[foreign-memaccess+abi] RFR: 8301360: Add dereference layout paths to memory layout API [v2]

[Jorn Vernee]

On Tue, 31 Jan 2023 10:20:15 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Now that we have a more general way to attach target layouts to address layouts (see https://git.openjdk.org/panama-foreign/pull/775), it might be useful to add support for layout paths which perform dereference of a given address value. Consider this case:
>> 
>> 
>> struct b {
>>    int x;
>> };
>> 
>> struct a {
>>     struct b *b;
>> };
>> 
>> 
>> Here, our goal is to construct a layout path for "a->b.x".
>> 
>> To do this, we use a *dereference path element* (see `PathElement#dereferenceElement`). These comes in two flavors:
>> 
>> * no argument factory - this assumes that the address layout being dereferenced has some target layout set;
>> * layout argument factory - this assumes that the address layout being dereferenced does *not* have a target layout set (e.g. it is not always possible to associate a target layout, especially in cyclic cases like a -> b -> a).
>> 
>> When a dereference element is applied to an existing path, a var handle that fetches the address pointed to by the current path is pushed on the stack, and a new layout path is started. This way, dereference path can be used to "chain" together multiple layout paths. All the intermediate memory read operations occur using a *plain* get. For more complex access operation (e.g. if atomic access is required for the whole dereference chain), multiple var handles have to be obtained.
>> 
>> It is an error to pass a dereference path to other layout operations such as `select`, `byte/bitOffset[Handle]`.
>
> Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Drop overload of PathElement.dereferenceElement
>   Address review comments

Marked as reviewed by jvernee (Committer).

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PR: https://git.openjdk.org/panama-foreign/pull/776