RFR: 8375621: Move RBTree implementation to inline file to minimize included headers

[Johan Sjölen]

On Mon, 19 Jan 2026 11:14:18 GMT, Casper Norrbin <cnorrbin at openjdk.org> wrote:

> Hi everyone,
> 
> The red-black tree implementation is split across `rbTree.hpp` and `rbTree.inline.hpp`. Some small `AbstractRBTree` functions are kept in `rbTree.hpp`, and also all of `RBTree`'s implementation. This makes the file less readable and requires extra header inclusions, notably the `os` class, which in turn includes additional unnecessary dependancies.
> 
> In this PR I have moved all implementation code `rbTree.inline.hpp` instead. As a result, `rbTree.hpp` now only contains declarations, making it cleaner and limiting header inclusions to what is strictly necessary. Implementation-specific headers are now only included in the inline file.
> 
> Additionally, I changed `RBTreeCHeapAllocator` to use `AllocateHeap`/`FreeHeap` from `allocation` instead of `os::malloc`/`os::free`, which allows us to get rid of the `os` dependancy altogether.
> 
> Note: This PR is dependant on #29082, which adds arena-style tree allocators, which this PR then moves to the inline file.
> 
> Testing:
> - Oracle tiers 1-5

Marked as reviewed by jsjolen (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/29295#pullrequestreview-3677843412