ok, so I was able to create a DWZ example and see how this is laid out and I now understand the format. I will see if I can modify this patch in a way that will work for DWZ and for .debug_types.
> On Apr 10, 2018, at 8:46 AM, Greg Clayton <clayb...@gmail.com> wrote: > > > >> On Apr 7, 2018, at 2:04 AM, Jan Kratochvil <jan.kratoch...@redhat.com >> <mailto:jan.kratoch...@redhat.com>> wrote: >> >> On Sat, 07 Apr 2018 00:52:26 +0200, Greg Clayton wrote: >>> Take look at how LLVM does it. I believe my changes mirror that approach. >> >> LLVM does not support partial units so there is nothing to look at there. >> >> >>> DWARFUnit should be the base class for anything that needs to hand out DIEs. >> >> That's OK for partial units. >> >> >>> Any specializations should be inheriting from DWARFUnit, like both >>> DWARFCompileUnit and DWARFTypeUnit. >> >> I see now the source of misunderstanding: >> >> DWARFCompileUnit = DW_TAG_compile_unit >> DWARFTypeUnit = DW_TAG_type_unit >> DWARFPartialUnit != DW_TAG_partial_unit >> ^^^^ >> BTW DW_TAG_imported_unit is importing (="caller") tag, not a unit >> (="callee"). >> >> DW_TAG_partial_unit gets read in (by >> DWARFDebugInfo::ParseCompileUnitHeadersIfNeeded) as DWARFCompileUnit because >> there is no quick enough way to find the difference. It would require >> reading >> the first DIE tag which means to read and decode .debug_abbrev for each unit >> being scanned. > > If there is no structural difference other than the first DW_TAG, is there a > reason you would need to know the difference? It would be fine to just add > extra methods on DWARFCompileUnit that do partial unit kind of things if it > is a partial unit? Let me know if I am missing something? > >> >> DWARFPartialUnit is used only as a remapping of DWARFCompileUnit to a new >> offset without any new data (there is stored only a new remapped offset whose >> value is only made up internally in LLDB) at the moment someone uses >> DW_TAG_imported_unit for it - at that moment we easily know that unit has to >> be DW_TAG_partial_unit. > > It is remapped to a new offset just to keep LLDB's lldb::user_id_t stuff > happy? A partial unit can refer to an external file on disk. The remapping is > solely making a unique offset by adding an offset to the offset of the > external file? > >> >> Therefore DWARFCompileUnit and DWARFTypeUnit both contain some their own >> data. >> But DWARFPartialUnit is just a remapping of DWARFCompileUnit (containing >> DW_TAG_partial_unit) to a new offset without any new data. Particularly >> m_die_array is not in DWARFPartialUnit. > > In reading the DWZ it sounded like you can only have 1 external debug info > file and that external debug info file can't itself refer to another? Is this > what the DWARFPartialUnit would be for and this is the only remapping that > needs to happen? > >> >> DWARFTypeUnit can be recognized easily as it is either in .debug_types >> (<=DWARF-4) or the unit header contains DW_UT_type (>=DWARF-5). >> DWARFPartialUnit (for DW_TAG_partial_unit) cannot be recognized easily first. >> Besides that one would need then some DWARFRemappedPartialUnit for what I use >> DWARFPartialUnit now. >> >> I have implemented it according to your advice from this mail - at least >> according to how I understood it: >> [lldb-dev] RFC for DWZ = DW_TAG_imported_unit + DWARF-5 supplementary >> files >> https://lists.llvm.org/pipermail/lldb-dev/2017-August/012664.html >> <https://lists.llvm.org/pipermail/lldb-dev/2017-August/012664.html> >> >> It does not try to share anything at AST level, it only shares DWARF data >> (and >> thus DWARFCompileUnit). Given that DWZ finds arbitrary unique DWARF subtrees >> I find more logical to decode it at DWARF (and not at AST) level. >> >> You wrote: >> # The drawback is this won't allow sharing /tmp/shared1 or /tmp/shared2 >> # between two different top level DWARF files, but it does allow one >> # clang::ASTContext to be used between all of them. >> >> In my implementation /tmp/shared1 >> (/usr/lib/debug/.dwz/coreutils-8.27-20.fc27.x86_64) is shared between >> multiple >> *.so files (which use DW_TAG_imported_unit) at DWARF level, also >> clang::ASTContext is shared. >> >> >> # SymbolFileDWARFDebugMap makes it lldb::user_id_t contain the CU index in >> the >> # top 32 bits, and the DIE offset within that .o file's DWARF in the bottom >> 32 >> # bits. You could do something similar in your case where the top 32 bits is >> # the index of the DWARF file in the "dwarf" array that would be maintained >> # in a new SymbolFileDWARFDWZ subclass. >> >> DW_TAG_imported_unit+DW_TAG_partial_unit