[foreign-jextract] RFR: 8277830: jextract generated code should not depend on Java layouts

[Athijegannathan Sundararajan]

On Mon, 29 Nov 2021 14:09:29 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> This patch tweaks jextract generation, so that a new constant class is emitted, namely `Constant$root`, which contains the definition of all basic C primitive types. These are declared with the correct alignment constraints, depending on platforms.
> 
> Every other layout created by jextract will depend on these shared layouts constants, so that jextract code doesn't depend on alignment choices in `ValueLayout` constants.
> 
> In the future, we should probably generalize this approach, so that e.g. layouts are emitted only once, and then referred to afterwards - not just for primitive types, but also for everything else (struct, typedef, etc.). This would minimize the amount of layouts we store in the constant classes.

approved.

build/test fine on Mac. samples run fine on Mac as well

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Marked as reviewed by sundar (Committer).

PR: https://git.openjdk.java.net/panama-foreign/pull/620