[Lldb-commits] [lldb] 9ed8896 - [lldb][DWARF] Support DW_AT_bit_size on type tags (#165686)

[via lldb-commits]

Author: Michael Buch
Date: 2025-10-30T16:30:59Z
New Revision: 9ed889631a61acc09a1086dc5e5298ec1cd69776

URL: https://github.com/llvm/llvm-project/commit/9ed889631a61acc09a1086dc5e5298ec1cd69776
DIFF: https://github.com/llvm/llvm-project/commit/9ed889631a61acc09a1086dc5e5298ec1cd69776.diff

LOG: [lldb][DWARF] Support DW_AT_bit_size on type tags (#165686)

One (DWARF-spec compliant) exmample is:
https://github.com/llvm/llvm-project/pull/164372, where we attach a
`DW_AT_bit_size` to `_BitInt` types that can't be exactly described by a
byte-size.

This patch adds support for `DW_AT_bit_size` to `DWARFASTParserClang`
when parsing type tags.

Note, we don't use this bit-size yet, but will do so in follow-up
patches.

Added: 
    

Modified: 
    lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.cpp
    lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.h
    lldb/unittests/SymbolFile/DWARF/DWARFASTParserClangTests.cpp

Removed: 
    


################################################################################
diff  --git a/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.cpp b/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.cpp
index 36bc17680f3fa..c049829f37219 100644
--- a/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.cpp
+++ b/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.cpp
@@ -450,6 +450,10 @@ ParsedDWARFTypeAttributes::ParsedDWARFTypeAttributes(const DWARFDIE &die) {
       byte_size = form_value.Unsigned();
       break;
 
+    case DW_AT_bit_size:
+      data_bit_size = form_value.Unsigned();
+      break;
+
     case DW_AT_alignment:
       alignment = form_value.Unsigned();
       break;

diff  --git a/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.h b/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.h
index da58f4c146226..f5f707129d67d 100644
--- a/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.h
+++ b/lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.h
@@ -574,6 +574,7 @@ struct ParsedDWARFTypeAttributes {
   lldb_private::plugin::dwarf::DWARFFormValue type;
   lldb::LanguageType class_language = lldb::eLanguageTypeUnknown;
   std::optional<uint64_t> byte_size;
+  std::optional<uint64_t> data_bit_size;
   std::optional<uint64_t> alignment;
   size_t calling_convention = llvm::dwarf::DW_CC_normal;
   uint32_t bit_stride = 0;

diff  --git a/lldb/unittests/SymbolFile/DWARF/DWARFASTParserClangTests.cpp b/lldb/unittests/SymbolFile/DWARF/DWARFASTParserClangTests.cpp
index 1abce6999874e..064ed6d1d3e58 100644
--- a/lldb/unittests/SymbolFile/DWARF/DWARFASTParserClangTests.cpp
+++ b/lldb/unittests/SymbolFile/DWARF/DWARFASTParserClangTests.cpp
@@ -1651,3 +1651,93 @@ TEST_F(DWARFASTParserClangTests, TestObjectPointer_IndexEncoding) {
     EXPECT_EQ(param_die, ast_parser.GetObjectParameter(sub2, context_die));
   }
 }
+
+TEST_F(DWARFASTParserClangTests, TestTypeBitSize) {
+  // Tests that we correctly parse DW_AT_bit_size of a DW_AT_base_type.
+
+  const char *yamldata = R"(
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_EXEC
+  Machine: EM_AARCH64
+DWARF:
+  debug_str:
+    - _BitInt(2)
+  debug_abbrev:
+    - ID:              0
+      Table:
+        - Code:            0x1
+          Tag:             DW_TAG_compile_unit
+          Children:        DW_CHILDREN_yes
+          Attributes:
+            - Attribute:       DW_AT_language
+              Form:            DW_FORM_data2
+        - Code:            0x2
+          Tag:             DW_TAG_base_type
+          Children:        DW_CHILDREN_no
+          Attributes:
+            - Attribute: DW_AT_name
+              Form:      DW_FORM_strp
+            - Attribute: DW_AT_encoding
+              Form:      DW_FORM_data1
+            - Attribute: DW_AT_byte_size
+              Form:      DW_FORM_data1
+            - Attribute: DW_AT_bit_size
+              Form:      DW_FORM_data1
+
+  debug_info:
+     - Version:  5
+       UnitType: DW_UT_compile
+       AddrSize: 8
+       Entries:
+
+# DW_TAG_compile_unit
+#   DW_AT_language [DW_FORM_data2]    (DW_LANG_C_plus_plus)
+
+        - AbbrCode: 0x1
+          Values:
+            - Value: 0x04
+
+#   DW_TAG_base_type
+#     DW_AT_name [DW_FORM_strp] ('_BitInt(2)')
+
+        - AbbrCode: 0x2
+          Values:
+            - Value: 0x0
+            - Value: 0x05
+            - Value: 0x01
+            - Value: 0x02
+...
+)";
+
+  YAMLModuleTester t(yamldata);
+
+  DWARFUnit *unit = t.GetDwarfUnit();
+  ASSERT_NE(unit, nullptr);
+  const DWARFDebugInfoEntry *cu_entry = unit->DIE().GetDIE();
+  ASSERT_EQ(cu_entry->Tag(), DW_TAG_compile_unit);
+  ASSERT_EQ(unit->GetDWARFLanguageType(), DW_LANG_C_plus_plus);
+  DWARFDIE cu_die(unit, cu_entry);
+
+  auto holder = std::make_unique<clang_utils::TypeSystemClangHolder>("ast");
+  auto &ast_ctx = *holder->GetAST();
+  DWARFASTParserClangStub ast_parser(ast_ctx);
+
+  auto type_die = cu_die.GetFirstChild();
+  ASSERT_TRUE(type_die.IsValid());
+  ASSERT_EQ(type_die.Tag(), DW_TAG_base_type);
+
+  ParsedDWARFTypeAttributes attrs(type_die);
+  EXPECT_EQ(attrs.byte_size.value_or(0), 1U);
+  EXPECT_EQ(attrs.data_bit_size.value_or(0), 2U);
+
+  SymbolContext sc;
+  auto type_sp =
+      ast_parser.ParseTypeFromDWARF(sc, type_die, /*type_is_new_ptr=*/nullptr);
+  ASSERT_NE(type_sp, nullptr);
+
+  EXPECT_EQ(llvm::expectedToOptional(type_sp->GetByteSize(nullptr)).value_or(0),
+            1U);
+}