[swift-api-digester] caching the equality comparison results among SDKNodes. (#5297)

During the lifetime of analysing source-breaking changes, we may compare
the equality of two SDKNodes more than once, which makes the comprison
results amenable for caching. This patch implemented it. NFC

Author: nkcsgexi

tools/swift-api-digester/swift-api-digester.cpp

@@ -808,17 +808,45 @@ NodeUniquePtr SDKNode::constructSDKNode(llvm::yaml::MappingNode *Node) {
   return Result;
 }
 
+/// This is for caching the comparison results between two SDKNodes.
+class SDKNodeEqualContext {
+  using NodePtrAndEqual = llvm::DenseMap<const SDKNode*, bool>;
+  llvm::DenseMap<const SDKNode*, llvm::DenseMap<const SDKNode*, bool>> Data;
+
+public:
+  Optional<bool> getEquality(const SDKNode* Left, const SDKNode* Right) {
+    auto &Map = Data.insert({Left, NodePtrAndEqual()}).first->getSecond();
+    if (Map.count(Right))
+      return Map[Right];
+    return None;
+  }
+
+  void addEquality(const SDKNode* Left, const SDKNode* Right, const bool Value) {
+    Data.insert(std::make_pair(Left, NodePtrAndEqual())).first->getSecond().
+      insert({Right, Value});
+  }
+};
+
 bool SDKNode::operator==(const SDKNode &Other) const {
+  static SDKNodeEqualContext EqualCache;
+  if (auto Cached = EqualCache.getEquality(this, &Other)) {
+    return Cached.getValue();
+  }
+  auto Exit = [&](const bool Result) {
+    EqualCache.addEquality(this, &Other, Result);
+    return Result;
+  };
+
   if (getKind() != Other.getKind())
-    return false;
+    return Exit(false);
 
   switch(getKind()) {
     case SDKNodeKind::TypeNominal:
     case SDKNodeKind::TypeFunc: {
       auto Left = this->getAs<SDKNodeType>();
       auto Right = (&Other)->getAs<SDKNodeType>();
-      return Left->getTypeAttributes().equals(Right->getTypeAttributes())
-        && Left->getPrintedName() == Right->getPrintedName();
+      return Exit(Left->getTypeAttributes().equals(Right->getTypeAttributes())
+        && Left->getPrintedName() == Right->getPrintedName());
     }
 
     case SDKNodeKind::Function:
@@ -828,9 +856,9 @@ bool SDKNode::operator==(const SDKNode &Other) const {
       auto Left = this->getAs<SDKNodeAbstractFunc>();
       auto Right = (&Other)->getAs<SDKNodeAbstractFunc>();
       if (Left->isMutating() ^ Right->isMutating())
-        return false;
+        return Exit(false);
       if (Left->isThrowing() ^ Right->isThrowing())
-        return false;
+        return Exit(false);
       SWIFT_FALLTHROUGH;
     }
     case SDKNodeKind::TypeDecl:
@@ -842,11 +870,11 @@ bool SDKNode::operator==(const SDKNode &Other) const {
           Children.size() == Other.Children.size()) {
         for (unsigned I = 0; I < Children.size(); ++ I) {
           if (*Children[I] != *Other.Children[I])
-            return false;
+            return Exit(false);
         }
-        return true;
+        return Exit(true);
       }
-      return false;
+      return Exit(false);
     }
   }
 }