Make iteration over the DeclContext::lookup_result safe.

The idiom:
```
DeclContext::lookup_result R = DeclContext::lookup(Name);
for (auto *D : R) {...}
```

is not safe when in the loop body we trigger deserialization from an AST file.
The deserialization can insert new declarations in the StoredDeclsList whose
underlying type is a vector. When the vector decides to reallocate its storage
the pointer we hold becomes invalid.

This patch replaces a SmallVector with an singly-linked list. The current
approach stores a SmallVector<NamedDecl*, 4> which is around 8 pointers.
The linked list is 3, 5, or 7. We do better in terms of memory usage for small
cases (and worse in terms of locality -- the linked list entries won't be near
each other, but will be near their corresponding declarations, and we were going
to fetch those memory pages anyway). For larger cases: the vector uses a
doubling strategy for reallocation, so will generally be between half-full and
full. Let's say it's 75% full on average, so there's N * 4/3 + 4 pointers' worth
of space allocated currently and will be 2N pointers with the linked list. So we
break even when there are N=6 entries and slightly lose in terms of memory usage
after that. We suspect that's still a win on average.

Thanks to @rsmith!

Differential revision: https://reviews.llvm.org/D91524

clang-tools-extra/clangd/unittests/TestTU.cpp

@@ -195,7 +195,7 @@ const NamedDecl &findDecl(ParsedAST &AST, llvm::StringRef QName) {
                            llvm::StringRef Name) -> const NamedDecl & {
     auto LookupRes = Scope.lookup(DeclarationName(&Ctx.Idents.get(Name)));
     assert(!LookupRes.empty() && "Lookup failed");
-    assert(LookupRes.size() == 1 && "Lookup returned multiple results");
+    assert(LookupRes.isSingleResult() && "Lookup returned multiple results");
     return *LookupRes.front();
   };
 

clang/include/clang/AST/ASTContext.h

@@ -604,6 +604,9 @@ class ASTContext : public RefCountedBase<ASTContext> {
   std::unique_ptr<interp::Context> InterpContext;
   std::unique_ptr<ParentMapContext> ParentMapCtx;
 
+  /// Keeps track of the deallocated DeclListNodes for future reuse.
+  DeclListNode *ListNodeFreeList = nullptr;
+
 public:
   IdentifierTable &Idents;
   SelectorTable &Selectors;
@@ -655,6 +658,24 @@ class ASTContext : public RefCountedBase<ASTContext> {
   }
   void Deallocate(void *Ptr) const {}
 
+  /// Allocates a \c DeclListNode or returns one from the \c ListNodeFreeList
+  /// pool.
+  DeclListNode *AllocateDeclListNode(clang::NamedDecl *ND) {
+    if (DeclListNode *Alloc = ListNodeFreeList) {
+      ListNodeFreeList = Alloc->Rest.dyn_cast<DeclListNode*>();
+      Alloc->D = ND;
+      Alloc->Rest = nullptr;
+      return Alloc;
+    }
+    return new (*this) DeclListNode(ND);
+  }
+  /// Deallcates a \c DeclListNode by returning it to the \c ListNodeFreeList
+  /// pool.
+  void DeallocateDeclListNode(DeclListNode *N) {
+    N->Rest = ListNodeFreeList;
+    ListNodeFreeList = N;
+  }
+
   /// Return the total amount of physical memory allocated for representing
   /// AST nodes and type information.
   size_t getASTAllocatedMemory() const {

clang/include/clang/AST/CXXInheritance.h

@@ -76,9 +76,8 @@ class CXXBasePath : public SmallVector<CXXBasePathElement, 4> {
 
   CXXBasePath() = default;
 
-  /// The set of declarations found inside this base class
-  /// subobject.
-  DeclContext::lookup_result Decls;
+  /// The declarations found inside this base class subobject.
+  DeclContext::lookup_iterator Decls;
 
   void clear() {
     SmallVectorImpl<CXXBasePathElement>::clear();

clang/include/clang/AST/Decl.h

@@ -579,6 +579,16 @@ class NamespaceDecl : public NamedDecl, public DeclContext,
     AnonOrFirstNamespaceAndInline.setInt(Inline);
   }
 
+  /// Returns true if the inline qualifier for \c Name is redundant.
+  bool isRedundantInlineQualifierFor(DeclarationName Name) const {
+    if (!isInline())
+      return false;
+    auto X = lookup(Name);
+    auto Y = getParent()->lookup(Name);
+    return std::distance(X.begin(), X.end()) ==
+      std::distance(Y.begin(), Y.end());
+  }
+
   /// Get the original (first) namespace declaration.
   NamespaceDecl *getOriginalNamespace();
 

clang/include/clang/AST/DeclBase.h

@@ -1220,65 +1220,110 @@ class PrettyStackTraceDecl : public llvm::PrettyStackTraceEntry {
 
   void print(raw_ostream &OS) const override;
 };
+} // namespace clang
 
-/// The results of name lookup within a DeclContext. This is either a
-/// single result (with no stable storage) or a collection of results (with
-/// stable storage provided by the lookup table).
-class DeclContextLookupResult {
-  using ResultTy = ArrayRef<NamedDecl *>;
-
-  ResultTy Result;
-
-  // If there is only one lookup result, it would be invalidated by
-  // reallocations of the name table, so store it separately.
-  NamedDecl *Single = nullptr;
-
-  static NamedDecl *const SingleElementDummyList;
+// Required to determine the layout of the PointerUnion<NamedDecl*> before
+// seeing the NamedDecl definition being first used in DeclListNode::operator*.
+namespace llvm {
+  template <> struct PointerLikeTypeTraits<::clang::NamedDecl *> {
+    static inline void *getAsVoidPointer(::clang::NamedDecl *P) { return P; }
+    static inline ::clang::NamedDecl *getFromVoidPointer(void *P) {
+      return static_cast<::clang::NamedDecl *>(P);
+    }
+    static constexpr int NumLowBitsAvailable = 3;
+  };
+}
 
