Relaxing in-order retirement
The only relaxation can be existent in the order of load and store instructions.
Result serialization as it is demanded by the serial instruction flow of the von Neumann architecture.
A fully parallel and highly speculative processor must look like a simple von Neumann processor as it was state-of-the-art in the fifties.
- Assume an instruction sequence A ends with a branch that predicts an instruction sequence B, and B is followed by a sequence C which is not dependent on B.
- Thus C is executed independently from the branch direction.
- Therefore, instructions in C can start to retire before B.