Relieving Register File and Instruction Window Pressure

Glenn Reinman, Brad Calder and Todd Austin
November 20, 2001

The reorder buffer and register file of a modern superscalar processor are both critical components of the processor pipeline that can have a large impact on processor performance. Larger reorder buffers and register files provide larger windows of speculation to the processor and allow greater levels of instruction-level parallelism. However, the access time to these critical path structures can grow quite large when these structures increase in size, especially considering the number of ports required on such structures. In this paper, we propose two architectures that are better able to scale to future technology sizes. First we examine a pipelined register file architecture that reduces the cost of bypassing. This architecture features a small operand file that is used to store register operands for instructions waiting in reservation stations. This operand file is also updated by the writeback stage of the process pipeline to provide register bypassing and can allow instructions to share stored register operands, helping to further hide the latency of the pipelined register file access. Second, we examine an architecture that decouples the reorder buffer and register file from the critical path of the processor, replacing them with a smaller, speculative register file and an operand file. The reorder buffer and larger register file are moved off the critical path of the processor and are used to still provide precise exceptions and a means to recover register state in the event of branch mispeculation. The speculative register file and operand file provide the low-latency, scalable performance to enable the execution core to better meet the demands of future cycle times. This technique also completely removes the need to perform conventional register renaming from the processor and the need to perform register map checkpointing.

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