RFR: 8203339: Add oopDesc::field_offset()

[Stefan Karlsson]

Thanks, Erik.

StefanK

On 2018-05-17 10:19, Erik Österlund wrote:
> Hi Stefan,
> 
> Looks good.
> 
> Thanks,
> /Erik
> 
> On 2018-05-17 09:23, Stefan Karlsson wrote:
>> Hi all,
>>
>> Please review this patch to add a oopDesc::field_offset() convenience 
>> function.
>>
>> http://cr.openjdk.java.net/~stefank/8203339/webrev.01/
>> https://bugs.openjdk.java.net/browse/JDK-8203339
>>
>> We use it in ZGC to extract the offset to oop fields in some of our 
>> OopClosures. The OopClosures provide the interior pointers to oop 
>> fields, and together with the object pointer the offset can be 
>> calculated.
>>
>> See the following ZGC use-case:
>>
>> http://hg.openjdk.java.net/zgc/zgc/file/3a52c8361e20/src/hotspot/share/gc/z/zHeapIterator.cpp#l87 
>>
>>
>> class ZHeapIteratorPushOopClosure : public ExtendedOopClosure {
>> private:
>>   ZHeapIterator* const _iter;
>>   const oop            _base;
>>
>> public:
>>   ZHeapIteratorPushOopClosure(ZHeapIterator* iter, oop base) :
>>       _iter(iter),
>>       _base(base) {}
>>
>>   void do_oop_nv(oop* p) {
>>     const oop obj = HeapAccess<ON_UNKNOWN_OOP_REF>::oop_load_at(_base, 
>> _base->field_offset(p));
>>     _iter->push(obj);
>>   }
>>
>> Thanks,
>> StefanK
>