If you want a exotic operation to make this better, then you would want a uninterruptible two-instruction sequence of: "atomic modify, then use old value as ind

If you want a exotic operation to make this better, then you would want a uninterruptible two-instruction sequence of: "atomic modify, then use old value as index for indexed store". So that would be a instruction with 4 parameters: memory location for atomic, atomic modify argument, base memory location for store, register value to store; and the instruction itself would encode the operation and store scale/stride. Importantly, the indexed store does not need to be atomic with respect to the first, it merely needs to be ordered after it with a guarantee that a context switch or interrupt can not result in partial completion. This is a much weaker requirement than making multiple cache-line loads/stores fully atomic with respect to each other. Such a operation would improve multi-producer queues with fixed-size elements smaller than a word and would likely be fairly trivial to implement, merely requiring fusion of two existing instructions in a way that prevents interruption between them.