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 668 matching lines @@
   __ 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.
   if (deopt == NULL) {
+#ifdef DEBUG

[Vyacheslav Egorov (Google), 2017/05/23 16:00:31]

I think this needs to go to constructor.

[erikcorry, 2017/05/24 13:37:15]

Done.

+    // The entry for Smi can be special, but all other entries have
+    // to match in the no-deopt case.
+    for (intptr_t i = 4; i < data.length(); i += 2) {
+      ASSERT(data[i + 1] == data[3]);
+    }
+#endif
+    // If the cid is not in the list, jump to the opposite label from the cids

[Vyacheslav Egorov (Google), 2017/05/23 16:00:31]

Please add a comment to the instruction in the intermediate_language.h that
documents this behavior (which is obscure to say the least)

[erikcorry, 2017/05/24 13:37:15]

Done.

+    // that are in the list.
     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