[cherry-pick][swift/releas/5.9] [lldb][DWARFASTParserClang] Don't create unnamed bitfields to account for vtable pointer

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

@@ -2839,10 +2839,9 @@ void DWARFASTParserClang::ParseSingleMember(
           die.GetCU()->Supports_unnamed_objc_bitfields();
 
     if (detect_unnamed_bitfields) {
-      llvm::Optional<FieldInfo> unnamed_field_info;
-      uint64_t last_field_end = 0;
-
-      last_field_end = last_field_info.bit_offset + last_field_info.bit_size;
+      std::optional<FieldInfo> unnamed_field_info;
+      uint64_t last_field_end =
+          last_field_info.bit_offset + last_field_info.bit_size;
 
       if (!last_field_info.IsBitfield()) {
         // The last field was not a bit-field...
@@ -2853,20 +2852,8 @@ void DWARFASTParserClang::ParseSingleMember(
           last_field_end += word_width - (last_field_end % word_width);
       }
 
-      // If we have a gap between the last_field_end and the current
-      // field we have an unnamed bit-field.
-      // If we have a base class, we assume there is no unnamed
-      // bit-field if this is the first field since the gap can be
-      // attributed to the members from the base class. This assumption
-      // is not correct if the first field of the derived class is
-      // indeed an unnamed bit-field. We currently do not have the
-      // machinary to track the offset of the last field of classes we
-      // have seen before, so we are not handling this case.
-      if (this_field_info.bit_offset != last_field_end &&
-          this_field_info.bit_offset > last_field_end &&
-          !(last_field_info.bit_offset == 0 &&
-            last_field_info.bit_size == 0 &&
-            layout_info.base_offsets.size() != 0)) {
+      if (ShouldCreateUnnamedBitfield(last_field_info, last_field_end,
+                                      this_field_info, layout_info)) {
         unnamed_field_info = FieldInfo{};
         unnamed_field_info->bit_size =
             this_field_info.bit_offset - last_field_end;
@@ -2907,8 +2894,10 @@ void DWARFASTParserClang::ParseSingleMember(
   // artificial member with (unnamed bitfield) padding.
   // FIXME: This check should verify that this is indeed an artificial member
   // we are supposed to ignore.
-  if (attrs.is_artificial)
+  if (attrs.is_artificial) {
+    last_field_info.SetIsArtificial(true);
     return;
+  }
 
   if (!member_clang_type.IsCompleteType())
     member_clang_type.GetCompleteType();
@@ -3660,3 +3649,35 @@ bool DWARFASTParserClang::CopyUniqueClassMethodTypes(
 
   return !failures.empty();
 }
+
+bool DWARFASTParserClang::ShouldCreateUnnamedBitfield(
+    FieldInfo const &last_field_info, uint64_t last_field_end,
+    FieldInfo const &this_field_info,
+    lldb_private::ClangASTImporter::LayoutInfo const &layout_info) const {
+  // If we have a gap between the last_field_end and the current
+  // field we have an unnamed bit-field.
+  if (this_field_info.bit_offset <= last_field_end)
+    return false;
+
+  // If we have a base class, we assume there is no unnamed
+  // bit-field if either of the following is true:
+  // (a) this is the first field since the gap can be
+  // attributed to the members from the base class.
+  // FIXME: This assumption is not correct if the first field of
+  // the derived class is indeed an unnamed bit-field. We currently
+  // do not have the machinary to track the offset of the last field
+  // of classes we have seen before, so we are not handling this case.
+  // (b) Or, the first member of the derived class was a vtable pointer.
+  // In this case we don't want to create an unnamed bitfield either
+  // since those will be inserted by clang later.
+  const bool have_base = layout_info.base_offsets.size() != 0;
+  const bool this_is_first_field =
+      last_field_info.bit_offset == 0 && last_field_info.bit_size == 0;
+  const bool first_field_is_vptr =
+      last_field_info.bit_offset == 0 && last_field_info.IsArtificial();
+
+  if (have_base && (this_is_first_field || first_field_is_vptr))
+    return false;
+
+  return true;
+}

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

@@ -206,19 +206,39 @@ class DWARFASTParserClang : public DWARFASTParser {
     uint64_t bit_size = 0;
     uint64_t bit_offset = 0;
     bool is_bitfield = false;
+    bool is_artificial = false;
 
     FieldInfo() = default;
 
     void SetIsBitfield(bool flag) { is_bitfield = flag; }
     bool IsBitfield() { return is_bitfield; }
 
+    void SetIsArtificial(bool flag) { is_artificial = flag; }
+    bool IsArtificial() const { return is_artificial; }
+
     bool NextBitfieldOffsetIsValid(const uint64_t next_bit_offset) const {
       // Any subsequent bitfields must not overlap and must be at a higher
       // bit offset than any previous bitfield + size.
       return (bit_size + bit_offset) <= next_bit_offset;
     }
   };
 
+  /// Returns 'true' if we should create an unnamed bitfield
+  /// and add it to the parser's current AST.
+  ///
+  /// \param[in] last_field_info FieldInfo of the previous DW_TAG_member
+  ///            we parsed.
+  /// \param[in] last_field_end Offset (in bits) where the last parsed field
+  ///            ended.
+  /// \param[in] this_field_info FieldInfo of the current DW_TAG_member
+  ///            being parsed.
+  /// \param[in] layout_info Layout information of all decls parsed by the
+  ///            current parser.
+  bool ShouldCreateUnnamedBitfield(
+      FieldInfo const &last_field_info, uint64_t last_field_end,
+      FieldInfo const &this_field_info,
+      lldb_private::ClangASTImporter::LayoutInfo const &layout_info) const;
+
   /// Parses a DW_TAG_APPLE_property DIE and appends the parsed data to the
   /// list of delayed Objective-C properties.
   ///

lldb/test/API/lang/cpp/bitfields/TestCppBitfields.py

@@ -168,3 +168,14 @@ def test_bitfield_behind_vtable_ptr(self):
                          result_children=with_vtable_and_unnamed_children)
         self.expect_var_path("with_vtable_and_unnamed",
                          children=with_vtable_and_unnamed_children)
+
+        derived_with_vtable_children = [
+            ValueCheck(name="Base", children=[
+              ValueCheck(name="b_a", value="2", type="uint32_t")
+            ]),
+            ValueCheck(name="a", value="1", type="unsigned int:1")
+        ]
+        self.expect_expr("derived_with_vtable",
+                         result_children=derived_with_vtable_children)
+        self.expect_var_path("derived_with_vtable",
+                         children=derived_with_vtable_children)

lldb/test/API/lang/cpp/bitfields/main.cpp

@@ -113,6 +113,12 @@ struct HasBaseWithVTable : BaseWithVTable {
 };
 HasBaseWithVTable base_with_vtable;
 
+struct DerivedWithVTable : public Base {
+  virtual ~DerivedWithVTable() {}
+  unsigned a : 1;
+};
+DerivedWithVTable derived_with_vtable;
+
 int main(int argc, char const *argv[]) {
   lba.a = 2;
 
@@ -153,5 +159,8 @@ int main(int argc, char const *argv[]) {
   base_with_vtable.b = 0;
   base_with_vtable.c = 5;
 
+  derived_with_vtable.b_a = 2;
+  derived_with_vtable.a = 1;
+
   return 0; // break here
 }