Adaptive pipeline structures for speculation control

Abstract

Pipelining is a common method for improving the throughput of a system, especially when the majority of the processing is sequential. Unfortunately when the sequentiality is broken, a pipelined system suffers additional delay and, most importantly for this work, energy waste which is roughly proportional to the pipeline depth. Standard pipelines cannot be modified once they are built so their depth is fixed. This paper proposes a method that allows the dynamic adaptation of the structure of an asynchronous pipeline, so that pipeline stages can be merged and split at run-time, allowing greater flexibility. It is based on novel latch controllers that can be configured dynamically as 'normal' or 'collapsed', i.e. keeping their latches permanently transparent. Using these controllers a model of AMULET3 was designed that is capable of changing its pipeline depth dynamically when branches are anticipated, in order to alleviate the energy loss when the branch finally arrives.