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101203.1 Kendrick Wong Describing Fortran descriptors Enhancement Withdrawn Kendrick Wong


Section 5.14, pg 102
Overview:
Adding DW_TAG_descriptor_type to describe descriptors-specific attributes.

Background:
Some languages, such as Fortran, provide types whose values may be dynamically 
allocated or associated with a variable under explicit program control. Currently, 
these attributes, such as DW_AT_allocated and DW_AT_associated are placed on the 
corresponding type DIE.  This approach forces creation of duplicate types for a
single Fortran type.

Consider this Fortran example:
type :: dt (l)
  integer, len :: l
  integer :: arr(l)
end type

integer :: n = 4
contains

subroutine s()
type (dt(n))               :: t1
type (dt(n)), pointer      :: t2
type (dt(n)), allocatable  :: t3
end subroutine

end

t1, t2 and t3 all share the same type 'dt'.  The current DWARF spec 
places all descriptor related attributes (DW_AT_data_location, DW_AT_associated
and DW_AT_allocated) on the DW_TAG_structure_type DIE for 'dt'.  However, we 
can't put all the descriptor specific attributes on the same DW_TAG_structure_type
DIE because there is a need to differentiate between t1, t2 and t3.  This forces 3
different 'dt' DW_TAG_structure_type to be created to describe this program.

Proposal:
This would modify the wording to 5.14 "Dynamic Type Properties".

A base or user-defined type may be accessed through a descriptor.  A descriptor 
is represented in DWARF by the debug information entry with the tag 
DW_TAG_descriptor_type. It has a DW_AT_type attribute, whose value is a reference 
to a debugging information entry describing a base type, a user-defined type or a 
type modifier.

5.14.1 Data Location
The DW_AT_data_location attribute may be used with DW_TAG_descriptor_type that 
provides one or more levels of hidden indirection and/or run-time parameters in its 
representation.  (Rest of text same as current text...)

5.14.2 Allocation and Association Status
The DW_AT_allocated attribute may optionally be used with DW_TAG_descriptor_type 
for which objects of the associated type can be explicitly allocated and deallocated.  
(Rest of text same as current text...)

The DW_AT_associated attribute may optionally be used with DW_TAG_descriptor_type 
for which objects of the associated type can be dynamically associated with other 
objects.  (Rest of text same as current text...)

The new DWARF for the example program would look like:

$11: DW_TAG_structure_type
      DW_AT_name (dt)
$12:  DW_TAG_member ...

$13: DW_TAG_descriptor_type        ! plain version
      DW_AT_data_location (...)
      DW_AT_type ($11)

$14: DW_TAG_descriptor_type
      DW_AT_data_location (...)
      DW_AT_associated (...)       ! 'pointer'
      DW_AT_type ($11)

$15: DW_TAG_descriptor_type
      DW_AT_data_location (...)
      DW_AT_allocated (...)        ! 'allocatable'
      DW_AT_type ($11)

$16: DW_TAG_variable
      DW_AT_name (t1)
      DW_AT_type ($13)
      DW_AT_location (...)
$17: DW_TAG_variable
      DW_AT_name (t2)
      DW_AT_type ($14)
      DW_AT_location (...)
$18: DW_TAG_variable
      DW_AT_name (t3)
      DW_AT_type ($15)
      DW_AT_location (...)

====

Possibly merge with 090824.1

12/27/12 -- Withdrawn, see 121112.1


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