On Thu, May 16, 2024 at 10:20 AM Mark Wielaard via Dwarf-discuss < dwarf-discuss@lists.dwarfstd.org> wrote:
> Hi, > > Section 6.2.4.1 Standard Content Descriptions under 1. DW_LNCT_path has > the following description on using DW_FORM_strx forms: > > In a .debug_line.dwo section, the forms DW_FORM_strx, DW_FORM_strx1, > DW_FORM_strx2, DW_FORM_strx3 and DW_FORM_strx4 may also be used. > These refer into the .debug_str_offsets.dwo section (and indirectly > also the .debug_str.dwo section) because no > .debug_line_str_offsets.dwo or .debug_line_str.dwo sections exist or > are defined for use in split objects. > > This confuses me a little, it is as if the strx forms are only allowed > when generating split dwarf. But that doesn't seem to be the intent, > because under 6.2.4.2 Vendor-defined Content Descriptions all DW_FORM_strx > variants are listed without special exceptions. > > So I assume the description under DW_LNCT_path is just to explain that > there isn't a .debug_line_str_offsets section that the DW_FORM_strx > forms can refer to. And that DW_FORM_strx forms are resolved as normal > and can be used in both .debug_line and .debug_line.dwo content > descriptions. > > This leaves the question how to determine the str_offsets_base to use. > This seems to require to lookup the CU DIE(s) that refers to the line > table and see if one of them has a DW_AT_str_offsets_base. > > While this is possible, it isn't very efficient when just parsing the > .debug_line table itself. > > Would it make sense to add the str_offsets_base to the line table > header (for DWARF6)? > > Or am I missing a better/simpler way to handle DW_FORM_strx in > .debug_line tables? Or maybe using DW_FORM_strx forms in the > .debug_line table is not something that is really done (I haven't > actually found any producer which does). > Yeah, I think it's not sometihng that's really done - it'd conflict with the "strip everything but .debug_line and .debug_line_str" that was enabled in DWARFv5, with the intent to allow string deduplication of path/directory/file names, while still ensuring that non-line debug info could be stripped while keeping the line table in tact/usable. (in DWARFv4 the line table's strings were in the .debug_line section, so they couldn't be deduplicated, they were moved out to allow deduplication but to a separate section so they could be preserved by stripping tools while still stripping all the other strings (which make up a lotl of debug info)) strx in non-dwo is there to reduce the number of relocations when referencing a string in a string pool many times - but the line table already has that functionality built in, by numbering the file/directories and referencing them by number in the line table state machine. So there isn't any/much motivation to use a strx encoding - and if there was, we'd want that strx to reference into the line_str section, but we probably wouldn't want to put the line_str_offsets table in a separate section, so it'd end up in the .debug_line section, and... we already have that numbering system in there. So, yeah, if it's not already clear enough -probably just forbidding strx in non-dwo line tables would be OK?
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