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

[Alex Buckley]

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