[lldb][DWARFUnit] Implement PeekDIEName query (llvm#78486)

This allows us to query the AT_Name of a DIE without parsing the entire
CU.

Part of the ongoing effort to support IDX_Parent in accelerator tables
[1].

[1]:
https://discourse.llvm.org/t/rfc-improve-dwarf-5-debug-names-type-lookup-parsing-speed/74151/44

lldb/source/Plugins/SymbolFile/DWARF/DWARFDebugInfo.cpp

@@ -191,3 +191,9 @@ DWARFDebugInfo::GetDIE(const DIERef &die_ref) {
     return cu->GetNonSkeletonUnit().GetDIE(die_ref.die_offset());
   return DWARFDIE(); // Not found
 }
+
+llvm::StringRef DWARFDebugInfo::PeekDIEName(const DIERef &die_ref) {
+  if (DWARFUnit *cu = GetUnit(die_ref))
+    return cu->GetNonSkeletonUnit().PeekDIEName(die_ref.die_offset());
+  return llvm::StringRef();
+}

lldb/source/Plugins/SymbolFile/DWARF/DWARFDebugInfo.h

@@ -43,6 +43,11 @@ class DWARFDebugInfo {
   bool ContainsTypeUnits();
   DWARFDIE GetDIE(const DIERef &die_ref);
 
+  /// Returns the AT_Name of this DIE, if it exists, without parsing the entire
+  /// compile unit. An empty is string is returned upon error or if the
+  /// attribute is not present.
+  llvm::StringRef PeekDIEName(const DIERef &die_ref);
+
   enum {
     eDumpFlag_Verbose = (1 << 0),  // Verbose dumping
     eDumpFlag_ShowForm = (1 << 1), // Show the DW_form type

lldb/source/Plugins/SymbolFile/DWARF/DWARFFormValue.cpp

@@ -18,8 +18,6 @@
 #include "DWARFFormValue.h"
 #include "DWARFUnit.h"
 
-class DWARFUnit;
-
 using namespace lldb_private;
 using namespace lldb_private::dwarf;
 using namespace lldb_private::plugin::dwarf;
@@ -502,7 +500,8 @@ dw_addr_t DWARFFormValue::Address() const {
       &offset, index_size);
 }
 
-DWARFDIE DWARFFormValue::Reference() const {
+std::pair<DWARFUnit *, uint64_t>
+DWARFFormValue::ReferencedUnitAndOffset() const {
   uint64_t value = m_value.value.uval;
   switch (m_form) {
   case DW_FORM_ref1:
@@ -516,9 +515,9 @@ DWARFDIE DWARFFormValue::Reference() const {
     if (!m_unit->ContainsDIEOffset(value)) {
       m_unit->GetSymbolFileDWARF().GetObjectFile()->GetModule()->ReportError(
           "DW_FORM_ref* DIE reference {0:x16} is outside of its CU", value);
-      return {};
+      return {nullptr, 0};
     }
-    return const_cast<DWARFUnit *>(m_unit)->GetDIE(value);
+    return {const_cast<DWARFUnit *>(m_unit), value};
 
   case DW_FORM_ref_addr: {
     DWARFUnit *ref_cu =
@@ -527,24 +526,29 @@ DWARFDIE DWARFFormValue::Reference() const {
     if (!ref_cu) {
       m_unit->GetSymbolFileDWARF().GetObjectFile()->GetModule()->ReportError(
           "DW_FORM_ref_addr DIE reference {0:x16} has no matching CU", value);
-      return {};
+      return {nullptr, 0};
     }
-    return ref_cu->GetDIE(value);
+    return {ref_cu, value};
   }
 
   case DW_FORM_ref_sig8: {
     DWARFTypeUnit *tu =
         m_unit->GetSymbolFileDWARF().DebugInfo().GetTypeUnitForHash(value);
     if (!tu)
-      return {};
-    return tu->GetDIE(tu->GetTypeOffset());
+      return {nullptr, 0};
+    return {tu, tu->GetTypeOffset()};
   }
 
   default:
-    return {};
+    return {nullptr, 0};
   }
 }
 
+DWARFDIE DWARFFormValue::Reference() const {
+  auto [unit, offset] = ReferencedUnitAndOffset();
+  return unit ? unit->GetDIE(offset) : DWARFDIE();
+}
+
 uint64_t DWARFFormValue::Reference(dw_offset_t base_offset) const {
   uint64_t value = m_value.value.uval;
   switch (m_form) {

lldb/source/Plugins/SymbolFile/DWARF/DWARFFormValue.h

@@ -60,6 +60,12 @@ class DWARFFormValue {
                                              const DWARFUnit *u);
   std::optional<uint8_t> GetFixedSize() const;
   DWARFDIE Reference() const;
+
+  /// If this is a reference to another DIE, return the corresponding DWARFUnit
+  /// and DIE offset such that Unit->GetDIE(offset) produces the desired DIE.
+  /// Otherwise, a nullptr and unspecified offset are returned.
+  std::pair<DWARFUnit *, uint64_t> ReferencedUnitAndOffset() const;
+
   uint64_t Reference(dw_offset_t offset) const;
   bool Boolean() const { return m_value.value.uval != 0; }
   uint64_t Unsigned() const { return m_value.value.uval; }

lldb/source/Plugins/SymbolFile/DWARF/DWARFUnit.cpp

@@ -663,6 +663,30 @@ DWARFUnit::GetDIE(dw_offset_t die_offset) {
   return DWARFDIE(); // Not found
 }
 
