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

[Tobias Hartmann]

On Mon, 23 Oct 2023 12:41:23 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.

I'm seeing a build failure:


[2023-10-23T13:15:09,517Z] /workspace/open/src/hotspot/share/opto/type.hpp:896:8: error: 'eq' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]
[2023-10-23T13:15:09,517Z]   bool eq(const Type* other) const;
[2023-10-23T13:15:09,517Z]        ^
[2023-10-23T13:15:09,517Z] //workspace/open/src/hotspot/share/opto/type.hpp:162:16: note: overridden virtual function is here
[2023-10-23T13:15:09,517Z]   virtual bool eq( const Type *t ) const;
[2023-10-23T13:15:09,517Z]                ^
[2023-10-23T13:15:09,517Z] /workspace/open/src/hotspot/share/opto/type.hpp:898:8: error: 'hash' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]
[2023-10-23T13:15:09,517Z]   uint hash() const;
[2023-10-23T13:15:09,517Z]        ^
[2023-10-23T13:15:09,517Z] /workspace/open/src/hotspot/share/opto/type.hpp:358:16: note: overridden virtual function is here
[2023-10-23T13:15:09,517Z]   virtual uint hash() const;
[2023-10-23T13:15:09,517Z]                ^
[2023-10-23T13:15:09,517Z]    ... (rest of output omitted)

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PR Comment: https://git.openjdk.org/jdk/pull/16309#issuecomment-1775171494