[SampleFDO] Fix a bug when appending function symbol into the Callees set of Root node in ProfiledCallGraph.

[wmi]

Summary

In ProfiledCallGraph::addProfiledFunction, to add a function symbol into the ProfiledCallGraph, currently an uninitialized ProfiledCallGraphNode node is created and inserted into Callees set of Root node before the node is initialized. The Callees set use ProfiledCallGraphNodeComparer as its comparator so the uninitialized ProfiledCallGraphNode may fail to be inserted into Callees set if the uninitialized memory may happen to contain a name which has been inserted into the Callees set before. The problem will prevent some function symbols from being annotated with profiles and cause performance regression. The patch fixes the problem.

[wenlei]

good catch, thanks for the fix!

[hoy]

LGTM, thanks!

llvm/include/llvm/Transforms/IPO/ProfiledCallGraph.h

public:
 
   iterator begin() { return Root.Callees.begin(); }
   iterator end() { return Root.Callees.end(); }
   ProfiledCallGraphNode *getEntryNode() { return &Root; }
   void addProfiledFunction(StringRef Name) {
     if (!ProfiledFunctions.count(Name)) {
       // Link to synthetic root to make sure every node is reachable
       // from root. This does not affect SCC order.
-      Root.Callees.insert(&ProfiledFunctions[Name]);
       ProfiledFunctions[Name] = ProfiledCallGraphNode(Name);
+      Root.Callees.insert(&ProfiledFunctions[Name]);
     }
   }
 
   void addProfiledCall(StringRef CallerName, StringRef CalleeName) {
     assert(ProfiledFunctions.count(CallerName));
     auto CalleeIt = ProfiledFunctions.find(CalleeName);
     if (CalleeIt == ProfiledFunctions.end()) {
       return;