We present OptiTrust, an interactive framework for optimizing general-purpose C code via series of programmer-guided, source-to-source transformations. Optimization steps are described in transformation scripts, expressed as OCaml programs. At every step, the programmer may interactively visualize the effect of the transformation as the difference between two pieces of human-readable C code. OptiTrust has been previously employed to optimize numerical simulation code. In this work, we showcase how to use OptiTrust to optimize matrix multiplication. We compare against TVM, which also relies on programmer guidance, but which restricts the input language and lacks easily readable feedback.