A bug in 9.12.1 was reported and confirmed (ghc bug https://gitlab.haskell.org/ghc/ghc/-/issues/25653) which can lead to incorrect runtime results in compiled programs. In particular this seems to happen for sub-word divisions with a known divisor when optimizations are enabled and affects at least the x86 backend, although it’s likely that other backends are also affected. We plan to release a bug fix release 9.12.2 in the near future to address this issue and recommend people do not upgrade until then.