Simon Jakobi pushed to branch wip/sjakobi/udfm-placement at Glasgow Haskell Compiler / GHC Commits: 5d0ab71a by fendor at 2026-07-27T17:31:05-04:00 Introduce global unit database cache As a first step for better sharing of `UnitInfo` across `UnitEnv`, we introduce a new datatype called `ExternalUnitDatabases`. It primarily serves as an in-memory representation of *all* `UnitDatabase`s across `UnitEnv`. This means, if multiple `HomeUnitEnv`s depend on the same database, one way or another, we make sure that we don't parse from disk every time. Instead, we store the in-memory representation in `ExternalUnitDatabases`. `ExternalUnitDatabaseCache` is the equivalent of `ExternalUnitState` in the `UnitEnv`. It is a mutable variable wrapping `ExternalUnitDatabases`. The mutable `ExternalUnitDatabaseCache` is used in `initUnits` to make sure we don't parse the same unit database multiple times. Almost by accident, we change the semantics of `initUnits` to honour modifications to `packageDBFlags`. The inability to change `packageDBFlags` while also reusing the already parsed `UnitDatabase`s was reported in #26423 as a bug. Hence, we think this behaviour change is warranted and acceptable, especially since it comes with a breaking change to the `initUnits` API. Add regression test for #26423 Closes #26423 - - - - - 6cce494a by fendor at 2026-07-27T17:31:05-04:00 Introduce UnitIndex for global external unit caching `UnitInfo`s have been observed to cause a lot of memory usage in #27500. Especially with multiple home units, as the same (external) units are processed from scratch, even though most of the time we end up with exactly the same `UnitInfo`. We introduce a `UnitEnv` global cache that allows us to store external unit information that is used across all `HomeUnitEnv`s. The most important change in this commit is the introduction of the `UnitIndex`. It stores a global mapping of `UnitId` -> `UnitInfo`, and `initUnits` always uses the cached `UnitInfo` entry to populate each `HomeUnitEnv`'s `UnitState`. This allows us to ensure the following property:

Each `UnitInfo` should be alive exactly once in GHC.

All `UnitState`s should reference 'UnitInfo's stored in the 'UnitIndex'. This ensured by calling 'initUnits' with the 'UnitIndex'. In addition, the `ExternalUnitDatabases` may also hold a reference to each on-disk representation of `UnitInfo`. This means, we impose an hard upper bound on the number of `UnitInfo`s alive in the GHC session:

The number of alive `UnitInfo`s closure objects must be the sum of all loaded unit database times two.

We add performance regression tests that make sure the number of live `UnitInfo` cannot exceed this threshold. Closes #27500 ------------------------- Metric Decrease: MultiComponentModules MultiComponentModulesRecomp MultiComponentModulesRecomp100 mhu-perf LinkableUsage02 ------------------------- These metrics increases are especially notable, as we are not even sharing anything big but merely the global package database with 50 entries. It shows how careful sharing of `UnitInfo` can improve memory usage. We expect this to be much more notable when the whole cabal package database is shared across multiple home units. `LinkableUsage02` metric decreases on unreg and i386 platform, only. --- Technical details To share the `UnitInfo`s correctly, it is important that we extract the `WireMap` into the `UnitIndex`. At the moment of writing, `WireMap` must be globally the same for all `HomeUnitEnv`s. This is important, as we could otherwise not cache the "fully-resolved" `UnitInfo`, as we don't change the `UnitId` or `unitAbiHash` when resolving wired-in units. Thus, there could be ambiguities, when the `WireMap` is not the same for all `UnitState`s across the `UnitEnv`. We consider a `UnitInfo` fully-resolved, if wired-in units have been updated, the `UnitInfo` has been validated and variables in the unit config, such as `${pkgroot}` have been resolved. Updating the wired-in units requires the `WireMap` to be globally the same. - - - - - f8e3bee9 by Zubin Duggal at 2026-07-27T17:31:49-04:00 testsuite: skip