Unifying memory addresses and memory segments

[Maurizio Cimadamore]

What Paul says is correct, the existing benchmark showed no regression 
before/after the change.

If you know of benchmarks that show a different picture now would be the 
ideal time to share them :-)

I also note that, in your original message you wrote about a detrimental 
effect on "memory management". What do you mean by that? Perhaps you 
mean more objects being created on the heap?

In principle, you can view a MemoryAddress in the old API as a 
MemorySegment backed by the global memory session, whose base address is 
a certain native address, and its size is zero. So, if clients want to 
use a segment just to communicate an address in the new API, and they 
create such address-like segments with `MemorySegment.ofLong(long)`, all 
should be fine and escape analysis should work roughly the same as 
before. But again, if you have evidence of the contrary, please let us know.

Thanks
Maurizio

On 25/08/2022 21:59, Paul Sandoz wrote:
>
>> On Aug 25, 2022, at 12:51 AM, Glavo <zjx001202 at gmail.com> wrote:
>>
>> It looks good in terms of simplifying API design, but this seems to have some detrimental effects on performance optimization as well as memory management.
>>
> Can you please data you have on any performance regressions you are observing?
>
>
>> Do we have some benchmarks to show changes in performance and memory overhead (number of temporary objects created)?
>>
> I believe the existing micro benchmarks show no regressions:
>
>    https://github.com/openjdk/jdk/tree/master/test/micro/org/openjdk/bench/java/lang/foreign
>
> Paul.
>
>
>> On Tue, Jul 19, 2022 at 10:05 PM Maurizio Cimadamore <maurizio.cimadamore at oracle.com> wrote:
>> Hi,
>> The Java 19 Foreign Function and Memory API (FFM API) uses zero-length
>> memory segments to encode pointers that are temporally safe (this
>> replaces the NativeSymbol abstraction that was available in Java 18).
>> It turns out that there's more to this approach than meets the eye, as
>> memory segments can (with few tweaks) be used as a replacement for
>> memory address everywhere in the FFM API, which leads to a simpler and
>> more symmetric API.
>>
>> We have captured our findings in the following document:
>>
>> http://cr.openjdk.java.net/~mcimadamore/panama/segment_address.html
>>
>> Cheers
>> Maurizio
>>
>>