RFR (M): 8064458 OopMap class could be more compact

[Christian Thalinger]

One thing I mentioned to Rickard privately is that I would prefer to have the current OopMap be renamed to something else (TempOopMap?) and keep OopMap as the one used.

> On Apr 30, 2015, at 9:26 AM, Vladimir Kozlov <vladimir.kozlov at oracle.com> wrote:
> 
> It is better now. I am fine with inner Mapping class.
> 
> One thing left. I think only Mapping and fields could be private since only ImmutableOopMapBuilder accesses them:
> 
> 
> + class ImmutableOopMapBuilder {
> + public:
> +   class Mapping;
> +
> + public:
> +   const OopMapSet* _set;
> 
> thanks,
> Vladimir
> 
> On 4/30/15 7:18 AM, Rickard Bäckman wrote:
>> On 04/29, Vladimir Kozlov wrote:
>>> On 4/29/15 7:52 AM, Rickard Bäckman wrote:
>>>> On 04/28, Vladimir Kozlov wrote:
>>>>> You need closed SA changes.
>>>> 
>>>> I have them, just forgot to send the mail. Small changes too.
>>>> 
>>>>> 
>>>>> Style:
>>>>> 
>>>>> Move fields to the beginning of ImmutableOopMapBuilder and classes.
>>>>> Add comments describing each field in ALL new classes. Add comments
>>>>> to fields in old class. It will help next persone who will look on
>>>>> oop maps later.
>>>> 
>>>> All fields are at the beginning? Just following style and keeping
>>>> friends above fields as other classes in the same file has.
>>> 
>>>   friends
>>>   fields
>>>   methods
>>> 
>>> It is better to see fields before methods which access them. I am
>>> not sure what codding style you are talking about. All classes in
>>> oopMap.hpp has above layout.
>>> 
>>> I am asking to change layout of ImmutableOopMapBuilder and Mapping classes. Also why Mapping is inner class?
>> 
>> Ah, I was looking at oopMap.hpp and didn't see them. Fixed.
>> Mapping is an inner class because it is only used by the Builder and I
>> didn't want to clutter the namespace with something as general (and bad)
>> as Mapping. I can move it out if that is preferred.
>> 
>>> 
>>> 
>>>> 
>>>>> 
>>>>> Add ResourceMark into ImmutableOopMapSet::build_from() to free memory allocated in ImmutableOopMapBuilder().
>>> 
>>> no ResourceMark in 8064458.2
>> 
>> Added.
>> 
>>> 
>>>>> Why you need ImmutableOopMapBuilder to be friend of class ImmutableOopMap?
>>>> 
>>>> It makes a call to data_addr() which is private. Only for debug builds
>>>> so I wrapped it in DEBUG_ONLY.
>>> 
>>> Put ImmutableOopMapBuilder::verify() under #ifdef ASSERT and its declaration in DEBUG_ONLY.
>>> 
>> 
>> Done.
>> 
>> Updated webrev: http://cr.openjdk.java.net/~rbackman/8064458.4
>> 
>> Thanks
>> /R
>> 
>>> Thanks,
>>> Vladimir
>>> 
>>>> 
>>>>> 
>>>>> I think a simple loop in ImmutableOopMapBuilder::verify() would be faster than calling memcmp.
>>>> 
>>>> Fixed.
>>>> 
>>>>> 
>>>>> Field _end is not used.
>>>> 
>>>> Removed.
>>>> 
>>>>> 
>>>>> ImmutableOopMapBuilder() calls reset() and next called heap_size()
>>>>> calls reset() again. May be move reset() to the end of heap_size()
>>>>> so that you don't need to call it in fill().
>>>>> 
>>>> 
>>>> Better yet, the reset() method isn't required anymore.
>>>> 
>>>> Updated webrev: http://cr.openjdk.java.net/~rbackman/8064458.2
>>>> 
>>>> /R
>>>> 
>>>>> Thanks,
>>>>> Vladimir
>>>>> 
>>>>> On 4/28/15 3:03 AM, Rickard Bäckman wrote:
>>>>>> Hi all,
>>>>>> 
>>>>>> can I please have reviews for this change:
>>>>>> 
>>>>>> RFR: http://cr.openjdk.java.net/~rbackman/8064458.1/
>>>>>> RFE: http://bugs.openjdk.java.net/browse/JDK-8064458
>>>>>> 
>>>>>> While looking at OopMaps a while ago I noticed that there were a couple
>>>>>> of different fields that were unused after the OopMaps were finalised.
>>>>>> 
>>>>>> I took some time to investigate and rearrange the OopMaps. Since I
>>>>>> didn't want to change how the OopMaps are built I introduced new data
>>>>>> structures. ImmutableOopMapSet and ImmutableOopMap. The original OopMap
>>>>>> structures are used to build up the OopMaps and when finalised they are
>>>>>> copied into the Immutable variants.
>>>>>> 
>>>>>> The ImmutableOopMapSet contains a few fields [size, count] and then a
>>>>>> list of [pc, offset]. The offset points to the offset after the list
>>>>>> where the ImmutableOopMap is placed. By moving pc out from OopMap to be
>>>>>> part of the list we can now have multiple pcs with identical OopMaps
>>>>>> point to the same data.
>>>>>> 
>>>>>> We only keep 1 empty OopMap, and the other compaction that is done in
>>>>>> this change is to check if the OopMap is identical to the previous one
>>>>>> and then reuse that one. So no complete uniqueness check.
>>>>>> 
>>>>>> I ran a couple of small benchmarks and printed the size of the old
>>>>>> OopMaps vs the new. The new layout uses about 20 - 25% of the space on
>>>>>> the benchmarks I've run.
>>>>>> 
>>>>>> Tested by running through JPRT, running BigApps and NSK.quick.testlist
>>>>>> 
>>>>>> Thanks
>>>>>> /R
>>>>>>