+: ${RELEASE=NO}

+.. _release-9-12-4:

+

+Version 9.12.4

+==============

+

+The significant changes to the various parts of the compiler are listed in the

+following sections. See the `migration guide

+<https://gitlab.haskell.org/ghc/ghc/-/wikis/migration/9.12>`_ on the GHC Wiki

+for specific guidance on migrating programs to this release.

+

+Compiler

+~~~~~~~~

+

+- Fixed a bug in CSE where the in-scope set was not properly maintained (:ghc-ticket:`25468`)

+- Fixed ``matchExpectedFunTys`` to use ``tcMkScaledFunTys`` (:ghc-ticket:`26277`)

+- Fixed ``parenBreakableList`` usage in ``ppHsContext`` for better pretty-printing of contexts

+- Improved error messages for bad record updates to allow out-of-scope data constructors (:ghc-ticket:`26391`)

+- Fixed a missing InVar->OutVar lookup in ``SetLevels`` (:ghc-ticket:`26681`)

+- Fixed split sections on Windows (:ghc-ticket:`26696`, :ghc-ticket:`26494`)

+- Fixed split sections for the LLVM backend (:ghc-ticket:`26770`)

+- Don't re-use stack slots for growing registers (:ghc-ticket:`26668`)

+- Fixed cast worker/wrapper incorrectly firing on INLINE functions (:ghc-ticket:`26903`)

+- Fixed non-determinism in ``TyLitMap`` by using deterministic maps for strings (:ghc-ticket:`26846`)

+- Fixed non-determinism in ``WithHsDocIdentifiers`` binary instance by using a stable sort (:ghc-ticket:`26858`)

+- Added ``-mcmodel=medium`` module flag to generated LLVM IR on LoongArch

+- Pass the ``mcmodel=medium`` parameter to CC via GHC on LoongArch

+- Pass the ``+evex512`` attribute to LLVM 18+ when ``-mavx512f`` is set (:ghc-ticket:`26410`)

+- Improved error handling in ``getPackageArchives`` (:ghc-ticket:`26383`)

+- Fixed a shadowing bug in implicit parameters (:ghc-ticket:`26451`)

+- Fixed a subtle bug in ``GHC.Core.Utils.mkTick`` that could generate type-incorrect code (:ghc-ticket:`26772`)

+- Fixed a long-standing interaction between ticks and casts in ``Eliminate Identity Cases``

+- ``NamedDefaults``: require the class to be standard or have an in-scope default declaration (:ghc-ticket:`25775`, :ghc-ticket:`25778`)

+

+Runtime System

+~~~~~~~~~~~~~~

+

+- Fixed a deadlock with eventlog flush interval and RTS shutdown (:ghc-ticket:`26573`)

+- Fixed eager black holes: record mutated closure and fix assertion (:ghc-ticket:`26495`)

+- Fixed object file format detection in ``loadArchive`` (:ghc-ticket:`26630`)

+- Use ``INFO_TABLE_CONSTR`` for ``stg_dummy_ret_closure`` (:ghc-ticket:`26745`)

+- Fixed lost wakeups in ``threadPaused`` for threads blocked on black holes (:ghc-ticket:`26324`)

+- Fixed ``stg_AP_STACK`` to push the correct update frame (:ghc-ticket:`26324`)

+- Fixed potential loop in heap reservation logic on certain kernels (:ghc-ticket:`26151`)

+- Don't use CAS without ``PARALLEL_GC`` on

+- Switch prim to use modern atomic compiler builtins (:ghc-ticket:`26729`)

+- Removed obsolete ``CC_SUPPORTS_TLS``, ``HAS_VISIBILITY_HIDDEN``, ``COMPILING_WINDOWS_DLL``,

+  and ``__GNUC__``-related logic

+- Removed the ``-O3`` pragma hack in ``Hash.c``

+- Removed unnecessary Cabal flags

+

+Code Generation

+~~~~~~~~~~~~~~~

+

+- NCG for PPC: add pattern for ``CmmRegOff`` to ``iselExpr64`` (:ghc-ticket:`26828`)

+- PPC NCG: Use libcall for 64-bit ``cmpxchg`` on 32-bit PowerPC (:ghc-ticket:`23969`)

+

+Bytecode Compiler

+~~~~~~~~~~~~~~~~~

+

+- Use 32 bits for breakpoint index (:ghc-ticket:`26325`)

+

+``base`` library

+~~~~~~~~~~~~~~~~

+

+- Expose ``Backtraces`` constructor and fields (:ghc-ticket:`26049`)

