Abstract

Dynamic programming allows locally optimal instruction selection for expression trees. More importantly, the algorithm allows concise and elegant specification of code generators. Aho, Ganapathi, and Tjiang have built the Twig code-generator-generator, which produces dynamic-programming code-generators from grammar-like specifications. Encoding a complex architecture as a grammar for a dynamic-programming codegenerator -generator shows the expressive power of the technique. Each instruction, addressing mode, register and class can be expressed individually in the grammar. The grammar can be factored much more readily than with the Graham-Glanville LR(1) algorithm, so it can be much more concise. Twig specifications for the VAX and MC68020 are described, and the corresponding code generators select very good (and under the right assumptions, optimal) instruction sequences. Limitations and possible improvements to the specification language are discussed. 1. Introduction One of the last...