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From "Robin Garner" <robin.gar...@anu.edu.au>
Subject Re: [Fwd: Re: [DRLVM][JET] write barrier for Java (mmtk)]
Date Fri, 13 Oct 2006 12:36:56 GMT

Urgh.  In that case the array base offset should be specified in the
constructor of an MMType - JikesRVM doesn't have this problem, because the
object pointer always points to element 0 of an array irrespective of
header sizes and alignment issues (and hence the align and offset
parameters to 'alloc').  I was brought up to believe that a fixed offset
to the first data field was a performance win - has it been measured ?

I'll have a look at doing this in MMTk, but probably not until after OOPSLA.

cheers

> Weldon Washburn wrote:
>>> I actually stumbled on this bug during bringup.  I replaced
>>> ...plus(arrayOffset)... with ...plus(12)...  Its a hack.  But now it
>>> looks
>>> like I simply set ARRAY_BASE_OFFSET to 12 during <clinit>.  Does this
>>> seem
>>> right??
>
> By the way, the array base offset may differ depending on the platform and
> array type.
> As as I know, currently on 32 bit platform (ia32) the array base offset
> is 12 for types of size <= 4.
> Double[] and long[] arrays have array base offset 16.
>
> on 64 bit platforms, if the vtable pointers are compressed (they are now),
> the array base offset is the same as on 32 bit platforms.
>
> Should someone disable vtable reference compression, the array base offset
> will be always 16 (8 bytes vptr + 4 bytes flags + 4 bytes array size).
>
>
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