Retiring

Metric Description

Superscalar processors can be conceptually divided into the 'front-end', where instructions are fetched and decoded into the operations that constitute them; and the 'back-end', where the required computation is performed. Each cycle, the front-end generates up to four of these operations placed into pipeline slots that then move through the back-end. Thus, for a given execution duration in clock cycles, it is easy to determine the maximum number of pipeline slots containing useful work that can be retired in that duration. The actual number of retired pipeline slots containing useful work, though, rarely equals this maximum. This can be due to several factors: some pipeline slots cannot be filled with useful work, either because the front-end could not fetch or decode instructions in time ('Front-end bound' execution) or because the back-end was not prepared to accept more operations of a certain kind ('Back-end bound' execution). Moreover, even pipeline slots that do contain useful work may not retire due to bad speculation. Front-end bound execution may be due to a large code working set, poor code layout, or microcode assists. Back-end bound execution may be due to long-latency operations or other contention for execution resources. Bad speculation is most frequently due to branch misprediction.

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