A Register-Free Abstract Prolog Machine with Jumbo Instructions

The majority of current Prolog systems are based on the WAM, in which registers are used to pass procedure arguments and store temporary data. In this paper, we present a stack machine for Prolog, named

TOAM Jr

., which departs from the TOAM adopted in early versions of B-Prolog in that it employs no registers for temporary data and it offers variable-size instructions for encoding unification and procedure calls. TOAM Jr. is suitable for fast bytecode interpretation: the omission of registers facilitates instruction merging and the use of jumbo instructions results in more compact code and execution of fewer instructions than the use of fine-grained instructions. TOAM Jr. is employed in B-Prolog version 7.0. Benchmarking shows that TOAM Jr. helps significantly improve the performance: the execution speed is increased by 48% on a Windows PC and 77% on a Linux machine. Despite the overhead on standard Prolog programs caused by the adoption of a spaghetti stack to support event handling and constraint solving, B-Prolog version 7.0 compares favorably well with the state-of-the-art WAM-based Prolog systems.