Magnus pushed to branch ghc-9.12 at Glasgow Haskell Compiler / GHC Commits: 62f58b9e by mangoiv at 2026-06-24T16:58:14+02:00 rts: reapply fix eager black holes: record mutated closure and fix assertion This commit was previously reverted on the ghc 9.12. branch and is now reapplied. This fixes two problems with handling eager black holes, introduced by a1de535f. - the closure mutation must be recorded even for eager black holes, since the mutator has mutated it before calling threadPaused - The assertion that an unmarked eager black hole must be owned by the TSO calling threadPaused is incorrect, since multiple threads can race to claim the black hole. fixes #26495 (cherry picked from commit 3ba3d9f9) - - - - - 3c837d7f by Sebastian Graf at 2026-06-25T15:57:57+02:00 Desugar a `case` scrutinee only once (#27383, #20251) In `dsExpr` for `HsCase` we desugared the scrutinee /twice/: once to build the Core `case` itself, and again inside `matchWrapper`, which re-desugared the source scrutinee (via `addHsScrutTmCs`) purely to record long-distance information for the pattern-match checker. For a single `case` that is merely wasteful. But for nested cases it is catastrophic. Consider case (case (case e of ... ) of ... ) of ... Desugaring the outer scrutinee desugars the middle `case` twice, each of which desugars the inner `case` twice, and so on. The work doubles at every level, so desugaring takes O(2^n) time in the nesting depth. That is the blowup reported in #27383; it is also what makes the machine-generated program in #20251 take an age to compile. The fix is simple. `matchWrapper` is handed the scrutinee anyway, so we give it the Core expression we have /already/ desugared, and record the long-distance term constraint with `addCoreScrutTmCs` instead of re-desugaring from source. This is just what `matchSinglePatVar` already does for single-pattern matches. So: * `matchWrapper` now takes `Maybe [CoreExpr]` rather than `Maybe [LHsExpr GhcTc]`. * The `HsCase` equation of `dsExpr` passes the already-desugared `core_discrim`; the arrow desugarer passes its match variables. * `addHsScrutTmCs` had no other use, so it is gone. Desugaring is now linear in the nesting depth. (The coverage checker still runs `simpleOptExpr` over each scrutinee, which leaves the total at O(n^2); that is ample.) The long-distance information itself is unchanged: the checker sees precisely the Core that backs the generated code. Test: deSugar/should_compile/T27383 (cherry picked from commit 67d41299be96798702377e6e2b826f9c8e070821) - - - - - 71481e34 by Cheng Shao at 2026-07-01T15:21:19+02:00 compiler: fix missing handling of CmmUnsafeForeignCall node in LayoutStack This patch fixes missing handling of `CmmUnsafeForeignCall` middle node in the `LayoutStack` pass. Before proc-points splitting, this pass computes liveliness of local registers, and spills those alive across a Cmm native call onto the stack. It need to traverse all middle nodes in each block and check whether a local register is an assignee, if so then the previous mapping in `sm_regs` is invalidated and needs to be dropped. However, it didn't handle `CmmUnsafeForeignCall` node which may also assign to a local register. When proc-points splitting is enabled, this can produce an invalid basic block that doesn't properly backup the updated local register to the stack before doing a Cmm call, resulting in completely invalid runtime behavior. The patch also adds a `T27447` regression test. With no-TNTC or with LLVM backend, without the fix the test case would output a stale 0x1111111111111111 value, instead of the expected 0x2222222222222222 output. Fixes #27447. Co-authored-by: Codex <codex@openai.com> (cherry picked from commit 3f00f234d0d5b3b3b2a23a5dc70ce372eb9bbdb4) - - - - - a9197261 by Andreas Klebinger at 