Re: [rvm-research] spare one bit from the object header

Yes, you are right !
I checked that just now, all the objects whose spared bit is set 1 , it’s Forwarding flag is true.
Thank you very much.



> 在 2015年9月30日，下午3:51，Robin Garner <[email protected]> 写道：
> 
> OK, this may be obvious, but in a copying collector the status word is 
> also used to store the forwarding pointer, so its contents can only be 
> relied on until an object is copied.  This is why the copy status bits 
> are always the low-order 2 bits.
> 
> hth,
> Robin
> 
> On 29/09/15 21:36, 王晨曦 wrote:
>> Hi, every one.
>> 
>> 	I want to spare one bit as a flag to mark some particular object from the thin lock bits used in the object header.
>> In the production configuration ,the ADDRESS_BASED_HASHING is true.
>> so the object header looks like:
>>  *      TTTT TTTT TTTT TTTT TTTT TOHH AAAA AAAA
>> the bit marked by ‘O’ is the bit  I want to spare.
>> 
>>  I modified the thin lock definitions:
>>   /** How many bits are allocated to a thin lock? */
>>   public static final int NUM_THIN_LOCK_BITS = ADDRESS_BASED_HASHING ? 21 : 19;
>> 
>>   /** How many bits to shift to get the thin lock? */
>>   public static final int THIN_LOCK_SHIFT = ADDRESS_BASED_HASHING ? 11 : 13;
>> 
>> 
>> But ,In GenNurseryTraceLocal.java traceObject() method.
>> I find that ,the spared bit ‘O’  can be set by something I don’t know.
>> 
>> the check code in the traceObject():
>> 
>>         Word objWordVal = VM.objectModel.readAvailableBitsWord(object);
>>         Log.write("object header word value:");
>>         Log.writeln(objWordVal);
>> 
>> 
>> and the the print info is :
>>   1871 object header word value:0x6e00859f  //  ….   0101  1001  1111
>> 
>>   1876 object header word value:0x6e009dfb  //  ….   1101   1111  1011
>> 
>> we can see that  the spared bit may be set by something.
>> So,my question is who set this spared bit ?
>> I checked that the thin lock count bits, it is reduced 1 bit, and I think the thin lock will not use this bit.
>> 
>> Any response is helpful, thank you.
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> 
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