Phi-Predication for Light-Weight If-Conversion

Predicated execution can eliminate hard to predict branches and help to enable instruction level parallelism. Many current predication variants exist where the result update is conditional based upon the outcome of the guarding predicate. However, conditional writing of a register creates a naming problem for an out-of-order processor, and can stall the issuing of instructions. This problem arises from potential multiple predicated definitions reaching a use, which is unresolved until the prior predicate values are computed. In this paper we focus on a light-weight form of predication, Phi-Predication, where all predicated instructions write a result value to their register regardless of the predicate value (i.e. even if it is false). Therefore, the predicate does not guard the writing of the result register; it instead acts as a form of selection between two input registers. This eliminates the naming problem for an out-of-order processor. Our Phi-Predicated ISA is derived from the ...

• Type: Conference
• Year: 2003
• Where: CGO
• Authors: Weihaw Chuang, Brad Calder, Jeanne Ferrante
• Keywords: CGO 2003, Naming Problem, Predicate Value, Prior Predicate Values, Software Engineering