[clang] [WIP] Implement `-dump-deserialized-declaration-ranges` flag. (PR #133910)

[Ilya Biryukov via cfe-commits]

================
@@ -49,6 +51,150 @@ LLVM_INSTANTIATE_REGISTRY(FrontendPluginRegistry)
 
 namespace {
 
+/// DeserializedDeclsLineRangePrinter dumps ranges of deserialized declarations
+/// to aid debugging and bug minimization. It implements ASTConsumer and
+/// ASTDeserializationListener, so that an object of
+/// DeserializedDeclsLineRangePrinter registers as its own listener. The
+/// ASTDeserializationListener interface provides the DeclRead callback that we
+/// use to collect the deserialized Decls. Note that printing or otherwise
+/// processing them as this point is dangerous, since that could trigger
+/// additional deserialization and crash compilation. Therefore, we process the
+/// collected Decls in HandleTranslationUnit method of ASTConsumer. This is a
+/// safe point, since we know that by this point all the Decls needed by the
+/// compiler frontend have been deserialized. In case our processing causes
+/// further deserialization, DeclRead from the listener might be called again.
+/// However, at that point we don't accept any more Decls for processing.
+class DeserializedDeclsLineRangePrinter : public ASTDeserializationListener,
+                                          public ASTConsumer {
+public:
+  explicit DeserializedDeclsLineRangePrinter(
+      SourceManager &SM, std::unique_ptr<llvm::raw_fd_ostream> OS)
+      : ASTDeserializationListener(), SM(SM), OS(std::move(OS)) {}
+
+  void DeclRead(GlobalDeclID ID, const Decl *D) override {
+    if (!IsCollectingDecls) {
+      return;
+    }
+    if (!D || isa<TranslationUnitDecl>(D) || isa<LinkageSpecDecl>(D) ||
+        isa<NamespaceDecl>(D))
+      return;
+    if (auto *DC = D->getDeclContext(); !DC || !DC->isFileContext())
+      return;
+    PendingDecls.push_back(D);
+    ASTDeserializationListener::DeclRead(ID, D);
+  }
+
+  using Position = std::pair<unsigned, unsigned>;
+  struct RequiredRanges {
+    StringRef Filename;
+    std::vector<std::pair<Position, Position>> FromTo;
+  };
+  void HandleTranslationUnit(ASTContext &Context) override {
+    IsCollectingDecls = false;
+    std::vector<const Decl *> Decls = std::move(PendingDecls);
+    if (!PendingDecls.empty()) {
+      llvm::errs() << "Deserialized more decls while printing, total of "
+                   << PendingDecls.size() << "\n";
+      PendingDecls.clear();
+    }
+
+    // Merge ranges in each of the files. For simplicity, track lines and hope
+    // they do not break things.
+    struct FileData {
+      std::vector<std::pair<Position, Position>> FromTo;
+      std::vector<std::pair<unsigned, unsigned>> Columns;
+      OptionalFileEntryRef Ref;
+    };
+    llvm::DenseMap<const FileEntry *, FileData> FileToLines;
+    for (const Decl *D : Decls) {
+      CharSourceRange R = SM.getExpansionRange(D->getSourceRange());
+      if (!R.isValid())
+        continue;
+
+      auto *F = SM.getFileEntryForID(SM.getFileID(R.getBegin()));
+      if (F != SM.getFileEntryForID(SM.getFileID(R.getEnd())))
+        continue;
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ilya-biryukov wrote:

NIT: Add a comment explaining that handling these cases is too difficult and they should be rare.

https://github.com/llvm/llvm-project/pull/133910