RFR: 8308248: Revisit alignment of layout constants on 32-bit platforms [v2]

[Paul Sandoz]

On Wed, 17 May 2023 11:15:11 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> The FFM API exposes layout constants for Java primitives. Among those there are constants for `JAVA_LONG` and `JAVA_DOUBLE`. Currently, the alignment of these layouts is set the same as their size (e.g. 8 bytes).
>> 
>> This is obviously correct on 64-bit platforms, but on 32-bit platform it is not, as such platforms cannot guarantee that doubles and longs will be always 64-bit aligned. This will also result in problems when trying to use e.g. `JAVA_DOUBLE` to model a C double for the linker API on 32-bit platforms.
>> 
>> For these reasons, it would be preferable to define the alignment of `JAVA_LONG` and `JAVA_DOUBLE` constants as `ADDRESS.byteSize()`. 
>> 
>> This patch rectifies alignment of those layout constants to reflect platform-dependent constraints. It also fixes the maximum alignment constraint supported by heap segments, so that it is 4 for long[] and double[] on 32-bit platforms.
>
> Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix max alignment constants

Marked as reviewed by psandoz (Reviewer).

src/java.base/share/classes/java/lang/foreign/ValueLayout.java line 488:

> 486:     /**
> 487:      * A value layout constant whose size is the same as that of a Java {@code long},
> 488:      * bit alignment set to {@code ADDRESS.bitSize()}, and byte order set to {@link ByteOrder#nativeOrder()}.

Suggestion:

     * (platform dependent) bit alignment set to {@code ADDRESS.bitSize()}, and byte order set to {@link ByteOrder#nativeOrder()}.

?

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PR Review: https://git.openjdk.org/jdk/pull/14007#pullrequestreview-1431604601
PR Review Comment: https://git.openjdk.org/jdk/pull/14007#discussion_r1197038551