[trishume]

Except that LuaJIT does something close to just a pointer offset. The trick it uses is to use a hash function that is known by the compiler so when you have a constant key ("y" in this case) the hash table set is compiled down to 3 instructions (imagine hash("y") = 0xa7):

1. Load whatever necessary to check if x[0xa7] is occupied 2. Branch to slow path if occupied 3. Store x[0xa7] = 'z'

And a modern super-scalar CPU will even do some of this in parallel.