RFR: 8167200: AArch64: Broken stack pointer adjustment in interpreter

[Andrew Haley]

This is a thinko in the template interpreter.

AArch64 has two stack pointers: the system SP and the expression
SP. The expression SP must always point to an address greater than or
equal to the system SP.

When we allocate a new monitor in the interpreter we move the contents
of the entire operand stack and then insert a monitor between the
operand stack and the frame. So, we need to allocate two more words
(the size of a monitor) on the stack. The expression SP is adjusted to
allow for this, and we then do a comparison to see if the system SP
needs also to be moved. This is wrong: whenever we allocate a monitor
we should also unconditionally adjust the system SP.

The bug here is that we might decide that we do not need any more
space in the system stack but later push several items onto the
expression stack. In this case the expression SP might be lower than
the system SP, and method arguments are corrupted.

This patch changes the logic so that whenever we insert a monitor we
unconditionally adjust the system SP.

http://cr.openjdk.java.net/~aph/8167200/

Andrew.