[Dwarf-Discuss] Default Location List Entry Issue 130121.1

[Michael Eager]

On 04/02/14 03:43, Mark Wielaard wrote:
> On Tue, 2014-04-01 at 18:42 -0700, Michael Eager wrote:
>> On 04/01/14 13:54, Mark Wielaard wrote:
>>
>>> What about using the presence of a DW_AT_external attribute on the data
>>> object that has a single location expression to know whether the described
>>> location is valid/visible outside of the enclosing lexical scope?
>>>
>>> Using that or some new flag (DW_AT_global_scope) to mark a data object
>>> that has a single location description with global scope might be cheaper
>>> than encoding it with a location list pointer and a default entry.
>>
>> Both DW_AT_external attribute and a hypothetical DW_AT_global_scope
>> attribute would describe scope, not the storage life of the object.
>> C unfortunately has confabulated the two concepts.
>
> In practice I believe DW_AT_external does both and I have used it that
> way to know whether to trust a single location description is globally
> valid. But yes, just like lifetime both "external" and "scope" are again
> bad words to use in this case and don't really express the thing we want
> (they might only accidentally).

Section 3.3.1 describes the DW_AT_external with the following:

   If the name of the subroutine described by an entry with the tag
   DW_TAG_subprogram is visible outside of its containing compilation
   unit, that entry has a DW_AT_external attribute, which is a flag.

Section 4.1 contains the following (describing an object):

   2. A DW_AT_external attribute, which is a flag, if the name of a
   variable is visible outside of its enclosing compilation unit.

There is nothing about the lifetime of any object.  Confabulating that
the scope attributes in DWARF imply something different is not supported
by the DWARF standard and will lead to misinterpretation of the DWARF standard.

> What about we call the flag attribute DW_AT_global_visible or
> DW_AT_global_location then it would be clear I think because it signals
> this isn't about language or DWARF tree (lexical) scoping?

Symbol visibility is adequately described by the DW_AT_external attribute
as defined above.  Object locations are adequately described by location
expressions and location lists.

>> Objects which have only a single location can be described with a location
>> expression.  They don't need a location list with a default entry.
>
> For a data object that has a single location descriptor (DW_AT_location
> in exprloc class form), the valid range is given by the address ranges
> of the DIEs that own the data object. So it only lets me express the
> location in a restricted range. I do need to use a location list with
> (just) a default entry if I want to indicate that the location
> description has a valid global range.

No, this interpretation is incorrect and is not supported by reading the
DWARF Standard.  Please see Section 2.6.

Perhaps you are confused by the following from Section 2.6 (which I think
is unambiguous):

    1. Single location descriptions, which are a language independent
       representation of addressing rules of arbitrary complexity built
       from DWARF expressions and/or other DWARF operations specific to
       describing locations. They are sufficient for describing the location
       of any object as long as its lifetime is either static or the same as
       the lexical block that owns it, and it does not move during its lifetime.

You seem to have interpreted "either/or" in the second sentence to mean "and".

> Is the above correct? Or is there another way to express that a location
> description is globally valid? Or am I misunderstanding the purpose of
> having a default entry in a location list?

You do not need a location list for a static object.  A simple location
expression will suffice.

A default location list entry (as proposed in 130121.1) gives the location
of an object for address values which are not otherwise specified in the
location list.

-- 
Michael Eager	 eager at eagercon.com
1960 Park Blvd., Palo Alto, CA 94306  650-325-8077