Fix misoptimization of 'is' test

runtime/vm/intermediate_language_x64.cc

Index: runtime/vm/intermediate_language_x64.cc
diff --git a/runtime/vm/intermediate_language_x64.cc b/runtime/vm/intermediate_language_x64.cc
index 8f22f334899c7eff30a60b316c5d44a5b68c12b0..8796e59038ed86bf3e4b772c5a93afe1f3a1dddd 100644
--- a/runtime/vm/intermediate_language_x64.cc
+++ b/runtime/vm/intermediate_language_x64.cc
@@ -686,6 +686,15 @@ Condition TestCidsInstr::EmitComparisonCode(FlowGraphCompiler* compiler,
   }
   // 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);