[CGSCC] Skip SCC if we will revisit it later

llvm/include/llvm/Analysis/CGSCCPassManager.h

struct CGSCCUpdateResult {
   /// This is set when a graph refinement takes place an the "current" point in
   /// the graph moves "down" or earlier in the post-order walk. This will often
   /// cause the "current" SCC to be a newly created SCC object and the old one
   /// to be added to the above worklist. When that happens, this pointer is
   /// non-null and can be used to continue processing the "top" of the
   /// post-order walk.
   LazyCallGraph::SCC *UpdatedC;
 
+  bool SkipCurrentC;
+
   /// Preserved analyses across SCCs.
   ///
   /// We specifically want to allow CGSCC passes to mutate ancestor IR
   /// (changing both the CG structure and the function IR itself). However,
   /// this means we need to take special care to correctly mark what analyses
   /// are preserved *across* SCCs. We have to track this out-of-band here
   /// because within the main `PassManeger` infrastructure we need to mark
   /// everything within an SCC as preserved in order to avoid repeatedly

llvm/lib/Analysis/CGSCCPassManager.cpp

PassManager<LazyCallGraph::SCC, CGSCCAnalysisManager, LazyCallGraph &,
   // a pointer that we can update.
   LazyCallGraph::SCC *C = &InitialC;
 
   // Get Function analysis manager from its proxy.
   FunctionAnalysisManager &FAM =
       AM.getCachedResult<FunctionAnalysisManagerCGSCCProxy>(*C)->getManager();
 
   for (auto &Pass : Passes) {
+    if (UR.SkipCurrentC) {
+      UR.SkipCurrentC = false;
+      LLVM_DEBUG(dbgs() << "?\n");
+      break;
+    }
+
     // Check the PassInstrumentation's BeforePass callbacks before running the
     // pass, skip its execution completely if asked to (callback returns false).
     if (!PI.runBeforePass(*Pass, *C))
       continue;
 
     PreservedAnalyses PassPA;
     {
       TimeTraceScope TimeScope(Pass->name());
ModuleToPostOrderCGSCCPassAdaptor::run(Module &M, ModuleAnalysisManager &AM) {
   // Keep sets for invalidated SCCs and RefSCCs that should be skipped when
   // iterating off the worklists.
   SmallPtrSet<LazyCallGraph::RefSCC *, 4> InvalidRefSCCSet;
   SmallPtrSet<LazyCallGraph::SCC *, 4> InvalidSCCSet;
 
   SmallDenseSet<std::pair<LazyCallGraph::Node *, LazyCallGraph::SCC *>, 4>
       InlinedInternalEdges;
 
-  CGSCCUpdateResult UR = {
-      RCWorklist, CWorklist, InvalidRefSCCSet,         InvalidSCCSet,
-      nullptr,    nullptr,   PreservedAnalyses::all(), InlinedInternalEdges,
+  CGSCCUpdateResult UR = {RCWorklist,
+                          CWorklist,
+                          InvalidRefSCCSet,
+                          InvalidSCCSet,
+                          nullptr,
+                          nullptr,
+                          false,
+                          PreservedAnalyses::all(),
+                          InlinedInternalEdges,
                           {}};
 
   // Request PassInstrumentation from analysis manager, will use it to run
   // instrumenting callbacks for the passes later.
   PassInstrumentation PI = AM.getResult<PassInstrumentationAnalysis>(M);
 
   PreservedAnalyses PA = PreservedAnalyses::all();
   CG.buildRefSCCs();
   for (auto RCI = CG.postorder_ref_scc_begin(),
#endif
     if (InitialSCCIndex < NewSCCIndex) {
       // Put our current SCC back onto the worklist as we'll visit other SCCs
       // that are now definitively ordered prior to the current one in the
       // post-order sequence, and may end up observing more precise context to
       // optimize the current SCC.
       UR.CWorklist.insert(C);
       LLVM_DEBUG(dbgs() << "Enqueuing the existing SCC in the worklist: " << *C
                         << "\n");
+      UR.SkipCurrentC = true;
       // Enqueue in reverse order as we pop off the back of the worklist.
       for (SCC &MovedC : llvm::reverse(make_range(RC->begin() + InitialSCCIndex,
                                                   RC->begin() + NewSCCIndex))) {
         UR.CWorklist.insert(&MovedC);
         LLVM_DEBUG(dbgs() << "Enqueuing a newly earlier in post-order SCC: "
                           << MovedC << "\n");
       }
     }