[Git][ghc/ghc][wip/T22715] 3 commits: Allow waiting for timerfd to be interrupted during rts shutdown

Simon Peyton Jones (@simonpj) gitlab at gitlab.haskell.org
Wed Jan 25 08:14:39 UTC 2023



Simon Peyton Jones pushed to branch wip/T22715 at Glasgow Haskell Compiler / GHC


Commits:
e5383a29 by Wander Hillen at 2023-01-24T20:02:26-05:00
Allow waiting for timerfd to be interrupted during rts shutdown

- - - - -
1957eda1 by Ryan Scott at 2023-01-24T20:03:01-05:00
Restore Compose's Read/Show behavior to match Read1/Show1 instances

Fixes #22816.

- - - - -
d9e4143c by Simon Peyton Jones at 2023-01-25T08:15:20+00:00
Fix in-scope set in specImports

Nothing deep here; I had failed to bring some
floated dictionary binders into scope.

Exposed by -fspecialise-aggressively

Fixes #22715.

- - - - -


10 changed files:

- compiler/GHC/Core/Opt/Specialise.hs
- docs/users_guide/9.8.1-notes.rst
- libraries/base/Data/Functor/Compose.hs
- + libraries/base/tests/T22816.hs
- + libraries/base/tests/T22816.stdout
- libraries/base/tests/all.T
- rts/posix/ticker/Pthread.c
- + testsuite/tests/simplCore/should_compile/T22715_2.hs
- + testsuite/tests/simplCore/should_compile/T22715_2a.hs
- testsuite/tests/simplCore/should_compile/all.T


Changes:

=====================================
compiler/GHC/Core/Opt/Specialise.hs
=====================================
@@ -33,6 +33,7 @@ import GHC.Core.Utils     ( exprIsTrivial
 import GHC.Core.FVs
 import GHC.Core.TyCo.FVs ( tyCoVarsOfTypeList )
 import GHC.Core.Opt.Arity( collectBindersPushingCo )
+-- import GHC.Core.Ppr( pprIds )
 
 import GHC.Builtin.Types  ( unboxedUnitTy )
 
@@ -736,7 +737,8 @@ specImports top_env (MkUD { ud_binds = dict_binds, ud_calls = calls })
   = return ([], wrapDictBinds dict_binds [])
 
   | otherwise
-  = do { (_env, spec_rules, spec_binds) <- spec_imports top_env [] dict_binds calls
+  = do { let env_w_dict_bndrs = top_env `bringFloatedDictsIntoScope` dict_binds
+       ; (_env, spec_rules, spec_binds) <- spec_imports env_w_dict_bndrs [] dict_binds calls
 
              -- Don't forget to wrap the specialized bindings with
              -- bindings for the needed dictionaries.
@@ -752,6 +754,7 @@ specImports top_env (MkUD { ud_binds = dict_binds, ud_calls = calls })
 
 -- | Specialise a set of calls to imported bindings
 spec_imports :: SpecEnv          -- Passed in so that all top-level Ids are in scope
+                                 ---In-scope set includes the FloatedDictBinds
              -> [Id]             -- Stack of imported functions being specialised
                                  -- See Note [specImport call stack]
              -> FloatedDictBinds -- Dict bindings, used /only/ for filterCalls
@@ -781,6 +784,7 @@ spec_imports env callers dict_binds calls
            ; return (env, rules1 ++ rules2, spec_binds1 ++ spec_binds2) }
 
 spec_import :: SpecEnv               -- Passed in so that all top-level Ids are in scope
+                                     ---In-scope set includes the FloatedDictBinds
             -> [Id]                  -- Stack of imported functions being specialised
                                      -- See Note [specImport call stack]
             -> FloatedDictBinds      -- Dict bindings, used /only/ for filterCalls
@@ -806,23 +810,35 @@ spec_import env callers dict_binds cis@(CIS fn _)
        ; eps_rules <- getExternalRuleBase
        ; let rule_env = se_rules env `updExternalPackageRules` eps_rules
 
