Weird performance behavior involving VarHandles

[Jorn Vernee]

I can reproduce this locally:

Benchmark                               Mode  Cnt  Score   Error  Units 
ReproducerBenchmarks.control            avgt    5  1.280 ± 0.015  ns/op 
ReproducerBenchmarks.gwt2_methodhandle  avgt    5  1.690 ± 0.008  ns/op 
ReproducerBenchmarks.gwt_methodhandle   avgt    5  1.305 ± 0.038  ns/op

Disabling tiered compilation 'fixes' the performance of gwt2:

Benchmark                               Mode  Cnt  Score   Error  Units 
ReproducerBenchmarks.control            avgt    5  1.299 ± 0.016  ns/op 
ReproducerBenchmarks.gwt2_methodhandle  avgt    5  1.312 ± 0.030  ns/op 
ReproducerBenchmarks.gwt_methodhandle   avgt    5  1.303 ± 0.034  ns/op

In both cases the assembly looks identical though. So, this may just be 
up to a different code cache layout (or something like that).

Jorn

On 25/04/2024 00:28, Maurizio Cimadamore wrote:
>
> Cool benchmark/test case!
>
> I don't know off-hand where the difference could be coming from - but 
> just curious: did you try accessing in a loop (e.g. to see if checks 
> are hoisted as expected) ?
>
> I seem to recall that the lambda forms for guards-with-test are rather 
> complex, as they need to profile the various branches. I wonder if 
> some "leftover" from the profiling code stays there and pollutes the 
> benchmark?
>
> Maurizio
>
> On 24/04/2024 07:37, Remi Forax wrote:
>> I get
>>
>> Benchmark                               Mode  Cnt  Score   Error  Units
>> ReproducerBenchmarks.control            avgt    5  1.250 ± 0.024  ns/op
>> ReproducerBenchmarks.gwt2_methodhandle  avgt    5  1.852 ± 0.024  ns/op
>>
>> and I don't understand why there is a difference in performance because
>> for c2, the strings "x" and "y" are constant so the corresponding
>> VarHandles should be constant thus optimized the same way.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/hotspot-compiler-dev/attachments/20240425/edffbdd2/attachment.htm>