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May 2026

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release progress GHC 9.12.5
by Magnus Viernickel 26 May '26

26 May '26

Dear GHC release stakeholders, We are approaching a release candidate for GHC 9.12.5. Currently there's only one blocking patch (https://gitlab.haskell.org/ghc/ghc/-/merge_requests/15640) that still needs to be backported and the test-suite seems to be doing well. Here are some important notes for this release of GHC 9.12. 1. Our Darwin CI is now running on MacOS version 26 for the aarch64 runners and on MacOS version 15 for the x86_64 runner. The SDK version on the MacOS 15 runner is 26.2, the SDK version on the aarch64 runners is 26.4. Note that this may affect bindists. 2. See https://gitlab.haskell.org/ghc/ghc/-/issues/27286 for an exhaustive list of MRs that have been backported. 3. If you are a distributor of haskell packages like stackage or nixpkgs, I would appreciate testing an RC with you, as soon as it's there. Please reach out if that's the case. 4. In ncoordinatio with the cabal developers, the GHC team has decided to stop maintaining the V1 ghc semaphore protocol that had some fundamental architectural flaws. While this is technically a (performance) regression (cabal versions that only support v1 will fall back to -j1 even when passed -jsem with 9.12.5) and adds a significant feature, it is the most pragmatic approach to fixing the glaring issues with the previous semaphore implementation without having to maintain a fundamentally broken design. Please check out https://gitlab.haskell.org/ghc/ghc/-/merge_requests/15729 for further information. We expect that `-jsem` will work out of the box with the next released cabal version and 9.12.5. I expect a release candidate to appear this week, depending on progress of the blocking patch. Please don't hesitate to reach out if you have any questions or remarks. Best Magnus

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