---       ; debugTraceMsg (text "specImport1" <+> vcat [ppr fn, ppr good_calls
---                                                    , ppr (getRules rule_env fn), ppr rhs])
+--       ; debugTraceMsg (text "specImport1" <+> vcat
+--           [ text "function:" <+> ppr fn
+--           , text "good calls:" <+> ppr good_calls
+--           , text "existing rules:" <+> ppr (getRules rule_env fn)
+--           , text "rhs:" <+> ppr rhs
+--           , text "dict_binds:" <+> ppr dict_binds ])
+
        ; (rules1, spec_pairs, MkUD { ud_binds = dict_binds1, ud_calls = new_calls })
-            <- runSpecM $ specCalls True env dict_binds
-                                    (getRules rule_env fn) good_calls fn rhs
+            <- runSpecM $ specCalls True env (getRules rule_env fn) good_calls fn rhs
 
        ; let spec_binds1 = [NonRec b r | (b,r) <- spec_pairs]
-             -- After the rules kick in we may get recursion, but
-             -- we rely on a global GlomBinds to sort that out later
+             -- After the rules kick in, via fireRewriteRules, we may get recursion,
+             -- but we rely on a global GlomBinds to sort that out later
              -- See Note [Glom the bindings if imported functions are specialised]
+             -- Meanwhile, though, bring the binders into scope
 
              new_subst = se_subst env `Core.extendSubstInScopeList` map fst spec_pairs
              new_env   = env { se_rules = rule_env `addLocalRules` rules1
                              , se_subst = new_subst }
+                         `bringFloatedDictsIntoScope` dict_binds1
+
+       -- Now specialise any cascaded calls
+--       ; debugTraceMsg (text "specImport 2" <+> vcat
+--           [ text "function:" <+> ppr fn
+--           , text "rules1:" <+> ppr rules1
+--           , text "spec_binds1" <+> ppr spec_binds1
+--           , text "dict_binds1" <+> ppr dict_binds1
+--           , text "new_calls" <+> ppr new_calls ])
 
-              -- Now specialise any cascaded calls
---       ; debugTraceMsg (text "specImport 2" <+> (ppr fn $$ ppr rules1 $$ ppr spec_binds1))
        ; (env, rules2, spec_binds2)
             <- spec_imports new_env (fn:callers)
                                     (dict_binds `thenFDBs` dict_binds1)
@@ -1561,10 +1577,11 @@ specDefn :: SpecEnv
 specDefn env body_uds fn rhs
   = do { let (body_uds_without_me, calls_for_me) = callsForMe fn body_uds
              rules_for_me = idCoreRules fn
-             dict_binds   = ud_binds body_uds
+             -- Bring into scope the binders from the floated dicts
+             env_w_dict_bndrs = bringFloatedDictsIntoScope env (ud_binds body_uds)
 
-       ; (rules, spec_defns, spec_uds) <- specCalls False env dict_binds
-                                               rules_for_me calls_for_me fn rhs
+       ; (rules, spec_defns, spec_uds) <- specCalls False env_w_dict_bndrs
+                                                    rules_for_me calls_for_me fn rhs
 
        ; return ( fn `addIdSpecialisations` rules
                 , spec_defns
@@ -1580,7 +1597,6 @@ specDefn env body_uds fn rhs
 specCalls :: Bool              -- True  =>  specialising imported fn
                                -- False =>  specialising local fn
           -> SpecEnv
-          -> FloatedDictBinds  -- Just so that we can extend the in-scope set
           -> [CoreRule]        -- Existing RULES for the fn
           -> [CallInfo]
           -> OutId -> InExpr
@@ -1594,7 +1610,7 @@ type SpecInfo = ( [CoreRule]       -- Specialisation rules
                 , [(Id,CoreExpr)]  -- Specialised definition
                 , UsageDetails )   -- Usage details from specialised RHSs
 
-specCalls spec_imp env dict_binds existing_rules calls_for_me fn rhs
+specCalls spec_imp env existing_rules calls_for_me fn rhs
         -- The first case is the interesting one
   |  notNull calls_for_me               -- And there are some calls to specialise
   && not (isNeverActive (idInlineActivation fn))
@@ -1610,8 +1626,11 @@ specCalls spec_imp env dict_binds existing_rules calls_for_me fn rhs
 --      See Note [Inline specialisations] for why we do not
 --      switch off specialisation for inline functions
 
