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Predicated execution enables the removal of branches by converting segments of branching code into sequences of conditional operations. An important side effect of this transformation is that the compiler must unconditionally assign resources to predicated operations. However, a resource is only put to productive use when the predicate associated with an operation evaluates to True. To reduce this superfluous commitment of resources, we propose probabilistic predicate-aware scheduling to assign multiple operations to the same resource at the same time, thereby over-subscribing its use. Assignment is performed in a probabilistic manner using a combination of predicate profile information and predicate analysis aimed at maximizing the benefits of over-subscription in view of the expected degree of conflict. Conflicts occur when two or more operations assigned to the same resource have their predicates evaluate to True. A predicate-aware VLIW processor pipeline detects such conflicts, recovers, and correctly executes the conflicting operations. By increasing the effective throughput of a fixed set of resources, probabilistic predicate-aware scheduling provided an average of 20% performance gain in our evaluations on a 4-issue processor, and 8% gain on a 6-issue processor.
Date of Conference: 20-24 March 2004