Life with Unix says: Wrote yacc, pcc (Portable C Compiler), lint and early versions of spell. Assisted Dennis in one of first ports of UNIX (to Interdata 8/32).
A Research Unix Reader says: Steve (Stephen C.) Johnson's yacc reduced Al (Alfred V.) Aho's expertise in language theory
to practice. Upon that base Steve built the portable C compiler that was used to port the system itself and to evaluate candidate instruction sets for unbuilt machines. Johnson made the first spell, worked on computer algebra, and devised languages for VLSI layout.