GitLab

Duncan Coutts pushed to branch wip/dcoutts/signals-cleanup at Glasgow Haskell Compiler / GHC

Commits:

234bab08

by Vladislav Zavialov at 2026-09-29T17:22:54-04:00

Add regression test for #7803

Commit 2effe18ab5, first shipped with GHC 8.2.1, fixed #7803 "Superclass
methods are left unspecialized".

This change turns the example from the ticket description into a regression
test, checking that foo1 is fully specialised at Double, with no calls to the
overloaded exp5Tail.

Closes #7803

Assisted-by: Claude Opus 5

c406135c

by Andreas Klebinger at 2026-10-01T06:57:28-04:00

RTS: Update flag parsing/handling:

Using 0 as argument to any of these flags now resets the flags setting to default:

    - ``-n0`` disables nursery chunking.
    - ``-K0`` (or a bare ``-K``) removes the stack size limit.
    - ``-M0`` removes the heap size limit.
    - ``-H0`` resets the suggested heap size, including the nursery scaling set by ``-H``.
    - ``-AL0`` makes the large object allocation limit follow ``-A`` again.

GC Generations are now limited to 64.

-n larger than -A now emits a warning and disables chunking.

Fixes #27830 #27829

984ee82b

by Andreas Klebinger at 2026-10-01T06:58:10-04:00

Fix MultiLayerModules timeout multiplier.

We accidentally used the compile multiplier for ghci commands which
use the run timeout setting to set the timeout.

Fixes #26263 and #27796.

50a9cf26
by Andreas Klebinger at 2026-10-01T06:58:10-04:00
```
testsuite: Include runtime in JUnit report.
```

e358619f

by Alan Zimmerman at 2026-10-01T06:59:01-04:00

EPA: Use EpaLocation in EpVirtualBraces

Haddock processing uses EpLayout.EpVirtualBraces to record the current
layout column, for allocating haddock comments correctly.

For exact printing, we need the line as well as the col, so we know
how many lines to skip when the indented code block starts.

This commit updates EpLayout.EpVirtualBraces so that it carries an
EpaLocation instead of just the Int column, to allow it to be used for
both purposes.

It is a simple mechanical change, with no other impact, but it allows
us to use EpLayout instead of AnnListLayout in AnnList, in the next
commit in this series.

1d2a26ee

by Andrew Lelechenko at 2026-10-01T20:22:04-04:00

Remove bitSize from MINIMAL pragma of class Bits

Cf. https://github.com/haskell/core-libraries-committee/issues/441

The function is deprecated for 12 years and has a default implementation,
yet it was never removed from the MINIMAL pragma, putting library authors
in a very awkward position. Let's finally remove bitSize from MINIMAL.

3cc21e4c
by Hannes Siebenhandl at 2026-10-01T20:22:59-04:00
```
Add regression test for #27641
```

c35096f5

by Hannes Siebenhandl at 2026-10-01T20:23:00-04:00

Don't `:add` modules the loader will reject

Only home modules can be actually added to the loader context, thus we
only accept the module if it belongs to a home unit.

a563cb27

by Simon Jakobi at 2026-10-02T17:37:36-04:00

ci: accept modified changelog.d entries in lint-changelog

The changelog guide says to extend an existing entry rather than add a
new one when the feature is already documented. lint_changelog only
counted added files, so such MRs failed before the validator ran.

Fixes #27779.

Assisted-by: Claude Fable 5.1

03082cd4

by Simon Jakobi at 2026-10-02T17:38:17-04:00

changelog.d: Fix warning flag name in the semaphore-v2 entry

The entry refers to -Wsemaphore-version-mismatch, which does not exist.
The warning GHC emits when it cannot use a -jsem semaphore, including
on a protocol version mismatch, is -Wsemaphore-open-failure.

Context: !15729, #25087

Assisted-by: Claude Fable 5.1

a93705ee

by Simon Jakobi at 2026-10-02T17:38:17-04:00

changelog.d: Fix the CLC proposal link in the T27455 entry

The entry used the Sphinx :ref: role with a URL as its target. :ref:
resolves only internal labels, so the generated release notes would
show the proposal name without a link. Use the plain hyperlink form
like the other entries.

Context: !16274, #27455

Assisted-by: Claude Fable 5.1

54381079

by Hannes Siebenhandl at 2026-10-02T17:39:01-04:00

Extend the `TrustOverlay` for wired-in units

The TrustOverlay only contained the `UnitId` of all on-disk units.
However, wired-in units are referred to by their wired-in unit id, which
is not the same as the on-disk unit id.

Thus, trust checks for wired-in units failed because the lookup was
using the wired-in unit, which wasn't present in the `TrustOverlay`.

We fix #27834, by copying the trust info for wired-in unit ids.

f8cfb1a5

by Hécate Kleidukos at 2026-10-04T19:36:39+02:00

[CLC 341] Strict variants of mutable types

This commit is the implementation of CLC Proposal 341
https://github.com/haskell/core-libraries-committee/issues/341

575c4bc6

by Rodrigo Mesquita at 2026-10-05T12:32:02-04:00

ttg: HsModuleName in the AST, ModuleName in GHC

This commit makes the AST fully independent from GHC by parsing a
"module name" as an `HsModuleName (= HText)` rather than constructing an
`ModuleName (= FastString)` straight away.

In the GHC side, everything remains as a `ModuleName`. We turn AST
`HsModuleName`s into fast strings with the cheap `mkFastStringShortText`
when we process the AST for manipulation within the compiler.

There are no performance regressions introduced by this approach.

This is the final piece of #21592

Alternative plans were discussed in
gitlab.haskell.org/ghc/ghc/-/merge_requests/16546#note_694647,
but the Simon suggested one won in simplicity and performance when
compared against my attempt at implementing my original plan of having
a family `ModuleNameP`.

b3b89d32

by mangoiv at 2026-10-05T22:03:32-04:00

hadrian: Persist the used flavour name in the build root (#20650)

This executes the idea proposed in #20650, teaching Hadrian to persist
the flavour used to build a build root. This both makes use of multiple
build roots more ergonomic (as the user no longer needs to repeat
themselves) and avoids a tiresome foot-gun (forgetting to specify a
flavour, often wholly invalidating a build root).

Closes #20650.

Co-authored-by: Sylvain Henry <sylvain@haskus.fr>

f977837b

by mangoiv at 2026-10-05T22:04:21-04:00

configure: fix sed script and subst tooldir script

Fixes #27672

Adds sanity checking for SUBST_TOOLDIR

Metric Decrease:
    LinkableUsage01
    MultiLayerModulesTH_OneShot
    T10547
    T12234
    T12425
    T13035
    T13820
    T18140
    T18730
    T18923
    T9198
    TcPlugin_RewritePerf
    mhu-perf

f1cb08be

by Hannes Siebenhandl at 2026-10-05T22:05:03-04:00

Prepend in-memory unit database for backpack

The packageDbFlags are stored in reverse order. Thus, prepending will
ensure that the in-memory unit database is the last item on the unit
database stack.

Further, the in-memory unit database needs to be able to shadow units
from earlier unit databases in the unit database stack.

