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

[David Blaikie]

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.")
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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