Superword-level parallelism in the presence of control flow

In this paper, we describe how to extend the concept of superword-level parallelization (SLP), used for multimedia extension architectures, so that it can be applied in the presence of control flow constructs. Superword-level parallelization involves identifying scalar instructions in a large basic block that perform the same operation, and, if dependences do not prevent it, combining them into a superword operation on a multi-word object. A key insight is that we can use techniques related to optimizations for architectures supporting predicated execution, even for multimedia ISAs that do not provide hardware predication. We derive large basic blocks with predicated instructions to which SLP can be applied. We describe how to minimize overheads for superword predicates and re-introduce control flow for scalar operations. We discuss other extensions to SLP to address common features of real multimedia codes. We present automatically-generated performance results on 8 multimedia codes to demonstrate the power of this approach. We observe speedups ranging from 1.97X to 15.07X as compared to both sequential execution and SLP alone.

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