Hello,
We would like to improve the readability and usability of the DWARF
debug information emitted for Ada programs. Right now, this
information contains a lot of GNAT-specific "encodings", which were
created to fill in the deficiencies of the stabs and DWARF-2
debugging formats when it comes to representing Ada types. With the
recent introduction of the DWARF 3 standard, debugging information is
now capable of handling complex Ada types without the use of the
"encodings". Therefore, our long-term plan is to totally get rid of
these encodings and make proper use of DWARF-3.
This work will probably take a long time. To make the transition as
easy and smooth as possible, in the short-term we would like to:
- improve the existing DWARF
- take advantage of the DWARF extensibility to create a GNAT
vendor extension
To illustrate what we would like to improve, consider, for instance,
the following code declaring an array type:
package hello is
type My_Array_Type is array (3 .. 42) of Integer;
For this type, the following dwarf-2 nodes are currently emitted:
Node 1: DW_AT_name = "hello__my_array_type"
Node 2: DW_AT_name = "hello__my_array_type___XA"
Node 3: DW_AT_name = "hello__Tmy_array_typeB___XUA"
Node 4: DW_AT_name = "hello__Tmy_array_typeB___XUB"
Node 5: DW_AT_name = "hello__Tmy_array_typeB___XUT"
Node 6: DW_AT_name = "hello__Tmy_array_typeB___XUT___XVE"
Node 7: DW_AT_name = "hello__Tmy_array_typeD1___XDLU_3__42"
In the above:
Node 1 describes the original type,
Node 2 is a "descriptive type" node, containing specific
information describing the original structure,
Nodes 3 through 7 describe types internally generated by the
compiler. Note the weird-looking suffixes ("XA", "XUA", "XUB", etc):
these are the "encodings", which are at the moment appended to the
name string.
Here is what we suggest to improve:
* Use the DW_AT_artificial flag to identify all types which are
generated internally by the compiler (as recommended by the DWARF 3
standard).
* Create a DWARF Attribute to link the type (Node 1) to the
"descriptive type" (Node 2)
* Create a DWARF Attribute which will contain the "encodings", so
that they do not pollute the DW_AT_name attribute.
* Change the contents of the DW_AT_name attribute, to use "." as
component separation, use the natural representations for operators,
and so forth.
In order to make the transition easier, and to allow time for other
tools (mainly gdb) to take advantage of these improvements at their
own schedule, we would like to introduce a command-line switch (-
gdwarf+), which will trigger the use of the GNAT vendor extension to
DWARF.
Before going ahead and submitting patches for these improvements, we
would be very interested in hearing your thoughts.
Nicolas
------------------------------------
-- GNAT vendor extension to DWARF --
------------------------------------
We are introducing two new DWARF attributes.
a) DW_AT_GNAT_encoding
----------------------
This attribute is encoded with value 0x2301.
Any debugging information entry representing a program entity,
named or implicit, may have a DW_AT_GNAT_encoding attribute. The
value of this attribute is a string representing the suffix
internally added by GNAT for various purposes, mainly for
representing debug information compatible with other formats. The
full description for each encoding can be found in exp_dbug.ads.
If a debugging information entry has multiple encodings, all of
them will be listed in DW_AT_GNAT_encoding. The separator for this
list is ':'.
b) DW_AT_GNAT_descriptive_type
-------------------------------
This attribute is encoded with value 0x2302.
Any debugging information entry representing a type may have a
DW_AT_GNAT_descriptive_type attribute whose value is a reference,
pointing to a debugging information entry representing another type
(also called "parallel type") associated to the type. The full
semantics for parallel types are described in exp_debug.ads.
