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

[Alexander Yermolovich via llvm-commits]

================
@@ -788,6 +792,136 @@ class RelroPaddingSection final : public SyntheticSection {
   void writeTo(uint8_t *buf) override {}
 };
 
+class DebugNamesSection final : public SyntheticSection {
+  // N.B. Everything in this class assumes that we are using DWARF32.
+  // If we move to DWARF64, most of this data will need to be re-sized,
+  // and the code that handles or manipulates it will need to be updated
+  // accordingly.
+
+public:
+  DebugNamesSection();
+  template <typename ELFT> static DebugNamesSection *create();
+  void writeTo(uint8_t *buf) override;
+  size_t getSize() const override { return sectionSize; }
+  bool isNeeded() const override;
+
+  void addSections(SmallVector<InputSectionBase *, 0> sec_list) {
+    inputDebugNamesSections = sec_list;
+  }
+
+  template <class ELFT> void writeToImpl(uint8_t *buf);
+
+  template <class ELFT, class RelTy>
+  void getNameRelocsImpl(InputSection *sec, ArrayRef<RelTy> rels,
+                         llvm::DenseMap<uint32_t, uint32_t> &relocs);
+
+  template <class ELFT>
+  void getNameRelocs(InputSectionBase *base,
+                     llvm::DenseMap<uint32_t, uint32_t> &relocs);
+
+  template <class ELFT>
+  void endianWrite(uint8_t size, uint8_t *buf_start, uint32_t offset,
+                   uint32_t data);
+
+  struct Abbrev : public llvm::FoldingSetNode {
+    uint32_t code;
+    uint32_t tag;
+    SmallVector<llvm::DWARFDebugNames::AttributeEncoding, 2> attributes;
+
+    void Profile(llvm::FoldingSetNodeID &id) const;
+  };
+
+  struct AttrValueData {
+    uint32_t attrValue;
+    uint8_t attrSize;
+  };
+
+  struct IndexEntry {
+    uint32_t abbrevCode;
+    uint32_t poolOffset;
+    union {
+      int32_t parentOffset = -1;
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ayermolo wrote:

Yep entry offsets are relative, with first one being zero for the purposes of parent. Discovered that when implementing support in BOLT. 

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