[Git][ghc/ghc][wip/marge_bot_batch_merge_job] 6 commits: Fix restarts in .ghcid

Marge Bot (@marge-bot) gitlab at gitlab.haskell.org
Sat Oct 14 20:57:30 UTC 2023



Marge Bot pushed to branch wip/marge_bot_batch_merge_job at Glasgow Haskell Compiler / GHC


Commits:
513861f9 by Sebastian Graf at 2023-10-14T16:57:01-04:00
Fix restarts in .ghcid

Using the whole of `hadrian/` restarted in a loop for me.

- - - - -
9882c7f1 by Sebastian Graf at 2023-10-14T16:57:01-04:00
CorePrep: Refactor FloatingBind (#23442)

A drastically improved architecture for local floating in CorePrep
that decouples the decision of whether a float is going to be let- or case-bound
from how far it can float (out of strict contexts, out of lazy contexts, to
top-level).

There are a couple of new Notes describing the effort:

  * `Note [Floating in CorePrep]` for the overview
  * `Note [BindInfo and FloatInfo]` for the new classification of floats
  * `Note [Floats and FloatDecision]` for how FloatInfo is used to inform
    floating decisions

This is necessary ground work for proper treatment of Strict fields and
unlifted values at top-level.

Fixes #23442.

NoFib results (omitted = 0.0%):
```
--------------------------------------------------------------------------------
        Program         Allocs    Instrs
--------------------------------------------------------------------------------
         pretty           0.0%     -1.6%
            scc           0.0%     -1.7%
--------------------------------------------------------------------------------
            Min           0.0%     -1.7%
            Max           0.0%     -0.0%
 Geometric Mean          -0.0%     -0.0%
```

- - - - -
4503c444 by Matthew Pickering at 2023-10-14T16:57:02-04:00
hadrian: Move ghcBinDeps into ghcLibDeps

This completes a5227080b57cb51ac34d4c9de1accdf6360b818b, the
`ghc-usage.txt` and `ghci-usage.txt` file are also used by the `ghc`
library so need to make sure they are present in the libdir even if we
are not going to build `ghc-bin`.

This also fixes things for cross compilers because the stage2
cross-compiler requires the ghc-usage.txt file, but we are using
the stage2 lib folder but not building stage3:exe:ghc-bin so
ghc-usage.txt was not being generated.

- - - - -
c219dfe7 by sheaf at 2023-10-14T16:57:04-04:00
Combine GREs when combining in mkImportOccEnv

In `GHC.Rename.Names.mkImportOccEnv`, we sometimes discard one import
item in favour of another, as explained in Note [Dealing with imports]
in `GHC.Rename.Names`. However, this can cause us to lose track of
important parent information.

Consider for example #24084:

  module M1 where { class C a where { type T a } }
  module M2 ( module M1 ) where { import M1 }
  module M3 where { import M2 ( C, T ); instance C () where T () = () }

When processing the import list of `M3`, we start off (for reasons that
are not relevant right now) with two `Avail`s attached to `T`, namely
`C(C, T)` and `T(T)`. We combine them in the `combine` function of
`mkImportOccEnv`; as described in Note [Dealing with imports] we discard
`C(C, T)` in favour of `T(T)`. However, in doing so, we **must not**
discard the information want that `C` is the parent of `T`. Indeed,
losing track of this information can cause errors when importing,
as we could get an error of the form

  ‘T’ is not a (visible) associated type of class ‘C’

We fix this by combining the two GREs for `T` using `plusGRE`.

Fixes #24084

- - - - -
b4a0deb6 by Ilias Tsitsimpis at 2023-10-14T16:57:06-04:00
hadrian: Pass -DNOSMP to C compiler when needed

Hadrian passes the -DNOSMP flag to GHC when the target doesn't support
SMP, but doesn't pass it to CC as well, leading to the following
compilation error on mips64el:

| Run Cc (FindCDependencies CDep) Stage1: rts/sm/NonMovingScav.c => _build/stage1/rts/build/c/sm/NonMovingScav.o.d
Command line: /usr/bin/mips64el-linux-gnuabi64-gcc -E -MM -MG -MF _build/stage1/rts/build/c/hooks/FlagDefaults.thr_debug_p_o.d -MT _build/stage1/rts/build/c/hooks/FlagDefaults.o -Irts/include -I_build/stage1/rts/build -I_build/stage1/rts/build/include -Irts/include -x c rts/hooks/FlagDefaults.c -Wall -Wextra -Wstrict-prototypes -Wmissing-prototypes -Wmissing-declarations -Winline -Wpointer-arith -Wmissing-noreturn -Wnested-externs -Wredundant-decls -Wundef -fno-strict-aliasing -DTHREADED_RTS -DDEBUG -fomit-frame-pointer -O2 -g -Irts -I_build/stage1/rts/build -DDEBUG -fno-omit-frame-pointer -g3 -O0
===> Command failed with error code: 1
In file included from rts/include/Stg.h:348,
                 from rts/include/Rts.h:38,
                 from rts/hooks/FlagDefaults.c:8:
rts/include/stg/SMP.h:416:2: error: #error memory barriers unimplemented on this architecture
  416 | #error memory barriers unimplemented on this architecture
      |  ^~~~~
rts/include/stg/SMP.h:440:2: error: #error memory barriers unimplemented on this architecture
  440 | #error memory barriers unimplemented on this architecture
      |  ^~~~~
rts/include/stg/SMP.h:464:2: error: #error memory barriers unimplemented on this architecture
  464 | #error memory barriers unimplemented on this architecture
      |  ^~~~~

The old make system correctly passed this flag to both GHC and CC [1].

Fix this error by passing -DNOSMP to CC as well.

[1] https://gitlab.haskell.org/ghc/ghc/-/blob/00920f176b0235d5bb52a8e054d89a664f8938fe/rts/ghc.mk#L407

Closes #24082

- - - - -
cc5c6853 by John Ericson at 2023-10-14T16:57:06-04:00
Users Guide: Drop dead code for Haddock refs to `parallel`

I noticed while working on !11451 that `@LIBRARY_parallel_UNIT_ID@` was
not substituted. It is dead code -- there is no `parallel-ref` usages
and it doesn't look like there ever was (going back to
3e5d0f188d6c8633e55e9ba6c8941c07e459fa4b), so let's delete it.

