Re-land "Add LazyCallGraph API to add function to RefSCC"

This re-commits https://reviews.llvm.org/D70927, which I reverted in
https://reviews.llvm.org/rG28213680b2a7d1fdeea16aa3f3a368879472c72a due
to a buildbot error:
http://lab.llvm.org:8011/builders/clang-cmake-x86_64-avx2-linux/builds/13251

I no longer include a test case that appears to crash when built with the
buildbot's compiler, GCC 5.4.0.

llvm/include/llvm/Analysis/LazyCallGraph.h

@@ -1061,6 +1061,10 @@ public:
   /// Introduce a node for the function \p NewF in the SCC \p C.
   void addNewFunctionIntoSCC(Function &NewF, SCC &C);
 
+  /// Introduce a node for the function \p NewF, as a single node in a
+  /// new SCC, in the RefSCC \p RC.
+  void addNewFunctionIntoRefSCC(Function &NewF, RefSCC &RC);
+
   ///@}
 
   ///@{
@@ -1167,6 +1171,13 @@ private:
   /// Helper to update pointers back to the graph object during moves.
   void updateGraphPtrs();
 
+  /// Helper to insert a new function, add it to the NodeMap, and populate its
+  /// node.
+  Node &createNode(Function &F);
+
+  /// Helper to add the given Node \p N to the SCCMap, mapped to the SCC \p C.
+  void addNodeToSCC(SCC &C, Node &N);
+
   /// Allocates an SCC and constructs it using the graph allocator.
   ///
   /// The arguments are forwarded to the constructor.

llvm/lib/Analysis/LazyCallGraph.cpp

@@ -1567,12 +1567,17 @@ void LazyCallGraph::removeDeadFunction(Function &F) {
 }
 
 void LazyCallGraph::addNewFunctionIntoSCC(Function &NewF, SCC &C) {
-  Node &CGNode = get(NewF);
-  CGNode.DFSNumber = CGNode.LowLink = -1;
-  CGNode.populate();
-  C.Nodes.push_back(&CGNode);
-  SCCMap[&CGNode] = &C;
-  NodeMap[&NewF] = &CGNode;
+  addNodeToSCC(C, createNode(NewF));
+}
+
+void LazyCallGraph::addNewFunctionIntoRefSCC(Function &NewF, RefSCC &RC) {
+  Node &N = createNode(NewF);
+
+  auto *C = createSCC(RC, SmallVector<Node *, 1>());
+  addNodeToSCC(*C, N);
+
+  RC.SCCIndices[C] = RC.SCCIndices.size();
+  RC.SCCs.push_back(C);
 }
 
 LazyCallGraph::Node &LazyCallGraph::insertInto(Function &F, Node *&MappedN) {
@@ -1589,6 +1594,21 @@ void LazyCallGraph::updateGraphPtrs() {
     RC->G = this;
 }
 
+LazyCallGraph::Node &LazyCallGraph::createNode(Function &F) {
+  assert(!lookup(F) && "node already exists");
+
+  Node &N = get(F);
+  NodeMap[&F] = &N;
+  N.DFSNumber = N.LowLink = -1;
+  N.populate();
+  return N;
+}
+
+void LazyCallGraph::addNodeToSCC(LazyCallGraph::SCC &C, Node &N) {
+  C.Nodes.push_back(&N);
+  SCCMap[&N] = &C;
+}
+
 template <typename RootsT, typename GetBeginT, typename GetEndT,
           typename GetNodeT, typename FormSCCCallbackT>
 void LazyCallGraph::buildGenericSCCs(RootsT &&Roots, GetBeginT &&GetBegin,

llvm/unittests/Analysis/LazyCallGraphTest.cpp

@@ -2210,4 +2210,46 @@ TEST(LazyCallGraphTest, RemoveFunctionWithSpurriousRef) {
   EXPECT_EQ(&RC2, &*I++);
   EXPECT_EQ(CG.postorder_ref_scc_end(), I);
 }
+
+TEST(LazyCallGraphTest, AddNewFunctionIntoRefSCC) {
+  LLVMContext Context;
+  // Build and populate a graph composed of a single, self-referential node.
+  std::unique_ptr<Module> M = parseAssembly(Context, "define void @f() {\n"
+                                                     "entry:\n"
+                                                     "  call void @f()\n"
+                                                     "  ret void\n"
+                                                     "}\n");
+  LazyCallGraph CG = buildCG(*M);
+  CG.buildRefSCCs();
+
+  // At this point 'f' is in the call graph.
+  auto &F = lookupFunction(*M, "f");
+  LazyCallGraph::Node *FN = CG.lookup(F);
+  EXPECT_NE(FN, nullptr);
+
+  // And it has an SCC, of course.
+  auto *FSCC = CG.lookupSCC(*FN);
+  EXPECT_NE(FSCC, nullptr);
+
+  // Now, create a new function 'g'.
+  auto *G = Function::Create(F.getFunctionType(), F.getLinkage(),
+                             F.getAddressSpace(), "g", F.getParent());
+  BasicBlock::Create(F.getParent()->getContext(), "entry", G);
+
+  // Instruct the LazyCallGraph to create a new node for 'g', within the same
+  // RefSCC as 'f', but in a separate SCC.
+  CG.addNewFunctionIntoRefSCC(*G, FSCC->getOuterRefSCC());
+
+  // 'g' should now be in the call graph.
+  LazyCallGraph::Node *GN = CG.lookup(*G);
+  EXPECT_NE(GN, nullptr);
+  // 'g' should have an SCC, composed of the singular node 'g'.
+  // ('f' should not be included in the 'g' SCC.)
+  LazyCallGraph::SCC *GSCC = CG.lookupSCC(*GN);
+  EXPECT_NE(GSCC, nullptr);
+  EXPECT_EQ(GSCC->size(), 1);
+  EXPECT_NE(GSCC, FSCC);
+  // 'g' and 'f' should be part of the same RefSCC.
+  EXPECT_EQ(&GSCC->getOuterRefSCC(), &FSCC->getOuterRefSCC());
+}
 }