2026-07-10T17:21:16+02:00 Fix a profiling race condition resulting in segfaults. StgToCmm: Don't assume tagged FUN closures in closureCodeBody. When entering a closure the self/node pointer might not be tagged in some situations when a thunk is evaluated by multiple threads. So we most AND away the tag bits rather than subtracting an expected tag. Apply.cmm: Fix a race condition occuring when a thunk is mutated during GC. In stg_ap_0_fast when might need to run GC before entering a thunk. If this happens another thread or the GC itself might mutate the closure making entering it no longer valid. We now check for this. Add test and changelog for #27123 fixes. (cherry picked from commit ed09895d7de1ca116a561868c151fd825a16ad0c) - - - - - f0a42e63 by Zubin Duggal at 2026-07-10T17:21:16+02:00 Bump semaphore-compat submodule to 2.0.1 This versions includes some cruicial fixes for darwin (cherry picked from commit 4180af3f71754472dbd49b85179b25fd29bd9998) - - - - - 27022a1e by Zubin Duggal at 2026-07-10T17:21:16+02:00 CorePrep: Don't speculatively evaluate bindings that we have already discovered to be absent In #25924, we segfault because speculation forces a projection out of a RUBBISH dictionary (which we generated because it absent). Solution: Don't speculate on bindings we already know are absent. Fixes 25924 (cherry picked from commit 9b714c4c833461c621f0a050680848d7248aa57e) - - - - - f7e9b9c5 by Zubin Duggal at 2026-07-11T11:19:06+02:00 Don't make absent fillers for terminating types In #25924 we discovered that we could speculatively evaluate an absent filler for a dictionary, and project a field (a superclass selector) out of it, resulting in segfaults. Solution: Never make an absent filler or rubbish literal for a terminating type like a dictionary. mkAbsentFiller returns Nothing for isTerminatingType, so worker/wrapper and the specialiser keep the real argument instead. Some small metric decreases because we do a little less work in the simplifier now. Metric Decrease: T9872a T9872b T9872c TcPlugin_RewritePerf Metric Increase: T9872b_defer (cherry picked from commit 4a59b3eece9b7106fcbe73d2d06a49755be4ea8f) - - - - - abf7c3be by mangoiv at 2026-07-13T20:28:18+02:00 chore: update user guide to reflect changed library versions - - - - - 38 changed files: - + changelog.d/T27123.md - + changelog.d/fix-absent-dict-projection - + changelog.d/fix-exponential-case-desugar-27383 - + changelog.d/fix-layout-stack-fcall - changelog.d/semaphore-v2 - compiler/GHC/Cmm/LayoutStack.hs - compiler/GHC/Core/Make.hs - compiler/GHC/Core/Opt/Specialise.hs - compiler/GHC/Core/Opt/WorkWrap.hs - compiler/GHC/Core/Opt/WorkWrap/Utils.hs - compiler/GHC/CoreToStg/Prep.hs - compiler/GHC/HsToCore/Arrows.hs - compiler/GHC/HsToCore/Expr.hs - compiler/GHC/HsToCore/Match.hs - compiler/GHC/HsToCore/Match.hs-boot - compiler/GHC/HsToCore/Pmc.hs - compiler/GHC/StgToCmm/Bind.hs - compiler/GHC/Types/Literal.hs - docs/users_guide/9.12.5-notes.rst - hadrian/src/Settings/Warnings.hs - libraries/semaphore-compat - rts/Apply.cmm - rts/ThreadPaused.c - + testsuite/tests/cmm/should_run/T27447.hs - + testsuite/tests/cmm/should_run/T27447.stdout - + testsuite/tests/cmm/should_run/T27447_cmm.cmm - testsuite/tests/cmm/should_run/all.T - + testsuite/tests/core-to-stg/T25924/B.hs - + testsuite/tests/core-to-stg/T25924/Main.hs - + testsuite/tests/core-to-stg/T25924/all.T - + testsuite/tests/core-to-stg/T25924a.hs - + testsuite/tests/core-to-stg/T25924a.stdout - testsuite/tests/core-to-stg/all.T - + testsuite/tests/deSugar/should_compile/T27383.hs - testsuite/tests/deSugar/should_compile/all.T - testsuite/tests/dmdanal/should_compile/T18982.stderr - + testsuite/tests/rts/T27123.hs - testsuite/tests/rts/all.T The diff was not included because it is too large. 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