We present an approach for implementing a formally certified loop-invariant code motion optimization by composing an unrolling pass and a formally certified yet efficient global subexpression elimination.This approach is lightweight: each pass comes with a simple and independent proof of correctness.Experiments show the approach significantly narrows the performance gap between the CompCert certified compiler and state-of-the-art optimizing compilers.Our static analysis employs an efficient yet verified hashed set structure, resulting in fast compilation.