Reapply "Fix misoptimization of 'is' test""

runtime/vm/intermediate_language_dbc.cc

 // Copyright (c) 2016, 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/globals.h"  // Needed here to get TARGET_ARCH_DBC.
 #if defined(TARGET_ARCH_DBC)
 
 #include "vm/intermediate_language.h"
 
 #include "vm/cpu.h"
@@ skipping 628 matching lines @@
   ASSERT(num_cases <= 255);
   __ TestCids(value, num_cases);
 
   bool result = false;
   for (intptr_t i = 0; i < data.length(); i += 2) {
     const intptr_t test_cid = data[i];
     result = data[i + 1] == true_result;
     __ Nop(result ? 1 : 0, compiler->ToEmbeddableCid(test_cid, this));
   }
 
-  // No match found, deoptimize or false.
+  // No match found, deoptimize or default action.
   if (CanDeoptimize()) {
     compiler->EmitDeopt(deopt_id(), ICData::kDeoptTestCids,
                         licm_hoisted_ ? ICData::kHoisted : 0);
   } else {
+    // If the cid is not in the list, jump to the opposite label from the cids
+    // that are in the list.  These must be all the same (see asserts in the
+    // constructor).
     Label* target = result ? labels.false_label : labels.true_label;
     __ Jump(target);
   }
 
   return NEXT_IS_TRUE;
 }
 
 
 void TestCidsInstr::EmitBranchCode(FlowGraphCompiler* compiler,
                                    BranchInstr* branch) {
@@ skipping 1409 matching lines @@
   }
   __ IfULe(length, index);
   compiler->EmitDeopt(deopt_id(), ICData::kDeoptCheckArrayBound,
                       (generalized_ ? ICData::kGeneralized : 0) |
                           (licm_hoisted_ ? ICData::kHoisted : 0));
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_DBC