status of VM long loop optimizations - call for action

[Maurizio Cimadamore]

Quick update: one of the VM fixes:

https://bugs.openjdk.java.net/browse/JDK-8278784

Did not make it to Java 18. Without this fix, benchmarks are a bit ofa 
mixed bag, with some big gains and some losses (esp. under pollution).

For this reason I think it would be preferrable to leave 18 as is, wait 
for more VM fixes, and then drop the workarounds in 19.

Cheers
Maurizio

On 10/12/2021 10:55, Maurizio Cimadamore wrote:
> (resending since mailing lists were down yesterday - I apologize if 
> this results in duplicates).
>
> Hi,
> few days ago some VM enhancements were integrated [1, 2], so it is 
> time to take a look again at where we are.
>
> I put together a branch which removes all workarounds (both for long 
> loops and for alignment checks):
>
> https://github.com/mcimadamore/jdk/tree/long_loop_workarounds_removal
>
> I also ran memory access benchmarks before/after, to see what the 
> difference is like - here's a visual report:
>
> https://jmh.morethan.io/?gists=dfa7075db33f7e6a2690ac80a64aa252,7f894f48460a6a0c9891cbe3158b43a7 
>
>
> Overall, I think the numbers are solid. The branch w/o workarounds 
> keep up with mainline in basically all cases but one (UnrolledAccess - 
> this code pattern needs more work in the VM, but Roland Westrelin has 
> identified a possible fix for it). In some cases (parallel tests) we 
> see quite a big jump forward.
>
> I think it's hard to say how these results will translate in real 
> world - my gut feeling is that the simpler bound checking logic will 
> almost invariably result in performance improvements with more complex 
> code patterns, despite what synthetic benchmark might say (the current 
> logic in mainline is fragile as it has to guard against integer 
> overflow, which in turns sometimes kills BCE optimizations).
>
> So I'd be inclined to integrate these changes in 18.
>
> If you gave a project that works agaist the Java 18 API, it would be 
> very helpful for us if you could try it on the above branch and report 
> back. This will help us make a more informed decision.
>
> Cheers
> Maurizio
>
> [1] - https://bugs.openjdk.java.net/browse/JDK-8276116
> [2] - https://bugs.openjdk.java.net/browse/JDK-8277850
>
>
>