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

[Maurizio Cimadamore]

On Thu, 31 Jul 2025 09:45:10 GMT, Maurizio Cimadamore <mcimadamore 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`).

IMHO the more honest approach is to move these constants closer to the _h generated files that use them, as that's the way jextract intends them to be used.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25546#discussion_r2244894284