[turbofan] Fix control reducer for dead loops.

src/compiler/osr.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler.h"
 #include "src/compiler/all-nodes.h"
 #include "src/compiler/common-operator.h"
 #include "src/compiler/control-reducer.h"
 #include "src/compiler/frame.h"
 #include "src/compiler/graph.h"
@@ skipping 200 matching lines @@
   LoopTree::Loop* loop = loop_tree->ContainingLoop(osr_loop);
   if (loop->depth() > 0) {
     PeelOuterLoopsForOsr(graph, common, tmp_zone, dead, loop_tree, loop,
                          osr_normal_entry, osr_loop_entry);
   }
 
   // Replace the normal entry with {Dead} and the loop entry with {Start}
   // and run the control reducer to clean up the graph.
   osr_normal_entry->ReplaceUses(dead);
   osr_loop_entry->ReplaceUses(graph->start());
+
+  // Normally the control reducer removes loops whose first input is dead,
+  // but we need to avoid that because the osr_loop is reachable through
+  // the second input, so reduce it and its phis manually.
+  osr_loop->ReplaceInput(0, dead);
+  Node* node = ControlReducer::ReduceMerge(jsgraph, common, osr_loop);
+  if (node != osr_loop) osr_loop->ReplaceUses(node);
+
+  // Run the normal control reduction, which naturally trims away the dead
+  // parts of the graph.
   ControlReducer::ReduceGraph(tmp_zone, jsgraph, common);
 
   return true;
 }
 
 
 void OsrHelper::SetupFrame(Frame* frame) {
   // The optimized frame will subsume the unoptimized frame. Do so by reserving
   // the first spill slots.
   frame->ReserveSpillSlots(UnoptimizedFrameSlots());
   // The frame needs to be adjusted by the number of unoptimized frame slots.
   frame->SetOsrStackSlotCount(static_cast<int>(UnoptimizedFrameSlots()));
 }
 
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8