MemoryAddress object changes once put into memory

[Ty Young]

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?


>
> Maurizio
>
>
> On 14/01/2020 16:18, Ty Young wrote:
>> Hi,
>>
>>
>> As mentioned in a different thread, a MemoryAccess instance changes 
>> once put into memory. Here are all the relevant bits from the class:
>>
>>
>>     private final MemorySegment segment;
>>     private final SequenceLayout sequenceLayout;
>>
>>     public ArrayArray(long length)
>>     {
>>         this.sequenceLayout = MemoryLayout.ofSequence(length*24, 
>> MemoryLayouts.JAVA_LONG);
>>
>>         this.segment = 
>> MemorySegment.allocateNative(this.sequenceLayout);
>>     }
>>
>>
>>     @Override
>>     public void setValue(int index, Array<E> value)
>>     {
>>         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());
>>
>>         addressHandle.set(addressSegment.baseAddress(), 
>> value.getAddress());
>>         layoutHandle.set(layoutSegment.baseAddress(), 
>> value.getElementSequenceLayout().elementLayout().bitSize());
>>         lengthHandle.set(lengthSegment.baseAddress(), 
>> value.getLength().getAsLong());
>>     }
>>
>>
>>     ...
>>
>>
>> As shown here, the address of the array being set is being stored in 
>> the addressSegment. However, once read back into Java it changes so 
>> that both it's ID changes, has an offset, and its limit is zero when 
>> it should be 64 even when read back in using the same VarHandle in 
>> setValue method. Am I doing something wrong(like no rebasing?) or is 
>> there something else going on?
>>