Abstract Instruction selection is the primary task in automatic code generation. This paper proposes a practical system for performing optimal instruction selection based on tree pattern matching for expression trees. A significant feature of the system is its ability to perform code generation without requiring cost analysis at code generation time. The target machine instructions are specified as attributed production rules in a regular tree grammar augmented with cost information in Graham Glanville style. Instruction selection is modelled as a process of determining minimum cost derivation for a given expression tree. A matching automaton is used for instruction selection. Cost information is encoded into the states of this automaton so that cost analysis is not required at code generation time. The folding technique of table compression is extended to this automaton and two schemes of table compression based on cost information are proposed. Compilers Retargetable code generation Code-generator Code-generator-generator Tree-pattern matching Instruction selection Table compression