Add an intrinsified early out path for Dart timeline calls

runtime/vm/intrinsifier_arm64.cc

 // Copyright (c) 2014, 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/globals.h"  // Needed here to get TARGET_ARCH_ARM64.
 #if defined(TARGET_ARCH_ARM64)
 
 #include "vm/intrinsifier.h"
 
 #include "vm/assembler.h"
 #include "vm/dart_entry.h"
 #include "vm/flow_graph_compiler.h"
 #include "vm/object.h"
 #include "vm/object_store.h"
 #include "vm/regexp_assembler.h"
 #include "vm/symbols.h"
+#include "vm/timeline.h"
 
 namespace dart {
 
 // When entering intrinsics code:
 // R4: Arguments descriptor
 // LR: Return address
 // The R4 register can be destroyed only if there is no slow-path, i.e.
 // if the intrinsified method always executes a return.
 // The FP register should not be modified, because it is used by the profiler.
 // The PP and THR registers (see constants_arm64.h) must be preserved.
@@ skipping 2144 matching lines @@
   __ ret();
 }
 
 
 void Intrinsifier::Profiler_getCurrentTag(Assembler* assembler) {
   __ LoadIsolate(R0);
   __ ldr(R0, Address(R0, Isolate::current_tag_offset()));
   __ ret();
 }
 
+
+void Intrinsifier::Timeline_isDartStreamEnabled(Assembler* assembler) {
+  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));
+  __ 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