Reapply "Fix misoptimization of 'is' test""

runtime/vm/intermediate_language_x64.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_X64.
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/intermediate_language.h"
 
 #include "vm/compiler.h"
@@ skipping 666 matching lines @@
   __ testq(val_reg, Immediate(kSmiTagMask));
   __ j(ZERO, 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;
     __ cmpq(cid_reg, Immediate(test_cid));
     __ j(EQUAL, 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) {
       __ jmp(target);
     }
   } else {
     __ jmp(deopt);
   }
   // Dummy result as the last instruction is a jump, any conditional
   // branch using the result will therefore be skipped.
   return ZERO;
@@ skipping 6077 matching lines @@
   __ Drop(1);
   __ popq(result);
 }
 
 
 }  // namespace dart
 
 #undef __
 
 #endif  // defined TARGET_ARCH_X64