Moreover, a `-clear-package-db` would remove the in-memory database if
it weren't the last item on the stack.

Closes #27835

8d49ed42

by Kazu Yamamoto at 2026-10-06T19:37:25-04:00

Do not leave standard file descriptors free at startup (#24450)

A process may be started with stdout closed -- `prog >&-` from a shell does
exactly that.  Nothing then reserves descriptor 1, and open(), pipe() and
socket() hand out the lowest free descriptor, so the next descriptor the
runtime creates for itself takes it: the ticker's pipe, or a capability's IO
manager control pipe.

'GHC.Internal.IO.StdHandles.stdout' is a CAF, built the first time the program
writes to stdout, which is long after that has happened.  It builds a 'Handle'
over descriptor 1, which now belongs to the runtime, and nothing reports an
error because the descriptor is perfectly good.  The program then writes its
output into the runtime's own plumbing.

What that looks like from the outside is a hang.  The descriptor the Handle has
taken over is the read end of a pipe, so poll() for POLLOUT on it never says
yes and epoll_ctl() accepts a registration for it; the thread waiting to write
blocks for ever.  Closing the Handle at exit closes the runtime's descriptor,
which is worse: the ticker then fails on every tick.

'hs_init_ghc' now opens /dev/null onto whichever of descriptors 0, 1 and 2 are
closed, before the runtime creates any descriptor of its own.  Each is opened
for the access that descriptor would normally have, read-only for 0 and
write-only for 1 and 2, so that writing to 'stdin' or reading from 'stdout'
still fails rather than quietly succeeding against /dev/null.  See
Note [Standard file descriptors must be open].

Moving only the descriptors the runtime opens would not do: the first file the
program itself opened would land on the free number instead.  The fix has to
leave no free number.  A program run this way therefore finds stdout connected
to /dev/null rather than failing with EBADF on the first write, which is what
such a program would get from most other language runtimes.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

65bcf9e9

by Simon Jakobi at 2026-10-06T19:38:09-04:00

changelog: Repair the sentence describing #27477

The description read "programs using could crash", the object of
"using" having been lost. The synopsis already says which programs are
affected, so drop the dangling word.

Assisted-by: Claude Fable 5.1

15ece296

by mangoiv at 2026-10-08T15:14:56-04:00

ci: move hadrian/ghci into the lint category so it can be picked up by more runners

Fixes #27817

c0823135

by mangoiv at 2026-10-08T15:14:56-04:00

ci: make lint-adjacent jobs into platform independent

- move lint jobs into lint-anyarch tag
- unify lint jobs under the same platform independent container

f76464cf

by Zubin Duggal at 2026-10-08T15:15:39-04:00

DmdAnal: Make AbsDmd neutral in maxDmd

fixes #27845

ffa56ae4

by sheaf at 2026-10-09T17:15:59+02:00

Preserve sign of zero when parsing -0.0#

This commit leverages the recently added GHC.Types.Literal.Floating
module in order to preserve the sign of negative zero from
source-written floating-point literals.

Fixes #27545

d538cac9

by Facundo Domínguez at 2026-10-09T20:26:38-04:00

Propagate annotations to simple interfaces

This allows annotations to be visible to plugins when run by
haddock. The LiquidHaskell plugin needs the annotations, in particular.

Fixes #26761

32c4f4ab

by meooow25 at 2026-10-09T20:27:28-04:00

Bump containers submodule to 0.8.1

And fix affected tests.

* T4086a: containers depends on template-haskell-lift now instead of
  template-haskell.
* T22611: Updated to be less dependent on containers internals.

c786065e

by sheaf at 2026-10-09T20:28:20-04:00

Use an explicit pattern match in toIfaceIdDetails

The old wildcard pattern let this code bit rot: it did not properly account
for recently added 'IdDetails' such as 'RepPolyId', which resulted in
spurious 'pprTrace' messages that could even pollute HLS sessions.

Fixes #23785

e7cacf59

by sheaf at 2026-10-09T20:29:09-04:00

Take into account ifKind in freeNamesIfDecl

Commit 3c2f4bb4 added the 'ifKind' field to various constructors
of 'IfaceDecl', which preserves the original kind (before type synonym
expansion) for better error messages.

However, that commit forgot to update 'freeNamesIfDecl' correspondingly.
This could result in a "urk! lookup local fingerpring" crash when e.g.
a type's kind uses a type synonym defined later, as we failed to add a
dependency on the type synonym during dependency analysis.

Fixes #27916

41e80f41

by Simon Peyton Jones at 2026-10-11T14:38:51-04:00

Simplify the error-suppression mechanism in GHC.Tc.Errors

In `GHC.Tc.Errors.reportWanteds` we suppress some errors in favour of
others.  But the mechanism had grown crufty, and #27331 exposed a bug.

This MR makes it simpler and more uniform, by doing everything via
the `ei_suppress` field of the ErrorItem.

Claude then found another closely-related bug (see the review linked
on !16564).  That bug is immortalised in new test T18851d.

I'm still not happy with `ignoreConstraint` but we can worry about
that another day; I have not touched it.

Main changes

* Instead of /predicting/ when an error will be reported, we set
  `cec_error_reported` when an error is /actually/ reported.  (I renamed
  it from `cec_suppress` to reflect this change.)

  Now errors are suppressed only when a preceding error has actually been
  reported.  Simpler and more robust.

  See `Note [cec_error_reported: suppressing less-serious errors]`

  The big changes are in `reportWanteds` which looks very different.

* New `Note [ei_suppress: suppressing confusing errors]` describes the
  (quite separate) `ei_suppress` mechanism for suppressing confusing
  errors, mainly arising from non-empty rewriter sets

* In `tryTypeClassDefaulting`, instead of switching off type-class defaulting
  if there is an insoluble /anywhere/, we just switch it off locally when
  we find an insoluble sub-tree. See Note [When to do type-class defaulting]
  and (W5) in Note [ApproximateWC]

* Greatly simplify `insolubleWantedCt`.

* I made type-class-defaulting be a bit more eager, only switched off when
  a peer or ancestor (but not child) constraint is insoluble.  See
  Note [When to do type-class defaulting].

Small refactorings

* Rename `askNoErrs` to `askErrsFound` and flip its polarity

* The code for "check if ei_suppress is true of all items" code is
  moved to `mkErrorItems` rather than in the large `reportWanteds`.

* Code for `has_gadt_match` moves to `mkGivenErrorReporter`, rather than
  polluting the large `reportWanteds`.

* Many tests with `-fdefer-type-errors` get more warnings, especially
  about ambiguous (Num a) constraints. This is by-design: see
  Note [Always warn with -fdefer-type-errors].  Previously GHC was not
  honouring this Note but now it is, hence the extra errors.

f217fc31

by Cheng Shao at 2026-10-11T14:39:33-04:00

compiler: ensure cross ghc --info output target unit ids

Previously, for cross ghcs, the base/ghc/ghc-internal unit ids in
--info output do not match the target unit ids reported by ghc-pkg,
breaking cross ghc release builds with hash-unit-ids when being used
with cabal 3.18. This patch fixes it by making --info output target
unit ids. Fixes #27919.

