Stack allocation API

[Maurizio Cimadamore]

Hi Felix,
the API has changed quite a bit since the email you are referring to. We 
now have a SegmentAllocator API which can be used to define custom 
allocation strategies. Out of the box, the API gives you ability to do 
arena allocation (but in the heap, not in the stack), or to obtain a 
SegmentAllocator which will keep allocating over the same segment over 
and over (e.g. will return slices). Note that a SegmentAllocator is 
accepted by all the downcall method handle targeting native functions 
which return structs by value (and also by other API methods returning 
structs), so you can significantly alter the performance of the entire 
API by using a custom allocator, such as the one defined in [1].

We have plans, when Loom arrives, it might be possible to add another 
kind of ResourceScope - that is neither confined, nor shared: a 
"structured" scope. The way this scope works is that it creates a Loom 
scope locals, and then it allow access from all the threads that inherit 
that scope local. Since such a scope will only be usable in a lambda 
expression (try-with-resources will not be supported there), it will be 
effectively be a stack-bounded scope. This would open up the possiblity 
of enabling stack allocation for all memory segments created within that 
scope.

But even w/o going that far, I'd say there are plenty of things in the 
existing API to try and make memory allocation more performant.

Maurizio

[1] - https://github.com/openjdk/panama-foreign/pull/509


On 16/02/2022 11:34, Felix Cravic wrote:
> Hello,
> I was wondering if there is any plan to expose an API to do stack allocation, similarly to graal [1] but with the advantage of not depending on graal native (and the fact that project Leyden may come).
> I am mostly seeking it for its potential performance aspect, which has already been discussed [2] but I do not think that the "very promising direction" has been communicated. Have any benchmark/study been done to define the usefulness of such low-level API?
>
> [1] -
> https://www.graalvm.org/sdk/javadoc/index.html?org/graalvm/nativeimage/StackValue.html
> [2] -
> https://mail.openjdk.java.net/pipermail/panama-dev/2019-November/006745.html