Re: Speculative versus parallel execution
"You want to have a context switch every time a branch causes a cache miss? That would be a Bad Thing."
It would indeed. But that is not what I say. What I say is that there is a pipeline of instructions interleaved from two or more threads, each having their own registers. No state needs to be saved, and executing every second instruction from different thread is no more expensive than executing instructions from a single thread. The advantage is that functional units can be shared, and since independent threads do not have fine-grained dependencies between each other, instructions from one thread can easily execute in parallel with instructions from another.
This is not my idea -- it has been found for decades in processors (just look for "X threads per core" in specifications). IMO, it is a better approach than speculative execution since it does not waste work (all instructions that are executed will be needed by one thread or another) and it is not considerably more complex than having one thread per core. Note that out-of-order execution is not a problem: That also executes only instruction that are needed, it just does so out of sequence, which requires register renaming, but that is not a huge problem. The main cost is complex scheduling, which increases power use (OOO processors use more energy scheduling instructions than actually executing them).
What speculation gives that these do not is (potentially) much faster execution of a single thread. But to do so, it uses resources that could have been used to execute instructions that are definitely needed. So it improves latency at the cost of throughput. OOO execution improves both at a cost in complexity and power use, and multi-threading improves only throughput, at a small cost in latency, because the two (or more) threads are given equal priority, so each thread may have to wait for others to stop using functional units.
Biting the hand that feeds IT