Co-authored-by: Codex <codex@openai.com>

d07a5176

by Simon Peyton Jones at 2026-10-13T18:10:39-04:00

Remove the Eq and Ord instances for ForAllTyFlag

In Note [Comparing visibility] in GHC.Core.TyCo.Compare we read that
"we always compare ForAllTyFlags with `eqForAllVis` or `cmpForAllVis`"
But alas there was an Eq and Ord instance for ForAllTyFlag, which made it
impossible to check this claim.   So I made these changes:

* Move the definition of ForAllTyFlag and its related functions from
  L.H.S.Specificity (where it did not belong, since L.H.S didn't use it)
  into GHC.Types.Var.

* Remove the Eq and Ord instances for ForAllTyFlag

* Deal with the fallout, by using `eqForAllVis` or related functions.

* In interface files we use IfaceType as a key for a de-deduplication map
  See Note [Deduplication during iface binary serialisation]
  in GHC.Iface.Binary.  For this we need /exact/ comparison, so we use a
  newtype wrapper for IfaceForAllTyFlag, and give use `cmpForAllTyFlagExactly`
  to implement its Ord instance.  Tiresome but easy.

* Rename `toIfaceForAllBndrs` to `toIfaceForAllTyBndrs`

ea5c054d

by Simon Peyton Jones at 2026-10-13T18:10:39-04:00

Refactor type and coercion substitution

This MR refactors type and coercion substitution so that they use
TyCoMapper, rather than hand-rolled code.  Fundamentally it's a simple
idea, but I had to do quite a bit of refactoring to make it fly.

I got Claude to help me this this refactoring, but I paid careful
attention to every line of code.

Main changes:

* New module GHC.Core.TyCo.Make which defines TyCoMapper, and a bunch of other
  simple "smart constructors" for types and coercions.  See
  GHC.Core.TyCo.Rep Note [The Type-related module hierarchy], esp (TRM1)

* The new TyCoMapper has new fields `tcm_tcapp_ty` and `tcm_tcapp_co` to handle
  type-constructor applications.  These replace `tcm_tycon`.

* Rewrite GHC.Core.TyCo.Subst to use TyCoMapper

* Rewrite GHC.Core.Type.expandTypeSynonyms to use TyCoMapper.

* Implement `seqType` and `seqCo` with TyCoFolder. To to this I had to add
  a `tcf_lit` field to TyCoFolder.

* A new module GHC.Utils.StrictIdentity to define a strict identity monad,
  which itself is used for
      substTyCoMapper :: TyCoMapper Subst StrictIdentity

* I removed an apparently-unused instance for Binary Levity

Refactoring `seq` functions

* In GHC.Core.Seq, redefine `seqOneShot`, `seqRuleInfo`, and friends to return
  `SeqResult`, a new type defined in GHC.Types.Baseic, rather than `()`.  We
  can define a Monoid instance for `SeqResult` that does the right thing.

* This has knock-on effects across the compiler.

34165e23

by Simon Peyton Jones at 2026-10-13T18:10:39-04:00

Refactor TyCoMapper to take a single monad; add StrictReaderT

TyCoMapper used to be parameterised over both an environment and a
monad, with fields like
    tcm_tyvar :: env -> TyVar -> m Type
Now it is parameterised over just the monad:
    tcm_tyvar :: TyVar -> m Type
If the mapping needs an environment, `m` is a reader monad over that
environment.  Many uses (e.g. zonkTcTypeMapper, swizzleMapper) need no
environment at all, and used to pass a dummy ().  The binder field
becomes
    tcm_tycobinder :: forall r. TyCoVar -> ForAllTyFlag
                   -> (TyCoVar -> m r) -> m r
and runs the continuation in the extended environment.  mapTyCoX is gone;
only mapTyCo remains.

To make this efficient, the reader monad must use the one-shot trick and
be strict in the environment.  So:

* New module GHC.Utils.Monad.StrictReader defines
      newtype StrictReaderT r m a
  with hand-written instances.  The builder of the pattern synonym
  `StrictReaderT` is a smart constructor that adds both `oneShot` and a
  bang on the environment, so every StrictReaderT computation is strict
  in the environment.  See Note [Instances for StrictReaderT].

* ZonkT becomes  type ZonkT = StrictReaderT ZonkEnv
  ZonkM becomes  type ZonkM = StrictReaderT ZonkGblEnv IO
  replacing their own hand-written instances.

* Substitution uses
      type SubstM = StrictReaderT Subst StrictIdentity
      substTyCoMapper :: TyCoMapper SubstM
  and similarly expandTypeSynonymMapper.

* zonk_tycomapper :: TyCoMapper (ZonkT TcM) is now much simpler.

* swizzleMapper uses StrictIdentity rather than Identity.

* Module renaming, so that the strict monads are all peers in GHC.Utils.Monad:
      GHC.Utils.StrictIdentity      ==>  GHC.Utils.Monad.StrictIdentity
      GHC.Utils.Monad.State.Strict  ==>  GHC.Utils.Monad.StrictState
  alongside the new GHC.Utils.Monad.StrictReader.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

c0299e15

by Simon Jakobi at 2026-10-13T18:11:23-04:00

testsuite: Line-buffer the driver's stdout

Python block-buffers stdout when it is a pipe. This is the case for
example in CI. As a result there is often no indication of progress at
all until the run ends.

To fix this, we configure it explicitly to use line buffering instead.

stderr is reconfigured too, for consistency: Python line-buffers it by
default since 3.9.

The type-ignores are for mypy, which types these handles as TextIO — an
interface without reconfigure.

Fixes #12934.

Assisted-by: Claude Fable 5

9b5beaa2

by Florian Ragwitz at 2026-10-13T18:12:05-04:00

hpc: Improve coverage semantics for instance methods and instances

Users care about which of their explicitly written methods are or aren't
covered, not whether all of the inherited methods are covered as well. Those
should be covered where they are implemented.

We therefore skip generating top-level boxes for inherited methods which will
appear as uncovered with no clear indication as to why, and no way to fix other
than exercising code the user has not written.

Previously, top-level boxes for those inherited methods would use the instance
head as their source span. Now that those boxes are gone, we can use that
source span for another purpose:

We introduce a new top-level box for each class instance with its instance head
as the source span, and use that to indicate whether or not an instance has
been covered at all.

We do this by making any method of the instance, inherited/generated or
otherwise, tick the box for the instance head. Excluded from this are
methodless classes, which could never be covered using the current mechanism.
This could be improved upon further.

These changes apply to ticking for program coverage only, and don't affect
source notes. Additionally, we update the scope of record selector ticking to
be limited to program coverage as well, as the oversight to also extend it to
source notes has previously caused bugs and because record selector ticking is
coverage-specific.

Fixes #17155, #15932.

836ec50a

by Simon Peyton Jones at 2026-10-13T18:12:54-04:00

Refactor fundeps again

This MR fixes #27828; there was a bad scoping error in `trim_qtvs`.

The main changes are

* Refactor `trim_qtvs` completely to eliminate the bug.

* Remove a redundant `apartnessCheck` from `mkTopOpenFamEqFDs`.