- - - - -


17 changed files:

- .ghcid
- compiler/GHC/CoreToStg/Prep.hs
- compiler/GHC/Data/OrdList.hs
- compiler/GHC/Rename/Env.hs
- compiler/GHC/Rename/Names.hs
- compiler/GHC/Types/Avail.hs
- compiler/GHC/Types/Name/Reader.hs
- docs/users_guide/conf.py
- docs/users_guide/ghc_config.py.in
- hadrian/src/Base.hs
- hadrian/src/Builder.hs
- hadrian/src/Rules/Program.hs
- hadrian/src/Settings/Packages.hs
- + testsuite/tests/rename/should_compile/T24084.hs
- + testsuite/tests/rename/should_compile/T24084_A.hs
- + testsuite/tests/rename/should_compile/T24084_B.hs
- testsuite/tests/rename/should_compile/all.T


Changes:

=====================================
.ghcid
=====================================
@@ -2,4 +2,4 @@
 --reload compiler
 --reload ghc
 --reload includes
---restart hadrian/
+--restart hadrian/ghci


=====================================
compiler/GHC/CoreToStg/Prep.hs
=====================================
@@ -1,4 +1,5 @@
 {-# LANGUAGE BangPatterns #-}
+{-# LANGUAGE ViewPatterns #-}
 
 {-# OPTIONS_GHC -Wno-incomplete-uni-patterns #-}
 
@@ -97,7 +98,8 @@ The goal of this pass is to prepare for code generation.
     (The code generator can't deal with anything else.)
     Type lambdas are ok, however, because the code gen discards them.
 
-5.  [Not any more; nuked Jun 2002] Do the seq/par munging.
+5.  ANF-isation results in additional bindings that can obscure values.
+    We float these out; see Note [Floating in CorePrep].
 
 6.  Clone all local Ids.
     This means that all such Ids are unique, rather than the
@@ -165,7 +167,7 @@ Here is the syntax of the Core produced by CorePrep:
     Expressions
        body ::= app
               | let(rec) x = rhs in body     -- Boxed only
-              | case body of pat -> body
+              | case app of pat -> body
               | /\a. body | /\c. body
               | body |> co
 
@@ -217,7 +219,7 @@ corePrepPgm logger cp_cfg pgm_cfg
         binds_out = initUs_ us $ do
                       floats1 <- corePrepTopBinds initialCorePrepEnv binds
                       floats2 <- corePrepTopBinds initialCorePrepEnv implicit_binds
-                      return (deFloatTop (floats1 `appendFloats` floats2))
+                      return (deFloatTop (floats1 `zipFloats` floats2))
 
     endPassIO logger (cpPgm_endPassConfig pgm_cfg)
               binds_out []
@@ -244,7 +246,7 @@ corePrepTopBinds initialCorePrepEnv binds
                                  -- Only join points get returned this way by
                                  -- cpeBind, and no join point may float to top
                                floatss <- go env' binds
-                               return (floats `appendFloats` floatss)
+                               return (floats `zipFloats` floatss)
 
 mkDataConWorkers :: Bool -> ModLocation -> [TyCon] -> [CoreBind]
 -- See Note [Data constructor workers]
@@ -268,7 +270,40 @@ mkDataConWorkers generate_debug_info mod_loc data_tycons
              LexicalFastString $ mkFastString $ renderWithContext defaultSDocContext $ ppr name
            span1 file = realSrcLocSpan $ mkRealSrcLoc (mkFastString file) 1 1
 
-{-
+{- Note [Floating in CorePrep]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ANFisation risks producing a lot of nested lets that obscures values:
+  let v = (:) (f 14) [] in e
+  ==> { ANF in CorePrep }
+  let v = let sat = f 14 in (:) sat [] in e
+Here, `v` is not a value anymore, and we'd allocate a thunk closure for `v` that
+allocates a thunk for `sat` and then allocates the cons cell.
+Hence we carry around a bunch of floated bindings with us so that we again
+expose the values:
+  let v = let sat = f 14 in (:) sat [] in e
+  ==> { Float sat }
+  let sat = f 14 in
+  let v = (:) sat [] in e
+(We will not do this transformation if `v` does not become a value afterwards;
+see Note [wantFloatLocal].)
+If `v` is bound at the top-level, we might even float `sat` to top-level;
+see Note [Floating out of top level bindings].
+For nested let bindings, we have to keep in mind Note [Core letrec invariant]
+and may exploit strict contexts; see Note [wantFloatLocal].
+
+There are 3 main categories of floats, encoded in the `FloatingBind` type:
+
+  * `Float`: A floated binding, as `sat` above.
+    These come in different flavours as described by their `FloatInfo` and
+    `BindInfo`, which captures how far the binding can be floated and whether or
+    not we want to case-bind. See Note [BindInfo and FloatInfo].
+  * `UnsafeEqualityCase`: Used for floating around unsafeEqualityProof bindings;
+    see (U3) of Note [Implementing unsafeCoerce].
+    It's exactly a `Float` that is `CaseBound` and `LazyContextFloatable`
+    (see `mkNonRecFloat`), but one that has a non-DEFAULT Case alternative to
+    bind the unsafe coercion field of the Refl constructor.
+  * `FloatTick`: A floated `Tick`. See Note [Floating Ticks in CorePrep].
+
 Note [Floating out of top level bindings]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 NB: we do need to float out of top-level bindings
@@ -557,9 +592,9 @@ cpeBind top_lvl env (NonRec bndr rhs)
              floats1 | triv_rhs, isInternalName (idName bndr)
                      = floats
                      | otherwise
-                     = addFloat floats new_float
+                     = snocFloat floats new_float
 
-             new_float = mkFloat env dmd is_unlifted bndr1 rhs1
+             new_float = mkNonRecFloat env dmd is_unlifted bndr1 rhs1
 
        ; return (env2, floats1, Nothing) }
 
@@ -578,15 +613,21 @@ cpeBind top_lvl env (Rec pairs)
        ; stuff <- zipWithM (cpePair top_lvl Recursive topDmd False env')
                            bndrs1 rhss
 