-  = -- pprTrace "specCalls: some" (ppr fn $$ ppr calls_for_me $$ ppr existing_rules) $
-    foldlM spec_call ([], [], emptyUDs) calls_for_me
+  = do { -- debugTraceMsg (text "specCalls: some" <+> vcat
+         --   [ text "function" <+> ppr fn
+         --   , text "calls:" <+> ppr calls_for_me
+         --   , text "subst" <+> ppr (se_subst env) ])
+       ; foldlM spec_call ([], [], emptyUDs) calls_for_me }
 
   | otherwise   -- No calls or RHS doesn't fit our preconceptions
   = warnPprTrace (not (exprIsTrivial rhs) && notNull calls_for_me && not (isClassOpId fn))
@@ -1639,9 +1658,6 @@ specCalls spec_imp env dict_binds existing_rules calls_for_me fn rhs
     (rhs_bndrs, rhs_body) = collectBindersPushingCo rhs
                             -- See Note [Account for casts in binding]
 
-    -- Bring into scope the binders from the floated dicts
-    env_with_dict_bndrs = bringFloatedDictsIntoScope env dict_binds
-
     already_covered :: SpecEnv -> [CoreRule] -> [CoreExpr] -> Bool
     already_covered env new_rules args      -- Note [Specialisations already covered]
        = isJust (specLookupRule env fn args (beginPhase inl_act)
@@ -1667,22 +1683,22 @@ specCalls spec_imp env dict_binds existing_rules calls_for_me fn rhs
 
            ; ( useful, rhs_env2, leftover_bndrs
              , rule_bndrs, rule_lhs_args
-             , spec_bndrs1, dx_binds, spec_args) <- specHeader env_with_dict_bndrs
-                                                               rhs_bndrs all_call_args
-
---           ; pprTrace "spec_call" (vcat [ text "fun:       "  <+> ppr fn
---                                        , text "call info: "  <+> ppr _ci
---                                        , text "useful:    "  <+> ppr useful
---                                        , text "rule_bndrs:"  <+> ppr rule_bndrs
---                                        , text "lhs_args:  "  <+> ppr rule_lhs_args
---                                        , text "spec_bndrs1:" <+> ppr spec_bndrs1
---                                        , text "leftover_bndrs:" <+> pprIds leftover_bndrs
---                                        , text "spec_args: "  <+> ppr spec_args
---                                        , text "dx_binds:  "  <+> ppr dx_binds
---                                        , text "rhs_body"     <+> ppr rhs_body
---                                        , text "rhs_env2:  "  <+> ppr (se_subst rhs_env2)
---                                        , ppr dx_binds ]) $
---             return ()
+             , spec_bndrs1, dx_binds, spec_args) <- specHeader env rhs_bndrs all_call_args
+
+--           ; debugTraceMsg (text "spec_call" <+> vcat
+--                [ text "fun:       "  <+> ppr fn
+--                , text "call info: "  <+> ppr _ci
+--                , text "useful:    "  <+> ppr useful
+--                , text "rule_bndrs:"  <+> ppr rule_bndrs
+--                , text "lhs_args:  "  <+> ppr rule_lhs_args
+--                , text "spec_bndrs1:" <+> ppr spec_bndrs1
+--                , text "leftover_bndrs:" <+> pprIds leftover_bndrs
+--                , text "spec_args: "  <+> ppr spec_args
+--                , text "dx_binds:  "  <+> ppr dx_binds
+--                , text "rhs_bndrs"     <+> ppr rhs_bndrs
+--                , text "rhs_body"     <+> ppr rhs_body
+--                , text "rhs_env2:  "  <+> ppr (se_subst rhs_env2)
+--                , ppr dx_binds ]
 
            ; if not useful  -- No useful specialisation
                 || already_covered rhs_env2 rules_acc rule_lhs_args


