This implementation does not conform to the Java standard in that
finalize methods are called from whichever thread happens to be garbage
collecting, and that thread may hold locks, whereas the standard
guarantees that finalize will be run from a thread which holds no locks.
Also, an object will never be finalized more than once, even if its
finalize method "rescues" (i.e. makes reachable) the object such that it
might become unreachable a second time and thus a candidate for
finalization once more. It's not clear to me from the standard if this
is OK or not.
Nonwithstanding the above, this implementation is useful for "normal"
finalize methods which simply release resources associated with an
object.
The previous code relied on the invalid assumption that the thread-local
heaps for all threads would have been cleared immediately following a
garbage collection. However, the last thing the garbage collection
function does is run finalizers which may allocate new objects. This
can lead allocate3 to call allocateSmall with a size which is too large
to accomodate, overflowing the heap.
The solution is to iterate until there really is enough room for the
original allocation request.
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.
This helps us support the Java Memory Model without adding a memory
barrier to every object allocation. It's also potentially more
efficient, since we zero out each heap segment all at once instead of
bit-by-bit with each object allocation.
The trick is to make all destructors non-virtual. This is safe because
we never use the delete operator, which is the only case where virtual
destructors are relevant. This is a better solution than implementing
our own delete operator, because we want libraries loaded at runtime to
use the libstdc++ version, not ours.
This simplifies the JNI implementation for looking up methods. It also
fixes a bug where an applications calls GetStaticMethodID with class A
and then calls CallStatic<Type>Method with class B which extends A. The
old code would look in the wrong method table and thus call the wrong
method.
