RFR: 8329532: Add an internal MemoryLayout transformer

[Maurizio Cimadamore]

On Tue, 2 Apr 2024 15:04:40 GMT, Per Minborg <pminborg at openjdk.org> wrote:

> This PR proposes to add an internal layout transformer that could be used to transform MemoryLayout entities. For example, it would be possible to convert a struct layout to use network order ((big-endian) instead of native byte order (often little-endian). Another application is to remove naming recursively for composite layouts when caching shapes etc.

src/java.base/share/classes/jdk/internal/foreign/layout/LayoutTransformers.java line 83:

> 81:          * @param layout to transform
> 82:          */
> 83:         MemoryLayout transform(T layout);

Not too sure about having a distinction between transform and deepTransform, at least not for the use cases we have so far. I'd say just keep the semantics of deepTransform, so that this becomes a functional interface?

src/java.base/share/classes/jdk/internal/foreign/layout/LayoutTransformers.java line 179:

> 177:         private static <T extends MemoryLayout> MemoryLayout transformFlat(LayoutTransformer<T> transformer, MemoryLayout l) {
> 178:             LayoutTransformerImpl<T> t = (LayoutTransformerImpl<T>) transformer;
> 179:             return switch (t.type) {

Why the big switch here? Aren't all arms just passing the layouts to the transform? As the method is already unchecked, perhaps simplifying the code and using e.g. raw types might be a better option, as it would make the code more readable.

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PR Review Comment: https://git.openjdk.org/jdk/pull/18582#discussion_r1567116664
PR Review Comment: https://git.openjdk.org/jdk/pull/18582#discussion_r1567114126