RFR: 8309203: C2: remove copy-by-value of GrowableArray for InterfaceSet [v4]

[Roland Westrelin]

On Wed, 25 Oct 2023 15:33:48 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> The `InterfaceSet` component of types is immutable (once it is fully
>> constructed). In that way, it's similar to other type components. In
>> this change I propose making `InterfaceSet` a subtype of `Type` (and
>> renaming it `TypeInterfaces`). It's then feasible to `hashcons`
>> `TypeInterfaces` instances the way it's done for other `Type`
>> instance.  A single copy of a `TypeInterfaces` for a particular set of
>> interfaces is live during a compilation (and for the entire length of
>> the compilation) and pointer equality can be used to check two sets of
>> interfaces for equality. I think that change makes interface support
>> fit better with the type system implementation and has the added
>> benefits that there's no copy by value of the GrowableArray that holds
>> the set of interfaces anymore.
>> 
>> A couple of required virtual methods for `Type` are not implemented
>> for `TypeInterfaces`: `xmeet` and `singleton` (their body now calls
>> `ShouldNotReachHere`). They would need a way to tell whether
>> `TypeInterfaces` instances are above the center line or not. I thought
>> about (and tried) having a root class `Type` that would contain the
>> bare minimum for type that can be `hashcons`'ed so this method
>> wouldn't need to be implemented by `TypeInterfaces`. That proved to be
>> quite complicated and I gave up.
>
> Roland Westrelin has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains seven additional commits since the last revision:
> 
>  - ciKlass* -> ciInstanceKlass*
>  - review
>  - Merge branch 'master' into JDK-8309203
>  - Update src/hotspot/share/opto/type.cpp
>    
>    Co-authored-by: Tobias Hartmann <tobias.hartmann at oracle.com>
>  - Update src/hotspot/share/opto/type.cpp
>    
>    Co-authored-by: Tobias Hartmann <tobias.hartmann at oracle.com>
>  - build fix
>  - fix

Anyone else for a review of this change?

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PR Comment: https://git.openjdk.org/jdk/pull/16309#issuecomment-1804143981