Enhancing multiple-path speculative execution with predicate window shifting

Abstract

Abstract Speculative execution has long been used as an approach to exploit instruction level parallelism across basic block boundaries. Most existing speculative execution techniques only support speculating along a single control path, and heavily rely on branch prediction to choose the right control path. In this paper, we review the existing approaches for speculative execution and propose an extended predicated execution mechanism, called predicate shifting, to support speculating along multiple control paths. The predicate shifting mechanism maintains a condition/predicate window for each basic block. With the condition/predicate window, instructions can be guarded by predicates related to current or future branch conditions. The predicate shifting mechanism can reduce the number of required tag bits by shifting conditions/predicates out of the condition/predicate window whenever they are no longer in use. To incorporate the predicate shifting mechanism into a VLIW processor, a new result-buffering structure, called future buffer, is used to buffer uncommitted results and to evaluate predicates. The FIFO structure of the future buffer not only simplifies exception handling but also allows multiple uncommitted writes to the same register. Experimental results show that the predicate shifting mechanism can use predicate tag effectively and achieve 24% performance improvement over the previous predicating mechanism (H. Ando, C. Nakanishi, T. Hara, M. Nakaya, Unconstrained speculative execution with predicated state buffering, in: Proceedings of the 22nd International Symposium on Computer Architecture, 1995, pp. 126–137) using a small predicate tag.