This set of patches fixes a bug in the VM when breaking within a try-finally, and also follows up with optimisations that open up with the bug fixed.

The bug has to do with executing a break within the finally block of a try-finally statement. For example:

def f():
    for x in (1,):
        print('a', x)
        try:
            raise Exception
        finally:
            print(1)
            break
        print('b', x)
f()

Currently in uPy the above code will print:

a 1
1
1
segmentation fault (core dumped)  micropython

Not only is there a seg fault, but the "1" in the finally block is printed twice. This is because when the VM executes a finally block it doesn't really know if that block was executed due to a fall-through of the try (no exception raised), or because an exception is active. In particular, for nested finallys the VM has no idea which of the nested ones have active exceptions and which are just fall-throughs. So when a break (or continue) is executed it tries to unwind all of the finallys, when in fact only some may be active.

It's questionable whether break (or return or continue) should be allowed within a finally block, because they implicitly swallow any active exception, but nevertheless it's allowed by CPython (although almost never used in the standard library). And uPy should at least not crash in such a case.

It took me a while to find an acceptable solution to this bug. Initially I thought to just ban such a construct in the compiler, but eventually I came across the solution presented here. It relies on the fact that exception and finally handlers always appear in the bytecode after the try body.

Note: there was a similar bug with a return in a finally block, but that was previously fixed in b735208

To understand the fix it's best to look at each commit here independently. In the end the following statements can be made:

1. The bug with break/continue in (nested) finallys is fixed.

2. The opcodes POP_BLOCK and POP_EXCEPT are replaced by POP_EXCEPT_JUMP.

3. Generated bytecode is reduced by 2 bytes for each use of a try-except (due to point 2 above).

4. Code size is reduced:

   bare-arm:   -88 -0.133% 
minimal x86:  -200 -0.129% 
   unix x64:  -320 -0.065%
unix nanbox:  -412 -0.093%
      stm32:  -192 -0.052% 
     cc3200:  -128 -0.069% 
    esp8266:  -676 -0.104% 
      esp32:  -444 -0.040%