[ELF] - Simplify readAddressArea() implementation.

[grimar]

Summary

Approach significantly simplifies LLD .gdb_index code and makes it much faster.
Also it should resolve issues, like D33176 tries to address once and forever in a clean way.

LLC binary linking without patch and without --gdb-index: 1,599241063
LLC binary linking without patch and with --gdb-index: 6,064316262
LLC binary linking with patch and with --gdb-index: 4,116792104

Time spent for building gdbindex changes from (6,064316262 - 1,599241063 == 4,465075199)
to (4,116792104- 1,599241063 == 2,517551041).
That is 2,517551041/4,465075199 = 0,564 or about 44% speedup.

[ruiu]

This looks better, but how does it work? grep -w -r DWARFAddress . shows nothing in my LLVM repository.

[grimar]

D33184 was committed finally, so that should be ready to land. Rafael gave LGTM earlier. Rui do you have any objections or we can land it ?

[grimar]

Rui, could you look at this, please ?

[ruiu]

LGTM

[grimar]

Rafael, Rui, thanks for review.

During rebasing it I faced with 2 more issues I did not expect to see (otherwise I would rebase it earlier before asking for final reviews).
Fortunately they seems to be minor so I am going to commit this with fixes for them.

1. I had to add S->Live check. That is fine because was done by D33175 already, so I just added this check to support already existent case we support.
2. I had to skip ranges which has R.LowPC == R.HighPC before adding them to index. That allows to pass testcase introduced by D33176. That change is consistent with gold

behavior and correct because DWARF manual says that: "A range list entry (but not a base address selection or end of list entry) whose beginning and
ending addresses are equal has no effect because the size of the range covered by such an entry is zero.". So there is no much sence to add
empty ranges to index and our previous logic already did not do that implicitly.

These two changes are consistent with current LLD behavior already and I hope fine.

lld/trunk/ELF/SyntheticSections.cpp

 readAddressArea(DWARFContext &Dwarf, InputSection *Sec, size_t CurrentCU) {
   std::vector<AddressEntry> Ret;
 
   for (std::unique_ptr<DWARFCompileUnit> &CU : Dwarf.compile_units()) {
     DWARFAddressRangesVector Ranges;
     CU->collectAddressRanges(Ranges);
 
     ArrayRef<InputSectionBase *> Sections = Sec->File->getSections();
-    for (DWARFAddressRange &R : Ranges)
-      if (InputSection *S = findSection(Sections, R.LowPC))
-        Ret.push_back({S, R.LowPC - S->getOffsetInFile(),
-                       R.HighPC - S->getOffsetInFile(), CurrentCU});
+    for (DWARFAddressRange &R : Ranges) {
+      InputSectionBase *S = Sections[R.SectionIndex];
+      if (!S || S == &InputSection::Discarded || !S->Live)
+        continue;
+      // Range list with zero size has no effect.
+      if (R.LowPC == R.HighPC)
+        continue;
+      Ret.push_back({cast<InputSection>(S), R.LowPC, R.HighPC, CurrentCU});
+    }
     ++CurrentCU;
   }
   return Ret;
 }
 
 static std::vector<std::pair<StringRef, uint8_t>>
 readPubNamesAndTypes(DWARFContext &Dwarf, bool IsLE) {
   StringRef Data[] = {Dwarf.getGnuPubNamesSection(),
                       Dwarf.getGnuPubTypesSection()};
 
   std::vector<std::pair<StringRef, uint8_t>> Ret;
   for (StringRef D : Data) {
     DWARFDebugPubTable PubTable(D, IsLE, true);
     for (const DWARFDebugPubTable::Set &Set : PubTable.getData())
       for (const DWARFDebugPubTable::Entry &Ent : Set.Entries)
         Ret.push_back({Ent.Name, Ent.Descriptor.toBits()});
   }
   return Ret;
 }
 
-class ObjInfoTy : public llvm::LoadedObjectInfo {
-  uint64_t getSectionLoadAddress(const object::SectionRef &Sec) const override {
-    auto &S = static_cast<const object::ELFSectionRef &>(Sec);
-    if (S.getFlags() & ELF::SHF_ALLOC)
-      return S.getOffset();
-    return 0;
-  }
-
-  std::unique_ptr<llvm::LoadedObjectInfo> clone() const override { return {}; }
-};
-
 void GdbIndexSection::readDwarf(InputSection *Sec) {
   Expected<std::unique_ptr<object::ObjectFile>> Obj =
       object::ObjectFile::createObjectFile(Sec->File->MB);
   if (!Obj) {
     error(toString(Sec->File) + ": error creating DWARF context");
     return;
   }
 
-  ObjInfoTy ObjInfo;
-  DWARFContextInMemory Dwarf(*Obj.get(), &ObjInfo);
+  DWARFContextInMemory Dwarf(*Obj.get());
 
   size_t CuId = CompilationUnits.size();
   for (std::pair<uint64_t, uint64_t> &P : readCuList(Dwarf, Sec))
     CompilationUnits.push_back(P);
 
   for (AddressEntry &Ent : readAddressArea(Dwarf, Sec, CuId))
     AddressArea.push_back(Ent);