* Lots of improvements to the Notes in FunDeps.hs

I also did a couple of pure refactors:

* I realised that we were carefully caching `cab_incomps` in each
  `CoAxBranch`, to make it more efficient to reduce a type family application,
  but we were not doing the same thing when computing relevance for functional
  dependencies; see Note [Relevance] in GHC.Tc.Solver.FunDeps.  So I added
  `cab_overlaps`, which is now used in `noEarlierMatch`.

* Use a proper data types, `AmIUnifying` and `OuterKindsFlag`, for two
  previously-Boolean arguments to `tc_unify_tys`.  This is a pure refactor.

889ef371

by Simon Peyton Jones at 2026-10-13T18:13:36-04:00

Refactor and simplify solver's unification code

Ticket #27844 showed that GHC's constraint solver could miss an
occurs check, and thereby build an infinite type.

This patch significantly simplifes the code in
GHC.Tc.Solver.Equality.canEqCanLHSFinish_try_unification
GHC.Tc.Solver.Equality.canEqCanLHSFinish_no_unification

It is now significantly simpler, and requires much less subtle
reasoning to understand.

Main changes

* If `canEqCanLHSFinish_try_unification` fails to unify, it now /always/ calls
`canEqCanLHSFinish_no_unification` rather than trying to finish the job itself.

* `GHC.Tc.Solver.Monad.checkTypeEq` is moved from Monad into Equality, and renamed
to `GHC.Tc.Solver.Equality.checkTypeEqNoUnification`.

* `canEqCanLHSFinish_no_unification` calls `checkTypeEqNoUnification` to test
the invariants for an inert equality. Before this MR there was a special case for
`do_not_prevent_rewriting`, but that has gone; instead it's just a consequence of
the flags passed to `checkTyEqRhs`, documented in the `PuOK` case of
`canEqCanLHSFinish_no_unification`.

* There is only one bit of "swap back" code, in `finish_no_unify` in
`canEqCanLHSFinish_try_unification. Previously this code was duplicated.

* `kickOutAfterFillingCoercionHole` should account for equalities with a
TyFamLHS. See (KOC5) in Note [Kick out after filling a coercion hole]

* No need to gather coercion holes in the `PuOK` case of `canEqCanLHSFinish_no_unification`.
See documentation with that code.

f9193e53

by Simon Peyton Jones at 2026-10-13T18:13:37-04:00

One line fix to fallout from

...#27731, !16564, simplifying Tc.Errors machinery

There was an over-eager assert!

cdd56b7a

by Florian Ragwitz at 2026-10-13T18:14:19-04:00

Fix several sphinx errors and warnings from rendering the release notes

The extension references which are removed here are for extension which were
already previously undocumented, so just removing them seems fine for now.

4f00da25
by Florian Ragwitz at 2026-10-13T18:14:19-04:00
```
Link to QualifiedStrings documentation
```

97adaf89

by Simon Peyton Jones at 2026-10-14T00:07:27+01:00

Line up simpleUnifyCheck and check_ty_eq_rhs

Even after #26543 was allegedly fixed, the original repo case in the
Description continued to fail.  The reason was that the QuickLook
unifier, `qlUnify`, used `simpleUnifyCheck` for checking unification
invariants; and `simpleUnifyCheck` conservatively rejected any RHS
with a coercion hole in it.  The upshot was that QuickLook was not
as clever as it should be, wrongly failing to do an impredicative
instantiation.

Interestingly `check_ty_eq_rhs`, which does the same job during
constraint solving, was more liberal: it just looked at the free vars
of the coercion, and allowed coercion holes.

This MR lines them up, adding some careful notes.  See
    Note [simpleUnifyCheck]
    Note [Occurs check and coercions]
    Note [Unification preconditions] esp (COERCIONS)

Things are better than before, but I am still uncomfortable about the
possibilty that a hole might be filled with a coercion that mentions
the LHS type variable; for now I have left this discomfort documented
in Note [Occurs check and coercions].

Main changes

* `qlUnify` now only unifies /instantiation variables/ not regular unification
  variables.  This makes QuickLook more predictable.  See (UQL6) in
  Note [QuickLook unification].

* `qlUnify` no longer needs to call `simpleUnifyCheck`, so we can get rid of
  `UC_QuickLook` and its code in `simpleUnifyCheck`.

* I found I could get rid of `mkOccFolders` in favour of `anyFreeVarsOfType`.
  Hooray.

* Improve documentation of CycleBreakerTv, and rename it to GivenCycleBreakerTv
  because they are only uesd for Givens!

* I accepted a 9% worsening in compile time allocation on T5030
  because the newly-consistent story triggered #27934.

Other changes

* I refactored `atvFolder` so that it pairs the "interesting-var" function
  with the in-scope set, to avoid building a closure with a free variable.
  Now we get nice tight loops for `anyFreeVarsOfType` and friends.
  See Note [Any-free-var folder].

Metric Increase:
    T5030

5f7b3047

by Simon Peyton Jones at 2026-10-14T00:07:27+01:00

Fix order-dependence in QuickLook

Ticket #27917 pointed out that QuickLook was order-dependent
-- very bad!

The solution is in `qlUnify`: spot when we are unifying a type with a
definite monotype, and then monomorphise any free instantiation
variables of that type.

See (UQL8) and (UQL9) in Note [QuickLook unification].

c391c70c

by Simon Peyton Jones at 2026-10-14T00:07:28+01:00

Add some comments about RuntimeUnk

This commit just adds comments explaining why we don't need to worry about
RuntimeUnkTv in the constraint solver.

It also tightens up docmentation of earlier (overlapping) commits in
this MR.

7df3ba2f

by Andrea Vezzosi at 2026-10-14T06:15:48-04:00

add runGhcWithSignalHandlers and remove handling from runGhc[T]

it only ever makes sense to call `withSignalHandlers` from `main`, otherwise
signals can be too easily swallowed without notification to the rest of your
process.

This commit takes withSignalHandlers out of `runGhc/runGhcT` and provides
`runGhcWithSignalHandlers` for the old behaviour.

3a72b8a2

by Hannes Siebenhandl at 2026-10-14T13:48:32-04:00

Fix #27836, normalise unit database path in ExternalUnitDatabases

Add a regression test, proofing that all variations:

- local-unit-db/my-test.conf
- ./local-unit-db/my-test.conf
- ./local-unit-db/my-test.conf/

are parsed only once.

8ddbc6f9
by Florian Ragwitz at 2026-10-14T13:49:28-04:00
```
Remove obsolete HPC types

Fixes #27946.
```

f34c79ee

by Alan Zimmerman at 2026-10-15T04:54:47-04:00

EPA: Start using vocurly as basis for AnnListLayout

For the existing cases that populate AnnList.

This uses AnnListBraces when braces are explicitly supplied, and
AnnListLayout when vocurly is used

The change also shows up AnnList usages that can never capture layout,
these are cleared up later.

033ba59c

by sheaf at 2026-10-15T04:55:44-04:00

Make exprFreeVars deep

exprFreeVars used to be a shallow free-variable finder (it didn't look
into kinds), but most callers wanted it to be deep. In particular,
'exprFreeVars' is often used to seed an 'InScopeSet', and that must
contain the /deep/ free variables.

