Macro-op scheduling: relaxing scheduling loop constraints

Ensuring back-to-back execution of dependent instructions in a conventional out-of-order processor requires scheduling logic that wakes up and selects instructions at the same rate as they are executed. To sustain high performance, integer ALU instructions typically have single-cycle latency, consequently requiring scheduling logic with the same single-cycle latency. Prior proposals have advocated the use of speculation in either the wakeup or select phases to enable pipelining of scheduling logic to achieve higher clock frequency. In contrast, this paper proposes macro-op scheduling which systematically removes instructions with single-cycle latency from the machine by combining them into macro-ops, and performs nonspeculative pipelined scheduling of multi-cycle operations. Macro-op scheduling also increases the effective size of the scheduling window by enabling multiple instructions to occupy a single issue queue entry. We demonstrate that pipelined 2-cycle macro-op scheduling performs comparably or even better than atomic scheduling or prior proposals for select-free scheduling.