Idempotent processor architecture

Improving architectural energy efficiency is important to address diminishing energy efficiency gains from technology scaling. At the same time, limiting hardware complexity is also important. This paper presents a new processor architecture, the idempotent processor architecture, that advances both of these directions by presenting a new execution paradigm that allows speculative execution without the need for hardware checkpoints to recover from mis-speculation, instead using only re-execution to recover. Idempotent processors execute programs as a sequence of compiler-constructed idempotent (re-executable) regions. The nature of these regions allows precise state to be reproduced by re-execution, obviating the need for hardware recovery support. We build upon the insight that programs naturally decompose into a series of idempotent regions and that these regions can be large. The paradigm of executing idempotent regions, which we call idempotent processing, can be used to support various types of speculation, including branch prediction, dependence prediction, or execution in the presence of hardware faults or exceptions.