RFR: 8360025: (se) Convert kqueue Selector Implementation to use FFM APIs

[Maurizio Cimadamore]

On Fri, 30 May 2025 12:00:28 GMT, Darragh Clarke <dclarke at openjdk.org> wrote:

> This PR aims to Panamize the Java Kqueue implementation, This is based on the work that was previously shared in https://github.com/openjdk/jdk/pull/22307 , The main change since then is that this branch takes advantage of the changes made in https://github.com/openjdk/jdk/pull/25043 to allow for better performance during errno handling.
> 
> These changes feature a lot of Jextract generated files, though alterations have been made in relation to Errno handling and performance improvements.
> 
> I will update this description soon to include performance metrics on a few microbenchmarks, though currently it's roughly 2% to 3% slower with the changes, which is somewhat expected, though there are still a few ideas of possible performance improvements that could be tried. Any suggestions or comments in that area are more than welcome however.

src/java.base/share/classes/jdk/internal/ffi/util/FFMUtils.java line 51:

> 49:     public static final AddressLayout C_POINTER = ValueLayout.ADDRESS
> 50:             .withTargetLayout(MemoryLayout.sequenceLayout(Long.MAX_VALUE, JAVA_BYTE));
> 51:     public static final ValueLayout.OfLong C_LONG = (ValueLayout.OfLong) Linker.nativeLinker().canonicalLayouts().get("long");

This seems problematic. On Windows, the C type `long` has a `JAVA_INT` layout, because it only uses 32 bits. The reason jextract re-generates all these constants is that they can, in general, vary by platform, so my general feeling is that we can't just have a shared set of layout constants at these level -- the constants will need to be added in the platform-specific directories. If you want to make this layer more portable, then the `C_LONG` constant should be dropped, and clients should use either `C_INT` or `C_LONG_LONG` (which is what most portable C APIs end up doing anyway).

Another possible way to have a more robust shared layer is to use definitions in stdint.h -- e.g. not `C_INT`, but `C_INT32_T` (but then you might have issues, as the jextract-generated files you are importing depend on names like `C_INT`).

src/java.base/share/classes/jdk/internal/ffi/util/FFMUtils.java line 99:

> 97:             .or(Linker.nativeLinker().defaultLookup());
> 98: 
> 99:     public static void traceDowncall(String name, Object... args) {

The tracing downcal support is also something that might be moved directly to the Linker -- either as a Linker option, or as a global flag, similar to `Xcheck:jni` (not in this PR)

src/java.base/share/classes/jdk/internal/ffi/util/FFMUtils.java line 106:

> 104:     }
> 105: 
> 106:     public static MemorySegment findOrThrow(String symbol) {

Hopefully this can go away once we update jextract to a new version that doesn't require this (not in this PR):
https://github.com/openjdk/jextract/pull/280

src/java.base/share/classes/jdk/internal/ffi/util/FFMUtils.java line 118:

> 116:     }
> 117: 
> 118:     public static MemoryLayout align(MemoryLayout layout, long align) {

As observed in the past, this `align` method could probably just be added on `MemoryLayout` (not as part of this PR, this is just a general observation).

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PR Review Comment: https://git.openjdk.org/jdk/pull/25546#discussion_r2244889620
PR Review Comment: https://git.openjdk.org/jdk/pull/25546#discussion_r2244899130
PR Review Comment: https://git.openjdk.org/jdk/pull/25546#discussion_r2244904140
PR Review Comment: https://git.openjdk.org/jdk/pull/25546#discussion_r2244897270