This commit makes it deep, which allows us to get rid of calls to
'closeOverKinds' or other workarounds for shallowness in:

  - 'GHC.Core.Opt.Simplify.Utils.abstractFloats'
  - 'GHC.HsToCore.Binds.dsSpec_help'
  - 'GHC.Core.Opt.Arity.etaBodyForJoinPoint'
  - 'GHC.Core.FVs.freeVars'
  - 'GHC.Core.Lint.lintCoreRule'

We also remove 'deepExprsFreeVarsDSet' and 'closeOverKindsDSet'.

Other places needed a little bit more care, in particular to properly
handle coercion variables free in kinds:

  - 'GHC.Core.Opt.Specialise.specHeader' quantifies over the binder of
    an unspecialised argument directly, instead of computing the free Ids
    of the argument.
  - 'GHC.HsToCore.dsWarnRuleShadowing' filters out 'CoVar's.
  - 'GHC.Core.Opt.SpecConstr.callToPat' had some ad-hoc logic to compute
    argument pattern types and a workaround with 'expandTypeSynonyms'.
    That logic is removed in favour of a new 'argsToPats' function which
    directly computes the argument pattern types by instantiating the
    type of the head of the call pattern.

Fixes #27936

f93d523c

by Luite Stegeman at 2026-10-15T04:56:34-04:00

testsuite: Use mkWeakMVar in memo001/memo002

mkWeakPtr in the memo001/memo001 tests created weak pointers
with the MVar box as the key, which is fragile in the presence
of optimization. The fix is using mkWeakMVar, keying on the
primitive MVar# instead.

Fixes #27924 and #27396

8c947573

by Simon Peyton Jones at 2026-10-16T13:11:53+01:00

Improve exprOkForSpeculation

Fixes #27910, which exposed an infelicity in exprOkForSpeculation

The new story is carefully described in new Note [exprOkForSpeculation],
esp (a-c) and (EOKS1).

7dedcf13

by Alan Zimmerman at 2026-10-16T19:03:17-04:00

EPA: Add TTG fields to FamilyInfo

This commit is a simple add, with unused extension fields.

It lays the groundwork for using them for exact print annotations

26b9a8a0

by Arkissa at 2026-10-17T16:04:10+00:00

ghci: Prefer exact source spans in :type-at

Prefer expression types for identical source spans.

Fixes #27412.

70dc4e61

by sheaf at 2026-10-18T18:15:15-04:00

Carefully implement coercion zapping

This commit implements coercion zapping in the rewriter, controlled by
the -fzap-coercions flag (on by default). This significantly reduces
the size of coercions produced when rewriting type family applications,
resulting in much faster compilation. See Note [Coercion zapping]
in GHC.Tc.Solver.Rewrite for an explanation and further details.

The rewriter now keeps track of the `CoVar`s and `CoercionHole`s it uses
when rewriting, which allows computing the dependencies of a zapped
coercion without traversing the coercion the rewriter produced, as
explained in Note [Computing coercion dependencies] in GHC.Tc.Solver.Rewrite.

Accompanying changes:

  - Rework how dependencies of 'UnivCo's are tracked, purely for
    performance reasons.

    We now use the 'UnivCoDeps' datatype instead of '[Coercion]', and
    we now insist that 'uco_deps' also contains the CoVars that appear
    in the LHS and RHS types of the UnivCo (as a caching mechanism).

  - Several performance improvements to the rewriter (e.g. increased
    strictness).

  - Improvements to the family application cache, including:
    - Better documentation.
    - Only cache a whole chain of reduction steps.
    - Don't put coercion holes in the cache, fixing #27518.

  - Allow Core Lint to be called from within the typechecker (and from
    within the coercion optimiser).

  - Move out "annotation linting" from Core Lint into the general Core
    pipeline, to avoid the dependency GHC.Core.Coercion.Opt->GHC.Core.Lint
    causing a regression in CountDepsParser due to transitive
    dependencies of GHC.Core.Opt.Monad and GHC.Unit.Module.ModGuts.

Fixes #8095 #27336 #27518

Summary of performance numbers

  Compile-time byte allocated in GHC's testsuite (0 regressions)

          CoOpt_Singletons(normal) ghc/alloc    634,040,332    595,448,728  -6.1% GOOD
           FamAppCachePerf(normal) ghc/alloc    495,629,812    191,612,920 -61.3% GOOD
               LargeRecord(normal) ghc/alloc  1,226,806,434    966,260,976 -21.2% GOOD
             SimplCastPerf(normal) ghc/alloc    160,505,678    148,112,688  -7.7% GOOD
                    T12227(normal) ghc/alloc    244,668,982    233,037,216  -4.8% GOOD
                    T12545(normal) ghc/alloc    697,804,842    424,705,152 -39.1% GOOD
                    T13386(normal) ghc/alloc    422,871,076     35,213,656 -91.7% GOOD
                    T15703(normal) ghc/alloc    288,402,364    273,692,560  -5.1% GOOD
                    T26426(normal) ghc/alloc     86,002,052     78,152,104  -9.1% GOOD
                    T27336(normal) ghc/alloc    498,932,268     35,886,016 -92.8% GOOD
                     T5030(normal) ghc/alloc    117,057,706     88,785,648 -24.2% GOOD
                  T5321Fun(normal) ghc/alloc    252,552,628    243,820,176  -3.5% GOOD
                     T8095(normal) ghc/alloc    768,198,564    127,779,520 -83.4% GOOD
                    T9872a(normal) ghc/alloc  1,655,675,020  1,406,017,008 -15.1% GOOD
                    T9872b(normal) ghc/alloc  1,864,180,628  1,644,706,696 -11.8% GOOD
              T9872b_defer(normal) ghc/alloc  2,834,585,468  1,657,729,760 -41.5% GOOD
                    T9872c(normal) ghc/alloc  1,572,573,116  1,354,055,424 -13.9% GOOD
                    T9872d(normal) ghc/alloc    351,516,004    331,071,424  -5.8% GOOD
      TcPlugin_RewritePerf(normal) ghc/alloc  2,063,065,680  1,847,464,272 -10.5% GOOD

  Chesskell (github.com/Proxymoron461/chesskell/issues)
    Typechecking that the first N plies of kholmovVsBronsteinShort
    are a valid chess game:

      Plies | GHC 9.14 time/alloc | This commit time/alloc |
      -----:|---------------------|------------------------|
      2     | 41s/20GB            | 0.2s/19MB              |
      4     | 95s/49GB            | 0.3s/25MB              |
      6     | timed out           | 0.6s/522MB             |
      8     | timed out           | 7s/673MB               |
      10    | timed out           | timed out              |

  Compilation of singletons-base: compile time -28%, allocations -25%

-------------------------
Metric Decrease:
    CoOpt_Singletons
    FamAppCachePerf
    LargeRecord
    SimplCastPerf
    T12227
    T12545
    T13386
    T15703
    T24984
    T26426
    T27336
    T5030
    T5321Fun
    T8095
    T9872a
    T9872b
    T9872b_defer
    T9872c
    T9872d
    TcPlugin_RewritePerf
-------------------------

Co-Authored-By: Sylvain Henry <sylvain@haskus.fr>

3d326d7d

by Sylvain Henry at 2026-10-18T18:16:09-04:00

JS: fix stack corruption in h$ap_gen_fast's pushAllRegs (#27624)

The loop added in 9e46990976 ("JS: support more than 128 registers
(#26558)") pushed register N to stack[sp - N - 2] while the switch it
falls through into, and every reader of the h$ap_gen frame, uses
stack[sp - (N - 2)]. Registers >= R32 were therefore written 4 slots
too low: the slots the readers consult were left stale and the frame
below was silently corrupted.

