[Tracing] Embed V8 runtime call stats into tracing.

src/arguments.h

Index: src/arguments.h
diff --git a/src/arguments.h b/src/arguments.h
index 34fbca4feacda2ca3b7eec59b054b107bb32c5b4..eadd7354d78c23f55ae24c4664c3afe4515cc37c 100644
--- a/src/arguments.h
+++ b/src/arguments.h
@@ -88,8 +88,6 @@ double ClobberDoubleRegisters(double x1, double x2, double x3, double x4);
   V8_NOINLINE static Type Stats_##Name(int args_length, Object** args_object, \
                                        Isolate* isolate) {                    \
     RuntimeCallTimerScope timer(isolate, &RuntimeCallStats::Name);            \
-    TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.runtime"),                     \
-                 "V8.Runtime_" #Name);                                        \
     Arguments args(args_length, args_object);                                 \
     return __RT_impl_##Name(args, isolate);                                   \
   }                                                                           \
@@ -101,7 +99,10 @@ double ClobberDoubleRegisters(double x1, double x2, double x3, double x4);
       return Stats_##Name(args_length, args_object, isolate);                 \
     }                                                                         \
     Arguments args(args_length, args_object);                                 \
-    return __RT_impl_##Name(args, isolate);                                   \
+    TRACE_EVENT_RUNTIME_CALL_STATS_TRACING_SCOPED(                            \
+        isolate, &tracing::TraceEventStatsTable::Name);                       \

[Camillo Bruni, 2016/07/27 08:29:07]

Could you put the the scope in the Stats_##Name macro?
I'd like to keep as many timers out of the default runtime function as possible.

[lpy, 2016/07/27 19:58:37]

I understand the concern here. The reason we are putting it here is we don't
want the scope to be behind the --runtime_call_stats flag.

What about moving the scope to everywhere that uses RUNTIME_FUNCTION macro? I
think eventually we will want to replace the RuntimeCallTimerScope, so at that
point we can move the scope in Stats_##Name and remove the flag.

WDYT?

[Camillo Bruni, 2016/07/29 08:56:15]

Oh yes of course you don't want to be behind a flag :), but could you just OR
your checks whether the trace is enabled or not?
You will have to do this anyway to reduce the overhead when removing the
RuntimeCallTimerScope.
Since the scopes aren't optimized away and given that some runtime functions are
called really frequently I'd like to reduce the overhead to a minimum.

Pushing the uses down to the RUNTIME_FUNCTION wouldn't help in this case either,
it really has to be a guarded call to a separate non-inlined function.
Maybe use the V8_UNLIKELY() around the check as well, I forgot about that.

+    Type result = __RT_impl_##Name(args, isolate);                            \
+    return result;                                                            \
   }                                                                           \
                                                                               \
   static Type __RT_impl_##Name(Arguments args, Isolate* isolate)