Build GHCi libraries with `ghc --merge-objs` by andreabedini · Pull Request #12358 · haskell/cabal

andreabedini · 2026-09-15T06:16:50Z

The only thing Cabal does with ld is ld -r to produce the GHCi object. GHC has exposed exactly that as ghc --merge-objs since 9.4 (GHC#20712, added so Cabal could drop its copy of the logic). This makes Cabal use it.

When ghc advertises --merge-objs, the GHCi object is produced by ghc with the merge tool and flags it was configured with, and whether a GHCi library can be built at all follows from Merge objects command being set in the settings (empty on the Windows bindists, so GHCi libs stay off there as today). On those GHCs we also stop probing ld at configure time, which is what #10970 wanted rid of. GHCs without the mode keep the ld -r path unchanged.

Detection is by capability, not version: a small GhcFeature type in Distribution.Simple.Program.GHC maps features to the flag that signals them in ghc --show-options, queried once when ghc is configured and recorded as program properties. --merge-objs is the first member; other version checks can migrate later. This is what stalled #9226, which this supersedes.

Tested with 9.12.2 and with a wrapper ghc that hides --merge-objs to exercise the fallback; the new LibraryForGhci test covers both. Not tested on Windows.

Refs #7828, #9301, #12332, #10970. Supersedes #9226.

This PR was prepared with the help of an AI agent (Claude Code); I reviewed and tested the result.

🤖 Generated with Claude Code

Add GhcFeature: each constructor names the flag whose presence in `ghc --show-options` signals it. Configuring ghc queries once and records the answers as program properties; ghcSupports reads them back. This is detection by capability rather than by version, unlike GhcImplInfo. First feature: the --merge-objs mode (GHC >= 9.4). Add the matching GhcMode too. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Copilot

🔵 Needs a closer look

Two moderate issues remain around missing merge-tool handling and test coverage.

Pull request overview

This pull request updates Cabal to use ghc --merge-objs for GHCi libraries when supported, while retaining the ld -r fallback.

Changes:

Adds GHC capability detection and merge-mode support.
Avoids unnecessary ld probing when GHC handles merging.
Adds changelog and integration-test coverage.

File summaries

File	Summary	Review note
`changelog.d/merge-objs.md`	Documents the new merging behavior.	No final comment.
`Cabal/src/Distribution/Simple/Program/GHC.hs`	Detects GHC features and renders merge commands.	No final comment.
`Cabal/src/Distribution/Simple/Program/Builtin.hs`	Detects GHC capabilities during configuration.	No final comment.
`Cabal/src/Distribution/Simple/GHC/Internal.hs`	Avoids `ld` probing when appropriate.	No final comment.
`Cabal/src/Distribution/Simple/GHC/Build/Link.hs`	Uses GHC merging with an `ld` fallback.	No final comment.
`Cabal/src/Distribution/Simple/Configure.hs`	Determines GHCi library availability.	Moderate (1 vote): A missing `Merge objects command` can still enable GHCi libraries; it should be treated as unsupported.
`cabal-testsuite/PackageTests/LibraryForGhci/setup.test.hs`	Tests merge and fallback behavior.	Moderate (1 vote): The test skips the platform where the empty-setting case matters; add controlled fixture or Windows coverage.
`cabal-testsuite/PackageTests/LibraryForGhci/p.cabal`	Defines the test package.	No final comment.
`cabal-testsuite/PackageTests/LibraryForGhci/Lib/Other.hs`	Provides an additional test module.	No final comment.
`cabal-testsuite/PackageTests/LibraryForGhci/Lib.hs`	Provides test library code.	No final comment.

