Enhancing Multiple - Path

Speculative execution has long been used as an approach to exploit instruction level par-allelism across basic block boundaries. Most existing speculative execution techniques only support speculating along single control path, and heavily rely on branch prediction to choose the right control path. In this paper, we 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-buuering structure, call future buuer, is used to buuer uncommitted results and to evaluate predicates. The FIFO structure of the future buuer not only simpliies exception handling but also allows multiple uncommitted writes to the same register. Experimental results show that the predicate shifting mechanism can use predicate tag eeectively and achieve 24% performance improvement over the previous predicating mechanism 2] using a small predicate tag.