[LazyCallGraph] Ignore empty RefSCCs rather than shift RefSCCs when removing dead functions

This is in preparation for D115545 which attempts to delete discardable functions if they are unused. With that change, shifting RefSCCs becomes noticeable in compile time. This change makes the LCG update negligible again.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D116776

Author: aeubanks

llvm/include/llvm/Analysis/LazyCallGraph.h

@@ -884,7 +884,9 @@ class LazyCallGraph {
     RefSCC *RC = nullptr;
 
     /// Build the begin iterator for a node.
-    postorder_ref_scc_iterator(LazyCallGraph &G) : G(&G), RC(getRC(G, 0)) {}
+    postorder_ref_scc_iterator(LazyCallGraph &G) : G(&G), RC(getRC(G, 0)) {
+      incrementUntilNonEmptyRefSCC();
+    }
 
     /// Build the end iterator for a node. This is selected purely by overload.
     postorder_ref_scc_iterator(LazyCallGraph &G, IsAtEndT /*Nonce*/) : G(&G) {}
@@ -899,6 +901,17 @@ class LazyCallGraph {
       return G.PostOrderRefSCCs[Index];
     }
 
+    // Keep incrementing until RC is non-empty (or null).
+    void incrementUntilNonEmptyRefSCC() {
+      while (RC && RC->size() == 0)
+        increment();
+    }
+
+    void increment() {
+      assert(RC && "Cannot increment the end iterator!");
+      RC = getRC(*G, G->RefSCCIndices.find(RC)->second + 1);
+    }
+
   public:
     bool operator==(const postorder_ref_scc_iterator &Arg) const {
       return G == Arg.G && RC == Arg.RC;
@@ -908,8 +921,8 @@ class LazyCallGraph {
 
     using iterator_facade_base::operator++;
     postorder_ref_scc_iterator &operator++() {
-      assert(RC && "Cannot increment the end iterator!");
-      RC = getRC(*G, G->RefSCCIndices.find(RC)->second + 1);
+      increment();
+      incrementUntilNonEmptyRefSCC();
       return *this;
     }
   };

llvm/lib/Analysis/LazyCallGraph.cpp

@@ -1544,15 +1544,9 @@ void LazyCallGraph::removeDeadFunction(Function &F) {
   assert(C.size() == 1 && "Dead functions must be in a singular SCC");
   assert(RC.size() == 1 && "Dead functions must be in a singular RefSCC");
 
-  auto RCIndexI = RefSCCIndices.find(&RC);
-  int RCIndex = RCIndexI->second;
-  PostOrderRefSCCs.erase(PostOrderRefSCCs.begin() + RCIndex);
-  RefSCCIndices.erase(RCIndexI);
-  for (int i = RCIndex, Size = PostOrderRefSCCs.size(); i < Size; ++i)
-    RefSCCIndices[PostOrderRefSCCs[i]] = i;
-
   // Finally clear out all the data structures from the node down through the
-  // components.
+  // components. postorder_ref_scc_iterator will skip empty RefSCCs, so no need
+  // to adjust LazyCallGraph data structures.
   N.clear();
   N.G = nullptr;
   N.F = nullptr;