[lld] [llvm] [dyndbg][LLD][ELF] Initial LLD support for dynamic debugging (PR #214188)

[Fangrui Song via llvm-commits]

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@@ -1246,6 +1257,71 @@ void ObjFile<ELFT>::initializeSymbols(const object::ELFFile<ELFT> &obj) {
     sym->isUsedInRegularObj = true;
     sym->referenced = true;
   }
+
+  if (dynDbgSec)
+    initDynDbgSymbols();
+}
+
+// Add the undefined symbols of the embedded unoptimized dynamic debugging
+// object so that the outer link resolves the inner link's dependencies. Tag
+// those reached by an inner relocation against a SHT_PROGBITS SHF_ALLOC
+// section with `isDynDbgRef`; the rest are only needed by debug sections.
+template <class ELFT> void ObjFile<ELFT>::initDynDbgSymbols() {
+  MemoryBufferRef dbgMb(toStringRef(dynDbgSec->content()),
+                        mb.getBufferIdentifier());
+  std::unique_ptr<ELFFileBase> efb = createObjFile(ctx, dbgMb);
+  // Compare ekind (note ObjFile<ELFT>::classof only tests InputFile::kind()).
+  if (efb->ekind != ekind) {
+    Err(ctx) << this << ": " << dynDbgSecName
+             << " contains an incompatible ELF type";
+    return;
+  }
+  auto &dbgObj = cast<ObjFile<ELFT>>(*efb);
+  const object::ELFFile<ELFT> obj = dbgObj.getObj();
+
+  ArrayRef<Elf_Sym> dbgSyms = dbgObj.template getGlobalELFSyms<ELFT>();
+  SmallVector<bool, 0> globalUsed(dbgSyms.size());
+  auto setSymUsed = [&, firstGlobal = dbgObj.firstGlobal](uint32_t symIdx) {
+    if (symIdx >= firstGlobal)
+      globalUsed[symIdx - firstGlobal] = true;
+  };
+
+  for (const Elf_Shdr &sh : dbgObj.template getELFShdrs<ELFT>()) {
+    if (!isStaticRelSecType(sh.sh_type))
+      continue;
+    const Elf_Shdr &target = *CHECK2(obj.getSection(sh.sh_info), &dbgObj);
+    if (target.sh_type != SHT_PROGBITS || !(target.sh_flags & SHF_ALLOC))
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MaskRay wrote:

Why `target.sh_type != SHT_PROGBITS`? If the inner object's `.init_array` references the external symbol, is it ignored?

https://github.com/llvm/llvm-project/pull/214188