Reapply "Fix misoptimization of 'is' test""

runtime/vm/intermediate_language_mips.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/globals.h"  // Needed here to get TARGET_ARCH_MIPS.
 #if defined(TARGET_ARCH_MIPS)
 
 #include "vm/intermediate_language.h"
 
 #include "vm/compiler.h"
@@ skipping 876 matching lines @@
   __ beq(CMPRES1, ZR, result ? labels.true_label : labels.false_label);
 
   __ LoadClassId(cid_reg, val_reg);
   for (intptr_t i = 2; i < data.length(); i += 2) {
     const intptr_t test_cid = data[i];
     ASSERT(test_cid != kSmiCid);
     result = data[i + 1] == true_result;
     __ BranchEqual(cid_reg, Immediate(test_cid),
                    result ? labels.true_label : labels.false_label);
   }
-  // No match found, deoptimize or false.
+  // No match found, deoptimize or default action.
   if (deopt == NULL) {
+    // 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;
     if (target != labels.fall_through) {
       __ b(target);
     }
   } else {
     __ b(deopt);
   }
   // Dummy result as the last instruction is a jump or fall through.
   return Condition(CMPRES1, ZR, AL);
 }
@@ skipping 5170 matching lines @@
   compiler->GenerateRuntimeCall(TokenPosition::kNoSource, deopt_id(),
                                 kGrowRegExpStackRuntimeEntry, 1, locs());
   __ lw(result, Address(SP, 1 * kWordSize));
   __ addiu(SP, SP, Immediate(2 * kWordSize));
 }
 
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_MIPS