MemoryAddress object changes once put into memory

[Ty Young]

On 1/14/20 10:46 AM, Maurizio Cimadamore wrote:
>
> On 14/01/2020 16:38, Ty Young wrote:
>>
>> On 1/14/20 10:34 AM, Maurizio Cimadamore wrote:
>>> If I understand correctly your question, this is to be expected. 
>>> What is really happening is that you are serializing a MemoryAddress 
>>> into a 'long' (its address) and save it into native memory. Since 
>>> we're really only storing a 64-bit number, all the characteristics 
>>> of a MemoryAddress are lost after a store - such as the spatial and 
>>> temporal bounds.
>>>
>>> When you read that, later on, you just read a long, and the runtime 
>>> will construct a MemoryAddress instance on top of that. Since no 
>>> bound information is available, the runtime will resurrect that 
>>> MemoryAddress as a Nothing-backed MemoryAddress. Which means the 
>>> address will be unusable.
>>>
>>> But if you are serializing and deserializing segments you know, you 
>>> can do a rebase after the deserialization (the get() operation), so 
>>> that your address will become functionally equivalent to the one you 
>>> started with.
>>
>>
>> Ah, OK. Thanks for the clarification.
>>
>>
>> What would you use to rebase the segment with? Obviously when I go to 
>> reconstruct the array in Java all I have is what's stored at that 
>> same point in the larger segment:
>>
>>
>>     @Override
>>     public Array<E> getValue(int index)
>>     {
>>         MemorySegment elementSegment = 
>> this.segment.asSlice(index*192, 192);
>>
>>         MemorySegment addressSegment = elementSegment.asSlice(0, 64);
>>         MemorySegment layoutSegment = elementSegment.asSlice(64, 64);
>>         MemorySegment lengthSegment = elementSegment.asSlice(128, 64);
>>
>>         VarHandle addressHandle = 
>> MemoryHandles.varHandle(MemoryAddress.class, ByteOrder.nativeOrder());
>>         VarHandle layoutHandle = MemoryHandles.varHandle(long.class, 
>> ByteOrder.nativeOrder());
>>         VarHandle lengthHandle = MemoryHandles.varHandle(long.class, 
>> ByteOrder.nativeOrder());
>>
>>         MemoryAddress address = 
>> (MemoryAddress)addressHandle.get(addressSegment.baseAddress());
>>         long layout = 
>> (long)layoutHandle.get(layoutSegment.baseAddress());
>>         long length = 
>> (long)lengthHandle.get(lengthSegment.baseAddress());
>>
>>         return new MemoryArray(address, layout, length);
>>     }
>>
>>
>> So how do you go about rebasing something like this?
>
> In your case there's not much you can do, other than to rebase to an 
> unchecked segment - after all you know the length of the saved array 
> (because you are storing and recovering it), so you could do:
>
> MemoryAddress newAddress = 
> address.rebase(ForeignUnsafe.ofNativeUnchecked(address, length))


IIRC no decision has been made regarding the mechanism of making 
ForeignUnsafe available and Netbeans isn't suggesting a class import. 
Which module do I need to add for that?


Or is there any other "safe" way of implementing multi-dimensional 
arrays that I'm not aware of?


>
> Maurizio