The Horizon compiler was developed in the mid-80s at Colorado State University as a C compiler retargetable to bit-slice microarchitectures. Many of the algorithms and techniques used in Horizon were used in Rocket in the 90s. The best description of Horizon available currently is:
Mueller, R. A., Duda, M. R., Sweany, P. H., and Walicki, J. S. Horizon: A Retargetable Compiler for Horizontal Microarchitectures, IEEE Transactions on Software Engineering, 14.5, May, 1988.