A Treegion-based Unified Approach to Speculation and Predication in Global Instruction Scheduling

This paper presents a treegion-based global scheduling technique for wide issue VLIW/EPIC processors. A treegion is a single-entry/multiple-exit global scheduling scope that consists of basic blocks with control-flow forming a tree. We propose a two-phase approach to global scheduling within a treegion scope that enables speculative code motion in the first phase and uses predication of all instructions in the second phase. In the first scheduling phase, tree traversal scheduling (TTS) takes full advantage of speculation to speed up all possible paths in a treegion. Over-aggressive speculation is limited by scheduling block-ending branches as early as possible, enabled by downward code motion. A multiway branch transformation is also performed to reduce control dependence height. In the second scheduling phase, fully resolved predicates (FRPs) are used to enable branch barrier instructions, such as stores and subroutine calls, to move across branches. Selective if-conversion can also be applied to remove hard-to-predict branches in a treegion. The simulation results based on an 8-issue EPIC style machine model show an average speedup of 21% of TTS over BB scheduling, an additional speedup of 6.4% from multiway branch transformation, and another 1.9% speedup from FRP-guarded code motion. Other code transformations such as treegion code layout and the general operation combining are also presented in this paper.