debugger to go through the
> vtable.  I'll get on it.

> I'll also change the stabs output to give the offset within the vtable
> rather than the offset of the base in a complete object; gdb will still
> need to be clever enough to know what to do with it.

Please give this a spin; gdb doesn't like it, but I think it's correct.
Note that the offset given in the stabs output should be treated as
negative, so for V3 objects,

  BaseAddr = LeftAddr + *((*LeftAddr) - Offset)

The dwarf2 output is the above encoded in reverse Polish notation.  I
suppose we could rely on gdb to know the equation for dwarf, too, but that
would be nonconforming and only saves four bytes.

2001-11-28  Jason Merrill  <jason@redhat.com>

	* dwarf2out.c (add_data_member_location_attribute): Do the
	right thing for virtual bases.
	* dbxout.c (dbxout_type): For a virtual base, print the offset
	within the vtable.

*** dwarf2out.c.~1~	Fri Nov 23 17:27:50 2001
--- dwarf2out.c	Wed Nov 28 20:42:37 2001
*************** add_data_member_location_attribute (die,
*** 8407,8434 ****
       dw_die_ref die;
       tree decl;
  {
!   unsigned long offset;
!   dw_loc_descr_ref loc_descr;
!   enum dwarf_location_atom op;
  
    if (TREE_CODE (decl) == TREE_VEC)
!     offset = tree_low_cst (BINFO_OFFSET (decl), 0);
    else
      offset = field_byte_offset (decl);
  
!   /* The DWARF2 standard says that we should assume that the structure address
!      is already on the stack, so we can specify a structure field address
!      by using DW_OP_plus_uconst.  */
  
  #ifdef MIPS_DEBUGGING_INFO
!   /* ??? The SGI dwarf reader does not handle the DW_OP_plus_uconst operator
!      correctly.  It works only if we leave the offset on the stack.  */
!   op = DW_OP_constu;
  #else
!   op = DW_OP_plus_uconst;
  #endif
  
!   loc_descr = new_loc_descr (op, offset, 0);
    add_AT_loc (die, DW_AT_data_member_location, loc_descr);
  }
  
--- 8407,8471 ----
       dw_die_ref die;
       tree decl;
  {
!   long offset;
!   dw_loc_descr_ref loc_descr = 0;
  
    if (TREE_CODE (decl) == TREE_VEC)
!     {
!       /* We're working on the TAG_inheritance for a base class.  */
! 
!       if (TREE_VIA_VIRTUAL (decl))
! 	{
! 	  /* For C++ virtual bases we can't just use BINFO_OFFSET, as they
! 	     aren't at a fixed offset from all (sub)objects of the same type.
! 	     We need to extract the appropriate offset from our vtable.  */
! 
! 	  dw_loc_descr_ref tmp;
! 	  /* Make a copy of the object address.  */
! 	  tmp = new_loc_descr (DW_OP_dup, 0, 0);
! 	  add_loc_descr (&loc_descr, tmp);
! 	  /* Extract the vtable address.  */
! 	  tmp = new_loc_descr (DW_OP_deref, 0, 0);
! 	  add_loc_descr (&loc_descr, tmp);
! 	  /* Calculate the address of the offset.  */
! 	  offset = tree_low_cst (BINFO_VPTR_FIELD (decl), 0);
! 	  if (offset >= 0)
! 	    abort ();
! 	  tmp = int_loc_descriptor (-offset);
! 	  add_loc_descr (&loc_descr, tmp);
! 	  tmp = new_loc_descr (DW_OP_minus, 0, 0);
! 	  add_loc_descr (&loc_descr, tmp);
! 	  /* Extract the offset.  */
! 	  tmp = new_loc_descr (DW_OP_deref, 0, 0);
! 	  add_loc_descr (&loc_descr, tmp);
! 	  /* Add it to the object address.  */
! 	  tmp = new_loc_descr (DW_OP_plus, 0, 0);
! 	  add_loc_descr (&loc_descr, tmp);
! 	}
!       else
! 	offset = tree_low_cst (BINFO_OFFSET (decl), 0);
!     }
    else
      offset = field_byte_offset (decl);
  
!   if (! loc_descr)
!     {
!       enum dwarf_location_atom op;
! 
!       /* The DWARF2 standard says that we should assume that the structure address
! 	 is already on the stack, so we can specify a structure field address
! 	 by using DW_OP_plus_uconst.  */
  
  #ifdef MIPS_DEBUGGING_INFO
!       /* ??? The SGI dwarf reader does not handle the DW_OP_plus_uconst operator
! 	 correctly.  It works only if we leave the offset on the stack.  */
!       op = DW_OP_constu;
  #else
!       op = DW_OP_plus_uconst;
  #endif
  
!       loc_descr = new_loc_descr (op, offset, 0);
!     }
    add_AT_loc (die, DW_AT_data_member_location, loc_descr);
  }
  
*** dbxout.c.~1~	Thu Nov 15 16:59:31 2001
--- dbxout.c	Wed Nov 28 20:49:04 2001
*************** dbxout_type (type, full)
*** 1492,1499 ****
  		putc (TREE_VIA_VIRTUAL (child) ? '1' : '0', asmfile);
  		putc (TREE_VIA_PUBLIC (child) ? '2' : '0', asmfile);
  		CHARS (2);
! 		print_wide_int (tree_low_cst (BINFO_OFFSET (child), 0)
! 				* BITS_PER_UNIT);
  		putc (',', asmfile);
  		CHARS (1);
  		dbxout_type (BINFO_TYPE (child), 0);
--- 1492,1506 ----
  		putc (TREE_VIA_VIRTUAL (child) ? '1' : '0', asmfile);
  		putc (TREE_VIA_PUBLIC (child) ? '2' : '0', asmfile);
  		CHARS (2);
! 		if (TREE_VIA_VIRTUAL (child))
! 		  /* For a virtual base, print the (negative) offset within
! 		     the vtable where we must look to find the necessary
! 		     adjustment.  */
! 		  print_wide_int (-tree_low_cst (BINFO_VPTR_FIELD (child), 0)
! 				  * BITS_PER_UNIT);
! 		else
! 		  print_wide_int (tree_low_cst (BINFO_OFFSET (child), 0)
! 				  * BITS_PER_UNIT);
  		putc (',', asmfile);
  		CHARS (1);
  		dbxout_type (BINFO_TYPE (child), 0);