+llvm::StringRef DWARFUnit::PeekDIEName(dw_offset_t die_offset) {
+  DWARFDebugInfoEntry die;
+  if (!die.Extract(GetData(), this, &die_offset))
+    return llvm::StringRef();
+
+  // Does die contain a DW_AT_Name?
+  if (const char *name =
+          die.GetAttributeValueAsString(this, DW_AT_name, nullptr))
+    return name;
+
+  // Does its DW_AT_specification or DW_AT_abstract_origin contain an AT_Name?
+  for (auto attr : {DW_AT_specification, DW_AT_abstract_origin}) {
+    DWARFFormValue form_value;
+    if (!die.GetAttributeValue(this, attr, form_value))
+      continue;
+    auto [unit, offset] = form_value.ReferencedUnitAndOffset();
+    if (unit)
+      if (auto name = unit->PeekDIEName(offset); !name.empty())
+        return name;
+  }
+
+  return llvm::StringRef();
+}
+
 DWARFUnit &DWARFUnit::GetNonSkeletonUnit() {
   ExtractUnitDIEIfNeeded();
   if (m_dwo)

lldb/source/Plugins/SymbolFile/DWARF/DWARFUnit.h

@@ -187,6 +187,11 @@ class DWARFUnit : public UserID {
 
   DWARFDIE GetDIE(dw_offset_t die_offset);
 
+  /// Returns the AT_Name of the DIE at `die_offset`, if it exists, without
+  /// parsing the entire compile unit. An empty is string is returned upon
+  /// error or if the attribute is not present.
+  llvm::StringRef PeekDIEName(dw_offset_t die_offset);
+
   DWARFUnit &GetNonSkeletonUnit();
 
   static uint8_t GetAddressByteSize(const DWARFUnit *cu);

lldb/unittests/SymbolFile/DWARF/DWARFDIETest.cpp

@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "Plugins/SymbolFile/DWARF/DWARFDIE.h"
+#include "Plugins/SymbolFile/DWARF/DWARFDebugInfo.h"
 #include "TestingSupport/Symbol/YAMLModuleTester.h"
 #include "llvm/ADT/STLExtras.h"
 #include "gmock/gmock.h"
@@ -104,3 +105,85 @@ TEST(DWARFDIETest, ChildIteration) {
   DWARFDIE no_children_die(unit, die_child0);
   EXPECT_TRUE(no_children_die.children().empty());
 }
+
+TEST(DWARFDIETest, PeekName) {
+  const char *yamldata = R"(
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_EXEC
+  Machine: EM_386
+DWARF:
+  debug_str:
+    - 'NameType1'
+    - 'NameType2'
+  debug_abbrev:
+    - Table:
+        - Code:            0x00000001
+          Tag:             DW_TAG_compile_unit
+          Children:        DW_CHILDREN_yes
+          Attributes:
+            - Attribute:       DW_AT_language
+              Form:            DW_FORM_data2
+        - Code:            0x00000002
+          Tag:             DW_TAG_base_type
+          Children:        DW_CHILDREN_no
+          Attributes:
+            - Attribute:       DW_AT_name
+              Form:            DW_FORM_strp
+        - Code:            0x00000003
+          Tag:             DW_TAG_base_type
+          Children:        DW_CHILDREN_no
+          Attributes:
+            - Attribute:       DW_AT_abstract_origin
+              Form:            DW_FORM_ref1
+        - Code:            0x00000004
+          Tag:             DW_TAG_base_type
+          Children:        DW_CHILDREN_no
+          Attributes:
+            - Attribute:       DW_AT_specification
+              Form:            DW_FORM_ref1
+  debug_info:
+    - Version:         4
+      AddrSize:        8
+      Entries:
+        - AbbrCode:        0x00000001
+          Values:
+            - Value:           0x000000000000000C
+        - AbbrCode:        0x00000002
+          Values:
+            - Value:           0x0000000000000000 # Name = NameType1
+        - AbbrCode:        0x00000002
+          Values:
+            - Value:           0x000000000000000a # Name = NameType2
+        - AbbrCode:        0x00000003
+          Values:
+            - Value:           0x000000000000000e # Ref abstract origin to NameType1 DIE.
+        - AbbrCode:        0x00000004
+          Values:
+            - Value:           0x0000000000000013 # Ref specification to NameType2 DIE.
+        - AbbrCode:        0x00000000
+)";
+
+  YAMLModuleTester t(yamldata);
+  auto *symbol_file =
+      llvm::cast<SymbolFileDWARF>(t.GetModule()->GetSymbolFile());
+  auto &debug_info = symbol_file->DebugInfo();
+
+  DIERef first_die(std::nullopt, DIERef::Section::DebugInfo,
+                   11 /*FirstDIEOffset*/);
+  EXPECT_EQ(debug_info.PeekDIEName(first_die), "");
+
+  DIERef second_die(std::nullopt, DIERef::Section::DebugInfo, 14);
+  EXPECT_EQ(debug_info.PeekDIEName(second_die), "NameType1");
+
+  DIERef third_die(std::nullopt, DIERef::Section::DebugInfo, 19);
+  EXPECT_EQ(debug_info.PeekDIEName(third_die), "NameType2");
+
+  DIERef fourth_die(std::nullopt, DIERef::Section::DebugInfo, 24);
+  EXPECT_EQ(debug_info.PeekDIEName(fourth_die), "NameType1");
+
+  DIERef fifth_die(std::nullopt, DIERef::Section::DebugInfo, 26);
+  EXPECT_EQ(debug_info.PeekDIEName(fifth_die), "NameType2");
+}