Patent US 4,763,255

A method for improving the quality of code generated by a compiler or assembler, for a target machine that has short and long forms of some of its instructions with the short forms executing faster or occupying less space. The method first determines which bits of the result of each computational instruction are significant, by a backwards pass over the program that is similar to liveness analysis. Then the significant bits thus computed are used to guide the code selection process to select the most efficient instruction that computes the correct result in all the significant bit positions.