[foreign-memaccess] RFR: Add benchmark to measure performance of VH adapters

[Maurizio Cimadamore]

On Mon, 18 May 2020 13:55:13 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> This patch adds a straightforward benchmark to measure performances of var handle adapters.
>> 
>> The benchmark is set up to read values from an int array, both with a var handle(obtained from `MethodHandles`), an
>> associated `MethodHandle` (derived from the first var handle) and with a segment-based, memory access var handle.
>> Then the same test is repeated, but with an extra adaptation step inserted in the middle - rather than reading `int`
>> values directly, the adapter turns `ints` into instances of `IntBox` and the loop code doing the sum converts them back
>> into ints.
>> Numbers are extremeluy solid on my machine:
>> 
>> TestAdaptVarHandles.mh_box_loop       avgt   30  0.306 ? 0.009  ms/op
>> TestAdaptVarHandles.mh_loop           avgt   30  0.297 ? 0.009  ms/op
>> TestAdaptVarHandles.segment_box_loop  avgt   30  0.308 ? 0.009  ms/op
>> TestAdaptVarHandles.segment_loop      avgt   30  0.307 ? 0.008  ms/op
>> TestAdaptVarHandles.vh_box_loop       avgt   30  0.296 ? 0.005  ms/op
>> TestAdaptVarHandles.vh_loop           avgt   30  0.291 ? 0.003  ms/op
>> 
>> I thought it would have been nice to add this to our benchmark suites since we do not have anything that tests VH
>> adaptation directly.
>
> With the adapter creating IntBox, this benchmark is also testing whether C2 can eliminate the allocation of the box.
> 
> If the goal is to measure an adapted VarHandle against a non-adapted baseline VarHandle, I think it makes sense to have
> the adapters be the identity function, since then the _only_ difference is whether an adapter is present or not.
> WDYT?

I guess my (non stated) goal was to benchmark non trivial adaptation where users could create their own carriers on top
of the foreign memory access API. I wanted to make sure that the VH machinery didn't add extra cost when doing so.

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PR: https://git.openjdk.java.net/panama-foreign/pull/175