EnterUnwindContext pushes an unwind context (exception handler and/or
finalizer) onto a stack.
LeaveUnwindContext pops the unwind context from that stack.
Upon return to the interpreter loop we check whether the VM has an
exception pending. If no unwind context is available we return from the
loop. If an exception handler is available we clear the VM's exception,
put the exception value into the accumulator register, clear the unwind
context's handler and jump to the handler. If no handler is available
but a finalizer is available we save the exception value + metadata (for
later use by ContinuePendingUnwind), clear the VM's exception, pop the
unwind context and jump to the finalizer.
ContinuePendingUnwind checks whether a saved exception is available. If
no saved exception is available it jumps to the resume label. Otherwise
it stores the exception into the VM.
The Jump after LeaveUnwindContext could be integrated into the
LeaveUnwindContext instruction. I've kept them separate for now to make
the bytecode more readable.
> try { 1; throw "x" } catch (e) { 2 } finally { 3 }; 4
1:
[ 0] EnterScope
[ 10] EnterUnwindContext handler:@4 finalizer:@3
[ 38] EnterScope
[ 48] LoadImmediate 1
[ 60] NewString 1 ("x")
[ 70] Throw
<for non-terminated blocks: insert LeaveUnwindContext + Jump @3 here>
2:
[ 0] LoadImmediate 4
3:
[ 0] EnterScope
[ 10] LoadImmediate 3
[ 28] ContinuePendingUnwind resume:@2
4:
[ 0] SetVariable 0 (e)
[ 10] EnterScope
[ 20] LoadImmediate 2
[ 38] LeaveUnwindContext
[ 3c] Jump @3
String Table:
0: e
1: x
Instead of using Strings in the bytecode ops this adds a global string
table to the Executable struct which individual operations can refer
to using indices. This brings bytecode ops one step closer to being
pointer free.
This limits the size of each block (currently set to 1K), and gets us
closer to a canonical, more easily analysable bytecode format.
As a result of this, "Labels" are now simply entries to basic blocks.
Since there is no more 'conditional' jump (as all jumps are always
taken), JumpIf{True,False} are unified to JumpConditional, and
JumpIfNullish is renamed to JumpNullish.
Also fixes#7914 as a result of reimplementing the loop logic.
