[foreign-abi] RFR: JDK-8242127: reorganize ABI-dependent layout constants

[Athijegannathan Sundararajan]

On Fri, 3 Apr 2020 18:26:40 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> There are two main problems with the way in which ABI-dependent constants are dealt with:
> 
> * the SystemABI.Type enum abstraction is rarely used - code just prefers to go with layouts constants in MemoryLayouts
> * the current enum doesn't scale to model platform-dependent ABI types, so it just has to work with the set of types that
>   are supported in all platforms
> 
> This patch rearranges things a bit. First, it removes the SystemABI.Type abstraction, and associated
> SystemABI::layoutFor method. Then, it moves all ABI layout constants from MemoryLayouts to SystemABI. Instead of using
> the enum value as a classification value for the ABI, each ABI constant holder defines its own classification
> constants. So there is a SystemABI.Win64.ArgumentClass, and a SystemABI.SysV.ArgumentClass and so on and so forth. A
> notable fact is that the set of classification values that are required by the ABI is much, much smaller than the set
> of "interesting" C types.  I then adjusted the implementation code not to no longer depend on the SystemABI.Type enum;
> while doing so I realized (suggestion from Jorn) that the public ArgumentClass enums would actually, both in Windows
> and AArch be enough to also implement the internal classification system. On SysV, since the classification is quite
> convoluted, there is a number of 'synthetic' classification classes that are only introduced when classifying struct
> (e.g. SSE_UP) which would be bad to expose outside - but we need these classes to keep the classification algorithm
> going. So, I moved ArgumentClassImpl under the sysv folder and kept the code as it was.  The resulting code is, I think
> more direct, and more scalable. We can keep adding layouts to SystemABI - both platform-independent and
> platform-dependent ones - they are just different sets of constants.

Marked as reviewed by sundar (Committer).

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PR: https://git.openjdk.java.net/panama-foreign/pull/97