Fix the loop to use the same slot formula as the switch, and add a
regression test: an unknown call to a thunk with 35 argument register
slots, which goes through the generic apply.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

6bbca569

by Andreas Klebinger at 2026-10-21T10:01:10+00:00

Fix a few tag inference bugs.

* Treat bottoming functions and nullary join points as functions.
* When generating bytecode don't rely on imported tag sigs.

Fixes #27831, #27841 and #27842.

71808e29

by Sylvain Henry at 2026-10-22T04:50:24-04:00

Add GHC.Internal.Unboxed.Do: do-notation for unboxed values (#27944)

QualifiedDo operators can be representation-polymorphic: a do-block is
expanded into ordinary applications before typechecking, and the only
check on a bound variable is the deferred fixed-runtime-rep check. A
representation-polymorphic (>>=) cannot be defined directly, though,
because its body would bind a variable of unknown representation.

The new module provides such operators for State#-passing code as
methods of a class indexed by RuntimeRep, with one instance per
representation. With optimisation the dictionaries disappear and a
block compiles to the usual nested case expressions:

  Ubx.do { x <- readWordArray# mba 0#     -- x :: Word#
         ; writeWordArray# mba 1# x        -- unit actions are statements
         ; Ubx.pure x }

Also add users guide documentation and tests.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

33247780

by Sylvain Henry at 2026-10-22T04:50:24-04:00

Bignum: use Ubx.do for State#-passing code (#27944)

Rewrite the State#-passing code of the GHC.Internal.Bignum modules
(Primitives, WordArray, BigNat, Natural, Integer, Backend.Native,
Backend.GMP) with the qualified do-notation of GHC.Internal.Unboxed.Do.
Nested case chains such as

    case newWordArray# sz s of { (# s, mwa #) ->
    case act mwa s          of { s ->
    unsafeFreezeByteArray# mwa s }}

become

    Ubx.do
       mwa <- newWordArray# sz
       act mwa
       unsafeFreezeByteArray# mwa

The optimised Core is unchanged (compared per top-level binding).

BigNat now enables MonoLocalBinds: local loops written with Ubx.do
must stay monomorphic in the state type, otherwise GHC does not turn
them into join points.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

593f5f5b

by Sylvain Henry at 2026-10-22T04:51:16-04:00

JS: remove archive padding after wasm objects (#25665)

On Darwin hosts, emar (llvm-ar) writes archives in the Darwin format when
the first member is not a known object file, which is the case for the
archives of the JS backend. This format pads each member to 8 bytes with
'\n' and includes the padding in the member size. Wasm objects do not
accept trailing bytes: wasm-ld failed with "section too large".

The JS linker now removes this padding when it extracts a wasm object from
an archive. In addition, Hadrian passes --format=gnu to emar for the JS
target. See Note [Wasm objects in Darwin archives].

Also document the format of Cc (wasm) objects in Note [Cc object format].

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

13dff5d0

by L0neGamer at 2026-10-22T21:37:31-04:00

add options for resizing bytearrays (#27244)

add five mutable bytearray resizing operations
for various kinds of pinnedness and aligning.

  - prefer unpinned when allocating new array (``resizeMutableByteArrayPreferUnpinned#``; the ``resizeMutableByteArray#`` default)
  - prefer pinned when allocating new array (``resizeMutableByteArrayPreferPinned#``)
  - prefer aligned and pinned when allocating new array (``resizeMutableByteArrayPreferAlignedPinned#``)
  - prefer same as input bytearray when allocating new array (``resizeMutableByteArrayKeepPinnedness#``)
  - ensure the output array is aligned and pinned after operation (``resizeMutableByteArrayMakeAlignedPinned#``)

0c17201c

by Sylvain Henry at 2026-10-22T21:38:23-04:00

JS: implement performMinorGC and performBlockingMajorGC (#28012)

GHC.Internal.System.Mem imports three C entry points: performMajorGC,
performGC (for performMinorGC) and performBlockingMajorGC. The JS RTS
only defined h$performMajorGC, so calling performMinorGC or
performBlockingMajorGC threw "ReferenceError: h$performGC is not
defined" at runtime.

The JS RTS has a single non-generational collector, so both missing
entry points are now aliases of h$performMajorGC.

Add a test that calls every GC entry point of System.Mem.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

7a631ff2

by L0neGamer at 2026-10-23T09:11:39-04:00

Mon(a|oi)d of No (Return|Mappend) progress (!15795)

Make non-canonical definitions of return and mappend
non-suppressible errors, and make canonical definitions
of those methods warnings.

CLC proposal: https://github.com/haskell/core-libraries-committee/issues/328

14656fd4

by Simon Jakobi at 2026-10-23T09:12:27-04:00

Cmm: keep the CBE substitution idempotent

When blocks are merged over multiple rounds during common block
elimination, a block that won a merge in an earlier round can still
lose a merge in a later round. The substitution map (Subst) used to
record this as a chain: the earlier loser mapped to a block that was
itself eliminated. In PLT terminology, the substitution became
*non-idempotent*. Applying such a substitution with a single lookup
left edges to unreachable blocks in the Cmm graph, eventually leading
to a setInfoTableStackMap panic (#27368).

With this patch, each round's fresh substitutions are applied to the old
ones as they are added (extendSubst), so every eliminated block maps
directly to its final replacement and the substitution stays idempotent.

Fixes #27368.

Assisted-by: Claude Fable 5.1

92848e61

by Evgeny Malyshev at 2026-10-23T20:26:04-04:00

hadrian: Include transitive dependencies in Haddock builds

Haddock needs indirect dependency interfaces to resolve references to
definitions outside a package's direct dependencies. Expose Cabal's
configured dependency closure through ContextData and use it alongside
the direct list, preserving direct-interface precedence.

Bump Shake's cache version for the serialized oracle change. Add a
Hadrian selftest requiring template-haskell's indirect ghc-prim interface.

Fixes #25850.
Fixes #27786.

Assisted-By: OpenAI Codex

Metric Increase:
    haddock.Cabal

5e8ae81b

by Zubin Duggal at 2026-10-23T20:26:47-04:00

WinIO: Don't confuse Win32 errors and NTSTATUS codes

WinIO misinterpreted NTSTATUS codes as Win32 errors, and had some ad hoc logic to deal with this.
However, this logic was incomplete and resulted in the mishandling of results of certain I/O operations
(see testcase).

Now the `ErrCode` argument in `CompletionCallback` is always the Win32 error code,
and we convert the NTSTATUS using RtlNtStatusToDosError.

