Bending the CLOS Mop for Java-Style Single Dispatch

5 days ago

The author encountered performance issues when running Clojure on OpenLDK, a Common Lisp JVM, due to CLOS's multi-method dispatch being inefficient for Java's single-dispatch model.
The solution involved customizing the CLOS Meta-Object Protocol (MOP) to replace the dispatch machinery for Java methods with a single-dispatch model, significantly improving performance.
A new metaclass, `java-generic-function`, was created with hash-table caches for dispatch and `invokespecial` calls, along with a lock for thread safety.
The custom discriminating function for `java-generic-function` uses a hash-table lookup for O(1) dispatch after the first call for each receiver class.
The biggest performance win came from intercepting PCL's `update-dfun` to clear caches instead of rebuilding the discrimination net, reducing the overhead during Clojure bootstrap.
Pre-creating hot generic functions like `<init>()` and `clone()` with the `java-generic-function` metaclass ensured fast dispatch from the start.
A separate cache was added for Java's `invokespecial` instruction, which is used for constructor chaining and `super` calls, further optimizing performance.
The changes reduced Clojure bootstrap time from 2 hours 45 minutes to 2 minutes 40 seconds, demonstrating the effectiveness of the MOP customization.
The solution is concise (120 lines of code) and minimally invasive, leveraging the MOP as intended without modifying SBCL or the runtime.

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