=====================================
docs/users_guide/9.8.1-notes.rst
=====================================
@@ -10,4 +10,74 @@ Compiler
 ~~~~~~~~
 
 - Added a new warning :ghc-flag:`-Wterm-variable-capture` that helps to make code compatible with
-  the future extension ``RequiredTypeArguments``.
\ No newline at end of file
+  the future extension ``RequiredTypeArguments``.
+=======
+
+GHCi
+~~~~
+
+
+Runtime system
+~~~~~~~~~~~~~~
+
+- On POSIX systems that support timerfd, RTS shutdown no longer has to wait for
+  the next RTS 'tick' to occur before continuing the shutdown process. See #22692.
+
+``base`` library
+~~~~~~~~~~~~~~~~
+
+
+``ghc-prim`` library
+~~~~~~~~~~~~~~~~~~~~
+
+``ghc`` library
+~~~~~~~~~~~~~~~
+
+``ghc-heap`` library
+~~~~~~~~~~~~~~~~~~~~
+
+
+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.
+
+.. ghc-package-list::
+
+    libraries/array/array.cabal:             Dependency of ``ghc`` library
+    libraries/base/base.cabal:               Core library
+    libraries/binary/binary.cabal:           Dependency of ``ghc`` library
+    libraries/bytestring/bytestring.cabal:   Dependency of ``ghc`` library
+    libraries/Cabal/Cabal/Cabal.cabal:       Dependency of ``ghc-pkg`` utility
+    libraries/Cabal/Cabal-syntax/Cabal-syntax.cabal:  Dependency of ``ghc-pkg`` utility
+    libraries/containers/containers/containers.cabal: Dependency of ``ghc`` library
+    libraries/deepseq/deepseq.cabal:         Dependency of ``ghc`` library
+    libraries/directory/directory.cabal:     Dependency of ``ghc`` library
+    libraries/exceptions/exceptions.cabal:   Dependency of ``ghc`` and ``haskeline`` library
+    libraries/filepath/filepath.cabal:       Dependency of ``ghc`` library
+    compiler/ghc.cabal:                      The compiler itself
+    libraries/ghci/ghci.cabal:               The REPL interface
+    libraries/ghc-boot/ghc-boot.cabal:       Internal compiler library
+    libraries/ghc-boot-th/ghc-boot-th.cabal: Internal compiler library
+    libraries/ghc-compact/ghc-compact.cabal: Core library
+    libraries/ghc-heap/ghc-heap.cabal:       GHC heap-walking library
+    libraries/ghc-prim/ghc-prim.cabal:       Core library
+    libraries/haskeline/haskeline.cabal:     Dependency of ``ghci`` executable
+    libraries/hpc/hpc.cabal:                 Dependency of ``hpc`` executable
+    libraries/integer-gmp/integer-gmp.cabal: Core library
+    libraries/libiserv/libiserv.cabal:       Internal compiler library
+    libraries/mtl/mtl.cabal:                 Dependency of ``Cabal`` library
+    libraries/parsec/parsec.cabal:           Dependency of ``Cabal`` library
+    libraries/pretty/pretty.cabal:           Dependency of ``ghc`` library
+    libraries/process/process.cabal:         Dependency of ``ghc`` library
+    libraries/stm/stm.cabal:                 Dependency of ``haskeline`` library
+    libraries/template-haskell/template-haskell.cabal: Core library
+    libraries/terminfo/terminfo.cabal:       Dependency of ``haskeline`` library
+    libraries/text/text.cabal:               Dependency of ``Cabal`` library
+    libraries/time/time.cabal:               Dependency of ``ghc`` library
+    libraries/transformers/transformers.cabal: Dependency of ``ghc`` library
+    libraries/unix/unix.cabal:               Dependency of ``ghc`` library
+    libraries/Win32/Win32.cabal:             Dependency of ``ghc`` library
+    libraries/xhtml/xhtml.cabal:             Dependency of ``haddock`` executable


=====================================
libraries/base/Data/Functor/Compose.hs
=====================================
