Summary

The layout constants ValueLayout.JAVA_LONG and ValueLayout.JAVA_DOUBLE assume 8-byte alignment even on 32-bit platform, which is incorrect.

Problem

All layout constants in ValueLayout assume an alignment that is the same as their size. This holds for constants such as ValueLayout.JAVA_LONG and ValueLayout.JAVA_DOUBLE, which assume 8-byte alignment. This assumption is incorrect on 32-bit platform, as long[]</code> and <code class="prettyprint" >double[] arrays can only guarantee that their elements will be 4-byte aligned.

This also creates issues when using layouts such as JAVA_LONG and JAVA_DOUBLE to interact with the Linker on 32-bit platforms: C types in such platforms are typically only 4-byte aligned, which means the linker would detect an alignment mismatch.

Solution

The solution is to tweak the layout constants such as JAVA_LONG and JAVA_DOUBLE so that they honestly reflect whatever alignment constraint can be provided by a particular platform. A good way to specify these constants is to define their alignment to be the same as the size of the ADDRESS constant (as that is also platform-dependent).

Specification

A summary of the required changes is attached to this CSR (see 8308248_v1.zip). Links are also included below:

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