-       ; let (floats_s, rhss1) = unzip stuff
-             -- Glom all floats into the Rec, *except* FloatStrings which can
-             -- (and must, because unlifted!) float further.
-             (string_floats, all_pairs) =
-               foldrOL add_float (emptyFloats, bndrs1 `zip` rhss1)
-                                 (concatFloats floats_s)
+       ; let (zipManyFloats -> floats, rhss1) = unzip stuff
+             -- Glom all floats into the Rec, *except* FloatStrings; see
+             -- see Note [ANF-ising literal string arguments], Wrinkle (FS1)
+             is_lit (Float (NonRec _ rhs) CaseBound TopLvlFloatable) = exprIsTickedString rhs
+             is_lit _                                                = False
+             (string_floats, top) = partitionOL is_lit (fs_binds floats)
+                 -- Strings will *always* be in `top_floats` (we made sure of
+                 -- that in `snocOL`), so that's the only field we need to
+                 -- partition.
+             floats'   = floats { fs_binds = top }
+             all_pairs = foldrOL add_float (bndrs1 `zip` rhss1) (getFloats floats')
        -- use env below, so that we reset cpe_rec_ids
        ; return (extendCorePrepEnvList env (bndrs `zip` bndrs1),
-                 string_floats `addFloat` FloatLet (Rec all_pairs),
+                 snocFloat (emptyFloats { fs_binds = string_floats })
+                           (Float (Rec all_pairs) LetBound TopLvlFloatable),
                  Nothing) }
 
   | otherwise -- See Note [Join points and floating]
@@ -604,10 +645,11 @@ cpeBind top_lvl env (Rec pairs)
 
         -- Flatten all the floats, and the current
         -- group into a single giant Rec
-    add_float (FloatLet (NonRec b r)) (ss, prs2) = (ss, (b,r)    : prs2)
-    add_float (FloatLet (Rec prs1))   (ss, prs2) = (ss, prs1    ++ prs2)
-    add_float s at FloatString{}         (ss, prs2) = (addFloat ss s, prs2)
-    add_float b                       _          = pprPanic "cpeBind" (ppr b)
+    add_float (Float bind bound _) prs2
+      | bound /= CaseBound = case bind of
+          NonRec x e -> (x,e) : prs2
+          Rec prs1 -> prs1 ++ prs2
+    add_float f                       _    = pprPanic "cpeBind" (ppr f)
 
 ---------------
 cpePair :: TopLevelFlag -> RecFlag -> Demand -> Bool
@@ -620,7 +662,8 @@ cpePair top_lvl is_rec dmd is_unlifted env bndr rhs
     do { (floats1, rhs1) <- cpeRhsE env rhs
 
        -- See if we are allowed to float this stuff out of the RHS
-       ; (floats2, rhs2) <- float_from_rhs floats1 rhs1
+       ; let dec = want_float_from_rhs floats1 rhs1
+       ; (floats2, rhs2) <- executeFloatDecision dec floats1 rhs1
 
        -- Make the arity match up
        ; (floats3, rhs3)
@@ -629,8 +672,8 @@ cpePair top_lvl is_rec dmd is_unlifted env bndr rhs
                else warnPprTrace True "CorePrep: silly extra arguments:" (ppr bndr) $
                                -- Note [Silly extra arguments]
                     (do { v <- newVar (idType bndr)
-                        ; let float = mkFloat env topDmd False v rhs2
-                        ; return ( addFloat floats2 float
+                        ; let float = mkNonRecFloat env topDmd False v rhs2
+                        ; return ( snocFloat floats2 float
                                  , cpeEtaExpand arity (Var v)) })
 
         -- Wrap floating ticks
@@ -640,35 +683,9 @@ cpePair top_lvl is_rec dmd is_unlifted env bndr rhs
   where
     arity = idArity bndr        -- We must match this arity
 
-    ---------------------
-    float_from_rhs floats rhs
-      | isEmptyFloats floats = return (emptyFloats, rhs)
-      | isTopLevel top_lvl   = float_top    floats rhs
-      | otherwise            = float_nested floats rhs
-
-    ---------------------
-    float_nested floats rhs
-      | wantFloatNested is_rec dmd is_unlifted floats rhs
-                  = return (floats, rhs)
-      | otherwise = dontFloat floats rhs
-
-    ---------------------
-    float_top floats rhs
-      | allLazyTop floats
-      = return (floats, rhs)
-
-      | otherwise
-      = dontFloat floats rhs
-
-dontFloat :: Floats -> CpeRhs -> UniqSM (Floats, CpeBody)
--- Non-empty floats, but do not want to float from rhs
--- So wrap the rhs in the floats
--- But: rhs1 might have lambdas, and we can't
---      put them inside a wrapBinds
-dontFloat floats1 rhs
-  = do { (floats2, body) <- rhsToBody rhs
-        ; return (emptyFloats, wrapBinds floats1 $
-                               wrapBinds floats2 body) }
+    want_float_from_rhs floats rhs
+      | isTopLevel top_lvl = wantFloatTop floats
+      | otherwise          = wantFloatLocal is_rec dmd is_unlifted floats rhs
 
 {- Note [Silly extra arguments]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -754,14 +771,14 @@ cpeRhsE env (Let bind body)
        ; (body_floats, body') <- cpeRhsE env' body
        ; let expr' = case maybe_bind' of Just bind' -> Let bind' body'
                                          Nothing    -> body'
-       ; return (bind_floats `appendFloats` body_floats, expr') }
+       ; return (bind_floats `appFloats` body_floats, expr') }
 
 cpeRhsE env (Tick tickish expr)
   -- Pull out ticks if they are allowed to be floated.
   | tickishFloatable tickish
   = do { (floats, body) <- cpeRhsE env expr
          -- See [Floating Ticks in CorePrep]
-       ; return (unitFloat (FloatTick tickish) `appendFloats` floats, body) }
+       ; return (FloatTick tickish `consFloat` floats, body) }
   | otherwise
   = do { body <- cpeBodyNF env expr
        ; return (emptyFloats, mkTick tickish' body) }
@@ -805,12 +822,12 @@ cpeRhsE env (Case scrut bndr _ alts@[Alt con bs _])
        ; (floats_rhs, rhs) <- cpeBody env rhs
          -- ... but we want to float `floats_rhs` as in (U3) so that rhs' might
          -- become a value
-       ; let case_float = FloatCase scrut bndr con bs True
-         -- NB: True <=> ok-for-spec; it is OK to "evaluate" the proof eagerly.
+       ; let case_float = UnsafeEqualityCase scrut bndr con bs
+         -- NB: It is OK to "evaluate" the proof eagerly.
          --     Usually there's the danger that we float the unsafeCoerce out of
          --     a branching Case alt. Not so here, because the regular code path
          --     for `cpeRhsE Case{}` will not float out of alts.
-             floats = addFloat floats_scrut case_float `appendFloats` floats_rhs
+             floats = snocFloat floats_scrut case_float `appFloats` floats_rhs
        ; return (floats, rhs) }
 