Review details

Suppressed comments (2)

Cabal/src/Distribution/Simple/Configure.hs:963

When Merge objects command is absent, this expression yields Nothing, and the fromMaybe True at line 968 enables GHCi libraries anyway. That lets a GHC advertising --merge-objs proceed without a configured merge tool (the comment above notes that GHC then falls back to an archive), so a missing setting should be treated as unsupported rather than capable.

          not . null <$> Map.lookup "Merge objects command" (compilerProperties comp)

cabal-testsuite/PackageTests/LibraryForGhci/setup.test.hs:10

The test skips exactly the platform where the new empty Merge objects command case is expected to matter, so it never verifies that --enable-library-for-ghci is disabled instead of invoking ghc --merge-objs without a merge tool. Please add a controlled compiler/settings fixture (or Windows coverage) that exercises this branch and checks the resulting configuration/warning.

  skipIfWindows "GHCi libraries are not built with GHC's Windows toolchain"

Files reviewed: 10/10 changed files
Comments generated: 0
Review effort level: Lite

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ulysses4ever · 2026-09-15T11:13:41Z

This looks reasonable. Did Copilot say there’s an issue when no merge tool is available. Is this spurious?

We used to run `ld -r` ourselves, finding ld from the ghc settings and probing it for -r and -x. When ghc has --merge-objs, let it do the merging: it uses the tool and flags it was configured with and handles long argument lists. Whether a GHCi library can be built at all then follows from "Merge objects command" being set, not from probing `ld --help`. GHCs without --merge-objs keep the ld path. Revives haskell#9226 with feature detection instead of a version gate. Refs haskell#7828, haskell#9301, haskell#12332. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

With --merge-objs we never run ld, so skip the two probes at configure time: compiling a C file through ghc to test `ld -x` (not always possible, see haskell#10970) and `ld --help` for relocatable output. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

andreabedini · 2026-09-17T09:24:12Z

This looks reasonable. Did Copilot say there’s an issue when no merge tool is available. Is this spurious?

It wasn't going to be a problem but it was cheap to fix anyway. Any GHC with --merge-objs (9.4+) also reports Merge objects command (present since 9.2), so the "absent key" branch can't happen with a real GHC. I've made a missing setting count as "no merge tool" anyway, and the test now checks that GHCi libraries are on iff the merge tool is set, so it runs on Windows too instead of skipping it.

Mikolaj · 2026-09-17T17:41:34Z

+    -- When GHC can merge objects itself (@ghc --merge-objs@, see
+    -- 'GhcMergeObjs') Cabal never runs @ld@, so we skip probing it.
+    -- The probe below compiles a C file through GHC, which is not always
+    -- possible (#10970), and the @--help@ probe in 'ldProgram' is only


If it's not always possible, why do we return cp unconditionally? Or does ghcSupports check that, too?

Mikolaj · 2026-09-17T17:43:12Z

+-- Meant to be called from the program's 'programPostConf'. If GHC cannot be
+-- queried (e.g. it predates @--show-options@) every feature is recorded as
+-- unsupported.
+detectGhcFeatures :: Verbosity -> ConfiguredProgram -> IO ConfiguredProgram


Nice new functionality! Can we maybe use it in other places where we currently detect capabilities ad hoc?

angerman

Hooray!

Copilot AI lite review requested due to automatic review settings September 15, 2026 06:16

andreabedini force-pushed the andrea/wip/merge-objs branch from 2790548 to 7b4e71e Compare September 15, 2026 06:17

Copilot started reviewing on behalf of andreabedini September 15, 2026 06:17 View session

andreabedini mentioned this pull request Sep 15, 2026

Use ghc --merge-objs when available to join object files #9226

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andreabedini requested review from Mikolaj and angerman September 15, 2026 06:19

Copilot AI reviewed Sep 15, 2026

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This was referenced Sep 15, 2026

fix(Cabal): Use the C toolchain GHC was configured with #12333

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Cabal does not use the C toolchain GHC was configured with #12332

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andreabedini added the attention: needs-review label Sep 15, 2026

andreabedini and others added 2 commits September 17, 2026 17:20

andreabedini force-pushed the andrea/wip/merge-objs branch from 7b4e71e to c783c0c Compare September 17, 2026 09:20

Mikolaj reviewed Sep 17, 2026

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angerman approved these changes Sep 18, 2026

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Build GHCi libraries with `ghc --merge-objs` - #12358

Build GHCi libraries with `ghc --merge-objs`#12358
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