Replace guarded list length field loads with constants

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
diff --git a/runtime/vm/intermediate_language.cc b/runtime/vm/intermediate_language.cc
index b681cd70f7edf22162f8a9226765859c265c4a7c..e49542354c42d846cdd6a4b37bc55b2696eb5a99 100644
--- a/runtime/vm/intermediate_language.cc
+++ b/runtime/vm/intermediate_language.cc
@@ -1366,6 +1366,16 @@ Definition* LoadFieldInstr::Canonicalize(FlowGraph* flow_graph) {
       IsFixedLengthArrayCid(call->Type()->ToCid())) {
     return call->ArgumentAt(1);
   }
+  // For arrays with guarded lengths, replace the length load
+  // with a constant.
+  LoadFieldInstr* load_array = instance()->definition()->AsLoadField();
+  if (load_array != NULL) {
+    const Field* field = load_array->field();

[srdjan, 2013/08/28 15:15:08]

'field' can be null, do you need to check?

[Cutch, 2013/08/29 07:23:26]

Done. Conceptual/context question: What does a LoadFieldInstr with a NULL field
indicate?

+    if (field->guarded_list_length() >= 0) {
+      return flow_graph->GetConstant(
+          Smi::Handle(Smi::New(field->guarded_list_length())));
+    }
+  }
   return this;
 }