Also when an overlapped/async operation such as ReadFile fails immediately,
with a Win32 error such as ERROR_BROKEN_PIPE, we don't get a further completion
notification, so we never recorded the failure. This worked by coincidence
earlier, because `STATUS_PENDING` happened to have the same numerical code as
`ERROR_NO_MORE_ITEMS`, but it doesn't now that we translate `STATUS_PENDING`
into the correct Win32 error code (`ERROR_IO_PENDING`). Fix this by using
the Win32 error value we obtained when an operation returns `STATUS_PENDING`.

Fixes #27971

c42ed66a

by Martijn Bastiaan at 2026-10-24T16:00:42+02:00

Add single-threaded test for #27526 + #27123

Issue #27526 is a duplicate of #27123, which has already been fixed in
HEAD. The proposed patch in the former carries a "nicer" regression test
in that it doesn't rely on multi-threading to trigger.

Closes #27526

7f73f515

by Simon Peyton Jones at 2026-10-24T17:29:19-04:00

Refactor QuickLook a bit

This MR simply refactors QuickLook a bit.  In particular, it
simplifies `tcApp`, and moves shared code into `finishApp`.

It also adds `Note [QuickLook: arguments before result]`
responding to #26599.

There should be no user-visible changes, except perhaps in error messages,
which improve marginally.

For some reason T24984 compiles 8% faster. I'll take the win!

Metric Decrease:
    T24984

a4a3a964

by Alan Zimmerman at 2026-10-24T17:30:03-04:00

EPA: Use AnnList for XClosedTypeFamily

We now have TTG annotations for FamilyInfo.

Make use of it to store the AnnList annotations for a closed type
family.

953e4c7f

by sheaf at 2026-10-26T18:19:10-04:00

Rework LogQueue to be a proper queue

Now 'LogQueue' uses a 'TQueue' with a separate 'TVar' to indicate
whether the log queue is finished logging, instead of the previous weird
data representation using a reverse-order '[Maybe Msg]' with 'Nothing'
denoting the end of the queue.

ec29c5fa

by Rodrigo Mesquita at 2026-10-26T18:19:10-04:00

Refactor GHC.Driver.MakeAction

Pull out of 'runParPipelines' the logic for creating a 'MakeEnv' ready
to be used by multiple threads, as that will be useful for downsweep as
well (which doesn't fit the 'runPipelines' flow), rather than being just
for upsweep.

The code is moved and re-structured to match the export list, simplify
the sequentiality checks previously both in 'runPipelines' and
'runAllPipelines', which were weirdly similar and confusing; into the
two part step where we need these checks: (1) to construct the MakeEnv,
(2) to run the MakeActions in parallel. These two steps are separate and
used to be too mixed up.

Some additional little simplifications or clean ups here and there.

3fb22073

by Rodrigo Mesquita at 2026-10-26T18:19:11-04:00

Refactor GHC.Driver.Downsweep in preparation for parallel downsweep

Pure refactor to improve the code to facilitate implementing parallel
downsweep in the next commit.

This commit puts a MakeEnv into the DownsweepEnv, gives the fields
proper names and uses RecordWildcards to simplify, rather than passing
around all diagnostic wrappers, driver-message-things and using 10s of
positional fields.

No behavior changes here!

52f385a7

by sheaf at 2026-10-26T18:19:11-04:00

Driver: enableCodeGenForTH in load', not downsweep

enableCodeGenForTH modifies the module graph for planning, creating
fresh temporary paths. So it should only be run when we are about to
execute a build plan (e.g. in load'), so that consumers that want to
know facts about the module graph don't go looking for files that don't
exist yet (in particular 'haddock --no-compilation').

82402d14

by sheaf at 2026-10-26T18:19:11-04:00

2-phase Cache/Search Finder monad

This commit restructures the finder abstraction by introducing the
'FinderM' monad, which splits module lookup operations into two phases:

  - a cache-only phase, performing no filesystem access,
  - from the first cache miss onwards, a search action which may access
    the filesystem.

This allows consumers to distinguish between quick cached results versus
more expensive filesystem search operations.

5c302b40

by sheaf at 2026-10-26T18:19:11-04:00

Separate Home/External finder caches

The caches for home modules and modules from external units have
different lifetimes, needing to be invalidated at different points.

This commit moves the cache for modules from external units into the
'UnitState' type, which keeps the cache correct whenever package flags
or package databases change (setSessionDynFlags, :set -package ...,
Backpack's addUnit).

This also makes it easier to clear the cache of home modules, as they
are now stored separately (obviating the 'isUnitEnvInstalledModule'
check we used to perform).

605fbb96

by sheaf at 2026-10-26T18:19:11-04:00

Add known home modules to the finder cache

The finder looks in the file system for the on-disk source that
corresponds to a Haskell module name. There are two kinds of modules
where we need to bypass this search:

  - a file passed directly in the command line, which may not be in any
    search path and whose module name may differ from the file name;
  - Backpack signatures and modules, which do not exist as files at all.

This commit handles these two situations by adding an immutable
'KnownHomeModules' in the finder cache, which is always consulted first
before doing any search.

This removes the ad-hoc imperative logic which relied on mutating the
finder cache at careful points.

'setModuleGraph' now also takes the known home modules, so that the
module graph and the known home modules are always set consistently.

See Note [Known home modules] in GHC.Unit.Finder.Types.

82718f44

by sheaf at 2026-10-26T18:19:11-04:00

Make 'UnresolvedImport' into a suitable cache key

The goal of this commit is to make 'UnresolvedImport' suitable for use
as a cache key. Specifically, during downsweep we want to be able to
cache module resolution queries. We want to do this using some kind of
map, e.g. (roughly speaking):

  Map UnresolvedImport ResolvedImport

For this to work, 'UnresolvedImport' must contain **precisely** the
input data to each resolution computation. In other words:

  1. If something changes that materially affects resolution, it must
     be stored in the cache key.
  2. If changing some input does not materially affect the outcome,
     then it must not be stored in the cache key.

To satisfy (1), we change 'UnitNode' to also keep track of the home
unit in whose unit state the unit's dependencies were looked up.

For (2), 'ImportResolution' now stores a 'ModuleLookupScope' instead of an
'UnresolvedImportOrigin'. Plugin imports are expressed with a new
'LookupPlugin' scope. The source location of an import, which is
immaterial to the resolution, is stored out of band outside
'UnresolvedImport', in 'ModSummary'.

d4635c7c

by sheaf at 2026-10-26T18:19:11-04:00

Add GHC.Data.Dependent

This commit adds a minimal implementation of dependent sums
('Some', 'DSum') and dependent maps ('DMap').

The implementation is not entirely performance optimal, as there are a
few extraneous allocations compared to the full-blown implementation in
the 'dependent-map' package. For our use case (caching downsweep
computations), this does not matter much.

See GHC.Data.Dependent.

6759533e

by sheaf at 2026-10-26T18:19:11-04:00

Driver: structured concurrent worker abstraction

This commit introduces a structured concurrency framework in the style
of the 'ki' library: a collection of threads within a scope.

We implement two kinds of concurrent workers on top of this framework:

  - Independent workers cannot wait for one another at all. The only
    scheduling operation is to wait for quiescence.

