How does the Objective-C method cache actually work internally?

The method cache is a per-class hash table mapping SEL to IMP for O(1) lookup.

Each Objective-C class maintains a method cache. When objc_msgSend is called, runtime first checks this cache. If found, it jumps directly to IMP. If not found, it walks method lists, then inserts result into cache. Cache uses open addressing and is protected by atomic operations to avoid locking in fast path.