[lld] [lld][ELF] Implement merged .debug_names section. (PR #86508)

[Fangrui Song via llvm-commits]

================
@@ -2690,6 +2693,759 @@ static uint32_t computeGdbHash(StringRef s) {
   return h;
 }
 
+DebugNamesSection::DebugNamesSection()
+    : SyntheticSection(0, SHT_PROGBITS, 1, ".debug_names") {}
+
+template <class ELFT, class RelTy>
+void DebugNamesSection::getNameRelocsImpl(
+    InputSection *sec, ArrayRef<RelTy> rels,
+    llvm::DenseMap<uint32_t, uint32_t> &relocs) {
+  for (auto &rel : rels) {
+    Symbol &sym = sec->getFile<ELFT>()->getRelocTargetSym(rel);
+    relocs[rel.r_offset] = sym.getVA(getAddend<ELFT>(rel));
+  }
+}
+
+template <class ELFT>
+void DebugNamesSection::getNameRelocs(
+    InputSectionBase *base, llvm::DenseMap<uint32_t, uint32_t> &relocs) {
+  auto *sec = cast<InputSection>(base);
+  const RelsOrRelas<ELFT> rels = sec->template relsOrRelas<ELFT>();
+  if (rels.areRelocsRel())
+    getNameRelocsImpl<ELFT>(sec, rels.rels, relocs);
+  else
+    getNameRelocsImpl<ELFT>(sec, rels.relas, relocs);
+}
+
+void DebugNamesSection::writeTo(uint8_t *buf) { invokeELFT(writeToImpl, buf); }
+
+template <class ELFT> void DebugNamesSection::writeToImpl(uint8_t *buf) {
+  SmallVector<uint32_t, 0> mergedCuOffsets;
+  SmallVector<uint32_t, 0> mergedTuOffsets;
+  llvm::DenseMap<uint32_t, uint32_t> strOffsets;
+  SmallVector<llvm::DenseMap<uint32_t, uint32_t>, 0> chunksRelocs;
+  chunksRelocs.reserve(numChunks);
+
+  for (size_t i = 0; i < numChunks; ++i) {
+    DebugNamesOutputChunk &chunk = outputChunks[i];
+    InputSectionBase *base = inputDebugNamesSections[i];
+    llvm::DenseMap<uint32_t, uint32_t> relocs;
----------------
MaskRay wrote:

`DenseMap<uint32_t, uint32_t> &relocs = chunksRelocs.emplace_back();` to save a move.

Please follow the convention: we normally don't use `llvm::` in `.cpp` files. `llvm::DenseMap` is for headers.

https://github.com/llvm/llvm-project/pull/86508