+namespace clang {
+/// A list storing NamedDecls in the lookup tables.
+class DeclListNode {
+  friend class ASTContext; // allocate, deallocate nodes.
+  friend class StoredDeclsList;
 public:
-  DeclContextLookupResult() = default;
-  DeclContextLookupResult(ArrayRef<NamedDecl *> Result)
-      : Result(Result) {}
-  DeclContextLookupResult(NamedDecl *Single)
-      : Result(SingleElementDummyList), Single(Single) {}
-
-  class iterator;
-
-  using IteratorBase =
-      llvm::iterator_adaptor_base<iterator, ResultTy::iterator,
-                                  std::random_access_iterator_tag, NamedDecl *>;
-
-  class iterator : public IteratorBase {
-    value_type SingleElement;
+  using Decls = llvm::PointerUnion<NamedDecl*, DeclListNode*>;
+  class iterator {
+    friend class DeclContextLookupResult;
+    friend class StoredDeclsList;
 
+    Decls Ptr;
+    iterator(Decls Node) : Ptr(Node) { }
   public:
-    explicit iterator(pointer Pos, value_type Single = nullptr)
-        : IteratorBase(Pos), SingleElement(Single) {}
+    using difference_type = ptrdiff_t;
+    using value_type = NamedDecl*;
+    using pointer = void;
+    using reference = value_type;
+    using iterator_category = std::forward_iterator_tag;
+
+    iterator() = default;
 
     reference operator*() const {
-      return SingleElement ? SingleElement : IteratorBase::operator*();
+      assert(Ptr && "dereferencing end() iterator");
+      if (DeclListNode *CurNode = Ptr.dyn_cast<DeclListNode*>())
+        return CurNode->D;
+      return Ptr.get<NamedDecl*>();
     }
+    void operator->() const { } // Unsupported.
+    bool operator==(const iterator &X) const { return Ptr == X.Ptr; }
+    bool operator!=(const iterator &X) const { return Ptr != X.Ptr; }
+    inline iterator &operator++() { // ++It
+      assert(!Ptr.isNull() && "Advancing empty iterator");
+
+      if (DeclListNode *CurNode = Ptr.dyn_cast<DeclListNode*>())
+        Ptr = CurNode->Rest;
+      else
+        Ptr = nullptr;
+      return *this;
+    }
+    iterator operator++(int) { // It++
+      iterator temp = *this;
+      ++(*this);
+      return temp;
+    }
+    // Enables the pattern for (iterator I =..., E = I.end(); I != E; ++I)
+    iterator end() { return iterator(); }
   };
+private:
+  NamedDecl *D = nullptr;
+  Decls Rest = nullptr;
+  DeclListNode(NamedDecl *ND) : D(ND) {}
+};
+
+/// The results of name lookup within a DeclContext.
+class DeclContextLookupResult {
+  using Decls = DeclListNode::Decls;
 
+  /// When in collection form, this is what the Data pointer points to.
+  Decls Result;
+
+public:
+  DeclContextLookupResult() = default;
+  DeclContextLookupResult(Decls Result) : Result(Result) {}
+
+  using iterator = DeclListNode::iterator;
   using const_iterator = iterator;
-  using pointer = iterator::pointer;
   using reference = iterator::reference;
 
-  iterator begin() const { return iterator(Result.begin(), Single); }
-  iterator end() const { return iterator(Result.end(), Single); }
-
-  bool empty() const { return Result.empty(); }
-  pointer data() const { return Single ? &Single : Result.data(); }
-  size_t size() const { return Single ? 1 : Result.size(); }
-  reference front() const { return Single ? Single : Result.front(); }
-  reference back() const { return Single ? Single : Result.back(); }
-  reference operator[](size_t N) const { return Single ? Single : Result[N]; }
-
-  // FIXME: Remove this from the interface
-  DeclContextLookupResult slice(size_t N) const {
-    DeclContextLookupResult Sliced = Result.slice(N);
-    Sliced.Single = Single;
-    return Sliced;
+  iterator begin() { return iterator(Result); }
+  iterator end() { return iterator(); }
+  const_iterator begin() const {
+    return const_cast<DeclContextLookupResult*>(this)->begin();
+  }
+  const_iterator end() const { return iterator(); }
+
+  bool empty() const { return Result.isNull();  }
+  bool isSingleResult() const { return Result.dyn_cast<NamedDecl*>(); }
+  reference front() const { return *begin(); }
+
+  // Find the first declaration of the given type in the list. Note that this
+  // is not in general the earliest-declared declaration, and should only be
+  // used when it's not possible for there to be more than one match or where
+  // it doesn't matter which one is found.
+  template<class T> T *find_first() const {
+    for (auto *D : *this)
+      if (T *Decl = dyn_cast<T>(D))
+        return Decl;
+
+    return nullptr;
   }
 };