@@ -33,7 +33,7 @@ import Data.Coerce (coerce)
 import Data.Data (Data)
 import Data.Type.Equality (TestEquality(..), (:~:)(..))
 import GHC.Generics (Generic, Generic1)
-import Text.Read ()
+import Text.Read (Read(..), ReadPrec, readListDefault, readListPrecDefault)
 
 infixr 9 `Compose`
 
@@ -55,9 +55,14 @@ deriving instance Eq (f (g a)) => Eq (Compose f g a)
 -- | @since 4.18.0.0
 deriving instance Ord (f (g a)) => Ord (Compose f g a)
 -- | @since 4.18.0.0
-deriving instance Read (f (g a)) => Read (Compose f g a)
+instance Read (f (g a)) => Read (Compose f g a) where
+    readPrec = liftReadPrecCompose readPrec
+
+    readListPrec = readListPrecDefault
+    readList     = readListDefault
 -- | @since 4.18.0.0
-deriving instance Show (f (g a)) => Show (Compose f g a)
+instance Show (f (g a)) => Show (Compose f g a) where
+    showsPrec = liftShowsPrecCompose showsPrec
 
 -- Instances of lifted Prelude classes
 
@@ -72,8 +77,8 @@ instance (Ord1 f, Ord1 g) => Ord1 (Compose f g) where
 
 -- | @since 4.9.0.0
 instance (Read1 f, Read1 g) => Read1 (Compose f g) where
-    liftReadPrec rp rl = readData $
-        readUnaryWith (liftReadPrec rp' rl') "Compose" Compose
+    liftReadPrec rp rl =
+        liftReadPrecCompose (liftReadPrec rp' rl')
       where
         rp' = liftReadPrec     rp rl
         rl' = liftReadListPrec rp rl
@@ -83,12 +88,20 @@ instance (Read1 f, Read1 g) => Read1 (Compose f g) where
 
 -- | @since 4.9.0.0
 instance (Show1 f, Show1 g) => Show1 (Compose f g) where
-    liftShowsPrec sp sl d (Compose x) =
-        showsUnaryWith (liftShowsPrec sp' sl') "Compose" d x
+    liftShowsPrec sp sl =
+        liftShowsPrecCompose (liftShowsPrec sp' sl')
       where
         sp' = liftShowsPrec sp sl
         sl' = liftShowList sp sl
 
+-- The workhorse for Compose's Read and Read1 instances.
+liftReadPrecCompose :: ReadPrec (f (g a)) -> ReadPrec (Compose f g a)
+liftReadPrecCompose rp = readData $ readUnaryWith rp "Compose" Compose
+
+-- The workhorse for Compose's Show and Show1 instances.
+liftShowsPrecCompose :: (Int -> f (g a) -> ShowS) -> Int -> Compose f g a -> ShowS
+liftShowsPrecCompose sp d (Compose x) = showsUnaryWith sp "Compose" d x
+
 -- Functor instances
 
 -- | @since 4.9.0.0


=====================================
libraries/base/tests/T22816.hs
=====================================
@@ -0,0 +1,31 @@
+module Main (main) where
+
+import Data.Functor.Classes
+import Data.Functor.Compose
+import Text.ParserCombinators.ReadP as P
+import Text.ParserCombinators.ReadPrec (ReadPrec, lift, minPrec, readPrec_to_S)
+
+readEither' :: ReadPrec a -> String -> Either String a
+readEither' rp s =
+  case [ x | (x,"") <- readPrec_to_S read' minPrec s ] of
+    [x] -> Right x
+    []  -> Left "read1: no parse"
+    _   -> Left "read1: ambiguous parse"
+ where
+  read' =
+    do x <- rp
+       lift P.skipSpaces
+       return x
+
+-- | Like 'read', but tailored to 'Read1'.
+read1 :: (Read1 f, Read a) => String -> f a
+read1 s = either errorWithoutStackTrace id (readEither' readPrec1 s)
+
+exRead, exRead1 :: Compose Maybe Maybe Int
+exRead  = read  "Compose Nothing"
+exRead1 = read1 "Compose Nothing"
+
+main :: IO ()
+main = do
+  putStrLn $ showsPrec  0 exRead  ""
+  putStrLn $ showsPrec1 0 exRead1 ""


=====================================
libraries/base/tests/T22816.stdout
=====================================
@@ -0,0 +1,2 @@
+Compose Nothing
+Compose Nothing


=====================================
libraries/base/tests/all.T
=====================================
@@ -286,6 +286,7 @@ test('T18642',
 test('T19288', exit_code(1), compile_and_run, [''])
