Subzero: Improve the use of NodeList objects.

src/IceCfgNode.cpp

 //===- subzero/src/IceCfgNode.cpp - Basic block (node) implementation -----===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file implements the CfgNode class, including the complexities
@@ skipping 732 matching lines @@
        i = LiveInAndOut.find_next(i)) {
     Variable *Var = Liveness->getVariable(i, this);
     Var->addLiveRange(FirstInstNum, LastInstNum + 1, 1);
   }
 }
 
 // If this node contains only deleted instructions, and ends in an
 // unconditional branch, contract the node by repointing all its
 // in-edges to its successor.
 void CfgNode::contractIfEmpty() {
-  if (InEdges.size() == 0)
+  if (InEdges.empty())
     return;
   Inst *Branch = NULL;
   for (Inst *I : Insts) {
     if (I->isDeleted())
       continue;
     if (I->isUnconditionalBranch())
       Branch = I;
     else if (!I->isRedundantAssign())
       return;
   }
   Branch->setDeleted();
   assert(OutEdges.size() == 1);
-  // Repoint all this node's in-edges to this node's successor.
-  for (CfgNode *Pred : InEdges) {
-    for (auto I = Pred->OutEdges.begin(), E = Pred->OutEdges.end(); I != E;
-         ++I) {
-      if (*I == this) {
-        *I = OutEdges[0];
-        OutEdges[0]->InEdges.push_back(Pred);
+  // Repoint all this node's in-edges to this node's successor, unless
+  // this node's successor is actually itself (in which case the
+  // statement "OutEdges.front()->InEdges.push_back(Pred)" could
+  // invalidate the iterator over this->InEdges).
+  if (OutEdges.front() != this) {
+    for (CfgNode *Pred : InEdges) {
+      for (auto I = Pred->OutEdges.begin(), E = Pred->OutEdges.end(); I != E;
+           ++I) {
+        if (*I == this) {
+          *I = OutEdges.front();
+          OutEdges.front()->InEdges.push_back(Pred);
+        }
       }
-    }
-    for (Inst *I : Pred->getInsts()) {
-      if (!I->isDeleted())
-        I->repointEdge(this, OutEdges[0]);
+      for (Inst *I : Pred->getInsts()) {
+        if (!I->isDeleted())
+          I->repointEdge(this, OutEdges.front());
+      }
     }
   }
   InEdges.clear();
   // Don't bother removing the single out-edge, which would also
   // require finding the corresponding in-edge in the successor and
   // removing it.
 }
 
 void CfgNode::doBranchOpt(const CfgNode *NextNode) {
   TargetLowering *Target = Func->getTarget();
@@ skipping 204 matching lines @@
       if (!First)
         Str << ", ";
       First = false;
       Str << "%" << I->getName();
     }
     Str << "\n";
   }
 }
 
 } // end of namespace Ice