[Dwarf-Discuss] Segment selectors for the range list table.

[Xing GUO]

Hi David,

On 7/16/20, David Blaikie <dblaikie at gmail.com> wrote:
> Looking at how segment selectors work:
>
> DW_AT_segment: Applies to a DIE subtree, including any ranges, high/low pc,
> locations, labels, etc
> debug_range/loc (v4 and below): Doesn't seem to allow specifying segment
> variation - inherits from the segment given on the nearest parent DIE that
> refers to the entry
> debug_rnglist/loclist (v5): includes segment selector size in the header,
> but doesn't seem to use it - segment selection via addresses in the address
> pool (RLE/LLE_*x encodings) would allow fine-grained segment selection, but
> direct address forms don't seem to allow segment selection ("This operand
> is the
> 19 same size as used in DW_FORM_addr.")

Thanks for your explanation.

So, If we want to use segmented addresses in the debug_rnglist or
debug_loclist table, we should use encodings like: startx_endx,
startx_length, etc., to index the {segment/address} pairs in the
address table of the .debug_addr section rather than the direct
address forms like: start_end, start_length, etc., right?

Besides, the segment_selector_size is just an indicator that tells us
if the current debug_rnglist/debug_loclist table is using segmented
address and it should have no effects on parsing the encoding and
operands (e.g., {DW_RLE_startx_endx, operands0, operands1},
{DW_RLE_start_end, operands0, operands1}) in the entries.

> debug_addr: segment_size in header, then list of {segment selector,
> address}
> debug_aranges: segment_size in header says, then the list contains triples
> of {segment selector, start address, length}
> debug_line: v5 encodes the address and segment selector size in the header,
> but I'm not sure if/how it's used. The DW_LNE_set_address operation says:
> "The DW_LNE_set_address opcode takes a single relocatable address as an
> operand. The size of the operand is the size of an address on the target
> machine. It sets the address register to the value given by the relocatable
> address and sets the op_index register to 0." - doesn't sound like it's
> reading the segment selector there.
>
> So... I don't think DWARFv5 made anything worse - if anything it did enable
> /a/ way to use fine grained segment selectors in range lists and location
> lists that doesn't appear, to me, to have been provided before. (it could
> be needed if you had some functions in some segment and some functions in
> another segment (which could be represented at the subprogram DIE level -
> DW_AT_segment 1 on one DW_TAG_subprogram, DW_AT_segment 2 on another
> DW_TAG_subprogram - but how would you represent the DW_AT_ranges for this
> CU (in DWARFv4, or in DWARFv5 without using addrx encodings)? I don't know
> how, because I think debug_ranges could describe one range list entry as
> being from one segment, and another range list entry as being in another
> segment - they would all be in whatever segment was in DW_AT_segment on the
> CU)
>
> does that make sense? Have I missed something about how you could use
> segment selectors in a debug_loc, debug_ranges, or loclist/rnglist that
> isn't using an addrx encoding?
>
> On Wed, Jul 15, 2020 at 6:37 AM Robinson, Paul via Dwarf-Discuss <
> dwarf-discuss at lists.dwarfstd.org> wrote:
>
>>
>>
>> > -----Original Message-----
>> > From: Dwarf-Discuss <dwarf-discuss-bounces at lists.dwarfstd.org> On
>> > Behalf
>> > Of Xing GUO via Dwarf-Discuss
>> > Sent: Tuesday, July 14, 2020 10:39 PM
>> > To: dwarf-discuss at lists.dwarfstd.org
>> > Subject: [Dwarf-Discuss] Segment selectors for the range list table.
>> >
>> > Hi there,
>> >
>> > The DWARFv5 spec mentioned that there might be segment selectors in
>> > the range list entries and when the segment_selector_size is 0, the
>> > segment selectors are omitted from the range list entries. However, it
>> > didn't mention how the segment selector should be encoded when the
>> > segment_selector_size isn't 0. Can anyone help me figure it out?
>> > Thanks a lot!
>>
>> Hi Xing,
>>
>> The segment selectors in the range list would be encoded the same way
>> as they would be in the main .debug_info section.  Range lists and
>> location lists are essentially extensions to .debug_info, for cases
>> where the range or location cannot be represented by simple DW_AT_*
>> attribute values.
>>
>> The specifics of encoding the segment selector would be whatever is
>> appropriate to the target.  DWARF does not specify these details.
>>
>> Best Regards,
>> --paulr
>>
>>
>> >
>> > 7.28 (page 243)
>> > The segment size is given by the segment_selector_size field of the
>> > header, and the address size is given by the address_size field of the
>> > header. If the segment_selector_size field in the header is zero, the
>> > segment selector is omitted from the range list entries.
>> >
>> > --
>> > Cheers,
>> > Xing
>> > _______________________________________________
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>


-- 
Cheers,
Xing