  - Coordinating workers declare an STM readiness condition (waiting on
    other workers to complete) which gates their start.

See Note [Deterministic concurrent workers] in GHC.Driver.Concurrency.

This commit ports upsweep to this new framework, with downsweep being
left as subsequent work.

Further changes along the way:

  - Refactoring of how concurrency is acquired to avoid the footgun of
    trying to use a no-op 'AbstractSem' as a lock in the serial case.

  - The "re-run with -j1" logic for semaphore opening errors no longer
    triggers on late semaphore failures (part-way through a lengthy
    computation).

  - Logger threads are properly cleaned up on exception, with each
    concurrent worker's log queue and local TmpFs properly bracketed.

  - The 'GhcMessage -> AnyGhcDiagnostic' and 'Maybe Messager'
    arguments of 'depanalE', 'depanalPartial' and 'downsweep', which
    were all dead in practice, have been dropped.

  - Concurrent logging has been completely overhauled. Each worker has
    its own log queue. Workers whose logs are printed as we go are
    spawned in the order in which their logs must be printed, now using
    a TQueue instead of the old complicated LogQueueQueue mechanism.

    'logThread' is replaced by 'withLogPrinter', which runs the thread
    that prints the log queues within a concurrent worker scope.

    These changes allow us to fix issues with the exception handling:
    we now always flush remaining log queues before exiting (even when
    the action throws an exception), and exceptions that arise while
    printing are properly rethrown. This fixes #28009.

38a99006

by sheaf at 2026-10-26T18:19:11-04:00

Rule-based deterministic concurrent downsweep

This commit rewrites downsweep as a single query-answering rule
(see 'DownsweepRule') that can be executed by concurrent worker threads.
The design allows every expensive operation (preprocessing files with CPP,
parsing headers, reading interfaces) to be performed concurrently
according to the -j<N>/-jsem flags.
See Note [Rules-based downsweep] in GHC.Driver.Downsweep.

The rules are run by "GHC.Driver.Concurrency.runRules" in a demand-driven
way: each worker can demand other workers to run, but never wait on
another worker. Every piece of work is done at most once.

Fixes #27514

Memory usage can increase due to doing more work concurrently:

-------------------------
Metric Increase:
    MultiComponentModulesRecomp
    MultiComponentModulesRecomp100
    mhu-perf
-------------------------

9fa7b7be

by sheaf at 2026-10-26T18:19:11-04:00

Acquire/release semaphore tokens more readily

This commit reworks GHC.Driver.MakeSem to use the scoped worker
abstraction of GHC.Utils.Concurrent.Scope. This allows us to get rid of
a lot of the tricky logic in GHC.Driver.MakeSem involving manual
exception handling, thread spawning and lifetime management, etc.

The main user-visible change is that token acquisition is no longer
rate limited (due to a bug, there previously was a 1s rate limit on
acquisition of semaphore tokens), with the release debounce period
shortened from 1s to 10ms.

Other changes:

  - With -jsem, the number of capabilities is now capped at the number
    of processors, bringing it in line with the -jN behaviour.
  - Pending jobs whose wait for a token is interrupted by an exception
    are now withdrawn from the jobserver, instead of acquiring a token
    that it hung onto without doing useful work.

Fixes #27763

c83c6733

by Alan Zimmerman at 2026-10-26T18:19:55-04:00

EPA: Update AnnClsInstDecl to contain AnnList

It was already separately tracking the '{', '}' and leading ';' values.
This aligns it with layout-introducing parser productions ('vocurly'),
so the layout processing can be handled uniformly

ac870ad5

by Simon Jakobi at 2026-10-28T04:40:42-04:00

X86 NCG: use 32-bit POPCNT for sub-word popcounts

A 32-bit write zero-extends the result to the full register, so we
don't need to emit additional code for this.

We also skip emitting zero-extension instructions for the argument
when it is already visibly zero-extended.

For the byte-wise popcount loop from #24813, this shrinks the popcount
sequence from 5 instructions to 3.

Assisted-by: Claude Fable 5.1

53087373

by Facundo Domínguez at 2026-10-28T04:41:32-04:00

Use a ShortByteString in Serialized instead of [Word8]

This makes the in-memory representation 40x more compact for all values
stored in annotations.

A test is added to measure the representation size of annotations
specified in the compiler input.

The GHC API is modified. All functions that previously handled annotations
in [Word8] form now use ShortByteString. The following definitions have
modified types:

  * GHC.findGlobalAnns
  * GHC.Tc.Gen.Splice.getAnnotationsByTypeRep
  * GHC.Core.Opt.Monad.getAnnotations
  * GHC.Core.Opt.Monad.getFirstAnnotations
  * GHC.Types.Annotations.deserializeAnns
  * GHC.Types.Annotations.findAnns
  * GHC.Types.Annotations.findAnnsByTypeRep
  * GHC.Serialized.Serialized
  * GHC.Serialized.toSerialized
  * GHC.Serialized.fromSerialized
  * GHC.Serialized.serializeWithData
  * GHC.Serialized.deserializeWithData
  * GHCi.Message.ReifyAnnotations

Fixes #28002.

65fd0e31

by Duncan Coutts at 2026-10-29T12:10:21+00:00

Move idle GC tracking to its own file, step 1 of 2

Add a new IdleGC.{c,h} module. Move the RecentActivity type,
recent_activity variable and {get,set}RecentActivity wrappers.

6e3916bc

by Duncan Coutts at 2026-10-29T12:10:21+00:00

Move idle GC functionality to its own file, step 2 of 2

We can now fully encapsulate the recent_activity state, exposing just
three hooks, and one query used in the scheduler and timer tick.

This is a prelude to making changes to the implementation of tracking
when to perform an idle gc.

c455f7cd

by Duncan Coutts at 2026-10-29T12:10:21+00:00

Correct comments in idle GC code

The comments still referred to stopTimer/startTimer. These were
replaced recently by pauseTimer/unpauseTimer, but the comment was not
updated.

e5fe6cc3

by Duncan Coutts at 2026-10-29T12:10:21+00:00

Make signal handling be a responsibility of the I/O manager(s)

Previously it was scattered between I/O managers and the scheduler, and
especially the scheduler's deadlock detection.

Previously the scheduler would poll for pending signals each iteration
of the scheduler loop. The scheduler also had some hairy signal
functionality in the deadlock detection: in the non-threaded RTS (only)
if there were still no threads running after deadlock detection then it
would block waiting for signals.

But signals can and (in my opinion) should be thought of as just a funny
kind of I/O, and thus should be a responsibility of the I/O manager.

So now we have the I/O managers poll for signals when they are polling
for I/O completion (and removing the separate poll in the scheduler).
And when I/O managers block waiting for I/O then they now also start
signal handlers if they get interrupted by a signal. Crucially, if there
is no pending I/O or timers, the awaitCompletedTimeoutsOrIO will still
block waiting for signals.

This patch puts us into an intermediate state: it temporarily breaks
deadlock detection in the non-threaded RTS. The waiting on I/O currently
happens before deadlock detection. This means we'll now wait forever on
signals before doing deadlock detection. We need to move waiting after
deadlock detection. We'll do that in a later patch.

46dc27d5