Software-based branch predication for AMD GPUs

Abstract

Branch predication is a program transformation technique that combines instructions of multiple branches of an if statement into a straight-line sequence and associates each instruction of the sequence with a predicate. The branch predication improves the execution of branch statements on processors that support predicated execution of instruction, e.g., Intel IA-64, because such transformation improves the instruction scheduling and might help cache performance. This paper proposes a novel software-based branch predication technique for GPU. The main motivation is that branch instructions can easily become a performance bottleneck for a GPU program because of the cost of branch instructions compared to ALU instructions and the possibility of low ALU utilization due to separation of ALU instructions within control flow blocks. Due to the SIMD nature and massive multi-threading architecture of the GPU, branching can be costly if more than one path is taken by a set of concurrent threads in a kernel. In this paper we reveal that branch predication can enable instruction packing, a VLIW-like GPU feature that is designed to increase the parallel execution of independent instructions, and can also decrease the number of control flow instructions thereby improving the performance of GPU kernels with both single and multiple branch paths. The key of our novel branch predication technique is a set of transformation rules that takes into consideration the specialties of the GPU architecture and implements software-based predicated execution of instruction on the GPU with little to no overhead. Furthermore, we identify architectural and program factors that affect the effectiveness of our technique and build a benefit analysis model for the transformation. The implementation of our technique on synthetic benchmarks and real-world application proves its effectiveness.