+- Store ``StackTrace`` and ``StackSnapshot`` in ``Backtraces``, deferring decoding until display

+- Evaluate backtraces for ``error`` exceptions at the moment they are thrown

+  (`CLC proposal #383 <https://github.com/haskell/core-libraries-committee/issues/383>`__,

+  :ghc-ticket:`26751`)

+

+``ghc-experimental`` library

+~~~~~~~~~~~~~~~~~~~~~~~~~~~~

+

+- Fixed ``GHC.Exception.Backtrace.Experimental`` module

+- Added ability to customise the collection of exception annotations

+

+``ghc-pkg``

+~~~~~~~~~~~~

+

+- Removed ``traceId`` from ``ghc-pkg`` executable

+

+``ghc-toolchain``

+~~~~~~~~~~~~~~~~~

+

+- Dropped ``ld.gold`` from merge object command

+

+Build System

+~~~~~~~~~~~~

+

+- Added ``hpc`` to release script

+- Use a response file to invoke GHC when analysing dependencies

+- Fixed ``GHC.Platform.Host`` generation for cross stage1 (:ghc-ticket:`26449`)

+- Fixed runtime error during ``users_guide`` build with Sphinx 9.1.0 (:ghc-ticket:`26810`)

+- Added ``ghc-{experimental,internal}.cabal`` to the list of dependencies of the doc target (:ghc-ticket:`26738`)

+

+Wasm Backend

+~~~~~~~~~~~~

+

+- Fixed dyld handling for forward declared ``GOT.func`` items (:ghc-ticket:`26430`)

+- Ensure ``setKeepCAFs()`` is called in GHCi (:ghc-ticket:`26106`)

+- Prevent bundlers from resolving ``import("node:timers")``

+- Use ``import.meta.main`` for proper distinction of Node.js main modules (:ghc-ticket:`26916`)

+

+Included libraries

+~~~~~~~~~~~~~~~~~~

+

+The package database provided with this distribution also contains a number of

+packages other than GHC itself. See the changelogs provided with these packages

+for further change information.

+

+.. ghc-package-list::

+

+    compiler/ghc.cabal:                                  The compiler itself

+    libraries/array/array.cabal:                         Dependency of ``ghc`` library

+    libraries/base/base.cabal:                           Core library

+    libraries/binary/binary.cabal:                       Dependency of ``ghc`` library

+    libraries/bytestring/bytestring.cabal:               Dependency of ``ghc`` library

+    libraries/Cabal/Cabal/Cabal.cabal:                   Dependency of ``ghc-pkg`` utility

+    libraries/Cabal/Cabal-syntax/Cabal-syntax.cabal:     Dependency of ``ghc-pkg`` utility

+    libraries/containers/containers/containers.cabal:    Dependency of ``ghc`` library

+    libraries/deepseq/deepseq.cabal:                     Dependency of ``ghc`` library

+    libraries/directory/directory.cabal:                 Dependency of ``ghc`` library

+    libraries/exceptions/exceptions.cabal:               Dependency of ``ghc`` and ``haskeline`` library

+    libraries/file-io/file-io.cabal:                     Dependency of ``directory`` library

+    libraries/filepath/filepath.cabal:                   Dependency of ``ghc`` library

+    libraries/ghc-boot/ghc-boot.cabal:                   Internal compiler library

+    libraries/ghc-boot-th/ghc-boot-th.cabal:             Internal compiler library

+    libraries/ghc-compact/ghc-compact.cabal:             Core library

+    libraries/ghc-experimental/ghc-experimental.cabal:   Core library

+    libraries/ghc-heap/ghc-heap.cabal:                   GHC heap-walking library

+    libraries/ghci/ghci.cabal:                           The REPL interface

+    libraries/ghc-internal/ghc-internal.cabal:           Core library

+    libraries/ghc-platform/ghc-platform.cabal:           Internal library

+    libraries/ghc-prim/ghc-prim.cabal:                   Core library

+    libraries/haskeline/haskeline.cabal:                 Dependency of ``ghci`` executable

+    libraries/hpc/hpc.cabal:                             Dependency of ``hpc`` executable

+    libraries/integer-gmp/integer-gmp.cabal:             Core library

+    libraries/mtl/mtl.cabal:                             Dependency of ``Cabal`` library

+    libraries/os-string/os-string.cabal:                 Dependency of ``filepath`` library

+    libraries/parsec/parsec.cabal:                       Dependency of ``Cabal`` library

+    libraries/pretty/pretty.cabal:                       Dependency of ``ghc`` library

