Hi Simon, On Fri, 2020-11-13 at 10:41 -0500, Simon Marchi wrote: > So in the end the logical thing to do when encountering a > DW_FORM_rnglistx in a split-unit, in order to support everybody, is > probably to go to the .debug_rnglists.dwo section, if there's one, > disregarding the (inherited) DW_AT_rnglists_base. If there isn't, then > try the linked file's .debug_rnglists section, using > DW_AT_rnglists_base. If there isn't, then something is malformed.
Yes, I think that makes sense. > > I interpreted it as when there is a base attribute in the (skeleton) > > unit, then the corresponding section (index table) can be found in the > > main object file. > > That doesn't work with how clang produces it, AFAIU. There is a > DW_AT_rnglists_base attribute in the skeleton and a .debug_rnglists in > the linked file, which is used for the skeleton's DW_AT_ranges > attribute. And there is also .debug_rnglists.dwo sections in the DWO > files. So DW_FORM_rnglistx values in the skeleton use the > .debug_rnglists in the linked file, while the DW_FORM_rnglistx values > in the DWO file use the .debug_rnglists.dwo in that file (even though > there is a DW_AT_rnglists_base in the skeleton). I would have expected the skeleton's DW_AT_ranges to use DW_FORM_secoffset, not DW_FORM_rnglistx. Precisely because you would then get an ambiguity. But it would indeed be good to handle that situation. > > I think it depends on who exactly you ask and what their specific > > goals/setups are. Both things, reducing the number of relocations and > > moving data out of the main object file, are independently useful in > > different context. But I think it is mainly reducing the number of > > relocations that is beneficial. For example clang (but not yet gcc) > > supports having the .dwo sections themselves in the main object file > > (using SHF_EXCLUDED for the .dwo sections, so the linker will still > > skip them). Which is also a possibility that the spec describes and > > which really makes split DWARF much more usable, because then you don't > > need to change your build system to deal with multiple output files. > > Not sure I understand. Does that mean that the .dwo sections are > emitted in the .o files, and that's the end of the road for them? The > DW_AT_dwo_name attributes of the skeletons then refer to the .o files? Yes, precisely. I am not sure whether it is already in any released clang, but if it is you could try -gsplit-dwarf=single to see an example. Note that elfutils libdw doesn't yet handle that variant. Luckily not because of a design issue, but because there are some sanity checks that trigger when seeing a .debug_xxx and .debug_xxx.dwo section in the same file. I have a partial patch to fix that and make it so that you can explicitly open a file as either a "main" Dwarf or "split" Dwarf. The only thing it doesn't do yet is share the file handle between the Dwarf object (which isn't strictly needed, but would be a nice optimization). I actually think having a "single" split-dwarf file (.o == .dwo) is the best way to support Split Dwarf more generically because then it would simply work without having to adjust all build systems to work with/around separate .dwo files. Cheers, Mark