runtime stats tests on debugged compilers Debugged flavours build the boot libraries without optimisation, so the runtime numbers do not match the baselines. - - - - - 1e326770 by Zubin Duggal at 2026-07-27T17:31:50-04:00 testsuite: mark #20706 tests fragile rather than broken Whether the static linux linker issues manifest depends on the host toolchain. - - - - - c0b13cbe by Zubin Duggal at 2026-07-27T17:31:50-04:00 testsuite: exclude libnuma from mostly-static It needs static system libraries (libnuma.a) that many platforms do not ship. Fixes #26914 - - - - - bee1913d by Alan Zimmerman at 2026-07-28T16:42:29-04:00 EPA: ClsInstDecl with decls as [LHsDecl GhcPs] in GhcPs Similar to 4fdfe75731e01dad7d7fa474c2703d0d3965afb1, this commit changes the as-parsed representation of class instance declarations to [LHsDecl GhcPs], and only separates them by type from the renamer onward. This also allows us to remove all the AnnSortKey machinery for exact printing, as it is now no longer needed. - - - - - 72c55eee by Cheng Shao at 2026-07-28T16:43:11-04:00 hadrian: implement and use writeFileAtomic to fix race condition This patch implements `writeFileAtomic` in hadrian and change all invocations of shake non-atomic `writeFile'` to use `writeFileAtomic`, to avoid multiple hadrian concurrent invocations overwriting the same in-tree generated file not in the build root directory. Fixes #27536. Additional notes: - `writeFileChanged`/`writeFileChangedBS` cannot be made atomic since it involves reading the file's older version, so their uses are left alone. It doesn't affect #27536 given their outputs are contained in the build root directory. - It's possible to shrink this patch by only making writes outside the build root directory atomic. But I think it's not worth the effort for fine grained distinction here, and atomic writes within the build root directory should also improve robustness of a hadrian build. - In the longer term we do want to make a ghc build only generate files within the build root directory, though that's a lot of work and outside the scope of this particular bugfix. Co-authored-by: Codex <codex@openai.com> - - - - - 46d4f963 by Sylvain Henry at 2026-07-29T06:38:40-04:00 RTS: correctly mark slop bytes when shrinking large arrays (#19048) Correctly mark slop bytes even when profiling is off so that heap census doesn't traverse garbage-collected closures. - - - - - 4762a8bf by Simon Jakobi at 2026-07-29T06:39:23-04:00 Add -XLazyFieldAnnotations (GHC proposal 752) Unbundle the prefix `~` lazy field annotation syntax from StrictData. The new LazyFieldAnnotations extension controls whether `~` is accepted on constructor fields. StrictData (and Strict, transitively) imply the new extension. See https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0752-la.... Closes #24455. Assisted-by: Claude Opus 4.8 - - - - - 0b6dcc84 by Simon Jakobi at 2026-07-29T06:40:04-04:00 testsuite: Relax T24471 residency tolerance T24471 peak residency fluctuates enough on i386 to cause spurious failures. Use the standard residency tolerance while retaining the existing allocation threshold. See https://gitlab.haskell.org/ghc/ghc/-/work_items/24471#note_682303. Assisted-by: gpt-5.6-sol via Codex CLI - - - - - 90e95b34 by Cheng Shao at 2026-07-29T06:40:45-04:00 compiler: fix missing top-level procedure labels in cmm dumps This patch fixes missing top-level procedure labels in some intermediate Cmm pass dumps. Fixes #27553. Co-authored-by: Codex <codex@openai.com> - - - - - 360a5946 by sheaf at 2026-07-29T06:41:35-04:00 Add some type-family-heavy performance tests FamAppCachePerf stress-tests the performance of lookups in the type family application cache. T27336 is a minimisation extracted from the reported reproducer. SimplCastPerf is a measure of coercion growth due to the simplifier calling mkTransCo without re-optimising the result. - - - - - 3ec9e2b9 by Mike Pilgrem at 2026-07-31T08:21:33-04:00 GHC Guide: Improve docs on response files - - - - - e5b2a1f7 by sheaf at 2026-07-31T08:22:23-04:00 Disable Core Lint for TcPlugin_RewritePerf This is a compiler