vwithfield and ValueType.findWither

Frederic Parain frederic.parain at oracle.com
Thu Jun 15 14:06:46 UTC 2017



On 06/15/2017 05:36 AM, Maurizio Cimadamore wrote:
>
>
> On 15/06/17 09:46, John Rose wrote:
>> On Jun 14, 2017, at 2:20 PM, Maurizio Cimadamore
>> <maurizio.cimadamore at oracle.com> wrote:
>>> Another possible story is that we always create values through the
>>> VCC constructor, and then unbox. If we think we're happy with that,
>>> that might be a fine choice too.
>> That's not a bad story for the JIT with aggressive scalarization,
>> and was my first idea about this, but it is painful in the interpreter.
>>
> In which ways it's bad for the interpreter? Note that the current
> implementation of the 'modified' findWither will do just that:
>
> * box incoming value
> * set field (with Unsafe.putXYZ)
> * unbox resulting reference back to a value
>
> So, it seems to me that, to set 5 fields, the above idiom will do
> box/unbox 5 times, while if we just create a VCC with the fields we want
> and then unbox, that's just one unboxing step.
>
> What am I missing?

With the vwithfield bytecode, the interpreter can avoid Java heap
allocations, with the box/Unsafe/unbox it cannot.

Just a performance/memory issue.

Fred


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