[Attributor][NFC] Update description for the dependency graph

[bbn]

Summary

The word "dependency graph" is a bit misleading. When there is an
edge from node A to B (A -> B), it actually mean that B depends on
A and when the state of A is updated, B should also be updated. So
I update the comment to make the description clearer.

[jdoerfert]

LGTM

[kuter]

Thank you.

llvm/include/llvm/Transforms/IPO/Attributor.h

 
 /// The data structure for the nodes of a dependency graph
 struct AADepGraphNode {
 public:
   virtual ~AADepGraphNode(){};
   using DepTy = PointerIntPair<AADepGraphNode *, 1>;
 
 protected:
-  /// Set of dependency graph nodes which this one depends on.
-  /// The bit encodes if it is optional.
+  /// Set of dependency graph nodes which should be updated if this one
+  /// is updated. The bit encodes if it is optional.
   TinyPtrVector<DepTy> Deps;
 
   static AADepGraphNode *DepGetVal(DepTy &DT) { return DT.getPointer(); }
   static AbstractAttribute *DepGetValAA(DepTy &DT) {
     return cast<AbstractAttribute>(DT.getPointer());
   }
 
   operator AbstractAttribute *() { return cast<AbstractAttribute>(this); }
public:
 
   virtual void print(raw_ostream &OS) const { OS << "AADepNode Impl\n"; }
   TinyPtrVector<DepTy> &getDeps() { return Deps; }
 
   friend struct Attributor;
   friend struct AADepGraph;
 };
 
+/// The data structure for the dependency graph
+///
+/// Note that in this graph if there is an edge from A to B (A -> B),
+/// then it means that B depends on A, and when the state of A is
+/// updated, node B should also be updated
 struct AADepGraph {
   AADepGraph() {}
   ~AADepGraph() {}
 
   using DepTy = AADepGraphNode::DepTy;
   static AADepGraphNode *DepGetVal(DepTy &DT) { return DT.getPointer(); }
   using iterator =
       mapped_iterator<TinyPtrVector<DepTy>::iterator, decltype(&DepGetVal)>;
 
   /// There is no root node for the dependency graph. But the SCCIterator
   /// requires a single entry point, so we maintain a fake("synthetic") root
   /// node that depends on every node.
   AADepGraphNode SyntheticRoot;
-
   AADepGraphNode *GetEntryNode() { return &SyntheticRoot; }
 
   iterator begin() { return SyntheticRoot.child_begin(); }
   iterator end() { return SyntheticRoot.child_end(); }
 
   void viewGraph();
 
   /// Dump graph to file