# Critical code generation bug with GHC 9.12.3

**URL:** <https://discourse.haskell.org/t/critical-code-generation-bug-with-ghc-9-12-3/13505>\
**Category:** Announcements\
**Created:** [January 8, 2026, 9:58am UTC](https://discourse.haskell.org/t/critical-code-generation-bug-with-ghc-9-12-3/13505 "2026-01-08T09:58:44Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![wz1000](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/wz1000/32/1245_2.png) [@wz1000](https://discourse.haskell.org/u/wz1000)\
**Post date:** [January 8, 2026, 9:58am UTC](https://discourse.haskell.org/t/critical-code-generation-bug-with-ghc-9-12-3/13505/1 "2026-01-08T09:58:45Z")

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Unfortunately we discovered a major code generation regression  
in GHC 9.12.3 that results in incorrect runtime results for  
certain operations involving sub-word divisions.

We recommend avoiding upgrading to this minor release at this  
time.

See [Sub-word division is broken in GHC 9.12.3 (again) (#26711) · Issues · Glasgow Haskell Compiler / GHC · GitLab](https://gitlab.haskell.org/ghc/ghc/-/issues/26711) for more  
details.

This is the same bug that affected GHC 9.12.1. A patch re-introducing  
this bug was accidently backported onto the 9.12.3 branch  
and went unnoticed throughout the testing period.

We are taking steps to ensure similar situations are less likely  
to occur again, including looking at ways to improve our testing procedure  
for minor GHC release, potentially using the Stackage infrastructure  
to run the testsuites of a broad range of packages to detect  
such issues.

We are also starting to prepare GHC 9.12.4 with a fix, which  
we hope to release soon. However, a significant amount of time  
has passed since the initial set of backports to GHC 9.12.3  
were completed, which means that a number of merge requests  
addressing issues of various severities has been marked for  
backporting to the GHC 9.12 branch. We are in the process of  
triaging this list and preparing a robust and reliable GHC 9.12.4  
release.

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**Author:** ![Kleidukos](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/kleidukos/32/1213_2.png) [@Kleidukos](https://discourse.haskell.org/u/Kleidukos)\
**Post date:** [January 8, 2026, 10:01am UTC](https://discourse.haskell.org/t/critical-code-generation-bug-with-ghc-9-12-3/13505/2 "2026-01-08T10:01:34Z")

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Thanks a lot for your work Zubin!

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**Author:** ![juhp](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.haskell.org/juhp/32/323_2.png) [@juhp](https://discourse.haskell.org/u/juhp)\
**Post date:** [January 8, 2026, 4:38pm UTC](https://discourse.haskell.org/t/critical-code-generation-bug-with-ghc-9-12-3/13505/3 "2026-01-08T16:38:35Z")

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I already patched the Fedora ghc9.12 package btw (the update is in the testing repos for a few more days).