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

[Fangrui Song via llvm-commits]

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@@ -2690,6 +2693,756 @@ static uint32_t computeGdbHash(StringRef s) {
   return h;
 }
 
+template <class ELFT>
+DebugNamesSection<ELFT>::DebugNamesSection()
+    : SyntheticSection(0, SHT_PROGBITS, 1, ".debug_names") {}
+
+template <class ELFT>
+template <class RelTy>
+void DebugNamesSection<ELFT>::getNameRelocsImpl(
+    InputSection *sec, ArrayRef<RelTy> rels,
+    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<ELFT>::getNameRelocs(
+    InputSectionBase *base, DenseMap<uint32_t, uint32_t> &relocs) {
+  auto *sec = cast<InputSection>(base);
+  const RelsOrRelas<ELFT> rels = sec->template relsOrRelas<ELFT>();
+  if (rels.areRelocsRel())
+    getNameRelocsImpl(sec, rels.rels, relocs);
+  else
+    getNameRelocsImpl(sec, rels.relas, relocs);
+}
+
+template <class ELFT> void DebugNamesSection<ELFT>::writeTo(uint8_t *buf) {
+  SmallVector<uint32_t, 0> mergedCuOffsets;
+  SmallVector<uint32_t, 0> mergedTuOffsets;
+  DenseMap<uint32_t, uint32_t> strOffsets;
+  SmallVector<DenseMap<uint32_t, uint32_t>, 0> chunksRelocs;
+  chunksRelocs.reserve(numChunks);
+
+  for (size_t i = 0, e = numChunks; i != e; ++i) {
+    DebugNamesOutputChunk &chunk = outputChunks[i];
+    InputSectionBase *base = inputDebugNamesSections[i];
+    DenseMap<uint32_t, uint32_t> &relocs = chunksRelocs.emplace_back();
+    getNameRelocs(base, relocs);
+
+    // Update CuOffsets list with new data
+    for (uint32_t cuOffset : chunk.compilationUnits)
+      mergedCuOffsets.push_back(chunk.sec->outSecOff + cuOffset);
+
+    // TODO: Update TuOffsets list with new data
+  }
+
+  // Update the entries with the relocated string offsets.
+  for (NamedEntry &stringEntry : mergedEntries) {
+    uint32_t oldOffset = stringEntry.stringOffsetOffset;
+    uint32_t idx = stringEntry.chunkIdx;
+    stringEntry.relocatedEntryOffset = chunksRelocs[idx][oldOffset];
----------------
MaskRay wrote:

This step and `relocatedEntryoffset` are unneeded. Just compute the relocated offset at write time.

Fixed in my branch.

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