[lldb][DWARFUnit] Implement PeekDIEName query

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

Author: felipepiovezan

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

@@ -191,3 +191,10 @@ 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/DWARFUnit.cpp

@@ -663,6 +663,14 @@ DWARFUnit::GetDIE(dw_offset_t die_offset) {
   return DWARFDIE(); // Not found
 }
 
+llvm::StringRef DWARFUnit::PeekDIEName(dw_offset_t die_offset) {
+  const DWARFDataExtractor &data = GetData();
+  DWARFDebugInfoEntry die;
+  if (!die.Extract(data, this, &die_offset))
+    return llvm::StringRef();
+  return die.GetName(this);
+}
+
 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 +105,61 @@ 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
+  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:        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");
+}