RFR (S): 8211394: CHECK_ must be used in the rhs of an assignment statement within a block

[David Holmes]

Hi Ioi,

tl;dr http://cr.openjdk.java.net/~dholmes/8211394/webrev.v2/

On 9/10/2018 8:58 AM, Ioi Lam wrote:
> On 10/8/18 3:25 PM, David Holmes wrote:
>> On 9/10/2018 8:04 AM, Ioi Lam wrote:
>>>> On Oct 8, 2018, at 2:19 PM, David Holmes <david.holmes at oracle.com> 
>>>> wrote:
>>>>
>>>> Hi Ioi,
>>>>
>>>>> On 9/10/2018 1:55 AM, Ioi Lam wrote:
>>>>> I am not sure if this change is really required:
>>>>> Symbol* MethodFamily::generate_no_defaults_message(TRAPS) const {
>>>>> - return SymbolTable::new_symbol("No qualifying defaults found", 
>>>>> THREAD);
>>>>>    + Symbol* s = SymbolTable::new_symbol("No qualifying defaults 
>>>>> found", CHECK_NULL);
>>>>>    + return s;
>>>>> I think the rationale is "since SymbolTable is a different class, 
>>>>> we don't know what value it returns when an exception is thrown."
>>>>
>>>> Yes. Seems important to try and establish some consistency on how to 
>>>> handle this.
>>>>
>>>>> However, the return value is a pointer type. Can we except anything 
>>>>> other than NULL in case of an exception? I think using the 
>>>>> CHECK_NULL here will make the code bigger and slower, and also 
>>>>> harder to read.
>>>>
>>>> We shouldn't expect anything other than an "error return value" if 
>>>> an exception is pending, but that relies on who/how the target code 
>>>> was coded.
>>>>
>>>> I don't think the code change will be at all noticeable in terms of 
>>>> size of speed. Readability is subjective.
>>>>
>>>
>>> Do we actually have a use case where the returned value is 
>>> significant when an exception is thrown? That would seem pretty 
>>> fragile to me.
>>
>> No it would be an error. The point is that unless you go and check 
>> that the called method has correctly handled exceptions, you don't 
>> know that. So rather than assume, or make the effort to establish, be 
>> conservative.
>>
> 
> Both the caller and callee have TRAPS in their argument, which means (to 
> me)
> 
>         "the returned value is value is valid only if no exception has 
> been thrown".
> 
> exceptions.hpp says
> 
>      // Macro naming conventions: Macros that end with _ require a 
> result value to be returned. They
>      // are for functions with non-void result type. The result value is 
> usually ignored because of
>      // the exception and is only needed for syntactic correctness. The 
> _0 ending is a shortcut for
>      // _(0) since this is a frequent case.
> 
> 
> by doing the CHECK_NULL in the caller, all you're ensuring is
> 
>      "when this function has encountered an exception, it will always 
> return NULL"
> 
> but this function's declaration already says the return value should be 
> ignored on exception, so you're simply fixing a value that no one should 
> read.
> 
> So, this seems to me like a pointless ritual.

I agree that all functions should return an "error value" when an 
exception is detected. I also agree that the return value of any 
exception-throwing function should be examined only if there is no 
exception pending.

I was on the fence as to whether one part of the code should assume the 
other part of the code was written correctly or whether it should be 
defensive. Paul (who filed the bug) wanted defensive. You object 
strongly to it. I've gone back to the review thread for JDK-8062808 
which fixed some return CHECK to return THREAD and no concerns were 
raised there about the change. It seems inappropriate for me to overrule 
all the previous reviewers when I'm on the fence anyway.

So version 2:

http://cr.openjdk.java.net/~dholmes/8211394/webrev.v2/

Thanks,
David

> Thanks
> - Ioi
> 
> 
>> David
>>
>>> If not, then returning whatever the caller returns seems good enough, 
>>> and less code to write.
>>>
>>> Thanks
>>> Ioi
>>>
>>>
>>>> Thanks,
>>>> David
>>>> -----
>>>>
>>>>> (BTW, we have at 150+ occurrencesof 'return.*THREAD[)];' in the code.)
>>>>
>>>>
>>>>
>>>>> Thanks
>>>>> - Ioi
>>>>>> On 10/7/18 3:08 PM, David Holmes wrote:
>>>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8211394
>>>>>> webrev: http://cr.openjdk.java.net/~dholmes/8211394/webrev/
>>>>>>
>>>>>> If a CHECK_ macro is used on a function call that is part of a 
>>>>>> return statement i.e.
>>>>>>
>>>>>> return foo(CHECK_NULL);
>>>>>>
>>>>>> then it expands into an unreachable if-statement that checks the 
>>>>>> exception state:
>>>>>>
>>>>>> return foo();
>>>>>> if (EXCEPTION_OCCURRED)
>>>>>>    return NULL;
>>>>>>
>>>>>> This is obviously a programming error, but unfortunately not 
>>>>>> something our often pedantic compilers complain about.
>>>>>>
>>>>>> There are two ways to fix:
>>>>>>
>>>>>> 1. Convert to assignment:
>>>>>>
>>>>>> T* t = foo(CHECK_NULL);
>>>>>> return t;
>>>>>>
>>>>>> 2. If the method is local and its exception behaviour easily 
>>>>>> discernible and matches the expected behaviour, then change CHECK_ 
>>>>>> to THREAD
>>>>>>
>>>>>> return foo(THREAD);
>>>>>>
>>>>>> Both fixes are applied as appropriate. As per the bug report I 
>>>>>> also revisited an earlier fix in this area - JDK-8062808 - and 
>>>>>> made adjustments.
>>>>>>
>>>>>> Thanks,
>>>>>> David
>>>
>