Implementing Thread.getStackTrace is tricky. A thread may interrupt
another thread at any time to grab a stack trace, including while the
latter is executing Java code, JNI code, helper thunks, VM code, or
while transitioning between any of these.
To create a stack trace we use several context fields associated with
the target thread, including snapshots of the instruction pointer,
stack pointer, and frame pointer. These fields must be current,
accurate, and consistent with each other in order to get a reliable
trace. Otherwise, we risk crashing the VM by trying to walk garbage
stack frames or by misinterpreting the size and/or content of
legitimate frames.
This commit addresses sensitive transition points such as entering the
helper thunks which bridge the transitions from Java to native code
(where we must save the stack and frame registers for use from native
code) and stack unwinding (where we must atomically update the thread
context fields to indicate which frame we are unwinding to). When
grabbing a trace for another thread, we determine what kind of code we
caught the thread executing in and use that information to choose the
thread context values with which to begin the trace. See
MyProcessor::getStackTrace::Visitor::visit for details.
In order to atomically update the thread context fields, we do the
following:
1. Create a temporary "transition" object to serve as a staging area
and populate it with the new field values.
2. Update a transition pointer in the thread object to point to the
object created above. As long as this pointer is non-null,
interrupting threads will use the context values in the staging
object instead of those in the thread object.
3. Update the fields in the thread object.
4. Clear the transition pointer in the thread object.
We use a memory barrier between each of these steps to ensure they are
made visible to other threads in program order. See
MyThread::doTransition for details.
The SingleRead::successor field is used (when non-null) to further
constrain the SiteMask in SingleRead::intersect based on reads of
successor values (as in the cases of moves and condensed-addressing
combine and translate instructions).
We now create a unique thunk for each vtable position so as to avoid
relying on using the return address to determine what method is to be
compiled and invoked, since we will not have the correct return address
in the case of a tail call. This required refactoring how executable
memory is allocated in order to keep AOT compilation working. Also, we
must always use the same register to hold the class pointer when
compiling virtual calls, and ensure that the pointer stays there until
the call instruction is executed so we know where to find it in the
thunk.