+    libraries/process/process.cabal:                     Dependency of ``ghc`` library

+    libraries/semaphore-compat/semaphore-compat.cabal:   Dependency of ``ghc`` library

+    libraries/stm/stm.cabal:                             Dependency of ``haskeline`` library

+    libraries/template-haskell/template-haskell.cabal:   Core library

+    libraries/terminfo/terminfo.cabal:                   Dependency of ``haskeline`` library

+    libraries/text/text.cabal:                           Dependency of ``Cabal`` library

+    libraries/time/time.cabal:                           Dependency of ``ghc`` library

+    libraries/transformers/transformers.cabal:           Dependency of ``ghc`` library

+    libraries/unix/unix.cabal:                           Dependency of ``ghc`` library

+    libraries/Win32/Win32.cabal:                         Dependency of ``ghc`` library

+    libraries/xhtml/xhtml.cabal:                         Dependency of ``haddock`` executable

+    utils/haddock/haddock-api/haddock-api.cabal:         Dependency of ``haddock`` executable

+    utils/haddock/haddock-library/haddock-library.cabal: Dependency of ``haddock`` executable

+   9.12.4-notes

+version:        4.21.2.0

+## 4.21.2.0 *March 2026*

+  * Expose `Backtraces` constructor and fields ([CLC proposal #199](https://github.com/haskell/core-libraries-committee/issues/199), [#26049](https://gitlab.haskell.org/ghc/ghc/-/issues/26049))

+  * Store `StackTrace` and `StackSnapshot` in `Backtraces`, deferring decoding until display

+  * Evaluate backtraces for "error" exceptions at the moment they are thrown ([CLC proposal #383](https://github.com/haskell/core-libraries-committee/issues/383), [#26751](https://gitlab.haskell.org/ghc/ghc/-/issues/26751))

+  data Backtraces = Backtraces {btrCostCentre :: GHC.Internal.Maybe.Maybe (GHC.Internal.Ptr.Ptr GHC.Internal.Stack.CCS.CostCentreStack), btrHasCallStack :: GHC.Internal.Maybe.Maybe GHC.Internal.Stack.Types.CallStack, btrExecutionStack :: GHC.Internal.Maybe.Maybe GHC.Internal.ExecutionStack.Internal.StackTrace, btrIpe :: GHC.Internal.Maybe.Maybe GHC.Internal.Stack.CloneStack.StackSnapshot}

+  data Backtraces = Backtraces {btrCostCentre :: GHC.Internal.Maybe.Maybe (GHC.Internal.Ptr.Ptr GHC.Internal.Stack.CCS.CostCentreStack), btrHasCallStack :: GHC.Internal.Maybe.Maybe GHC.Internal.Stack.Types.CallStack, btrExecutionStack :: GHC.Internal.Maybe.Maybe GHC.Internal.ExecutionStack.Internal.StackTrace, btrIpe :: GHC.Internal.Maybe.Maybe GHC.Internal.Stack.CloneStack.StackSnapshot}

+  data Backtraces = Backtraces {btrCostCentre :: GHC.Internal.Maybe.Maybe (GHC.Internal.Ptr.Ptr GHC.Internal.Stack.CCS.CostCentreStack), btrHasCallStack :: GHC.Internal.Maybe.Maybe GHC.Internal.Stack.Types.CallStack, btrExecutionStack :: GHC.Internal.Maybe.Maybe GHC.Internal.ExecutionStack.Internal.StackTrace, btrIpe :: GHC.Internal.Maybe.Maybe GHC.Internal.Stack.CloneStack.StackSnapshot}

+  collectBacktraces :: (?callStack::GHC.Internal.Stack.Types.CallStack) => GHC.Types.IO Backtraces

+  collectExceptionAnnotation :: GHC.Internal.Stack.Types.HasCallStack => GHC.Types.IO GHC.Internal.Exception.Context.SomeExceptionAnnotation

+  getBacktraceMechanismState :: BacktraceMechanism -> GHC.Types.IO GHC.Types.Bool

+  getCollectExceptionAnnotationMechanism :: GHC.Types.IO CollectExceptionAnnotationMechanism

+  setBacktraceMechanismState :: BacktraceMechanism -> GHC.Types.Bool -> GHC.Types.IO ()

+  setCollectExceptionAnnotation :: forall a. GHC.Internal.Exception.Context.ExceptionAnnotation a => (GHC.Internal.Stack.Types.HasCallStack => GHC.Types.IO a) -> GHC.Types.IO ()