RFR (XS) JDK-8006280 - structure packing for 64-bit (resend)

[David Holmes]

Hi Ioi,

On 24/01/2013 1:07 PM, Ioi Lam wrote:
> (re-sending with public-accessible bug URL).
>
> WebRev:
> http://cr.openjdk.java.net/~iklam/8006280/reorder_metadata_001/
>
> Bug: Need to reorder metadata structures to reduce size (64-bit)
>       http://bugs.sun.com/view_bug.do?bug_id=8006280
>
> Sponsor: Jiangli
>
> Summary:
>
> On x64:
>
>    + Klass: 8 byte reduction

I assume this is only in product mode? And we gain an extra 4 bytes as 
there is no need to padd out to 64-bit alignment.

>    + InstanceKlass: 8 byte reduction (in addition to reduction in Klass)

Ditto re padding.

>    + Method: 8 byte reduction

Ditto re padding.

>    This is verified by using the "pahole" script (as described in bug report)
>
> On ARM-EABI:
>
>    I also manually verify that this change for ARM-EABI (which requires
>    8-byte alignment for jlong but not pointers):

32-bit ARM requires 4 byte pointer alignment. I'm not sure what point 
you are making here. Is gcc performing automatic 8-byte alignment for 
"jlong" class members? And is this different to other platforms?

>    + Klass: 8 byte reduction
>    + InstanceKlass: no change (apart from Klass reduction) -- no jlong fields

What changes that you made to instanceKlass relate to jlong fields ??

>    + Method: no change -- no jlong fields

Only because TIERED is not enabled?

Are the C++ compilers actually required to layout class instances in 
declaration order? I thought they might optimize some of this themselves :(

Thanks,
David


>    Before: (linux_armvfp_2.6-productEmb/jre/lib/arm/minimal/libjvm.diz
>
>      (gdb) p&((Klass*)0)->_alloc_count
>      $1 = (juint *) 0x58
>      (gdb) p&((Klass*)0)->_last_biased_lock_bulk_revocation_time
>      $2 = (jlong *) 0x60
>      (gdb) p sizeof(Klass)
>      $3 = 120
>
>      The "hole" was at address 0x5c
>
>    After:
>
>      (gdb) p&((Klass*)0)->_access_flags
>      $7 = (AccessFlags *) 0x54
>      (gdb) p&((Klass*)0)->_last_biased_lock_bulk_revocation_time
>      $8 = (jlong *) 0x58
>      (gdb) p sizeof(Klass)
>      $6 = 112
>
> Tests run:
>
> + JPRT -- (hotspot only, on hotspotwest queue, including embedded builds)
>
> + UTE/vm.quick.testlist, 100% passed
>
>
> Thanks
> Ioi
>
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