Pattern match on generated code to find edge counters.

runtime/vm/intermediate_language.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/intermediate_language.h"
 
 #include "vm/bigint_operations.h"
 #include "vm/bit_vector.h"
 #include "vm/dart_entry.h"
 #include "vm/flow_graph_allocator.h"
@@ skipping 1646 matching lines @@
                                    deopt_id_,
                                    Scanner::kDummyTokenIndex);
   }
   if (HasParallelMove()) {
     compiler->parallel_move_resolver()->EmitNativeCode(parallel_move());
   }
 }
 
 
 LocationSummary* TargetEntryInstr::MakeLocationSummary() const {
+  // FlowGraphCompiler::EmitInstructionPrologue is not called for block
+  // entry instructions, so this function is unused.  If it becomes
+  // reachable, note that the deoptimization descriptor in unoptimized code
+  // comes after the point of local register allocation due to pattern
+  // matching the edge counter code backwards (as a code reuse convenience
+  // on some platforms).
   UNREACHABLE();
   return NULL;
 }
 
 
 LocationSummary* PhiInstr::MakeLocationSummary() const {
   UNREACHABLE();
   return NULL;
 }
 
@@ skipping 1010 matching lines @@
       return kCosRuntimeEntry;
     default:
       UNREACHABLE();
   }
   return kSinRuntimeEntry;
 }
 
 #undef __
 
 }  // namespace dart