RFR: 8213411: JDK-8209189 incorrect for Big Endian (JVM crashes)

[Erik Österlund]

Hi Vladimir,

Thanks for the review.

/Erik

On 2018-11-07 17:43, Vladimir Kozlov wrote:
> Looks good.
>
> Thanks,
> Vladimir
>
> On 11/7/18 5:45 AM, Erik Österlund wrote:
>> Hi Vladimir,
>>
>> Thank you for the review. I made an abstraction for the bit fiddling 
>> as suggested to make it more readable.
>> There is now an AllStatic IsUnloadingState class that has a mask and 
>> shift for each field, expresses getters and setters using them, and a 
>> create function for setting some passed in fields into a new bit field.
>>
>> Webrev:
>> http://cr.openjdk.java.net/~eosterlund/8213411/webrev.01/
>>
>> Thanks,
>> /Erik
>>
>> On 2018-11-06 17:21, Vladimir Kozlov wrote:
>>> Hi Erik,
>>>
>>> It would be nice if you use enum for values, masks and shift (like 
>>> for markOop). It will become more readable.
>>>
>>> Thanks,
>>> Vladimir
>>>
>>> On 11/6/18 2:34 AM, Erik Österlund wrote:
>>>> Hi,
>>>>
>>>> In my patch for 8209189 I assumed that the following struct 
>>>> containing a bit field using 3 bits, would be packed to one byte:
>>>>
>>>> struct IsUnloadingStruct {
>>>>    unsigned int _is_unloading:1;
>>>>    unsigned int _unloading_cycle:2;
>>>> };
>>>>
>>>> Turns out though that the size of this 3 bit bitfield is typically 
>>>> 4 bytes, causing trouble on big endian machines. The compilers can 
>>>> be tamed to turn this to a byte (as expected) by using struct packing.
>>>>
>>>> So the two most obvious solutions are either
>>>> 1) Make a struct packing macro, and trust the compilers won't mess 
>>>> up then.
>>>> 2) Do manual bit fiddling because we do not trust compilers.
>>>>
>>>> Martin convinced me to go with #2.
>>>>
>>>> Bug:
>>>> https://bugs.openjdk.java.net/browse/JDK-8213411
>>>>
>>>> Webrev:
>>>> http://cr.openjdk.java.net/~eosterlund/8213411/webrev.00/
>>>>
>>>> Running a bunch of tests while waiting for review.
>>>>
>>>> Thanks,
>>>> /Erik
>>