Abstract
The role of the instruction scheduler is to supply instructions to functional units in a timely manner so as to avoid data and structural hazards. Current schedulers are based on the broadcast of result register numbers to all instructions waiting in the issue queue and on a global arbiter to select ready instructions from that queue. This approach called broadcast scheduling does not scale well due to its complexity. To reduce the complexity of the broadcast schedulers, data-flow pre-scheduling has been proposed. The basic idea is to predict the issue time of instructions based on the availability of operands and then time them down until they are ready to issue. However, resource conflicts for issue slots and functional units delay the issue time of conflicted instructions, and cause a large amount of replays. We propose to add instruction pre-selection to data-flow pre-schedulers for accurate instruction pre-scheduling. Our pre-scheduler keeps track of the allocation status of resources so that re source conflicts are eliminated. Pre-scheduled instructions are stored in an issue buffer until their issue delay elapses and then issue automatically. Our analysis shows that pre-schedulers with pre-selection result in performance improvements of 60% over current broadcast schedulers in pipeline designs where the scheduler is the bottleneck. In future technologies we expect this result to hold as logic intensive designs with short wires will be preferable to de signs with long wire delays.
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Choi, W., Park, S., Dubois, M.: Accurate Instruction Pre-scheduling in Dynamically Scheduled Processors,” Technical Report #CENG-2007-3, Depart¬ment of Electrical Engineering - Systems, University of Southern California (March 2007)
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Choi, W., Park, SJ., Dubois, M. (2009). Accurate Instruction Pre-scheduling in Dynamically Scheduled Processors. In: Stenström, P. (eds) Transactions on High-Performance Embedded Architectures and Compilers II. Lecture Notes in Computer Science, vol 5470. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-00904-4_7
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