Phi-predication for light-weight if-conversion

Abstract

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. 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 predicated features of the Multiflow ISA, with extensions to efficiently predicate complex control flow. Our compiler modifications also expand upon prior techniques to provide efficient code generation. We examine the use of phi-predication for an in-order and out-of-order architecture and compare its performance to using select-op and IA64 ISA predication.