Output Value Placement in Moded Logic Programs

Most implementations of logic programming languages treat input and output arguments to procedures in a fundamentally asymmetric way: input values are passed in registers, but output values are returned in memory. In some cases, placing the outputs in memory is useful to preserve the opportunity for tail call optimization. In other cases, this asymmetry can lead to a large number of unnecessary memory references and adversely affect performance. When input/output modes for arguments are known it is often possible to avoid much of this unnecessary memory traffic via a form of interprocedural register allocation. In this paper we discuss how this problem may be addressed by returning output values in registers where it seems profitable to do so. The techniques described have been implemented in the jc system, but are also applicable to other moded logic programming languages, such as Parlog, as well as languages like Prolog when input/output modes have been determined via dataflow analysis.