[foreign-memaccess] RFR 8230449: Simplify creation of handles for memory slices

[Maurizio Cimadamore]

New webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8230449_v2/

Cheers
Maurizio

On 03/09/2019 15:55, Maurizio Cimadamore wrote:
>
> On 03/09/2019 15:25, Jorn Vernee wrote:
>>
>> Hi,
>>
>> Some comments:
>> - MemoryLayout.java; The javadoc should be updated from "if the step 
>> factor is < 0" -> "is 0", since it can also be negative.
>> - LayoutPathImpl.java; Could use a helper method to do the sequence 
>> index check (code is duplicated between 2 `sequenceElement` methods). 
>> Also, the `step == 0` check is done in the 
>> PathElement::sequenceElement already.
>>
> Well spotted - will fix
>>
>> Lastly, I'm not so sure about the last check for invalid step factor. 
>> It would still be valid to access a single element when the 
>> coordinate passed to the VH is 0, but this check blocks that.
>>
> The idea is that if you pass a step factor that is bigger than the 
> selected sequence slice.
>
> It's true that, in general, we provide no guarantee for out-of-bound 
> indices in array-like VH - but I guess I saw this as a user-mistake 
> and wanted to prevent it.
>
> Now that I think more, I guess I see that, by removing that check, we 
> would actually be able to produce VH which are capable of accessing 
> exactly ONE element, which I guess can come in handy in certain cases.
>
> Maurizio
>
>> Rest looks good.
>>
>> Cheers,
>> Jorn
>>
>> On 02/09/2019 17:27, Maurizio Cimadamore wrote:
>>> Hi,
>>> the foreign memory API is pretty good at expressing access to 
>>> multi-dimensional data, e.g. by allowing to define custom strides 
>>> and offsets. There are however some limitations that limit the 
>>> expressiveness of the API when it comes to denote complex 
>>> VarHandle-based views of contiguous chunks of memory:
>>>
>>> 1) It is not possible to use negative strides (which makes it 
>>> impossible e.g. to create a VarHandle which returns the contents of 
>>> the memory region "in reverse order")
>>> 2) there's no way in the PathElement API to select a 'slice' of a 
>>> sequence layout (e.g. from start given start index, with given step)
>>>
>>> The following webrev aims at closing this gap:
>>>
>>> http://cr.openjdk.java.net/~mcimadamore/panama/8230449/
>>>
>>> P.S.
>>> To allow negative strides I had to remove a validation of the stride 
>>> in the combinator API which ensured that the stride was greater than 
>>> the offset + carrierSize. I believe this restriction to be a tad 
>>> obscure and not a fundamental one (this was also pointed out by Jorn 
>>> when I added the restriction in the first place), so I'm ok to drop 
>>> it in name for more expressiveness.
>>>
>>> Maurizio
>>>