[PATCH] D116776: [LazyCallGraph] Ignore empty RefSCCs rather than shift RefSCCs when removing dead functions

[Arthur Eubanks via Phabricator via llvm-commits]

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This is in preparation for D115545 <https://reviews.llvm.org/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.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D116776

Files:
  llvm/include/llvm/Analysis/LazyCallGraph.h
  llvm/lib/Analysis/LazyCallGraph.cpp


Index: llvm/lib/Analysis/LazyCallGraph.cpp
===================================================================
--- llvm/lib/Analysis/LazyCallGraph.cpp
+++ llvm/lib/Analysis/LazyCallGraph.cpp
@@ -1544,15 +1544,9 @@
   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;
Index: llvm/include/llvm/Analysis/LazyCallGraph.h
===================================================================
--- llvm/include/llvm/Analysis/LazyCallGraph.h
+++ llvm/include/llvm/Analysis/LazyCallGraph.h
@@ -884,7 +884,9 @@
     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 @@
       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 @@
 
     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;
     }
   };


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