On 2020-11-13 10:18 a.m., Mark Wielaard wrote: > That too, but I was actually referring to the sections that define > Range List and Location List Tables (7.28 and 7.29) which define the > meaning of DW_AT_rnglists_base and DW_AT_loclists_base. But you could > also look at 3.1.3 Split Full Compilation Unit Entries which says that > those base attributes are inherited from the corresponding skeleton > compilation unit for a split unit.
Hmm, indeed, if we interpret that sentence in 3.1.3 to the letter, it suggests that the the DW_FORM_rnglistx attributes in the DWO are meant to point in the linked file's .debug_rnglists. Otherwise, inheriting DW_AT_rnglists_base wouldn't be meaningful. But when DWO files use a .debug_rnglists.dwo section, it doesn't make sense to consider the inherited DW_AT_rnglists_base. 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. >> What I understand from this is that the rnglist class and >> DW_AT_rnglists_base attribute help reduce the number of relocations in >> the non-split case (it removes the need for relocations from >> DW_AT_ranges attribute values in .debug_info to .debug_rnglists). I >> don't understand it as saying anything about where to put the rnglist >> data in the split-unit case. > > 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). > At least that is how elfutils libdw interprets the > base attributes, not just for rnglists_base, but also str_offsets_base, > addr_base, etc. And that is also how/when GCC emits them. > >>> So I believe both encodings are valid according to the spec. It just >>> depends on what you are optimizing for, small main object file size or >>> smallest encoding with least number of indirections. >> >> So, if I understand correctly, gcc's way of doing things (putting all >> the rnglists in a common .debug_rnglists section) reduces the overall >> size of debug info since the rnglists can use the direct addressing >> rnglists entries (e.g. DW_RLE_start_end) rather than the indirect ones >> (e.g. DW_RLE_startx_endx). But this come at the expense of a lot of >> relocations in the rnglists themselves, since they refer to addresses >> directly. > > Yes, and it reduces the number of .debug_addr entries that need > relocations. > >> I thought that the main point of split-units was to reduce the number of >> relocations processed by the linker and data moved around by the linker, >> to reduce link time and provide a better edit-build-debug cycle. Is >> that the case? > > 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? Simon