[Dwarf-Discuss] Add file attributes to debug-line file table (proposal 130701.1)

Michael Eager eager at eagercon.com
Tue Mar 18 08:32:32 PDT 2014


Hi Mark --

I'll pass your comments on to the DWARF Committee.

On 03/18/14 07:33, Mark Wielaard wrote:
> Hi,
>
> I was reading the suggestion for adding MD5 digests to the .debug_line
> program header. http://dwarfstd.org/ShowIssue.php?issue=130701.1
>
> Adding more attributes of files seems like a good thing, but as
> specified this isn't extensible without changing the version number
> again and defining new formats. Would it be possible to make this a
> little more generic and vendor extensible?
>
> The .debug_macro proposal includes some language for defining
> extensibility that could possibly be used.
> http://dwarfstd.org/ShowIssue.php?issue=110722.1
>
> So instead of a fixed file_entry_format (ubyte) field that just allows
> one attribute (group) to be defined we could change it to something
> like:
>
> file_attributes (ubyte)
>    - Possibly zero, number of attributes added to each file in the
>      file_names table.
> file_attribute_description (sequence of attribute descriptions)
>    - Entries in this description describe which attributes and in what
>      format those attributes are encoded for each file entry. It has
>      file_attributes entries. Each entry consists of:
>      - file_attribute (ubyte). One of DW_LNF_timestamp,
>        DW_LNF_length, DW_LNF_MD5, ...
>      - file_attribute_format. A uleb128 describing the number of
>        arguments for the format followed a single byte describing the
>        form of the argument. The allowed values are DW_FORM_data1,
>        DW_FORM_data2, DW_FORM_data4, DW_FORM_data8, DW_FORM_sdata, DW_FORM_udata,
>        DW_FORM_block, DW_FORM_block1, DW_FORM_block2, DW_FORM_block4, DW_FORM_flag,
>        DW_FORM_string, DW_FORM_strp and DW_FORM_sec_offset.
>
> file_names (sequence of file entries)
>    - Each entry consists of the following values:
>      - A null-terminated string containing the full or relative path name
>        of a source file.
>      - An unsigned LEB128 number representing the directory index of a
>        directory in the include_directories section.
>      - For each file_attribute_format described in the
>        file_attribute_description entries the value encoded in the format
>        given by file_attribute_format.
>
> For example DW_LNF_timestamp and DW_LNF_length would have as format 1
> DW_FORM_udata. DW_LNF_MD5 could by described by 2 DW_FORM_data8.
>
> That would allow extending the number of attributes in later DWARF
> versions or as vendor extensions and give consumers a way to skip over
> any unknown attributes.
>
> Would something like the above suggestion be useful? Then I can work it
> out a bit more. Did I miss any subtle corner cases? What is the status
> of the .debug_macro proposal (110722.1)? It would be good to match the
> descriptions of both extension mechanisms as much as possible.
>
> Thanks,
>
> Mark
>
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-- 
Michael Eager	 eager at eagercon.com
1960 Park Blvd., Palo Alto, CA 94306  650-325-8077



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