ASTScope: Small optimization to ASTScopeImpl memory layout

Combine the wasExpanded flag with the parent pointer, and remove
the haveAddedCleanup flag since it's no longer necessary now that
"re-expansion" is gone.

Author: slavapestov

include/swift/AST/ASTScope.h

@@ -136,18 +136,16 @@ class ASTScopeImpl {
   /// storage declaration or is directly descended from it.
 
 private:
-  /// Always set by the constructor, so that when creating a child
-  /// the parent chain is available.
-  ASTScopeImpl *parent = nullptr; // null at the root
+  /// The pointer:
+  /// - Always set by the constructor, so that when creating a child
+  ///   the parent chain is available. Null at the root.
+  /// The int:
+  /// - A flag indicating if the scope has been expanded yet or not.
+  llvm::PointerIntPair<ASTScopeImpl *, 1> parentAndWasExpanded;
 
   /// Child scopes, sorted by source range.
   Children storedChildren;
 
-  bool wasExpanded = false;
-
-  /// Can clear storedChildren, so must remember this
-  bool haveAddedCleanup = false;
-
   mutable Optional<CharSourceRange> cachedCharSourceRange;
 
 #pragma mark - constructor / destructor
@@ -177,8 +175,12 @@ class ASTScopeImpl {
 
 #pragma mark - tree declarations
 protected:
-  NullablePtr<ASTScopeImpl> getParent() { return parent; }
-  NullablePtr<const ASTScopeImpl> getParent() const { return parent; }
+  NullablePtr<ASTScopeImpl> getParent() {
+    return parentAndWasExpanded.getPointer();
+  }
+  NullablePtr<const ASTScopeImpl> getParent() const {
+    return parentAndWasExpanded.getPointer();
+  }
 
   const Children &getChildren() const { return storedChildren; }
 
@@ -247,13 +249,13 @@ class ASTScopeImpl {
 
 #pragma mark - Scope tree creation
 public:
-  /// Expand the scope. Asserts if it was already expanded.
+  /// Expand the scope if unexpanded.
   ASTScopeImpl *expandAndBeCurrent(ScopeCreator &);
 
-  bool getWasExpanded() const { return wasExpanded; }
+  bool getWasExpanded() const { return parentAndWasExpanded.getInt(); }
 
 protected:
-  void setWasExpanded() { wasExpanded = true; }
+  void setWasExpanded() { parentAndWasExpanded.setInt(1); }
   virtual ASTScopeImpl *expandSpecifically(ScopeCreator &) = 0;
 
 public:
@@ -354,7 +356,7 @@ class ASTScopeImpl {
   /// what obtaines for scoping. However, guards are different. The scope after
   /// the guard else must hop into the innermoset scope of the guard condition.
   virtual NullablePtr<const ASTScopeImpl> getLookupParent() const {
-    return parent;
+    return getParent();
   }
 
 #pragma mark - - lookup- local bindings

lib/AST/ASTScopeCreation.cpp

@@ -579,19 +579,17 @@ ScopeCreator::addPatternBindingToScopeTree(PatternBindingDecl *patternBinding,
 
 void ASTScopeImpl::addChild(ASTScopeImpl *child, ASTContext &ctx) {
   ASTScopeAssert(!child->getParent(), "child should not already have parent");
-  child->parent = this;
+  child->parentAndWasExpanded.setPointer(this);
 
 #ifndef NDEBUG
   checkSourceRangeBeforeAddingChild(child, ctx);
 #endif
 
   // If this is the first time we've added children, notify the ASTContext
   // that there's a SmallVector that needs to be cleaned up.
-  // FIXME: If we had access to SmallVector::isSmall(), we could do better.
-  if (storedChildren.empty() && !haveAddedCleanup) {
+  if (storedChildren.empty())
     ctx.addDestructorCleanup(storedChildren);
-    haveAddedCleanup = true;
-  }
+
   storedChildren.push_back(child);
 }