8379744: Add a way to update the key of a red-black tree node

[caspernorrbin]

Hi everyone,

For more advanced users of the red-black tree, we should add a way to update the key of an existing node. This can be needed if, for example, a node represents a range that has been expanded. Updating the key avoids having to remove and re-insert the node, both of which can trigger tree balancing. In contrast, updating the key can be done efficiently with only a lookup.

Assuming the tree is not ill-formed, it is sufficient to validate the new key against the node's local neighbours to ensure that we do not violate ordering. We assert for this in debug builds. It is the user's responsibility to ensure that the new key does not break tree invariants. The tree performs no additional operations besides updating the key.

Testing:

• Oracle tier 1
• 2 new gtests covering both success and failure cases

---

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Issue

• JDK-8379744: Add a way to update the key of a red-black tree node (Enhancement - P4)

Reviewers

• Joel Sikström (@jsikstro - Reviewer)

Reviewing

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8379744: Add a way to update the key of a red-black tree node

Reviewed-by: jsikstro

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At the time when this comment was updated there had been 64 new commits pushed to the master branch:

• 1ea8ef9: 8379781: G1: Full GC does not print partial array task stats
• 713664f: 8379804: Refactor jdk/com/sun tests to use JUnit
• fec7229: 8379625: PPC: cleanup C2 OptoAssembly
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[mlbridge]

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• 01: Full - Incremental (8cef375c)
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[jsikstro]

Looks good. This is useful in use-cases we have in ZGC where we would like to avoid calls to malloc/free when we know by invariant that we could just update a node's key.

It's worth mentioning that this might not be the case for other usages of the tree, and using this function can, as you say, break the invariants. I think the asserts are an OK approach to maintain the tree's invariants, but can definitely shoot you in the leg if used incorrectly.

[jsikstro]

Suggested change

	// Updates the key in the given the node or node cursor.
	// Updates the key in the given node or node cursor.