2.4. Version 9.2.4 — Glasgow Haskell Compiler 9.2.7 User's Guide

# 2.4. Version 9.2.4

The significant changes to the various parts of the compiler are listed in the following sections.

The `LLVM backend` of this release is to be used with LLVM 9, 10, 11, or 12.

## 2.4.1. Compiler

- Add the `DeepSubsumption` language extension which reverses the effects of the Simplified Subsumption Proposal introduced in GHC 9.0. This is an attempt to make GHC 9.2.4 more backwards compatible with GHC 8.10 and eases migration for users who depended on this feature.

  This extension is enabled by default with the `Haskell2010` and `Haskell98` languages but disabled with the `GHC2021` language originally introduced in GHC 9.2.1.

  See the Deep Subsumption Proposal for more details.

- The flag `-ddump-llvm` doesn’t imply `-fllvm` any more (#21776).
- Fix a compiler panic due to a bug in the simplifier (#21694).
- Fix a bug where the flag `Werror=unrecognised-warning-flags` did not behave as expected (#21682).
- Fix miscompilations on AArch64 (#21624, #21773, #20735).
- Fix a miscompilation bug that manifests due to improper handling of name shadowing during common subexpression elimination (#21685).
- Fix a space leak that may manifest using the `extendMG` function (#21818).
- Ensure types from record dot syntax are stored in the HIE file AST (#21797).
- Fix a compiler panic when importing “wrapper” with `-XCApiFFI` (#20272).
- Enable `-Wunicode-bidirectional-format-characters` by default. This was meant to be enabled previously but was not due to a botched backport (#21865).

## 2.4.2. Runtime system

- Fix segfaults that may arise due to a bug in the implementation of the `keepAlive#` primop. This may regress performance for certain programs which use this primop or functions which use the primop, such as `withForeignPtr`. These regressions are mostly small, but can be larger in certain edge cases. Judicious use of `unsafeWithForeignPtr` when its argument is known not to statically diverge can mitigate these in many cases. It is our judgement that the critical correctness issues justify the regression in performance and that it is important to get a release out with the fix while we work on a better approach which will improve performance for future releases (#21708).
- Fix a segfault that may arise using LDV profiling (#21880).
- Fix a bug in the nonmoving GC which resulted in segfaults due to early GC (#21885).
- Fix accounting for copied bytes during sequential garbage collections (#21745).
- Allow passing``-po`` flag to set output during non-profiled builds (#21445).
- Respect the `-po` flag to set output while heap profiling (#21446).

## 2.4.3. GHCi

- Allow CApi FFI calls in GHCi (#7388).
- Fix behaviour of Ctrl-C on Windows in GHCi (#21889).

## 2.4.4. Core libraries

- Bump `base` to 4.16.3.0.
- Ensure `hGetBufNonBlocking` doesn’t block on Windows (#21665).

## 2.4.5. Build system and packaging

- Don’t override linker on Darwin during `configure` (#21712).
- Fix a hadrian bug to do with building profiled executables without corresponding libraries (#19624).
- Fix a panic on FreeBSD when building with hadrian due to incorrect arch triple (#15718).

## 2.4.6. 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.

Package | Version | Reason for inclusion
--- | --- | ---
ghc | 9.2.7 | The compiler itself
Cabal | 3.6.3.0 | Dependency of `ghc-pkg` utility
Win32 | 2.12.0.1 | Dependency of `ghc` library
array | 0.5.4.0 | Dependency of `ghc` library
base | 4.16.4.0 | Core library
binary | 0.8.9.0 | Dependency of `ghc` library
bytestring | 0.11.4.0 | Dependency of `ghc` library
containers | 0.6.5.1 | Dependency of `ghc` library
deepseq | 1.4.6.1 | Dependency of `ghc` library
directory | 1.3.6.2 | Dependency of `ghc` library
exceptions | 0.10.4 | Dependency of `ghc` and `haskeline` library
filepath | 1.4.2.2 | Dependency of `ghc` library
ghc-boot-th | 9.2.7 | Internal compiler library
ghc-boot | 9.2.7 | Internal compiler library
ghc-compact | 0.1.0.0 | Core library
ghc-heap | 9.2.7 | GHC heap-walking library
ghc-prim | 0.8.0 | Core library
ghci | 9.2.7 | The REPL interface
haskeline | 0.8.2 | Dependency of `ghci` executable
hpc | 0.6.1.0 | Dependency of `hpc` executable
integer-gmp | 1.1 | Core library
libiserv | 9.2.7 | Internal compiler library
mtl | 2.2.2 | Dependency of `Cabal` library
parsec | 3.1.15.0 | Dependency of `Cabal` library
pretty | 1.1.3.6 | Dependency of `ghc` library
process | 1.6.16.0 | Dependency of `ghc` library
stm | 2.5.0.2 | Dependency of `haskeline` library
template-haskell | 2.18.0.0 | Core library
terminfo | 0.4.1.5 | Dependency of `haskeline` library
text | 1.2.5.0 | Dependency of `Cabal` library
time | 1.11.1.1 | Dependency of `ghc` library
transformers | 0.5.6.2 | Dependency of `ghc` library
unix | 2.7.2.2 | Dependency of `ghc` library
xhtml | 3000.2.2.1 | Dependency of `haddock` executable