Half_Life version 0.1

Half_Life (460K) is a packaged version of the Life compiler as it existed when DEC PRL was closed in September 1994. Half_Life was written primarily by Richard Meyer and Bruno Dumant with guidance from Peter Van Roy and support from the Paradise and Proteus projects at DEC PRL. The development of Half_Life was partially funded by the ESPRIT project ACCLAIM (PE7195). Half_Life is copyright 1994 Digital Equipment Corporation and DFKI (Deutsches Forschungszentrum fuer Kuenstliche Intelligenz). Non-commercial use is free and encouraged.

Half_Life essentially implements the Login subset of Life (i.e., Prolog with psi-terms). Functions are supported syntactically, that is, they can be defined, but they are called with unification instead of matching. Half_Life compiles small programs correctly, but no guarantees are given as to its suitability for anything except research into implementation technology.

Half_Life has three innovative characteristics:

It uses a novel technique, the RISC-macro technique, to generate portable C code that approaches the speed of assembly code. The idea is to generate macros that syntactically resemble RISC instructions. The macros are defined in such a way that they are compiled as RISC instructions. Control flow (functions and jumps) are implemented with low overhead (<5%) and the generated C procedure sizes are bounded, to be acceptable to the C compiler. GCC's global register allocation ability is used to assign processor registers to Life's run-time structures.

It implements psi-term unification with performance competitive to Prolog. It does this by providing two representations for psi-terms, a static one (corresponding to Prolog's) and a dynamic one (dynamically resizable hash table). The choice of representation is made by the system at run-time, i.e., the representation starts as the static one, and flips to the dynamic one if features (record fields) are added to a term. Technically, Half_Life provides constructive proof that open feature structures can be implemented as efficiently as closed feature structures.

It is written using the Wild_Life preprocessor described in the Wild_Life user manual. This preprocessor has been an enormous help in reducing the drudgery of compiler writing. The preprocessor is a generalization of the Extended DCG preprocessor used to build Aquarius Prolog.

Short questions about the Half_Life system can be addressed to myself or to Richard Meyer at richard.meyer@eqtg.sbi.com. Questions about the parser and the Wild_Life preprocessor can be addressed to Bruno Dumant at bruno.dumant@inria.fr.

Please send all comments to vanroy@dfki.uni-sb.de.

Last modified on Feb. 23, 1995.