Add an intrinsified early out path for Dart timeline calls

runtime/vm/intrinsifier_mips.cc

Index: runtime/vm/intrinsifier_mips.cc
diff --git a/runtime/vm/intrinsifier_mips.cc b/runtime/vm/intrinsifier_mips.cc
index 2398a7b957d512cc3a278c788e4733e2fa33f164..d42bcde0b45e5a20e846cba9ab24383079641d1b 100644
--- a/runtime/vm/intrinsifier_mips.cc
+++ b/runtime/vm/intrinsifier_mips.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 {
 
@@ -2208,6 +2209,28 @@ void Intrinsifier::Profiler_getCurrentTag(Assembler* assembler) {
   __ delay_slot()->lw(V0, Address(V0, Isolate::current_tag_offset()));
 }
 
+
+void Intrinsifier::Timeline_isDartStreamEnabled(Assembler* assembler) {
+  if (!FLAG_support_timeline) {
+    __ LoadObject(V0, Bool::False());
+    __ Ret();
+    return;
+  }
+  Label true_label;
+  // Load TimelineStream*.
+  __ lw(V0, Address(THR, Thread::dart_stream_offset()));
+  // Load uintptr_t from TimelineStream*.
+  __ lw(V0, Address(V0, TimelineStream::enabled_offset()));

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

__ lw(T0, Address(V0, ...));
__ LoadObject(V0, Bool::False());
__ LoadObject(V1, Bool::True());
__ Ret();
__ delay_slot->movz(V0, V1, T0);  // mov if T0 != 0.

+  __ bne(V0, ZR, &true_label);
+  // Not enabled.
+  __ LoadObject(V0, Bool::False());
+  __ Ret();
+  // Enabled.
+  __ Bind(&true_label);
+  __ LoadObject(V0, Bool::True());
+  __ Ret();
+}
+
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_MIPS