performance test, but the test source hard-coded -dcore-lint, defeating the measurement. ------------------------- Metric Decrease: TcPlugin_RewritePerf ------------------------- - - - - - 85b10c00 by Alan Zimmerman at 2026-07-31T22:09:47+01:00 EPA: Remove LocatedP from OverlapMode We have type LocatedP = GenLocated SrcSpanAnnP type SrcSpanAnnP = EpAnn AnnPragma As the first step in removing this in favour of LocatedA which only captures location, comments and trailing annotations, we remove it from OverlapMode We do this by moving the AnnPragma into the TTG extension point instead. - - - - - c9a34a00 by Viktor Dukhovni at 2026-08-02T04:34:17-04:00 Fix note typo - - - - - 4f2a21f7 by Andreas Klebinger at 2026-08-02T22:46:46-04:00 Apply oneShot Monad trick to STG LintM - - - - - 21e4b89d by Andreas Klebinger at 2026-08-02T22:46:46-04:00 stgLint: Use a single reader env for read only arguments. - - - - - d415f38a by Alan Zimmerman at 2026-08-02T22:47:27-04:00 EPA: Remove LocatedP from CType The next step of removing use of LocatedP by moving the AnnPragma for CType into its TTG extension point instead. - - - - - e6b94f45 by Simon Jakobi at 2026-08-04T16:57:12+02:00 Word64Map: add compareSize compareSize m c compares the size of a map to an Int, but unlike compare (size m) c it stops traversing the map once the outcome is determined. Based on https://github.com/haskell/containers/pull/1139 Assisted-by: Claude Opus 5 - - - - - a39e93e5 by Simon Jakobi at 2026-08-04T16:58:46+02:00 Use a pigeonhole sort for deterministic UniqDFM iteration Deterministic UniqDFM iteration used a list mergesort, allocating O(n log n) cons cells and contributing significantly to compiler allocations (#27459). Use a pigeonhole sort where appropriate, while retaining the mergesort fallback. See Note [Sorting a UDFM] and Note [Cost of deterministic iteration]. ------------------------- Metric Decrease: InstanceMatching InstanceMatching1 ManyAlternatives T12707 T13379 T13719 T24471 T5321FD T5321Fun T783 ------------------------- Assisted-by: gpt-5.6-sol via Codex CLI - - - - - 153 changed files: - + changelog.d/T26423 - + changelog.d/fix-cmm-dump-labels - + changelog.d/fix-heap-census-large-arrays-19048 - + changelog.d/lazy-field-annotations - + changelog.d/unit-index - compiler/GHC.hs - compiler/GHC/Cmm/Pipeline.hs - compiler/GHC/Data/Word64Map/Internal.hs - compiler/GHC/Data/Word64Map/Lazy.hs - compiler/GHC/Data/Word64Map/Strict.hs - compiler/GHC/Data/Word64Map/Strict/Internal.hs - compiler/GHC/Driver/Backpack.hs - compiler/GHC/Driver/Env.hs - compiler/GHC/Driver/Flags.hs - compiler/GHC/Driver/Main/Hsc.hs - compiler/GHC/Driver/Main/Interactive.hs - compiler/GHC/Driver/Main/Passes.hs - compiler/GHC/Driver/Session/Units.hs - compiler/GHC/Hs/Decls.hs - compiler/GHC/Hs/Decls/Overlap.hs - compiler/GHC/Hs/Instances.hs - compiler/GHC/Hs/Stats.hs - compiler/GHC/Hs/Utils.hs - compiler/GHC/HsToCore/Docs.hs - compiler/GHC/HsToCore/Quote.hs - compiler/GHC/Iface/Ext/Ast.hs - compiler/GHC/Parser.y - compiler/GHC/Parser/Annotation.hs - compiler/GHC/Parser/PostProcess.hs - compiler/GHC/Rename/Module.hs - compiler/GHC/Rename/Names.hs - compiler/GHC/Stg/Lint.hs - compiler/GHC/Tc/Deriv.hs - compiler/GHC/Tc/Errors/Ppr.hs - compiler/GHC/Tc/Errors/Types.hs - compiler/GHC/Tc/TyCl.hs - compiler/GHC/Tc/TyCl/Class.hs - compiler/GHC/Tc/TyCl/Instance.hs - compiler/GHC/Tc/Utils/Env.hs - compiler/GHC/Tc/Utils/Instantiate.hs - compiler/GHC/ThToHs.hs - compiler/GHC/Types/ForeignCall.hs - compiler/GHC/Types/Unique.hs - compiler/GHC/Types/Unique/DFM.hs - compiler/GHC/Unit/Env.hs - + compiler/GHC/Unit/External/Database.hs - + compiler/GHC/Unit/External/Index.hs - + compiler/GHC/Unit/External/ModuleOrigin.hs - + compiler/GHC/Unit/External/Providers.hs - + compiler/GHC/Unit/External/Query.hs - + compiler/GHC/Unit/External/Substitution.hs - + compiler/GHC/Unit/External/Validate.hs - + compiler/GHC/Unit/External/Visibility.hs - + compiler/GHC/Unit/External/Wired.hs - compiler/GHC/Unit/Home/Graph.hs - compiler/GHC/Unit/Info.hs - compiler/GHC/Unit/State.hs - compiler/GHC/Unit/State.hs-boot - compiler/GHC/Unit/Types.hs - compiler/Language/Haskell/Syntax/Decls.hs - compiler/ghc.cabal.in - docs/users_guide/exts/strict.rst - docs/users_guide/using.rst - ghc/GHCi/UI.hs - hadrian/src/Hadrian/Builder/Ar.hs - hadrian/src/Hadrian/Utilities.hs - hadrian/src/Rules/BinaryDist.hs - hadrian/src/Rules/CabalReinstall.hs - hadrian/src/Rules/Documentation.hs - hadrian/src/Rules/Generate.hs - hadrian/src/Rules/Library.hs - hadrian/src/Rules/Nofib.hs - hadrian/src/Rules/Program.hs - hadrian/src/Rules/Register.hs - hadrian/src/Rules/Rts.hs - hadrian/src/Rules/SourceDist.hs - hadrian/src/Rules/Test.hs - hadrian/src/Rules/ToolArgs.hs - libraries/ghc-internal/src/GHC/Internal/Ix.hs - libraries/ghc-internal/src/GHC/Internal/LanguageExtensions.hs - rts/Apply.cmm - rts/ZeroSlop.c → rts/MarkSlop.c - rts/PrimOps.cmm - rts/Printer.c - rts/ProfHeap.c - rts/RtsFlags.c - rts/ThreadPaused.c - rts/include/Cmm.h - rts/include/rts/storage/ClosureMacros.h - rts/rts.cabal - rts/sm/NonMovingMark.c - rts/sm/Sanity.c - rts/sm/Storage.c - testsuite/driver/testlib.py - testsuite/tests/count-deps/CountDepsParser.stdout - + testsuite/tests/deSugar/should_run/LazyFieldAnnotationsSemantics.hs - + testsuite/tests/deSugar/should_run/LazyFieldAnnotationsSemantics.stdout - testsuite/tests/deSugar/should_run/all.T - + testsuite/tests/driver/T26423/Hello.hs - + testsuite/tests/driver/T26423/Makefile - + testsuite/tests/driver/T26423/T26423.hs - + testsuite/tests/driver/T26423/T26423.stderr - + testsuite/tests/driver/T26423/T26423.stdout - + testsuite/tests/driver/T26423/all.T - + testsuite/tests/driver/T26423/test/Test.hs - + testsuite/tests/driver/T26423/test/test.pkg - testsuite/tests/driver/T4437.hs - + testsuite/tests/driver/TUnitInfo/Foo.hs - + testsuite/tests/driver/TUnitInfo/Makefile - + testsuite/tests/driver/TUnitInfo/all.T - + testsuite/tests/driver/TUnitInfo/genMhu.sh - + testsuite/tests/driver/TUnitInfo/generic-unit-info-space-mhu.stdout - + testsuite/tests/driver/TUnitInfo/generic-unit-info-space-single.stdout - + testsuite/tests/driver/TUnitInfo/generic-unit-info-space.hs - + testsuite/tests/driver/TUnitInfo/generic-unit-info-space.stdout - testsuite/tests/driver/all.T - testsuite/tests/driver/mostly-static/Makefile - testsuite/tests/ghci/T13786/all.T - testsuite/tests/ghci/linking/all.T - testsuite/tests/ghci/linking/dyn/all.T - testsuite/tests/haddock/should_compile_flag_haddock/T17544.stderr - testsuite/tests/interface-stability/template-haskell-exports.stdout - testsuite/tests/package/T20010/all.T - testsuite/tests/parser/should_compile/DumpRenamedAst.stderr - + testsuite/tests/perf/compiler/FamAppCachePerf.hs - + testsuite/tests/perf/compiler/SimplCastPerf.hs - + testsuite/tests/perf/compiler/T27336.hs - testsuite/tests/perf/compiler/all.T - testsuite/tests/plugins/all.T - testsuite/tests/printer/Test24533.stdout - + testsuite/tests/rts/T19048.hs - + testsuite/tests/rts/T19048.stdout - testsuite/tests/rts/all.T - testsuite/tests/rts/linker/all.T - testsuite/tests/tcplugins/TcPlugin_RewritePerf.hs - testsuite/tests/tcplugins/TcPlugin_RewritePerf.stderr - + testsuite/tests/typecheck/should_compile/LazyFieldAnnotations.hs - testsuite/tests/typecheck/should_compile/all.T - testsuite/tests/typecheck/should_fail/LazyFieldsDisabled.stderr - + testsuite/tests/typecheck/should_fail/LazyFieldsDisabledStrictData.hs - + testsuite/tests/typecheck/should_fail/LazyFieldsDisabledStrictData.stderr - testsuite/tests/typecheck/should_fail/all.T - utils/check-exact/ExactPrint.hs - utils/check-exact/Utils.hs - utils/haddock/haddock-api/src/Haddock.hs - utils/haddock/haddock-api/src/Haddock/Backends/Hoogle.hs - utils/haddock/haddock-api/src/Haddock/Backends/LaTeX.hs - utils/haddock/haddock-api/src/Haddock/Backends/Xhtml/Decl.hs - utils/haddock/haddock-api/src/Haddock/Convert.hs - utils/haddock/haddock-api/src/Haddock/GhcUtils.hs - utils/haddock/haddock-api/src/Haddock/Interface/Create.hs - utils/haddock/haddock-api/src/Haddock/Interface/Rename.hs - utils/haddock/haddock-api/src/Haddock/Types.hs The diff was not included because it is too large. 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