 test('T19719', normal, compile_and_run, [''])
 test('T20107', extra_run_opts('+RTS -M50M'), compile_and_run, ['-package bytestring'])
+test('T22816', normal, compile_and_run, [''])
 test('trace', normal, compile_and_run, [''])
 test('listThreads', js_broken(22261), compile_and_run, [''])
 test('inits1tails1', normal, compile_and_run, [''])


=====================================
rts/posix/ticker/Pthread.c
=====================================
@@ -43,6 +43,7 @@
 #include "Proftimer.h"
 #include "Schedule.h"
 #include "posix/Clock.h"
+#include <sys/poll.h>
 
 #include <time.h>
 #if HAVE_SYS_TIME_H
@@ -95,28 +96,53 @@ static OSThreadId thread;
 // file descriptor for the timer (Linux only)
 static int timerfd = -1;
 
+// pipe for signaling exit
+static int pipefds[2];
+
 static void *itimer_thread_func(void *_handle_tick)
 {
     TickProc handle_tick = _handle_tick;
     uint64_t nticks;
+    ssize_t r = 0;
+    struct pollfd pollfds[2];
+
+#if USE_TIMERFD_FOR_ITIMER
+    pollfds[0].fd = pipefds[0];
+    pollfds[0].events = POLLIN;
+    pollfds[1].fd = timerfd;
+    pollfds[1].events = POLLIN;
+#endif
 
     // Relaxed is sufficient: If we don't see that exited was set in one iteration we will
     // see it next time.
     TSAN_ANNOTATE_BENIGN_RACE(&exited, "itimer_thread_func");
     while (!RELAXED_LOAD(&exited)) {
         if (USE_TIMERFD_FOR_ITIMER) {
-            ssize_t r = read(timerfd, &nticks, sizeof(nticks));
-            if ((r == 0) && (errno == 0)) {
-               /* r == 0 is expected only for non-blocking fd (in which case
-                * errno should be EAGAIN) but we use a blocking fd.
-                *
-                * Due to a kernel bug (cf https://lkml.org/lkml/2019/8/16/335)
-                * on some platforms we could see r == 0 and errno == 0.
-                */
-               IF_DEBUG(scheduler, debugBelch("read(timerfd) returned 0 with errno=0. This is a known kernel bug. We just ignore it."));
+            if (poll(pollfds, 2, -1) == -1) {
+                sysErrorBelch("Ticker: poll failed: %s", strerror(errno));
             }
-            else if (r != sizeof(nticks) && errno != EINTR) {
-               barf("Ticker: read(timerfd) failed with %s and returned %zd", strerror(errno), r);
+
+            // We check the pipe first, even though the timerfd may also have triggered.
+            if (pollfds[0].revents & POLLIN) {
+                // the pipe is ready for reading, the only possible reason is that we're exiting
+                exited = true; // set this again to make sure even RELAXED_LOAD will read the proper value
+                // no further action needed, skip ahead to handling the final tick and then stopping
+            }
+            else if (pollfds[1].revents & POLLIN) { // the timerfd is ready for reading
+                r = read(timerfd, &nticks, sizeof(nticks)); // this should never block now
+
+                if ((r == 0) && (errno == 0)) {
+                   /* r == 0 is expected only for non-blocking fd (in which case
+                    * errno should be EAGAIN) but we use a blocking fd.
+                    *
+                    * Due to a kernel bug (cf https://lkml.org/lkml/2019/8/16/335)
+                    * on some platforms we could see r == 0 and errno == 0.
+                    */
+                   IF_DEBUG(scheduler, debugBelch("read(timerfd) returned 0 with errno=0. This is a known kernel bug. We just ignore it."));
+                }
+                else if (r != sizeof(nticks) && errno != EINTR) {
