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

Volker Simonis volker.simonis at gmail.com
Mon Jan 18 10:53:54 UTC 2016

Hi Mikael,

your change builds and runs fine on ppc64.

Thanks for considering ppc64 in your patch,

On Fri, Jan 15, 2016 at 6:04 PM, Mikael Gerdin <mikael.gerdin at oracle.com> 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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