On Tue, Sep 1, 2020 at 6:59 AM David Anderson <dave...@linuxmail.org> wrote:
> On 8/31/20 8:39 PM, David Blaikie wrote: > > Hmm... yeah. I guess LLVM implements rnglistx /rnglist_base the same > > as strx/str_offsets_base. Where it assumes that any *x encoding refers > > to entities in the .dwo, even in the absence of a > > rnglists_base/str_offsets_base in the split full unit. I had thought > > we'd implemented it to emit a rnglists_base in the split full unit, > > which would've been in contrast to the str_offsets_base - so my > > mistake/apologies for the previous description. > > So the base addresses are in the skeleton and the actual section > (rnglists/loclists/str_offsets/str) > can go with Split Full (i.e, in a .dwo) if it has no addresses but must > go with the skeleton if has addresses. > Sorry, I missed a step/not sure I understand this ^ comment - could you rephrase/expound/clarify a bit? I'm suggesting there are two possible ways we could spec this: 1) all loclistx, rnglistx, strx in .dwo are required/guaranteed/defined to always refer to debug_loclists.dwo, debug_rnglists.dwo, debug_str_offsets.dwo In this model, there's no way to -> this is how parts of the spec seem to be already defined, and how strx/loclistx work in the DWARFv4 GNU extension Split DWARF implementation (there's no loclists_base or str_offsets_base - the strx/loclistx in debug_info.dwo is assumed to refer to the str_offsets.dwo/loc.dwo sections) 2) allow/require a *x encoding in a split full unit to refer (when combined with a *_base attribute on the skeleton CU) a split full unit's *_rnglist/loclist/str_offsets (non-dwo) contributions if the split full unit has no *_base attributes. The split full unit's *_base attribute could then be optionally specified to say "resolve *x encodings relative to/in the split unit, instead of searching back up into the skeleton unit". (1) is what LLVM's implemented at the moment, and changing that to (2) wouldn't be too hard (we'd just always emit a *_base attribute in the split full unit any time we were using the corresponding *x forms in the split full unit) > > Ok. > > This way the standard is not in error as written. I think there's still some contradictions - the two bits I quoted previously: "The DW_AT_addr_base and DW_AT_str_offsets_base attributes provide context that may be necessary to interpret the contents of the corresponding split DWARF object file." "The following attributes are not part of a split full compilation unit entry but instead are inherited (if present) from the corresponding skeleton compilation unit: DW_AT_low_pc, DW_AT_high_pc, DW_AT_ranges, DW_AT_stmt_list, DW_AT_comp_dir, DW_AT_str_offsets_base, DW_AT_addr_base and DW_AT_rnglists_base." If we're going with model (1), then the first of those two quotations should remove "and DW_AT_str_offsets_base" and the second should carveout a special case for str_offsets_base and rnglists_base to say they cannot be specified on a skeleton unit, but also are /not/ inherited by the split full compilation unit. (essentially the split full compilation unit has implicit *_base attributes (if you think of them more like the (2) model above) equal to the size of the contribution headers for those 3 sections) > This understanding > restricts what information can be derived from > the Split Full CU by itself (ie, without the skeleton) a bit since the > base addresses are not in the Split Full CU DIE. > That confuses me a bit. If you have rnglists.dwo, for example - you can't use the actual rnglists_base from the skeleton CU to find the rnglists.dwo contribution (because the rnglists_base will be relocated in the final executable, to a value that has nothing to do with the rnglists.dwo contribution location in the dwo or the index-relative location in the dwp). So having the skeleton CU shouldn't make you any more or less able to parse/dump/etc *x forms in a split full unit if we're using (1). If we're using (2), then the wording needs to change to say that you must specify *_base on the split full unit if you want to resolve *x forms in the split full unit into rnglists.dwo/loclists.dwo/str_offsets.dwo references, and in the absence of *_base on the split full unit, such *x forms would use the *_base in the skeleton unit and the rnglists/loclists/str_offsets (non-dwo) in the linked executable. > > Mike Eager: please delete the new issue 200831.1 as it is simply wrong. > > DavidA > > > > >
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