 cpeRhsE env (Case scrut bndr ty alts)
@@ -859,7 +876,7 @@ cpeBody :: CorePrepEnv -> CoreExpr -> UniqSM (Floats, CpeBody)
 cpeBody env expr
   = do { (floats1, rhs) <- cpeRhsE env expr
        ; (floats2, body) <- rhsToBody rhs
-       ; return (floats1 `appendFloats` floats2, body) }
+       ; return (floats1 `appFloats` floats2, body) }
 
 --------
 rhsToBody :: CpeRhs -> UniqSM (Floats, CpeBody)
@@ -882,7 +899,7 @@ rhsToBody expr@(Lam {})   -- See Note [No eta reduction needed in rhsToBody]
   | otherwise                   -- Some value lambdas
   = do { let rhs = cpeEtaExpand (exprArity expr) expr
        ; fn <- newVar (exprType rhs)
-       ; let float = FloatLet (NonRec fn rhs)
+       ; let float = Float (NonRec fn rhs) LetBound TopLvlFloatable
        ; return (unitFloat float, Var fn) }
   where
     (bndrs,_) = collectBinders expr
@@ -1125,7 +1142,8 @@ cpeApp top_env expr
         :: CorePrepEnv
         -> [ArgInfo]                  -- The arguments (inner to outer)
         -> CpeApp                     -- The function
-        -> Floats
+        -> Floats                     -- INVARIANT: These floats don't bind anything that is in the CpeApp!
+                                      -- Just stuff floated out from the head of the application.
         -> [Demand]
         -> Maybe Arity
         -> UniqSM (CpeApp
@@ -1170,7 +1188,7 @@ cpeApp top_env expr
                    (ss1 : ss_rest, False) -> (ss1,    ss_rest)
                    ([],            _)     -> (topDmd, [])
         (fs, arg') <- cpeArg top_env ss1 arg
-        rebuild_app' env as (App fun' arg') (fs `appendFloats` floats) ss_rest rt_ticks (req_depth-1)
+        rebuild_app' env as (App fun' arg') (fs `zipFloats` floats) ss_rest rt_ticks (req_depth-1)
 
       CpeCast co
         -> rebuild_app' env as (Cast fun' co) floats ss rt_ticks req_depth
@@ -1182,7 +1200,7 @@ cpeApp top_env expr
            rebuild_app' env as fun' floats ss (tickish:rt_ticks) req_depth
         | otherwise
         -- See [Floating Ticks in CorePrep]
-        -> rebuild_app' env as fun' (addFloat floats (FloatTick tickish)) ss rt_ticks req_depth
+        -> rebuild_app' env as fun' (snocFloat floats (FloatTick tickish)) ss rt_ticks req_depth
 
 isLazyExpr :: CoreExpr -> Bool
 -- See Note [lazyId magic] in GHC.Types.Id.Make
@@ -1261,8 +1279,7 @@ Other relevant Notes:
  * Note [runRW arg] below, describing a non-obvious case where the
    late-inlining could go wrong.
 
