[intrinsics] performance improvements for the intrinsified version of Objects::hash

[Brian Goetz]

We are delegating hashing to a method:

     int hash(int[] numbers)

which means that while we may be eliminating primitive boxing, we are 
still paying for array boxing.  Using either MH loop combinators or 
hand-unrolled variants of the above should unlock more speedup on the 
small (and most common) cases.

On 2/27/2019 3:24 PM, Alex Buckley wrote:
> On 2/26/2019 5:49 PM, Vicente Romero wrote:
>> In the last performance measurement we found a noticeable
>> degradation in performance for large number of arguments (~100), even
>> for primitive types. Patch [1] improves the performance for both
>> primitive and reference types with the difference that now the
>> performance is much better than vanilla JDK13 for primitive types but it
>> is still worst than vanilla for reference types. Although we are in
>> better shape now compared to the state as of 02/22. Keep tuned :)
>
> Previous intrinsification effort, relative to vanilla JDK 13:
>
>                  Intrinsified  Vanilla  Speedup
> testHash1IntVariable    42564    42799       1x
> testHash2IntVariables   41573     9019       5x
> testHash100IntVariables     4       27       0.15x
>
> New intrinsification effort, relative to vanilla JDK 13:
>
>                  Intrinsified  Vanilla  Speedup
> testHash1IntVariable    41149    42799       1x
> testHash2IntVariables   19075     9019       2x
> testHash100IntVariables      697       27      26x
>
> I note that the speedup of the 2IntVariables case is cut from 5x to 
> 2x. That seems like quite a penalty for speeding up the 
> 100IntVariables case (admittedly by a lot). But maybe what's happening 
> is that the speedup improves as more variables are hashed. I wonder if 
> it's fair to say for Objects::hash that vanilla invocation has a high 
> fixed cost (box box box) and low variable costs (but who cares, 
> because they're overwhelmed by the fixed cost) ... while intrinsified 
> invocation has a low fixed cost (just run the BSM) but higher variable 
> costs -- you pay for the hashing you get.
>
> Alex