From 7b7a21b1920864b8b80e8984a9d698992921aba0 Mon Sep 17 00:00:00 2001
From: Chandler Carruth <chandlerc@gmail.com>
Date: Fri, 25 Apr 2014 09:08:10 +0000
Subject: [PATCH] [LCG] During the incremental re-build of an SCC after
 removing an edge, remove the nodes in the SCC from the SCC map entirely prior
 to the DFS walk. This allows the SCC map to represent both the state of
 not-yet-re-added-to-an-SCC and added-back-to-this-SCC independently. The
 first is being missing from the SCC map, the second is mapping back to
 'this'. In a subsequent commit, I'm going to use this property to simplify
 the new node list for this SCC.

In theory, I think this also makes the contract for orphaning a node
from the graph slightly less confusing. Now it is also orphaned from the
SCC graph. Still, this isn't quite right either, and so I'm not adding
test cases here. I'll add test cases for the behavior of orphaning nodes
when the code *actually* supports it. The change here is mostly
incidental, my goal is simplifying the algorithm.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207213 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Analysis/LazyCallGraph.cpp | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/lib/Analysis/LazyCallGraph.cpp b/lib/Analysis/LazyCallGraph.cpp
index bcefd4f24d5..6fb62703f94 100644
--- a/lib/Analysis/LazyCallGraph.cpp
+++ b/lib/Analysis/LazyCallGraph.cpp
@@ -191,9 +191,10 @@ LazyCallGraph::SCC::removeInternalEdge(LazyCallGraph &G, Node &Caller,
   SmallVector<Node *, 1> Worklist;
   Worklist.swap(Nodes);
   for (Node *N : Worklist) {
-    // Clear these to 0 while we re-run Tarjan's over the SCC.
+    // The nodes formerly in this SCC are no longer in any SCC.
     N->DFSNumber = 0;
     N->LowLink = 0;
+    G.SCCMap.erase(N);
   }
 
   // The callee can already reach every node in this SCC (by definition). It is
@@ -230,9 +231,8 @@ LazyCallGraph::SCC::removeInternalEdge(LazyCallGraph &G, Node &Caller,
       Node &ChildN = *I;
       // If this child isn't currently in this SCC, no need to process it.
       // However, we do need to remove this SCC from its SCC's parent set.
-      SCC &ChildSCC = *G.SCCMap.lookup(&ChildN);
-      if (&ChildSCC != this) {
-        ChildSCC.ParentSCCs.erase(this);
+      if (SCC *ChildSCC = G.SCCMap.lookup(&ChildN)) {
+        ChildSCC->ParentSCCs.erase(this);
         continue;
       }
 
@@ -298,6 +298,7 @@ LazyCallGraph::SCC::removeInternalEdge(LazyCallGraph &G, Node &Caller,
     N->DFSNumber = -1;
     N->LowLink = -1;
     Nodes.push_back(N);
+    G.SCCMap.insert(std::make_pair(N, this));
     for (Node &ChildN : *N) {
       if (NewNodes.count(&ChildN))
         continue;