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

[Maurizio Cimadamore]

On Mon, 30 Jan 2023 15:25:54 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]`.

This pull request has now been integrated.

Changeset: a943c2eb
Author:    Maurizio Cimadamore <mcimadamore at openjdk.org>
URL:       https://git.openjdk.org/panama-foreign/commit/a943c2eb10ba40c36d5a6e874160d0a747457510
Stats:     246 lines in 3 files changed: 233 ins; 0 del; 13 mod

8301360: Add dereference layout paths to memory layout API

Reviewed-by: jvernee

-------------

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