[DWARF] Use sequential integers for the IDs of the SymbolFileDWOs

[labath]

Summary

Instead of using the offset of the contained compile unit, we use it's
ID. The goal of this change is two-fold:

• free up space in the user_id_t representation to enable storing the debug-info-carrying section (debug_types/debug_info) without decreasing the amount of debug info we can address (as would be the case with D61503).
• be a step towards supporting DWO files containing more than one unit (important for debug_types+dwo, but can also happen with regular dwo+lto). For this part to fully work we'd still need to add a way to lookup the SymbolFileDWO without going through GetCompileUnitAtIndex, but making sure things don't accidentally work because the SymbolFile ID is the same as compile unit offset is a step towards that.

[clayborg]

Might want the person that did DWO support to chime in here if that wasn't you?

[labath]

It was @tberghammer. I'm cc-ing him to these patches, but I'm not sure what's his state these days...

source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.cpp

SymbolFileDWARF::DecodedUID SymbolFileDWARF::DecodeUID(lldb::user_id_t uid) {
   // references to other DWARF objects and we must be ready to receive a
   // "lldb::user_id_t" that specifies a DIE from another SymbolFileDWARF
   // instance.
   if (SymbolFileDWARFDebugMap *debug_map = GetDebugMapSymfile()) {
     SymbolFileDWARF *dwarf = debug_map->GetSymbolFileByOSOIndex(
         debug_map->GetOSOIndexFromUserID(uid));
     return {dwarf, {DW_INVALID_OFFSET, dw_offset_t(uid)}};
   }
-  return {this, {dw_offset_t(uid >> 32), dw_offset_t(uid)}};
+  uint32_t dwarf_id = uid >> 32;
+  dw_offset_t die_offset = uid;
+
+  if (die_offset == DW_INVALID_OFFSET)
+    return {nullptr, DIERef()};
+
+  SymbolFileDWARF *dwarf = this;
+  if (DebugInfo()) {
+    if (DWARFUnit *unit = DebugInfo()->GetCompileUnitAtIndex(dwarf_id)) {
+      if (unit->GetDwoSymbolFile())
+        dwarf = unit->GetDwoSymbolFile();
+    }
+  }
+  return {dwarf, {DW_INVALID_OFFSET, die_offset}};
 }
 
 DWARFDIE
 SymbolFileDWARF::GetDIE(lldb::user_id_t uid) {
   // This method can be called without going through the symbol vendor so we
   // need to lock the module.
   std::lock_guard<std::recursive_mutex> guard(GetModuleMutex());
 

source/Plugins/SymbolFile/DWARF/SymbolFileDWARFDwo.cpp

 
 using namespace lldb;
 using namespace lldb_private;
 
 SymbolFileDWARFDwo::SymbolFileDWARFDwo(ObjectFileSP objfile,
                                        DWARFUnit *dwarf_cu)
     : SymbolFileDWARF(objfile.get()), m_obj_file_sp(objfile),
       m_base_dwarf_cu(dwarf_cu) {
-  SetID(((lldb::user_id_t)dwarf_cu->GetOffset()) << 32);
+  SetID(((lldb::user_id_t)dwarf_cu->GetID()) << 32);
 }
 
 void SymbolFileDWARFDwo::LoadSectionData(lldb::SectionType sect_type,
                                          DWARFDataExtractor &data) {
   const SectionList *section_list =
       m_obj_file->GetSectionList(false /* update_module_section_list */);
   if (section_list) {
     SectionSP section_sp(section_list->FindSectionByType(sect_type, true));

 
 TypeSystem *
 SymbolFileDWARFDwo::GetTypeSystemForLanguage(LanguageType language) {
   return GetBaseSymbolFile()->GetTypeSystemForLanguage(language);
 }
 
 DWARFDIE
 SymbolFileDWARFDwo::GetDIE(const DIERef &die_ref) {
-  lldbassert(m_base_dwarf_cu->GetOffset() == die_ref.cu_offset);
+  lldbassert(die_ref.cu_offset == m_base_dwarf_cu->GetOffset() ||
+             die_ref.cu_offset == DW_INVALID_OFFSET);
   return DebugInfo()->GetDIEForDIEOffset(die_ref.die_offset);
 }