Track async causal stack traces

runtime/vm/intrinsifier_x64.cc

 // Copyright (c) 2013, 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_X64.
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/intrinsifier.h"
 
 #include "vm/assembler.h"
@@ skipping 2261 matching lines @@
   __ j(NOT_ZERO, &true_label, Assembler::kNearJump);
   // Not enabled.
   __ LoadObject(RAX, Bool::False());
   __ ret();
   // Enabled.
   __ Bind(&true_label);
   __ LoadObject(RAX, Bool::True());
   __ ret();
 }
 
+
+void Intrinsifier::ClearAsyncThreadStackTrace(Assembler* assembler) {
+  __ LoadObject(RAX, Object::null_object());
+  __ movq(Address(THR, Thread::async_stack_trace_offset()), RAX);
+  __ ret();
+}
+
+
+void Intrinsifier::SetAsyncThreadStackTrace(Assembler* assembler) {
+  __ movq(Address(THR, Thread::async_stack_trace_offset()), RAX);
+  __ LoadObject(RAX, Object::null_object());

[siva, 2017/02/08 18:46:29]

why does RAX need to be reset to the null object?

[rmacnak, 2017/02/08 18:52:49]

The corresponding native function returns null. Intrinsics should always match
the behavior of the regular path.

[siva, 2017/02/08 18:59:12]

True, why does the native function return null could be some random object
right?

+  __ ret();
+}
+
 #undef __
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64