can be also used for optimization of >> a single file (without /usr/lib/debug/.dwz/* file which is used exclusively >> for DW_TAG_partial_unit entries). Additionally the tags can be also used for >> recursive inclusion. I haven't found how to use the top 32 bits for that. >> I just reserve new remapped offset space for each DW_TAG_imported_unit (in >> the >> bottom 32 bits). >> >> I tried first to implement dw_offset_t caller (=unit with >> DW_TAG_imported_unit) to be tracked along any dw_offset_t DIE offset but that >> would require huge changes of the DWARF parsing code everywhere. Also it >> cannot work well given the inclusion is recursive (so we would need >> std::vector<dw_offset_t> of the callers stack). >> >> >> >>> I have no idea what are in your other patches >> >> OK, so there was a gross misunderstanding and my DWARFUnit implemented >> something very different from what you expected+approved. I am sure fine with >> that but I needed to understand first why you did that big screw up of my >> carefully coded patch. >> >> >>>> This is how DWARFPartialUnit works, it is only a DWARFCompileUnit remapped >>>> to >>>> new offset. I do not see how to implement it transparently without the >>>> accessor (and without needlessly copying all the data fields many times >>>> into >>>> each DWARFPartialUnit instance). >>> >>> What extra functions are needed for a partial unit that can't be done in >>> a DWARFCompileUnit? Seems like they both contain things, but the partial >>> unit can be referenced from compile units. >> >> DWARFPartialUnit is only a remapping, not really a representation of DWARF >> file data. So DWARFPartialUnit cannot contain its own m_die_array, m_version >> and other data members you have moved back to DWARFUnit. >> >> >>> As we are saying, we are trying to make the layering more like LLVM's >>> layering, so that is what I meant by "fix the layering". I believe we should >>> strive for being more like LLVM so that any transition can happen without >>> major re-organization of the DWARF code later. So I would like the get the >>> ok the revert the revert if you on board with what my suggestions are in the >>> paragraph. I know this will require modifications to your patches and >>> apologize for that. >>> >>> Let me know what you think, >> >> I would like to know what is the approved plan / upstreaming order regarding >> all the planned changes: >> >> (1) Your DWARFTypeUnit patch - it makes it more aligned to LLVM DWARFUnits. >> (2) My DWZ patch - it is a new feature with no counterpart in LLVM >> DWARFUnits. >> (3) Replacement of LLDB DWARFUnits with LLVM DWARFUnits. >> >> I can sure work even on (3) but after half a year of work on DWZ support >> which >> completely blocks LLDB for Red Hat usage (as Red Hat requires "upstream >> first" >> to prevent heavy forks like what happened for Red Hat GDB) it makes the DWZ >> upstreaming possibility too far for me to start refactoring LLDB for (3) >> first >> - before upstreaming (2). >> >> >> Thanks, >> Jan > > We need to solve (1) first so we can move onto (2). (3) can wait IMHO, but if > someone really wants to tackle that it is fine. > > I think I need to see any example of this DWZ so I can intelligently comment > on thing. The brief description of DWZ at > http://www.dwarfstd.org/ShowIssue.php?issue=120604.1&type=open > <http://www.dwarfstd.org/ShowIssue.php?issue=120604.1&type=open> doesn't help > me see how this is actually laid out on disk and who refers to who and how > things are laid out in the DWARF itself. Can you somehow make some simple > executables available to me so I can see the DWARF in the main compile unit, > the partial unit and the info in the external DWZ file? I know you know what > you have done and fully understand this feature, but I would like to take a > look at an example so I can see how we can make both the .debug_types and > your feature work using similar work arounds. > > One idea that might work is to expand the DWARFDataExtractor so it can > contain enough information to create a lldb::user_id_t that would work for > .debug_info, .debug_types and for the alternate debug file. It would also > need to be able to get us to the right debug info when a SymbolFileDWARF API > was handed a lldb::user_id_t and be able to turn that back into something. > The current offset hack that was used for .debug_types could be used for this > purpose as well. But I really would like to see an example program that > refers to an external DWZ file so I can make sure what I recommend is viable. > > Greg Clayton
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