-
- Note [runRW arg]
+Note [runRW arg]
 ~~~~~~~~~~~~~~~~~~~
 Consider the Core program (from #11291),
 
@@ -1294,7 +1311,6 @@ the function and the arguments) will forgo binding it to a variable. By
 contrast, in the non-bottoming case of `hello` above  the function will be
 deemed non-trivial and consequently will be case-bound.
 
-
 Note [Simplification of runRW#]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Consider the program,
@@ -1408,8 +1424,7 @@ But with -O0, there is no FloatOut, so CorePrep must do the ANFisation to
     foo = Foo s
 
 (String literals are the only kind of binding allowed at top-level and hence
-their floats are `OkToSpec` like lifted bindings, whereas all other unlifted
-floats are `IfUnboxedOk` so that they don't float to top-level.)
+their `FloatInfo` is `TopLvlFloatable`.)
 
 This appears to lead to bad code if the arg is under a lambda, because CorePrep
 doesn't float out of RHSs, e.g., (T23270)
@@ -1432,24 +1447,13 @@ But actually, it doesn't, because "turtle"# is already an HNF. Here is the Cmm:
 
 Wrinkles:
 
-(FS1) It is crucial that we float out String literals out of RHSs that could
-      become values, e.g.,
-
-        let t = case "turtle"# of s { __DEFAULT -> MkT s }
-        in f t
-
-      where `MkT :: Addr# -> T`. We want
-
-        let s = "turtle"#; t = MkT s
-        in f t
-
-      because the former allocates an extra thunk for `t`.
-      Normally, the `case turtle# of s ...` becomes a `FloatCase` and
-      we don't float `FloatCase` outside of (recursive) RHSs, so we get the
-      former program (this is the 'allLazyNested' test in 'wantFloatNested').
-      That is what we use `FloatString` for: It is essentially a `FloatCase`
-      which is always ok-to-spec/can be regarded as a non-allocating value and
-      thus be floated aggressively to expose more value bindings.
+(FS1) We detect string literals in `cpeBind Rec{}` and float them out anyway;
+      otherwise we'd try to bind a string literal in a letrec, violating
+      Note [Core letrec invariant]. Since we know that literals don't have
+      free variables, we float further.
+      Arguably, we could just as well relax the letrec invariant for
+      string literals, or anthing that is a value (lifted or not).
+      This is tracked in #24036.
 -}
 
 -- This is where we arrange that a non-trivial argument is let-bound
@@ -1459,10 +1463,9 @@ cpeArg env dmd arg
   = do { (floats1, arg1) <- cpeRhsE env arg     -- arg1 can be a lambda
        ; let arg_ty      = exprType arg1
              is_unlifted = isUnliftedType arg_ty
-             want_float  = wantFloatNested NonRecursive dmd is_unlifted
-       ; (floats2, arg2) <- if want_float floats1 arg1
-                            then return (floats1, arg1)
-                            else dontFloat floats1 arg1
+             dec         = wantFloatLocal NonRecursive dmd is_unlifted
+                                                  floats1 arg1
+       ; (floats2, arg2) <- executeFloatDecision dec floats1 arg1
                 -- Else case: arg1 might have lambdas, and we can't
                 --            put them inside a wrapBinds
 
@@ -1474,8 +1477,8 @@ cpeArg env dmd arg
          else do { v <- newVar arg_ty
                  -- See Note [Eta expansion of arguments in CorePrep]
                  ; let arg3 = cpeEtaExpandArg env arg2
-                       arg_float = mkFloat env dmd is_unlifted v arg3
-                 ; return (addFloat floats2 arg_float, varToCoreExpr v) }
+                       arg_float = mkNonRecFloat env dmd is_unlifted v arg3
+                 ; return (snocFloat floats2 arg_float, varToCoreExpr v) }
        }
 
 cpeEtaExpandArg :: CorePrepEnv -> CoreArg -> CoreArg
@@ -1508,20 +1511,6 @@ See Note [Eta expansion for join points] in GHC.Core.Opt.Arity
 Eta expanding the join point would introduce crap that we can't
 generate code for
 
-Note [Floating unlifted arguments]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Consider    C (let v* = expensive in v)
-
-where the "*" indicates "will be demanded".  Usually v will have been
-inlined by now, but let's suppose it hasn't (see #2756).  Then we
-do *not* want to get
-
-     let v* = expensive in C v
-
-because that has different strictness.  Hence the use of 'allLazy'.
-(NB: the let v* turns into a FloatCase, in mkLocalNonRec.)
-
-
 ------------------------------------------------------------------------------
 -- Building the saturated syntax
 -- ---------------------------------------------------------------------------
@@ -1714,7 +1703,9 @@ Since call-by-value is much cheaper than call-by-need, we case-bind arguments
 that are either
 
   1. Strictly evaluated anyway, according to the DmdSig of the callee, or
-  2. ok-for-spec, according to 'exprOkForSpeculation'
+  2. ok-for-spec, according to 'exprOkForSpeculation'.
+     This includes DFuns `$fEqList a`, for example.
+     (Could identify more in the future; see reference to !1866 below.)
 
 While (1) is a no-brainer and always beneficial, (2) is a bit
 more subtle, as the careful haddock for 'exprOkForSpeculation'
@@ -1791,159 +1782,262 @@ of the very function whose termination properties we are exploiting.
 It is also similar to Note [Do not strictify a DFun's parameter dictionaries],
 where marking recursive DFuns (of undecidable *instances*) strict in dictionary
 *parameters* leads to quite the same change in termination as above.
+
+Note [BindInfo and FloatInfo]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The `BindInfo` of a `Float` describes whether it will be case-bound or
+let-bound:
+
+  * `LetBound`: A let binding `let x = rhs in ...`, can be Rec or NonRec.
+  * `CaseBound`: A case binding `case rhs of x -> { __DEFAULT -> .. }`.
+                 (So always NonRec.)
+                 Some case-bound things (string literals, lifted bindings)
+                 can float to top-level (but not all), hence it is similar
+                 to, but not the same as `StrictContextFloatable :: FloatInfo`
+                 described below.
+
+This info is used in `wrapBinds` to pick the corresponding binding form.
+
+We want to case-bind iff the binding is (non-recursive, and) either
