RFR: 8203339: Add oopDesc::field_offset()

[Stefan Karlsson]

Thanks, Roman.

StefanK

On 2018-05-17 10:03, Roman Kennke wrote:
> 
>>
>> 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);
>>    }
>>
> 
> Seems useful. Patch looks good to me.
> 
> Roman
>