RFR: JDK-8214114: Switch expressions with try-catch statements

[Maurizio Cimadamore]

On 06/12/2018 23:24, Jan Lahoda wrote:
> On 6.12.2018 23:04, Maurizio Cimadamore wrote:
>> While I don't think the stack should be as what Vicente says (after all,
>> there's a Println object somewhere that should be tracked), I tend to
>> agree with Vicente that there's something fishy here - I looked at the
>> bug report and also run the example and the verifier crashes _before_
>> entering the exception handler - it crashes on the 'new' when the
>> exception is allocated. That doesn't seem to match what you said in the
>> RFR. Now, as I pointed out in my other message, I believe some stashing
>> is unavoidable as the VM will wipe out the stack, but it seems like
>> there could be more issues piled up here?
>
> Sorry, I should have been more precise. What I meant is that the 
> StackMap for catch handler must not have anything on stack (except for 
> the Exception) (because VM will wipe the stack, that is, I think, 
> another angle to look at this). That is not related only to athrow - 
> the exception may be coming from a method invocation, for example. The 
> reason the verifier barks on "new" is, I think, because the 
> instruction may throw an exception as well. So it checks the StackMap 
> for the exception handler, and finds out there's something on the 
> stack (in the StackMap) that should not be there, and fails (or, from 
> another angle, when checking the exception handler, the verifier finds 
> out the handler expects something on the stack, besides the exception, 
> which can't be there).

And sorry for the confusion - I realized I misread the verifier error - 
it's actually pointing at the handler entry (as javac generated both 
[PrintStream, Exception], while the VM expects just [Exception] - it 
being an handler). So what you did is correct and I think just fixing 
the stackmap is not a real solution as then we need a way to recover the 
values before leaving the switch.

Maurizio

>
> Jan
>
>>
>> Maurizio
>>
>> On 06/12/2018 21:50, Vicente Romero wrote:
>>> Hi Jan,
>>>
>>> Probably I'm missing something but shouldn't it be enough to generate
>>> the right stackframe for the exception handler? Which in this case
>>> should only contain:
>>>
>>> stack = [ class java/lang/Exception ]
>>>
>>> Thanks,
>>> Vicente
>>>
>>> On 12/6/18 11:51 AM, Jan Lahoda wrote:
>>>> Hi,
>>>>
>>>> Switch expressions that contain try-catch statements don't work
>>>> currently, as the catch handlers require that the stack is empty when
>>>> the handler is invoked, but this may not be true when the switch
>>>> expression is inside an expression evaluation (and so some values are
>>>> already on the stack).
>>>>
>>>> The proposed solution is to stash the stack content to local
>>>> variables before "running" the switch expression, and fill the stack
>>>> content back when breaking out of the switch expression. This is done
>>>> only if the try-catch statement is present in the switch expression,
>>>> as it produces longer bytecode.
>>>>
>>>> Also, it turned out variables inside switch expressions in field
>>>> initializers don't work properly (as their owner is the field, not a
>>>> method), attempted to fix that as well.
>>>>
>>>> JBS: https://bugs.openjdk.java.net/browse/JDK-8214114
>>>> Webrev: http://cr.openjdk.java.net/~jlahoda/8214114/webrev.00/
>>>>
>>>> How does this look?
>>>>
>>>> Thanks,
>>>>     Jan
>>>