+
+  * ok-for-spec-eval (and perhaps lifted, see Note [Speculative evaluation]), or
+  * unlifted, or
+  * strictly used
+
+The `FloatInfo` of a `Float` describes how far it can float without
+(a) violating Core invariants and (b) changing semantics.
+
+  * Any binding is at least `StrictContextFloatable`, meaning we may float it
+    out of a strict context such as `f <>` where `f` is strict.
+
+  * A binding is `LazyContextFloatable` if we may float it out of a lazy context
+    such as `let x = <> in Just x`.
+    Counterexample: A strict or unlifted binding that isn't ok-for-spec-eval
+                    such as `case divInt# x y of r -> { __DEFAULT -> I# r }`.
+                    Here, we may not foat out the strict `r = divInt# x y`.
+
+  * A binding is `TopLvlFloatable` if it is `LazyContextFloatable` and also can
+    be bound at the top level.
+    Counterexample: A strict or unlifted binding (ok-for-spec-eval or not)
+                    such as `case x +# y of r -> { __DEFAULT -> I# r }`.
+
+This meaning of "at least" is encoded in `floatsAtLeastAsFarAs`.
+Note that today, `LetBound` implies `TopLvlFloatable`, so we could make do with
+the the following enum (check `mkNonRecFloat` for whether this is up to date):
+
+   LetBoundTopLvlFloatable          (lifted or boxed values)
+  CaseBoundTopLvlFloatable          (strings, ok-for-spec-eval and lifted)
+  CaseBoundLazyContextFloatable     (ok-for-spec-eval and unlifted)
+  CaseBoundStrictContextFloatable   (not ok-for-spec-eval and unlifted)
+
+Although there is redundancy in the current encoding, SG thinks it is cleaner
+conceptually.
+
+See also Note [Floats and FloatDecision] for how we maintain whole groups of
+floats and how far they go.
+
+Note [Floats and FloatDecision]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+We have a special datatype `Floats` for modelling a telescope of `FloatingBind`
+and caching its "maximum" `FloatInfo`, according to `floatsAtLeastAsFarAs`
+(see Note [BindInfo and FloatInfo] for the ordering).
+There are several operations for creating and combining `Floats` that maintain
+scoping and the cached `FloatInfo`.
+
+When deciding whether we want to float out a `Floats` out of a binding context
+such as `let x = <> in e` (let), `f <>` (app), or `x = <>; ...` (top-level),
+we consult the cached `FloatInfo` of the `Floats`:
+
+  * If we want to float to the top-level (`x = <>; ...`), we check whether
+    we may float-at-least-as-far-as `TopLvlFloatable`, in which case we
+    respond with `FloatAll :: FloatDecision`; otherwise we say `FloatNone`.
+  * If we want to float locally (let or app), then the floating decision is
+    described in Note [wantFloatLocal].
+
+`executeFloatDecision` is then used to act on the particular `FloatDecision`.
 -}
 
+-- See Note [BindInfo and FloatInfo]
+data BindInfo
+  = CaseBound -- ^ A strict binding
+  | LetBound  -- ^ A lazy or value binding
+  deriving Eq
+
+-- See Note [BindInfo and FloatInfo]
+data FloatInfo
+  = TopLvlFloatable
+  -- ^ Anything that can be bound at top-level, such as arbitrary lifted
+  -- bindings or anything that responds True to `exprIsHNF`, such as literals or
+  -- saturated DataCon apps where unlifted or strict args are values.
+
+  | LazyContextFloatable
+  -- ^ Anything that can be floated out of a lazy context.
+  -- In addition to any 'TopLvlFloatable' things, this includes (unlifted)
+  -- bindings that are ok-for-spec that we intend to case-bind.
+
+  | StrictContextFloatable
+  -- ^ Anything that can be floated out of a strict evaluation context.
+  -- That is possible for all bindings; this is the Top element of 'FloatInfo'.
+
+  deriving Eq
+
+instance Outputable BindInfo where
+  ppr CaseBound = text "Case"
+  ppr LetBound  = text "Let"
+
+instance Outputable FloatInfo where
+  ppr TopLvlFloatable = text "top-lvl"
+  ppr LazyContextFloatable = text "lzy-ctx"
+  ppr StrictContextFloatable = text "str-ctx"
+
+-- See Note [Floating in CorePrep]
+-- and Note [BindInfo and FloatInfo]
 data FloatingBind
-  -- | Rhs of bindings are CpeRhss
-  -- They are always of lifted type;
-  -- unlifted ones are done with FloatCase
-  = FloatLet CoreBind
-
-  -- | Float a literal string binding.
-  -- INVARIANT: The `CoreExpr` matches `Lit (LitString bs)`.
-  --            It's just more convenient to keep around the expr rather than
-  --            the wrapped `bs` and reallocate the expr.
-  -- This is a special case of `FloatCase` that is unconditionally ok-for-spec.
-  -- We want to float out strings quite aggressively out of RHSs if doing so
-  -- saves allocation of a thunk ('wantFloatNested'); see Wrinkle (FS1)
-  -- in Note [ANF-ising literal string arguments].
-  | FloatString !CoreExpr !Id
-
-  | FloatCase
-       CpeBody         -- ^ Scrutinee
-       Id              -- ^ Case binder
-       AltCon [Var]    -- ^ Single alternative
-       Bool            -- ^ Ok-for-speculation; False of a strict,
-                       --   but lifted binding that is not OK for
-                       --   Note [Speculative evaluation].
-
-  -- | See Note [Floating Ticks in CorePrep]
+  = Float !CoreBind !BindInfo !FloatInfo
+  | UnsafeEqualityCase !CoreExpr !CoreBndr !AltCon ![CoreBndr]
   | FloatTick CoreTickish
 
-data Floats = Floats OkToSpec (OrdList FloatingBind)
+-- See Note [Floats and FloatDecision]
+data Floats
+  = Floats
+  { fs_info  :: !FloatInfo
+  , fs_binds :: !(OrdList FloatingBind)
+  }
 
 instance Outputable FloatingBind where
-  ppr (FloatLet b) = ppr b
-  ppr (FloatString e b) = text "string" <> braces (ppr b <> char '=' <> ppr e)
-  ppr (FloatCase r b k bs ok) = text "case" <> braces (ppr ok) <+> ppr r
+  ppr (Float b bi fi) = ppr bi <+> ppr fi <+> ppr b
+  ppr (FloatTick t) = ppr t
+  ppr (UnsafeEqualityCase scrut b k bs) = text "case" <+> ppr scrut
                                 <+> text "of"<+> ppr b <> text "@"
                                 <> case bs of
                                    [] -> ppr k
                                    _  -> parens (ppr k <+> ppr bs)
-  ppr (FloatTick t) = ppr t
 
 instance Outputable Floats where
-  ppr (Floats flag fs) = text "Floats" <> brackets (ppr flag) <+>
