[foreign-abi] [Rev 01] RFR: 8241017: Enhance AllocationScope to support "unbounded" mode

[Maurizio Cimadamore]

On Mon, 16 Mar 2020 15:43:00 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Looks good!
>
>> I have some disagreements on the implementation. The expected behaviour,
>> in my mind anyway, is that the returned MemoryAddress is *ALWAYS* just
>> an offset of a larger, always same MemorySegment,
> 
> The goal of AllocationScope is to provide some extra aid to those clients who need to allocate multiple segments which
> feature the same temporal bounds - e.g. so that they can be closed all at once. It gives you back an address to the
> allocated memory. How it's implemented inside, it's well, an implementation detail, and I don't really see why a client
> should assume certain behavior out of the addresses that it gets out of AllocationScope. Do you expect that all
> addresses given back to you via malloc are consecutive? I hope not!  As for resizing a segment dynamically, I've said
> many times that the design centre for memory segment API is that to have immutable segments, so that bound checks can
> be better optimized. If that's the constraint, then, in order to have unbounded allocation, you have to allocate
> multiple segments and keep them in a list. Maybe we can do some tuning on the threshold and make the impl smarter at
> reusing the memory, but this is mostly beyond the point - as this is not supposed to be an highly efficient allocator -
> this is just supposed to be an help for clients building native application - one thing that we found was pretty common
> for clients to do when we started porting more and more code using the new bindings.

/integrate

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PR: https://git.openjdk.java.net/panama-foreign/pull/53