+                   barf("Ticker: read(timerfd) failed with %s and returned %zd", strerror(errno), r);
+                }
             }
         } else {
             if (rtsSleep(itimer_interval) != 0) {
@@ -138,8 +164,10 @@ static void *itimer_thread_func(void *_handle_tick)
         }
     }
 
-    if (USE_TIMERFD_FOR_ITIMER)
+    if (USE_TIMERFD_FOR_ITIMER) {
         close(timerfd);
+    }
+
     return NULL;
 }
 
@@ -185,6 +213,10 @@ initTicker (Time interval, TickProc handle_tick)
     if (timerfd_settime(timerfd, 0, &it, NULL)) {
         barf("timerfd_settime: %s", strerror(errno));
     }
+
+    if (pipe(pipefds) < 0) {
+        barf("pipe: %s", strerror(errno));
+    }
 #endif
 
     /*
@@ -237,9 +269,21 @@ exitTicker (bool wait)
 
     // wait for ticker to terminate if necessary
     if (wait) {
+#if USE_TIMERFD_FOR_ITIMER
+        // write anything to the pipe to trigger poll() in the ticker thread
+        if (write(pipefds[1], "stop", 5) < 0) {
+            sysErrorBelch("Ticker: Failed to write to pipe: %s", strerror(errno));
+        }
+#endif
         if (pthread_join(thread, NULL)) {
             sysErrorBelch("Ticker: Failed to join: %s", strerror(errno));
         }
+#if USE_TIMERFD_FOR_ITIMER
+        // These need to happen AFTER the ticker thread has finished to prevent a race condition
+        // where the ticker thread closes the read end of the pipe before we're done writing to it.
+        close(pipefds[0]);
+        close(pipefds[1]);
+#endif
         closeMutex(&mutex);
         closeCondition(&start_cond);
     } else {


=====================================
testsuite/tests/simplCore/should_compile/T22715_2.hs
=====================================
@@ -0,0 +1,6 @@
+module T22715_2 where
+
+import T22715_2a
+
+debugTerminalKeys :: (forall m. CommandMonad m => m Char) -> Input IO Char
+debugTerminalKeys eval = runIdT eval


=====================================
testsuite/tests/simplCore/should_compile/T22715_2a.hs
=====================================
@@ -0,0 +1,29 @@
+{-# OPTIONS_GHC -Wno-missing-methods #-}
+
+module T22715_2a where
+
+newtype IdentityT m a = IdentityT (m a) deriving Functor
+newtype IdT m a = IdT {runIdT :: m a} deriving Functor
+
+class Functor m => SillyA m where
+  unused :: m a -> m a
+
+class SillyA m => SillyB m where
+  unused2 :: m a -> m a
+
+instance SillyA m => SillyA (IdentityT m) where
+instance SillyB m => SillyB (IdentityT m) where
+
+instance SillyA m => SillyA (IdT m) where
+instance SillyB m => SillyB (IdT m) where
+
+instance SillyA IO where
+instance SillyB IO where
+
+class Functor m => Special m
+instance Functor m => Special (IdT m)
+
+type Input m = IdentityT (IdentityT m)
+
+class (Special m, SillyB m) => CommandMonad m
+instance SillyB m => CommandMonad (IdT (Input m))


=====================================
testsuite/tests/simplCore/should_compile/all.T
=====================================
@@ -469,4 +469,4 @@ test('T22662', normal, compile, [''])
 test('T22725', normal, compile, ['-O'])
 test('T22502', normal, compile, ['-O'])
 test('T22611', [when(wordsize(32), skip), grep_errmsg(r'\$salterF') ], compile, ['-O -ddump-simpl -dsuppress-uniques -dsuppress-all'])
-
+test('T22715_2', normal, multimod_compile, ['T22715_2', '-v0 -O -fspecialise-aggressively'])



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/46cc07d133309fdabf846eb2547998eec20fed1c...d9e4143cd1d2984a18d85763b3d64509f8bae6e8

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/46cc07d133309fdabf846eb2547998eec20fed1c...d9e4143cd1d2984a18d85763b3d64509f8bae6e8
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