[lld] [lld][ELF] Add --debug-names to create merged .debug_names. (PR #86508)

[Fangrui Song via llvm-commits]

================
@@ -2711,6 +2714,609 @@ static uint32_t computeGdbHash(StringRef s) {
   return h;
 }
 
+// 4-byte alignment ensures that values in the hash lookup table and the name
+// table are aligned.
+DebugNamesBaseSection::DebugNamesBaseSection()
+    : SyntheticSection(0, SHT_PROGBITS, 4, ".debug_names") {}
+
+// Get the size of the .debug_names section header in bytes for DWARF32:
+static uint32_t getDebugNamesHeaderSize(uint32_t augmentationStringSize) {
+  return /* unit length */ 4 +
+         /* version */ 2 +
+         /* padding */ 2 +
+         /* CU count */ 4 +
+         /* TU count */ 4 +
+         /* Foreign TU count */ 4 +
+         /* Bucket Count */ 4 +
+         /* Name Count */ 4 +
+         /* Abbrev table size */ 4 +
+         /* Augmentation string size */ 4 +
+         /* Augmentation string */ augmentationStringSize;
+}
+
+static Expected<DebugNamesBaseSection::IndexEntry *>
+readEntry(uint64_t &offset, const DWARFDebugNames::NameIndex &ni,
+          uint64_t entriesBase, DWARFDataExtractor &namesExtractor,
+          const LLDDWARFSection &namesSec) {
+  auto ie = makeThreadLocal<DebugNamesBaseSection::IndexEntry>();
+  ie->poolOffset = offset;
+  Error err = Error::success();
+  uint64_t ulebVal = namesExtractor.getULEB128(&offset, &err);
+  if (err)
+    return createStringError(inconvertibleErrorCode(),
+                             "invalid abbrev code in entry: %s",
+                             toString(std::move(err)).c_str());
+  if (ulebVal <= UINT32_MAX)
+    ie->abbrevCode = static_cast<uint32_t>(ulebVal);
+  else
+    return createStringError(inconvertibleErrorCode(),
+                             "abbrev code in entry too large for DWARF32: %d",
+                             ulebVal);
+  auto it = ni.getAbbrevs().find_as(ie->abbrevCode);
+  if (it == ni.getAbbrevs().end())
+    return createStringError(inconvertibleErrorCode(),
+                             "entry abbrev code not found in abbrev table: %d",
+                             ie->abbrevCode);
+
+  DebugNamesBaseSection::AttrValue attr, cuAttr = {0, 0};
+  for (DWARFDebugNames::AttributeEncoding a : it->Attributes) {
+    if (a.Index == dwarf::DW_IDX_parent) {
+      if (a.Form == dwarf::DW_FORM_ref4) {
+        attr.attrValue = namesExtractor.getU32(&offset, &err);
+        attr.attrSize = 4;
+        ie->parentOffset = entriesBase + attr.attrValue;
+      } else if (a.Form != DW_FORM_flag_present)
+        return createStringError(inconvertibleErrorCode(),
+                                 "invalid form for DW_IDX_parent");
+    } else {
+      switch (a.Form) {
+      case DW_FORM_data1:
+      case DW_FORM_ref1: {
+        attr.attrValue = namesExtractor.getU8(&offset, &err);
+        attr.attrSize = 1;
+        break;
+      }
+      case DW_FORM_data2:
+      case DW_FORM_ref2: {
+        attr.attrValue = namesExtractor.getU16(&offset, &err);
+        attr.attrSize = 2;
+        break;
+      }
+      case DW_FORM_data4:
+      case DW_FORM_ref4: {
+        attr.attrValue = namesExtractor.getU32(&offset, &err);
+        attr.attrSize = 4;
+        break;
+      }
+      default:
+        return createStringError(
+            inconvertibleErrorCode(),
+            "unrecognized form encoding %d in abbrev table", a.Form);
+      }
+    }
+    if (err)
+      return createStringError(inconvertibleErrorCode(),
+                               "error while reading attributes: %s",
+                               toString(std::move(err)).c_str());
+    if (a.Index == DW_IDX_compile_unit)
+      cuAttr = attr;
+    else if (a.Form != DW_FORM_flag_present)
+      ie->attrValues.push_back(attr);
+  }
+
+  // Canonicalize abbrev by placing the CU/TU index at the end.
+  ie->attrValues.push_back(cuAttr);
+
----------------
MaskRay wrote:

Our convention does not add a blank line here

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