A computer, computer compiler and method for reducing the number of interferences between variables during graph coloring while maintaining the possibility that the instructions will be executed in parallel. A compiler, which converts into a machine language the source code of a program written in a programming language and optimizes the program includes: a directed acyclic graph DAG analysis unit 11 for constructing and analyzing a DAG for an instruction in a program to be processed; an interference graph construction unit 12 for employing the analysis results to construct an interference graph representing the probability that an interference will occur between variables used by the instructions; and a graph coloring unit 13 for allocating registers for the instruction based on the interference graph that is constructed.