Change heuristic to determine which fields contain modifieable double boxes: do it if field was ini…

runtime/vm/flow_graph_optimizer.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/flow_graph_optimizer.h"
 
 #include "vm/bit_vector.h"
 #include "vm/cha.h"
 #include "vm/compiler.h"
 #include "vm/cpu.h"
@@ skipping 33 matching lines @@
 DEFINE_FLAG(bool, truncating_left_shift, true,
     "Optimize left shift to truncate if possible");
 DEFINE_FLAG(bool, use_cha_deopt, true,
     "Use class hierarchy analysis even if it can cause deoptimization.");
 #if defined(TARGET_ARCH_ARM) || defined(TARGET_ARCH_IA32)
 DEFINE_FLAG(bool, trace_smi_widening, false, "Trace Smi->Int32 widening pass.");
 #endif
 
 DECLARE_FLAG(bool, polymorphic_with_deopt);
 DECLARE_FLAG(bool, source_lines);
+DECLARE_FLAG(bool, trace_field_guards);
 DECLARE_FLAG(bool, trace_type_check_elimination);
 DECLARE_FLAG(bool, warn_on_javascript_compatibility);
 
 // Quick access to the current isolate and zone.
 #define I (isolate())
 #define Z (zone())
 
 static bool ShouldInlineSimd() {
   return FlowGraphCompiler::SupportsUnboxedSimd128();
 }
@@ skipping 4525 matching lines @@
     // usage count of at least 1/kGetterSetterRatio of the getter usage count.
     // This is to avoid unboxing fields where the setter is never or rarely
     // executed.
     const Field& field = Field::ZoneHandle(Z, instr->field().raw());
     const String& field_name = String::Handle(Z, field.name());
     const Class& owner = Class::Handle(Z, field.owner());
     const Function& getter =
         Function::Handle(Z, owner.LookupGetterFunction(field_name));
     const Function& setter =
         Function::Handle(Z, owner.LookupSetterFunction(field_name));
-    bool result = !getter.IsNull()
-               && !setter.IsNull()
-               && (setter.usage_counter() > 0)
-               && (FLAG_getter_setter_ratio * setter.usage_counter() >=
-                   getter.usage_counter());
-    if (!result) {
-      if (FLAG_trace_optimization) {
+    bool unboxed_field = false;
+    if (!getter.IsNull() && !setter.IsNull()) {
+      if (field.is_double_initialized()) {
+        unboxed_field = true;
+      } else if ((setter.usage_counter() > 0) &&
+                 ((FLAG_getter_setter_ratio * setter.usage_counter()) >=
+                   getter.usage_counter())) {
+        unboxed_field = true;
+      }
+    }
+    if (!unboxed_field) {
+      if (FLAG_trace_optimization || FLAG_trace_field_guards) {
         ISL_Print("Disabling unboxing of %s\n", field.ToCString());
+        if (!setter.IsNull()) {
+          OS::Print("  setter usage count: %" Pd "\n", setter.usage_counter());
+        }
+        if (!getter.IsNull()) {
+          OS::Print("  getter usage count: %" Pd "\n", getter.usage_counter());
+        }
       }
       field.set_is_unboxing_candidate(false);
       field.DeoptimizeDependentCode();
     } else {
       FlowGraph::AddToGuardedFields(flow_graph_->guarded_fields(), &field);
     }
   }
 }
 
 
@@ skipping 4142 matching lines @@
 
   // Insert materializations at environment uses.
   for (intptr_t i = 0; i < exits_collector_.exits().length(); i++) {
     CreateMaterializationAt(
         exits_collector_.exits()[i], alloc, *slots);
   }
 }
 
 
 }  // namespace dart