[lld-macho] Skip over symbols in un-parsed debug info sections

[int3]

Summary

clang appears to emit symbols in __debug_aranges, at least
for arm64... in the examples I've seen, it doesn't seem like those
symbols are referenced outside of __DWARF, so I think they're safe to
ignore. But hopefully @clayborg can confirm.

[clayborg]

yes, anything in the debug_aranges are safe to ignore as dsymutil will create a new debug_aranges and doesn't end up using any of the debug_aranges sections from the .o files and nothing from debug_aranges ends up in the debug map.

[int3]

Thanks!

lld/MachO/InputFiles.cpp

for (size_t i = 0, n = nList.size(); i < n; ++i) {
 
     if ((sym.n_type & N_TYPE) != N_SECT) {
       symbols[i] = parseNonSectionSymbol(sym, name);
       continue;
     }
 
     const section_64 &sec = sectionHeaders[sym.n_sect - 1];
     SubsectionMap &subsecMap = subsections[sym.n_sect - 1];
-    assert(!subsecMap.empty());
+
+    // parseSections() may have chosen not to parse this section.
+    if (subsecMap.empty())
+      continue;
+
     uint64_t offset = sym.n_value - sec.addr;
 
     // If the input file does not use subsections-via-symbols, all symbols can
     // use the same subsection. Otherwise, we must split the sections along
     // symbol boundaries.
     if (!subsectionsViaSymbols) {
       symbols[i] = createDefined(sym, name, subsecMap[0], offset);
       continue;

lld/MachO/SyntheticSections.cpp

auto addSymbol = [&](std::vector<SymtabEntry> &symbols, Symbol *sym) {
     symbols.push_back({sym, strx});
   };
 
   // Local symbols aren't in the SymbolTable, so we walk the list of object
   // files to gather them.
   for (InputFile *file : inputFiles) {
     if (auto *objFile = dyn_cast<ObjFile>(file)) {
       for (Symbol *sym : objFile->symbols) {
+        if (sym == nullptr)
+          continue;
         // TODO: when we implement -dead_strip, we should filter out symbols
         // that belong to dead sections.
         if (auto *defined = dyn_cast<Defined>(sym)) {
           if (!defined->isExternal()) {
             StringRef name = defined->getName();
             if (!name.startswith("l") && !name.startswith("L"))
               addSymbol(localSymbols, sym);
           }

lld/test/MachO/stabs.s

.set Lset1, Lsection_abbrev-Lsection_abbrev ## Offset Into Abbrev. Section
   .quad  Lfunc_begin0            ## DW_AT_low_pc
 .set Lset3, Lfunc_end0-Lfunc_begin0     ## DW_AT_high_pc
   .long  Lset3
   .byte  0                       ## End Of Children Mark
 Ldebug_info_end0:
 .subsections_via_symbols
 .section  __DWARF,__debug_line,regular,debug
 
+.section  __DWARF,__debug_aranges,regular,debug
+ltmp1:
+  .byte 0
+
 #--- no-debug.s
 ## This file has no debug info.
 .text
 .globl _no_debug
 _no_debug:
   ret