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

[Rickard Bäckman]

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
> >>>
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