RFR: 8312522: Implementation of Foreign Function & Memory API [v22]

[Maurizio Cimadamore]

On Mon, 18 Sep 2023 14:17:30 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> This patch contains the implementation of the foreign linker & memory API JEP for Java 22. The initial patch is composed of commits brought over directly from the [panama-foreign repo](https://github.com/openjdk/panama-foreign). The main changes found in this patch come from the following PRs:
>> 
>> 1. https://github.com/openjdk/panama-foreign/pull/836 Where previous iterations supported converting Java strings to and from native strings in the UTF-8 encoding, we've extended the supported encoding to all the encodings found in the `java.nio.charset.StandardCharsets` class.
>> 2. https://github.com/openjdk/panama-foreign/pull/838 We dropped the `MemoryLayout::sequenceLayout` factory method which inferred the size of the sequence to be `Long.MAX_VALUE`, as this led to confusion among clients. A client is now required to explicitly specify the sequence size.
>> 3. https://github.com/openjdk/panama-foreign/pull/839 A new API was added: `Linker::canonicalLayouts`, which exposes a map containing the platform-specific mappings of common C type names to memory layouts.
>> 4. https://github.com/openjdk/panama-foreign/pull/840 Memory access varhandles, as well as byte offset and slice handles derived from memory layouts, now feature an additional 'base offset' coordinate that is added to the offset computed by the handle. This allows composing these handles with other offset computation strategies that may not be based on the same memory layout. This addresses use-cases where clients are working with 'dynamic' layouts, whose size might not be known statically, such as variable length arrays, or variable size matrices.
>> 5. https://github.com/openjdk/panama-foreign/pull/841 Remove this now redundant API. Clients can simply use the difference between the base address of two memory segments.
>> 6. https://github.com/openjdk/panama-foreign/pull/845 Disambiguate uses of `SegmentAllocator::allocateArray`, by renaming methods that both allocate + initialize memory segments to `allocateFrom`. (see the original PR for the problematic case)
>> 7. https://github.com/openjdk/panama-foreign/pull/846 Improve the documentation for variadic functions.
>> 8. https://github.com/openjdk/panama-foreign/pull/849 Several test fixes to make sure the `jdk_foreign` tests can pass on 32-bit machines, taking linux-x86 as a test bed.
>> 9. https://github.com/openjdk/panama-foreign/pull/850 Make the linker API required. The `Linker::nativeLinker` method is not longer allowed to throw an `UnsupportedO...
>
> Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Avoid eager use of LibFallback in FallbackLinker static block

src/java.base/share/classes/java/lang/foreign/Linker.java line 152:

> 150:  * <p>
> 151:  * The following table shows some examples of how C types are modelled in Linux/x64 according to the
> 152:  * "System V Application Binary Interface - AMD64 Architecture Processor Supplement" (all the examples provided

I have seen some discussion on this and I agree an authoritative link does not exist (I have searched for it in the past). I guess I'm not super wild about also including "AMD64 Architecture Processor Supplement". E.g. IMHO "System V Application Binary Interface" is more than enough?

src/java.base/share/classes/java/lang/foreign/Linker.java line 409:

> 407:  *
> 408:  * Variadic functions are C functions which can accept a variable number and type of arguments. They are declared with a
> 409:  * trailing ellipsis ({@code ...}) at the end of the formal parameter list, such as: {@code void foo(int x, ...);}.

Looking at the javadoc - it seems to me that the `;` after the declaration of `foo` leads to a bit of jarring as it is immediately followed by a period (`.`). Consider dropping that - or maybe put the declaration in a snippet.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/15103#discussion_r1334395919
PR Review Comment: https://git.openjdk.org/jdk/pull/15103#discussion_r1334399541