Formally Verifying Optimizations with Block Simulations

CompCert (ACM Software System Award 2021) is the first industrial-strength compiler with a mechanically checked proof of correctness. Yet, CompCert remains a moderately optimizing C compiler. Indeed, some optimizations of “gcc ‍-O1” such as Lazy Code Motion (LCM) or Strength Reduction (SR) were still missing: developing these efficient optimizations together with their formal proofs remained a challenge. Cyril Six et al. have developed efficient formally verified translation validators for certifying the results of superblock schedulers and peephole optimizations. We revisit and generalize their approach into a framework (integrated into CompCert) able to validate many more optimizations: an enhanced superblock scheduler, but also Dead Code Elimination (DCE), Constant Propagation (CP), and more noticeably, LCM and SR. In contrast to other approaches to translation validation, we co-design our untrusted optimizations and their validators. Our optimizations provide hints, in the forms of invariants or CFG morphisms , that help keep the formally verified validators both simple and efficient. Such designs seem applicable beyond CompCert.