unexpected allocation of Binding.Context in downcall method handle

[Rado Smogura]

Hi all,


Sorry if it will not be in thread, but I had to re-register. And thank 
you for fast response.

> Hi,
> following the discussion on [1], here is a quick patch which attempts to
> remove the additional allocation:
>
> https://github.com/mcimadamore/jdk/pull/new/avoid_context_alloc
>
> I'm not able to see any gain using synthetic benchmarks, but perhaps
> worth a try to confirm that this is really the issue we're seeing?
>
> Thanks
> Maurizio
I think this change could be hard to spot on benchmarks  (on debugger I 
don't see Binding.Context.ofAllocator called). I guess it's about few 
hardware instructions.

I slightly changed existing benchmark [1]

public void setup() {
     constString =MemorySegment.allocateNative(str.length() +1,ResourceScope.globalScope());
     constString.setUtf8String(0,str);
}

@Benchmark public int panama_strlen_noalloc()throws Throwable {
     return (int)STRLEN.invokeExact((Addressable)constString);
}

And results are as follow

After
Benchmark                         (size)  Mode  Cnt   Score   Error  Units
StrLenTest.panama_strlen_noalloc       5  avgt   20  12.680 ± 0.451  ns/op
StrLenTest.panama_strlen_noalloc      20  avgt   20  12.361 ± 0.026  ns/op
StrLenTest.panama_strlen_noalloc     100  avgt   20  12.510 ± 0.168  ns/op


Before (3f9638d124076019f49eb77bc3ff8b466e4beb53)

Benchmark                         (size)  Mode  Cnt   Score   Error  Units
StrLenTest.panama_strlen_noalloc       5  avgt   20  12.837 ± 0.515  ns/op
StrLenTest.panama_strlen_noalloc      20  avgt   20  12.471 ± 0.129  ns/op
StrLenTest.panama_strlen_noalloc     100  avgt   20  12.537 ± 0.395  ns/op

Theoretically it's 1% gain, but it's as well can be in error margin.

[1] https://github.com/rsmogura/panama-foreign/commit/e10e20f89767747c24f657d2cd5de6b7fc9a934c

Kind regards,
Rado