Branch penalty reduction on IBM Cell SPUs via software branch hinting

As power-efficiency becomes paramount concern in processor design, architectures are coming up that completely do away with hardware branch prediction, and rely solely on software branch hinting. A popular example is the Synergistic Processing Unit (SPU) in the IBM Cell processor. To be able to minimize the branch penalty using branch hint instructions, in addition to estimating the branch probabilities (which has been looked at before [6, 25, 24]), it is important to carefully insert branch hints. Towards this, in this paper, we i) construct a branch penalty model for compiler, ii) formulate the problem of minimizing branch penalty using branch hinting and iii) propose a heuristic to solve this problem. The heuristic is based on three basic techniques that we introduce in this paper: NOP padding, hint pipelining, and nested loop restructuring. Experimental results on several benchmarks show that our solution can reduce the branch penalty as much as 35.4% over the previous approach.