Re: DWARF-2 expression error in location list entries

Re: DWARF-2 expression error in location list entries

[Carl Burch]

Daniel-

> I believe the language was recently discussed on the dwarf workgroup
> list.  It's unclear, but the error is correct.  DW_OP_regx describes
> a location ("the value is in this register"), not a value ("the value is
> the contents of this register").  Therefore the value may not be
> stack-sized, and is not pushed on to the expression stack, so there's
> nothing for you to DW_OP_deref later.
> 
> >    The expression we have in mind to add is to describe Fortran reference
> > parameters in location list entries like :
> > 
> > { 0x4022720--0x4022724: DW_OP_regx  32 DW_OP_deref   }
> 
> Isn't that just "DW_OP_bregx 32 0"?

   Yes, I think so.  Thanks for the clear explanation.
   
   Any idea why the same situation in decode_locdesc() of dwarf2read.c is only
a complaint?

							- Carl Burch
							  HP WDB Team
							  

Re: DWARF-2 expression error in location list entries

[Daniel Jacobowitz]

On Tue, May 08, 2007 at 03:49:22PM -0700, Carl Burch wrote:
>    Yes, I think so.  Thanks for the clear explanation.
>    
>    Any idea why the same situation in decode_locdesc() of dwarf2read.c is only
> a complaint?

Probably a historical accident.

-- 
Daniel Jacobowitz
CodeSourcery

Re: DWARF-2 expression error in location list entries

[David Anderson]

>   A couple of us have looked for that first restriction in the DWARF3
> standard and not found it.  I also went back to the DWARF2 standard
> literally cited by the message and failed to find it there as well.  Can
> anyone point out where to read the restriction these error() calls refer to?

The clearest hint is in DWARF3 2.6.1: 
"The following DWARF operations can be used to name a register. They can be used only in
location expressions. Each register name operator must be used alone (as a DWARF expression
consisting of just that one operation)."
(goes on to mention DW_OP_reg*).

It's true that this is not as clear as it should be 
(DW_OP_piece is not mentioned in 2.6.1, for example).
2.6.2 does try to clarify this, but it's not easy to interpret.
But Daniel Jacobowitz is exactly right.  
These can only be used stand-alone (or with DW_OP_piece).

Jim Blandy suggested a nice little grammar
which made all this quite clear (March 8 2007, dwarf-discuss mailing list).

The lack of clarity issue got lost in a discussion about a proposal
on constant expressions in location lists.    I'll resurrect Jim's suggestion
for the DWARF committee.

Regards,
David Anderson

DWARF-2 expression error in location list entries

[Carl Burch]

   In dwarf2expr.c function execute_stack_op() there are two occurrences of
error() calls asserting :

         if (op_ptr != op_end && *op_ptr != DW_OP_piece)
            error (_("DWARF-2 expression error: DW_OP_reg operations must be "
                   "used either alone or in conjuction with DW_OP_piece."));

   A couple of us have looked for that first restriction in the DWARF3
standard and not found it.  I also went back to the DWARF2 standard
literally cited by the message and failed to find it there as well.  Can
anyone point out where to read the restriction these error() calls refer to?

   The expression we have in mind to add is to describe Fortran reference
parameters in location list entries like :

{ 0x4022720--0x4022724: DW_OP_regx  32 DW_OP_deref   }

Yes, we know that Gnu Fortran describes reference params in terms of passing
the underlying pointers by value instead of the original Fortran types by
reference, thereby avoiding this problem.  This is a case where we're trying
to be better instead of de facto standard.

						- Carl Burch
						  HP WDB Team

P.S. If anyone edits this file soon, you might want to fix the spelling of
"conjuction" in the message.

Re: DWARF-2 expression error in location list entries

[Daniel Jacobowitz]

On Tue, May 08, 2007 at 01:22:25PM -0700, Carl Burch wrote:
>    In dwarf2expr.c function execute_stack_op() there are two occurrences of
> error() calls asserting :
> 
>          if (op_ptr != op_end && *op_ptr != DW_OP_piece)
>             error (_("DWARF-2 expression error: DW_OP_reg operations must be "
>                    "used either alone or in conjuction with DW_OP_piece."));
> 
>    A couple of us have looked for that first restriction in the DWARF3
> standard and not found it.  I also went back to the DWARF2 standard
> literally cited by the message and failed to find it there as well.  Can
> anyone point out where to read the restriction these error() calls refer to?

I believe the language was recently discussed on the dwarf workgroup
list.  It's unclear, but the error is correct.  DW_OP_regx describes
a location ("the value is in this register"), not a value ("the value is
the contents of this register").  Therefore the value may not be
stack-sized, and is not pushed on to the expression stack, so there's
nothing for you to DW_OP_deref later.

>    The expression we have in mind to add is to describe Fortran reference
> parameters in location list entries like :
> 
> { 0x4022720--0x4022724: DW_OP_regx  32 DW_OP_deref   }

Isn't that just "DW_OP_bregx 32 0"?

-- 
Daniel Jacobowitz
CodeSourcery