Add an intrinsified early out path for Dart timeline calls

runtime/vm/intrinsifier_arm64.cc

Index: runtime/vm/intrinsifier_arm64.cc
diff --git a/runtime/vm/intrinsifier_arm64.cc b/runtime/vm/intrinsifier_arm64.cc
index 88512e81816c74ca911ac4b1e266215bb48cee89..7a95406e4e57e306debe81b53292055264544fb7 100644
--- a/runtime/vm/intrinsifier_arm64.cc
+++ b/runtime/vm/intrinsifier_arm64.cc
@@ -14,6 +14,7 @@
 #include "vm/object_store.h"
 #include "vm/regexp_assembler.h"
 #include "vm/symbols.h"
+#include "vm/timeline.h"
 
 namespace dart {
 
@@ -2178,6 +2179,29 @@ void Intrinsifier::Profiler_getCurrentTag(Assembler* assembler) {
   __ ret();
 }
 
+
+void Intrinsifier::Timeline_isDartStreamEnabled(Assembler* assembler) {
+  if (!FLAG_support_timeline) {
+    __ LoadObject(R0, Bool::False());
+    __ ret();
+    return;
+  }
+  Label true_label;
+  // Load TimelineStream*.
+  __ ldr(R0, Address(THR, Thread::dart_stream_offset()));
+  // Load uintptr_t from TimelineStream*.
+  __ ldr(R0, Address(R0, TimelineStream::enabled_offset()));
+  __ cmp(R0, Operand(0));

[zra, 2016/05/17 17:35:49]

__ cmp(R0, Operand(0));
__ LoadObject(R0, Bool::False());
__ LoadObject(TMP, Bool::True());
__ csel(R0, TMP, R0, NE);

+  __ b(&true_label, NE);
+  // Not enabled.
+  __ LoadObject(R0, Bool::False());
+  __ ret();
+  // Enabled.
+  __ Bind(&true_label);
+  __ LoadObject(R0, Bool::True());
+  __ ret();
+}
+
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_ARM64