-                         braces (vcat (map ppr (fromOL fs)))
-
-instance Outputable OkToSpec where
-  ppr OkToSpec    = text "OkToSpec"
-  ppr IfUnliftedOk = text "IfUnliftedOk"
-  ppr NotOkToSpec = text "NotOkToSpec"
-
--- Can we float these binds out of the rhs of a let?  We cache this decision
--- to avoid having to recompute it in a non-linear way when there are
--- deeply nested lets.
-data OkToSpec
-   = OkToSpec           -- ^ Lazy bindings of lifted type. Float as you please
-   | IfUnliftedOk       -- ^ A mixture of lazy lifted bindings and n
-                        -- ok-to-speculate unlifted bindings.
-                        -- Float out of lets, but not to top-level!
-   | NotOkToSpec        -- ^ Some not-ok-to-speculate unlifted bindings
-
-mkFloat :: CorePrepEnv -> Demand -> Bool -> Id -> CpeRhs -> FloatingBind
-mkFloat env dmd is_unlifted bndr rhs
-  | Lit LitString{} <- rhs = FloatString rhs bndr
-
-  | is_strict || ok_for_spec
-  , not is_hnf             = FloatCase rhs bndr DEFAULT [] ok_for_spec
-      -- See Note [Speculative evaluation]
-      -- Don't make a case for a HNF binding, even if it's strict
-      -- Otherwise we get  case (\x -> e) of ...!
-
-  | is_unlifted            = FloatCase rhs bndr DEFAULT [] True
-      -- we used to assertPpr ok_for_spec (ppr rhs) here, but it is now disabled
-      -- because exprOkForSpeculation isn't stable under ANF-ing. See for
-      -- example #19489 where the following unlifted expression:
-      --
-      --    GHC.Prim.(#|_#) @LiftedRep @LiftedRep @[a_ax0] @[a_ax0]
-      --                    (GHC.Types.: @a_ax0 a2_agq a3_agl)
-      --
-      -- is ok-for-spec but is ANF-ised into:
-      --
-      --    let sat = GHC.Types.: @a_ax0 a2_agq a3_agl
-      --    in GHC.Prim.(#|_#) @LiftedRep @LiftedRep @[a_ax0] @[a_ax0] sat
-      --
-      -- which isn't ok-for-spec because of the let-expression.
-
-  | is_hnf                 = FloatLet (NonRec bndr                       rhs)
-  | otherwise              = FloatLet (NonRec (setIdDemandInfo bndr dmd) rhs)
-                   -- See Note [Pin demand info on floats]
-  where
-    is_hnf      = exprIsHNF rhs
-    is_strict   = isStrUsedDmd dmd
-    ok_for_spec = exprOkForSpecEval (not . is_rec_call) rhs
-    is_rec_call = (`elemUnVarSet` cpe_rec_ids env)
+  ppr (Floats info binds) = text "Floats" <> brackets (ppr info) <> braces (ppr binds)
+
+lubFloatInfo :: FloatInfo -> FloatInfo -> FloatInfo
+lubFloatInfo StrictContextFloatable _                      = StrictContextFloatable
+lubFloatInfo _                      StrictContextFloatable = StrictContextFloatable
+lubFloatInfo LazyContextFloatable   _                      = LazyContextFloatable
+lubFloatInfo _                      LazyContextFloatable   = LazyContextFloatable
+lubFloatInfo TopLvlFloatable        TopLvlFloatable        = TopLvlFloatable
+
+floatsAtLeastAsFarAs :: FloatInfo -> FloatInfo -> Bool
+-- See Note [Floats and FloatDecision]
+floatsAtLeastAsFarAs l r = l `lubFloatInfo` r == r
 
 emptyFloats :: Floats
-emptyFloats = Floats OkToSpec nilOL
+emptyFloats = Floats TopLvlFloatable nilOL
 
 isEmptyFloats :: Floats -> Bool
-isEmptyFloats (Floats _ bs) = isNilOL bs
+isEmptyFloats (Floats _ b) = isNilOL b
 
-wrapBinds :: Floats -> CpeBody -> CpeBody
-wrapBinds (Floats _ binds) body
-  = foldrOL mk_bind body binds
-  where
-    mk_bind (FloatCase rhs bndr con bs _) body = Case rhs bndr (exprType body) [Alt con bs body]
-    mk_bind (FloatString rhs bndr)        body = Case rhs bndr (exprType body) [Alt DEFAULT [] body]
-    mk_bind (FloatLet bind)               body = Let bind body
-    mk_bind (FloatTick tickish)           body = mkTick tickish body
-
-addFloat :: Floats -> FloatingBind -> Floats
-addFloat (Floats ok_to_spec floats) new_float
-  = Floats (combine ok_to_spec (check new_float)) (floats `snocOL` new_float)
-  where
-    check FloatLet {}   = OkToSpec
-    check FloatTick{}   = OkToSpec
-    check FloatString{} = OkToSpec
-    check (FloatCase _ _ _ _ ok_for_spec)
-      | ok_for_spec     = IfUnliftedOk
-      | otherwise       = NotOkToSpec
-        -- The ok-for-speculation flag says that it's safe to
-        -- float this Case out of a let, and thereby do it more eagerly
-        -- We need the IfUnliftedOk flag because it's never ok to float
-        -- an unlifted binding to the top level.
-        -- There is one exception: String literals! But those will become
-        -- FloatString and thus OkToSpec.
-        -- See Note [ANF-ising literal string arguments]
+getFloats :: Floats -> OrdList FloatingBind
+getFloats = fs_binds
 
 unitFloat :: FloatingBind -> Floats
-unitFloat = addFloat emptyFloats
-
-appendFloats :: Floats -> Floats -> Floats
-appendFloats (Floats spec1 floats1) (Floats spec2 floats2)
-  = Floats (combine spec1 spec2) (floats1 `appOL` floats2)
-
-concatFloats :: [Floats] -> OrdList FloatingBind
-concatFloats = foldr (\ (Floats _ bs1) bs2 -> appOL bs1 bs2) nilOL
-
-combine :: OkToSpec -> OkToSpec -> OkToSpec
-combine NotOkToSpec _  = NotOkToSpec
-combine _ NotOkToSpec  = NotOkToSpec
-combine IfUnliftedOk _ = IfUnliftedOk
-combine _ IfUnliftedOk = IfUnliftedOk
-combine _ _            = OkToSpec
+unitFloat = snocFloat emptyFloats
+
+floatInfo :: FloatingBind -> FloatInfo
+floatInfo (Float _ _ info)     = info
+floatInfo UnsafeEqualityCase{} = LazyContextFloatable -- See Note [Floating in CorePrep]
+floatInfo FloatTick{}          = TopLvlFloatable      -- We filter these out in cpePair,
+                                                      -- see Note [Floating Ticks in CorePrep]
+
+-- | Append a `FloatingBind` `b` to a `Floats` telescope `bs` that may reference any
+-- binding of the 'Floats'.
+snocFloat :: Floats -> FloatingBind -> Floats
+snocFloat floats fb =
+  Floats { fs_info  = lubFloatInfo (fs_info floats) (floatInfo fb)
+         , fs_binds = fs_binds floats `snocOL` fb }
+
+-- | Cons a `FloatingBind` `b` to a `Floats` telescope `bs` which scopes over
+-- `b`.
+consFloat :: FloatingBind -> Floats -> Floats
+consFloat fb floats =
+  Floats { fs_info  = lubFloatInfo (fs_info floats) (floatInfo fb)
+         , fs_binds = fb `consOL`  fs_binds floats }
+
