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

[Maurizio Cimadamore]

On Tue, 16 May 2023 11:18:09 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.

Javadoc: https://cr.openjdk.org/~mcimadamore/jdk/8308248/8308248/v1/javadoc/java.base/module-summary.html
Specdiff: https://cr.openjdk.org/~mcimadamore/jdk/8308248/8308248/v1/specdiff_out/overview-summary.html

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PR Comment: https://git.openjdk.org/jdk/pull/14007#issuecomment-1551095329