Make v8::Isolate::SetRAILMode thread safe and remove the

PERFORMANCE_DEFAULT mode.

BUG=

Review-Url: https://codereview.chromium.org/2108503003
Cr-Commit-Position: refs/heads/master@{#37340}

include/v8.h

@@ -5378,16 +5378,13 @@ struct JitCodeEvent {
  * profile/evaluate-performance/rail
  */
 enum RAILMode {
-  // Default performance mode: V8 will optimize for both latency and
-  // throughput in this mode.
-  PERFORMANCE_DEFAULT,
   // Response performance mode: In this mode very low virtual machine latency
   // is provided. V8 will try to avoid JavaScript execution interruptions.
   // Throughput may be throttled.
   PERFORMANCE_RESPONSE,
   // Animation performance mode: In this mode low virtual machine latency is
   // provided. V8 will try to avoid as many JavaScript execution interruptions
-  // as possible. Throughput may be throttled
+  // as possible. Throughput may be throttled. This is the default mode.
   PERFORMANCE_ANIMATION,
   // Idle performance mode: The embedder is idle. V8 can complete deferred work
   // in this mode.

src/isolate.cc

@@ -1839,7 +1839,7 @@ Isolate::Isolate(bool enable_serializer)
       // TODO(bmeurer) Initialized lazily because it depends on flags; can
       // be fixed once the default isolate cleanup is done.
       random_number_generator_(NULL),
-      rail_mode_(PERFORMANCE_DEFAULT),
+      rail_mode_(PERFORMANCE_ANIMATION),
       serializer_enabled_(enable_serializer),
       has_fatal_error_(false),
       initialized_from_snapshot_(false),
@@ -2995,9 +2995,9 @@ void Isolate::CheckDetachedContextsAfterGC() {
 }
 
 void Isolate::SetRAILMode(RAILMode rail_mode) {
-  rail_mode_ = rail_mode;
+  rail_mode_.SetValue(rail_mode);
   if (FLAG_trace_rail) {
-    PrintIsolate(this, "RAIL mode: %s\n", RAILModeName(rail_mode_));
+    PrintIsolate(this, "RAIL mode: %s\n", RAILModeName(rail_mode));
   }
 }
 

src/isolate.h

@@ -1253,8 +1253,6 @@ class Isolate {
 
   const char* RAILModeName(RAILMode rail_mode) const {
     switch (rail_mode) {
-      case PERFORMANCE_DEFAULT:
-        return "DEFAULT";
       case PERFORMANCE_RESPONSE:
         return "RESPONSE";
       case PERFORMANCE_ANIMATION:
@@ -1263,8 +1261,6 @@ class Isolate {
         return "IDLE";
       case PERFORMANCE_LOAD:
         return "LOAD";
-      default:
-        UNREACHABLE();
     }
     return "";
   }
@@ -1315,7 +1311,7 @@ class Isolate {
   DateCache* date_cache_;
   CallInterfaceDescriptorData* call_descriptor_data_;
   base::RandomNumberGenerator* random_number_generator_;
-  RAILMode rail_mode_;
+  base::AtomicValue<RAILMode> rail_mode_;
 
   // Whether the isolate has been created for snapshotting.
   bool serializer_enabled_;