RFR(M) 8147461: Use byte offsets for vtable start and vtable length offsets

Edward Nevill edward.nevill at gmail.com
Tue Jan 19 10:07:05 UTC 2016

Hi Mikael,

I tried a release and fastdebug build on aarch64. The release build builds fine and passes a basic smoke test but the fastdebug build fails with the following errors.

/home/ed/new_jdk9/hs-comp/hotspot/src/share/vm/jvmci/jvmciCompilerToVM.cpp: In static member function 'static void CompilerToVM::Data::initialize()':
/home/ed/new_jdk9/hs-comp/hotspot/src/share/vm/jvmci/jvmciCompilerToVM.cpp:153:37: error: cannot convert 'ByteSize' to 'int' in assignment
   InstanceKlass_vtable_start_offset = InstanceKlass::vtable_start_offset();
/home/ed/new_jdk9/hs-comp/hotspot/src/share/vm/jvmci/jvmciCompilerToVM.cpp:154:38: error: cannot convert 'ByteSize' to 'int' in assignment
   InstanceKlass_vtable_length_offset = InstanceKlass::vtable_length_offset() * HeapWordSize;
After fixing these the fastdebug build completed and passed the same basic tests.

All the best,

On Fri, 2016-01-15 at 18:04 +0100, Mikael Gerdin wrote:
> Hi all,
> As per the previous discussion in mid-December[0] about moving the 
> _vtable_length field to class Klass, here's the first RFR and webrev, 
> according to my suggested plan[1]:
> > My current plan is to first modify the vtable_length_offset accessor to
> > return a byte offset (which is what it's translated to by all callers).
> >
> > Then I'll tackle moving the _vtable_len field to Klass.
> >
> > Finally I'll try to consolidate the vtable related methods to Klass,
> > where they belong.
> This change actually consists of three changes:
> * modifying InstanceKlass::vtable_length_offset to become a byte offset 
> and use the ByteSize type to communicate the scaling.
> * modifying InstanceKlass::vtable_start_offset to become a byte offset 
> and use the ByteSize type, for symmetry reasons mainly.
> * adding a vtableEntry::size_in_bytes() since in many places the vtable 
> entry size is used in combination with the vtable start to compute a 
> byte offset for vtable lookups.
> I don't foresee any issues with the fact that the byte offset is 
> represented as an int, for two reasons:
> 1) If the offset of any of these grows to over 2 gigabytes then we have 
> a huge footprint problem with InstanceKlass
> 2) The offsets are converted to byte offsets and stored in ints already 
> in the cpu specific code I've modified.
> Bug link: https://bugs.openjdk.java.net/browse/JDK-8147461
> Webrev: http://cr.openjdk.java.net/~mgerdin/8147461/webrev.0/
> Testing: JPRT on Oracle supported platforms, testing on AARCH64 and 
> PPC64 would be much appreciated, appropriate mailing lists have been 
> CC:ed to notify them of the request.
> [0] 
> http://mail.openjdk.java.net/pipermail/hotspot-dev/2015-December/021152.html
> [1] 
> http://mail.openjdk.java.net/pipermail/hotspot-dev/2015-December/021224.html
> Thanks!
> /Mikael

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