RFR: JDK-8153203: Remove liveRange.hpp

Bengt Rutisson bengt.rutisson at oracle.com
Thu Mar 31 19:05:57 UTC 2016

Hi Jesper,

On 31/03/16 20:08, Jesper Wilhelmsson wrote:
> Hi Bengt,
> Nice cleanup!

Thanks! And thanks for looking at this!

> The comment in psMarkSweepDecorator.cpp:294 is slightly confusing: 
> "The first dead object should contain a pointer to the first live object"
> Does it mean "The first object is dead and should contain a pointer to 
> the first live object"?

It actually means that the first dead object is no longer an object. 
Instead, at that memory address, there is just a pointer to the first 
live object that the previous phase found. So, I think the comment is 
correct, but I understand why it is confusing. "First dead" here, refers 
to the variable _first_dead.

Maybe this would be clearer:

"The first dead object is no longer an object. At that memory address, 
there is a pointer to the first live object that the previous phase found."


> Besides that the change looks good.
> /Jesper
> Den 31/3/16 kl. 17:46, skrev Bengt Rutisson:
>> Hi everyone,
>> Could I have a couple of reviews for this change?
>> http://cr.openjdk.java.net/~brutisso/8153203/webrev.00/
>> https://bugs.openjdk.java.net/browse/JDK-8153203
>> The methods CompactibleSpace::scan_and_forward() and
>> PSMarkSweepDecorator::precompact() are more or less copy-paste 
>> versions of each
>> other. Both these use the LiveRange class.
>> The LiveRange class is used to write to the mark word of a dead 
>> object. But no
>> one really cares that the LiveRange class is used. Instead it just 
>> gives an
>> extra level of indirection to already complicated code. We also do 
>> unnecessary
>> work to keep track of the end of the range even though no one ever 
>> gets the end
>> value.
>> We also do duplicate stores to _first_dead in these methods.
>> The methods that consume the values from the LiveRange
>> (CompactibleSpace::scan_and_adjust_pointers() and
>> PSMarkSweepDecorator::adjust_pointers()) don't use the LiveRange 
>> class. Instead
>> they use oop()->mark()->decode_pointer(), which is kind of odd 
>> considering that
>> this is normally used for forwarded objects.
>> The code would be simpler if we just store and load directly from the 
>> memory
>> addresses we are working with.
>> Improving the assert at the end of the
>> CompactibleSpace::scan_and_adjust_pointers() and
>> PSMarkSweepDecorator::adjust_pointers() methods to log the values of 
>> q and
>> prev_q will hopefully improve the chances of understanding 
>> JDK-8073321 if that
>> ever happens again.
>> Thanks,
>> Bengt

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