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

[David Holmes]

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.

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
>