[clang] [WIP] [Modules] Delay reading type source info of the preferred_name attribute. (PR #122250)

[Ilya Biryukov via cfe-commits]

================
@@ -3129,42 +3129,86 @@ class AttrReader {
   OMPTraitInfo *readOMPTraitInfo() { return Reader.readOMPTraitInfo(); }
 
   template <typename T> T *readDeclAs() { return Reader.readDeclAs<T>(); }
+
+  AttributeCommonInfo readAttributeCommonInfo() {
+    IdentifierInfo *AttrName = readIdentifier();
+    IdentifierInfo *ScopeName = readIdentifier();
+    SourceRange AttrRange = readSourceRange();
+    SourceLocation ScopeLoc = readSourceLocation();
+    unsigned ParsedKind = readInt();
+    unsigned Syntax = readInt();
+    unsigned SpellingIndex = readInt();
+    bool IsAlignas = (ParsedKind == AttributeCommonInfo::AT_Aligned &&
+                      Syntax == AttributeCommonInfo::AS_Keyword &&
+                      SpellingIndex == AlignedAttr::Keyword_alignas);
+    bool IsRegularKeywordAttribute = readBool();
+
+    AttributeCommonInfo Info(AttrName, ScopeName, AttrRange, ScopeLoc,
+                             AttributeCommonInfo::Kind(ParsedKind),
+                             {AttributeCommonInfo::Syntax(Syntax),
+                              SpellingIndex, IsAlignas,
+                              IsRegularKeywordAttribute});
+    return Info;
+  }
+
+  std::optional<attr::Kind> readAttrKind() {
+    auto V = readInt();
+    if (!V)
+      return {};
+
+    // Kind is stored as a 1-based integer because 0 is used to indicate a null
+    // Attr pointer.
+    return static_cast<attr::Kind>(V - 1);
+  }
+
+  Attr *createAttribute(attr::Kind Kind, AttributeCommonInfo Info) {
+    ASTContext &Context = Reader.getContext();
+    Attr *New = nullptr;
+    auto Record = *this;
+#include "clang/Serialization/AttrPCHRead.inc"
+
+    assert(New && "Unable to decode attribute?");
+    return New;
+  }
 };
-}
+} // namespace
 
 Attr *ASTRecordReader::readAttr() {
   AttrReader Record(*this);
-  auto V = Record.readInt();
-  if (!V)
+  attr::Kind Kind;
+  if (auto KindOpt = Record.readAttrKind(); !KindOpt)
     return nullptr;
+  else
+    Kind = *KindOpt;
 
-  Attr *New = nullptr;
-  // Kind is stored as a 1-based integer because 0 is used to indicate a null
-  // Attr pointer.
-  auto Kind = static_cast<attr::Kind>(V - 1);
-  ASTContext &Context = getContext();
-
-  IdentifierInfo *AttrName = Record.readIdentifier();
-  IdentifierInfo *ScopeName = Record.readIdentifier();
-  SourceRange AttrRange = Record.readSourceRange();
-  SourceLocation ScopeLoc = Record.readSourceLocation();
-  unsigned ParsedKind = Record.readInt();
-  unsigned Syntax = Record.readInt();
-  unsigned SpellingIndex = Record.readInt();
-  bool IsAlignas = (ParsedKind == AttributeCommonInfo::AT_Aligned &&
-                    Syntax == AttributeCommonInfo::AS_Keyword &&
-                    SpellingIndex == AlignedAttr::Keyword_alignas);
-  bool IsRegularKeywordAttribute = Record.readBool();
-
-  AttributeCommonInfo Info(AttrName, ScopeName, AttrRange, ScopeLoc,
-                           AttributeCommonInfo::Kind(ParsedKind),
-                           {AttributeCommonInfo::Syntax(Syntax), SpellingIndex,
-                            IsAlignas, IsRegularKeywordAttribute});
+  AttributeCommonInfo Info = Record.readAttributeCommonInfo();
----------------
ilya-biryukov wrote:

Instead of doing this, could we have a single function (e.g. `readAttrImpl`) that handles both cases and just asserts `Decl*` is not null whenever it sees the `PreferredName`?

That way, we only have one copy of the code and `readAttributeCommonInfo` is not really needed.

https://github.com/llvm/llvm-project/pull/122250