Bug in FieldsAllocationStyle=2 logic

[Vladimir Kozlov]

I have nothing to add to Kris's nice explanation.
I just want to say that you should check if it really can help your application. 
It may help GC but it may also harm execution performance since different layout 
of fields may increase data cache misses.

Vladimir

Krystal Mok wrote:
> Hi David,
> 
> I'll let Vladimir, the original author, give the definitive answer. :-)
> 
> Anyway, here's my guess:
> Packing oop fields together makes iterating oops in an object faster. 
> For example, the marking phase of GC would benefit from this.
> 
> Supposed we had a few classes as follows:
> 
> class Base {
>   Object o1;
>   int i;
> }
> 
> class Derived extends Base {
>   Object o2;
>   float f;
> }
> 
> An instance of Derived would contain a Base subobject within itself.
> It's straightforward to keep the Base's field layout in the subobject in 
> an Derived instance; in other words, Base::o1 and Base::i would be at 
> the same offset in a Base instance and in a Derived instance.
> 
> So, to pack oops in Base and Derived together while keeping Base's 
> layout intact in Derived, a reasonable solution is to alternate between 
> FieldsAllocationStyle=1 and FieldsAllocationStyle=0 in an inheritance 
> chain. That's exactly what FieldsAllocationStyle is doing.
> 
> But if we push hard to pack all oop fields in an object together, we'd 
> probably end up using a bidirectional object layout, like the one used 
> in SableVM (http://www.sablevm.org/features.html). It'd be really fun to 
> see this implemented in HotSpot VM sometime in the future.
> 
> This paper is worth reading if you're interested in object layouts:
> A Comprehensive Evaluation of Object Scanning 
> Techniques, http://users.cecs.anu.edu.au/~steveb/downloads/pdf/scan-ismm-2011.pdf
> 
> Regards,
> Kris Mok
> 
> On Thu, Jun 23, 2011 at 2:29 AM, David Dabbs <dmdabbs at gmail.com 
> <mailto:dmdabbs at gmail.com>> wrote:
> 
> 
> 
>      > -----Original Message-----
>      > From: hotspot-dev-bounces at openjdk.java.net
>     <mailto:hotspot-dev-bounces at openjdk.java.net> [mailto:hotspot-dev-
>     <mailto:hotspot-dev->
>      > bounces at openjdk.java.net <mailto:bounces at openjdk.java.net>] On
>     Behalf Of Vladimir Kozlov
>      > Sent: Wednesday, June 22, 2011 12:39 PM
>      > To: Krystal Mok
>      > Cc: hotspot-dev
>      > Subject: Re: Bug in FieldsAllocationStyle=2 logic
>      >
>      > Thank you for such detail analysis and suggested fix.
>      > I filed bug and I will fix it in 7u2.
>      >
>      > 7058036: FieldsAllocationStyle=2 does not work in 32-bit VM
>      >
>      > Thanks,
>      > Vladimir
>      >
>      > PS: if possible, please, report problems to hotspot-
>      > dev at openjdk.java.net <mailto:dev at openjdk.java.net> alias
>      > so all Hotspot developers can see it. I did original changes but I am
>      > in
>      > Compiler (JIT) group.
>      >
>      > Krystal Mok wrote:
>      > > Hi all,
>      > >
>      > > I think I've found a bug in ClassFileParser::parseClassFile() that
>      > deals
>      > > with FieldsAllocationStyle=2, which was introduced about a year
>      > > ago: http://hg.openjdk.java.net/jdk6/jdk6/hotspot/rev/b9d85fcdf743
>      > >
>      > > int map_size = super_klass->nonstatic_oop_map_size();
>      > > OopMapBlock* first_map =
>     super_klass->start_of_nonstatic_oop_maps();
>      > > OopMapBlock* last_map = first_map + map_size - 1;
>      > >
>      > > This code accidentally works on LP64 systems because it takes 1
>     word
>      > per
>      > > OopMapBlock, so nonstatic_oop_map_size() and
>      > nonstatic_oop_map_count()
>      > > would actually return the same value.
>      > >
>      > > But on 32-bit systems, an OopMapBlock takes 2 words, which
>      > > makes nonstatic_oop_map_size() == nonstatic_oop_map_count() *
>     2, and
>      > > breaks the code above.
>      > >
>      > > The code should really be:
>      > >
>      > > int map_count = super_klass->nonstatic_oop_map_count();
>      > > OopMapBlock* first_map =
>     super_klass->start_of_nonstatic_oop_maps();
>      > > OopMapBlock* last_map = first_map + map_count - 1;
>      > >
>      > > I found this because FieldsAllocationStyle=2 doesn't work for me on
>      > > 32-bit Windows (JDK6u25 and 6u26), but works on 64-bit Ubuntu
>      > JDK6u25.
>      > > Here's a min repro of my test: https://gist.github.com/1037866
>      > >
>      > > Could anybody please verify this?
>      > > Just checked the current tip of hsx/hotspot-rt, and it still
>     has this
>      > > behavior:
>      > > http://hg.openjdk.java.net/hsx/hotspot-
>      >
>     rt/hotspot/file/1744e37e032b/src/share/vm/classfile/classFileParser.cpp
>      > > line 3290
>      > >
>      > > Regards,
>      > > Kris Mok
> 
> 
>     What would be the use case/work load for choosing this option?
> 
> 
>     Thanks,
> 
>     David
> 
> 
>