+-- | Append two telescopes, nesting the right inside the left.
+appFloats :: Floats -> Floats -> Floats
+appFloats outer inner =
+  Floats { fs_info  = lubFloatInfo (fs_info outer) (fs_info inner)
+         , fs_binds = fs_binds outer `appOL` fs_binds inner }
+
+-- | Zip up two `Floats`, none of which scope over the other
+zipFloats :: Floats -> Floats -> Floats
+-- We may certainly just nest one telescope in the other, so appFloats is a
+-- valid implementation strategy.
+zipFloats = appFloats
+
+-- | `zipFloats` a bunch of independent telescopes.
+zipManyFloats :: [Floats] -> Floats
+zipManyFloats = foldr zipFloats emptyFloats
+
+mkNonRecFloat :: CorePrepEnv -> Demand -> Bool -> Id -> CpeRhs -> FloatingBind
+mkNonRecFloat env dmd is_unlifted bndr rhs = -- pprTraceWith "mkNonRecFloat" ppr $
+  Float (NonRec bndr' rhs) bound info
+  where
+    bndr' = setIdDemandInfo bndr dmd -- See Note [Pin demand info on floats]
+    (bound,info)
+      | is_lifted, is_hnf        = (LetBound, TopLvlFloatable)
+          -- is_lifted: We currently don't allow unlifted values at the
+          --            top-level or inside letrecs
+          --            (but SG thinks that in principle, we should)
+      | is_data_con bndr         = (LetBound, TopLvlFloatable)
+          -- We need this special case for unlifted DataCon workers/wrappers
+          -- until #17521 is fixed
+      | exprIsTickedString rhs   = (CaseBound, TopLvlFloatable)
+          -- String literals are unboxed (so must be case-bound) and float to
+          -- the top-level
+      | is_unlifted, ok_for_spec = (CaseBound, LazyContextFloatable)
+      | is_lifted,   ok_for_spec = (CaseBound, TopLvlFloatable)
+          -- See Note [Speculative evaluation]
+          -- Ok-for-spec-eval things will be case-bound, lifted or not.
+          -- But when it's lifted we are ok with floating it to top-level
+          -- (where it is actually bound lazily).
+      | is_unlifted || is_strict = (CaseBound, StrictContextFloatable)
+          -- These will never be floated out of a lazy RHS context
+      | otherwise                = assertPpr is_lifted (ppr rhs) $
+                                   (LetBound, TopLvlFloatable)
+          -- And these float freely but can't be speculated, hence LetBound
+
+    is_lifted   = not is_unlifted
+    is_hnf      = exprIsHNF rhs
+    is_strict   = isStrUsedDmd dmd
+    ok_for_spec = exprOkForSpecEval (not . is_rec_call) rhs
+    is_rec_call = (`elemUnVarSet` cpe_rec_ids env)
+    is_data_con = isJust . isDataConId_maybe
 
+-- | Wrap floats around an expression
+wrapBinds :: Floats -> CpeBody -> CpeBody
+wrapBinds floats body
+  = -- pprTraceWith "wrapBinds" (\res -> ppr floats $$ ppr body $$ ppr res) $
+    foldrOL mk_bind body (getFloats floats)
+  where
+    -- See Note [BindInfo and FloatInfo] on whether we pick Case or Let here
+    mk_bind f@(Float bind CaseBound _) body
+      | NonRec bndr rhs <- bind
+      = mkDefaultCase rhs bndr body
+      | otherwise
+      = pprPanic "wrapBinds" (ppr f)
+    mk_bind (Float bind _ _) body
+      = Let bind body
+    mk_bind (UnsafeEqualityCase scrut b con bs) body
+      = mkSingleAltCase scrut b con bs body
+    mk_bind (FloatTick tickish) body
+      = mkTick tickish body
+
+-- | Put floats at top-level
 deFloatTop :: Floats -> [CoreBind]
--- For top level only; we don't expect any FloatCases
-deFloatTop (Floats _ floats)
-  = foldrOL get [] floats
+-- Precondition: No Strict or LazyContextFloatable 'FloatInfo', no ticks!
+deFloatTop floats
+  = foldrOL get [] (getFloats floats)
   where
-    get (FloatLet b)               bs = get_bind b                 : bs
-    get (FloatString body var)     bs = get_bind (NonRec var body) : bs
-    get (FloatCase body var _ _ _) bs = get_bind (NonRec var body) : bs
-    get b _ = pprPanic "corePrepPgm" (ppr b)
+    get (Float b _ TopLvlFloatable) bs
+      = get_bind b : bs
+    get b _  = pprPanic "corePrepPgm" (ppr b)
 
     -- See Note [Dead code in CorePrep]
     get_bind (NonRec x e) = NonRec x (occurAnalyseExpr e)
@@ -1951,25 +2045,113 @@ deFloatTop (Floats _ floats)
 
 ---------------------------------------------------------------------------
 
-wantFloatNested :: RecFlag -> Demand -> Bool -> Floats -> CpeRhs -> Bool
-wantFloatNested is_rec dmd rhs_is_unlifted floats rhs
-  =  isEmptyFloats floats
-  || isStrUsedDmd dmd
-  || rhs_is_unlifted
-  || (allLazyNested is_rec floats && exprIsHNF rhs)
-        -- Why the test for allLazyNested?
-        --      v = f (x `divInt#` y)
-        -- we don't want to float the case, even if f has arity 2,
-        -- because floating the case would make it evaluated too early
-
-allLazyTop :: Floats -> Bool
-allLazyTop (Floats OkToSpec _) = True
-allLazyTop _                   = False
-
-allLazyNested :: RecFlag -> Floats -> Bool
-allLazyNested _      (Floats OkToSpec    _)  = True
-allLazyNested _      (Floats NotOkToSpec _)  = False
-allLazyNested is_rec (Floats IfUnliftedOk _) = isNonRec is_rec
+{- Note [wantFloatLocal]
+~~~~~~~~~~~~~~~~~~~~~~~~
+Consider
+  let x = let y = e1 in e2
+  in e
+Similarly for `(\x. e) (let y = e1 in e2)`.
+Do we want to float out `y` out of `x`?
+(This is discussed in detail in the paper
+"Let-floating: moving bindings to give faster programs".)
+
+`wantFloatLocal` is concerned with answering this question.
+It considers the Demand on `x`, whether or not `e2` is unlifted and the
+`FloatInfo` of the `y` binding (e.g., it might itself be unlifted, a value,
+strict, or ok-for-spec).
+
+We float out if ...
+  1. ... the binding context is strict anyway, so either `x` is used strictly
+     or has unlifted type.
+     Doing so is trivially sound and won`t increase allocations, so we
+     return `FloatAll`.
+     This might happen while ANF-ising `f (g (h 13))` where `f`,`g` are strict:
+       f (g (h 13))
+       ==> { ANF }
+       case (case h 13 of r -> g r) of r2 -> f r2
+       ==> { Float }
+       case h 13 of r -> case g r of r2 -> f r2
+     The latter is easier to read and grows less stack.
+  2. ... `e2` becomes a value in doing so, in which case we won't need to
+     allocate a thunk for `x`/the arg that closes over the FVs of `e1`.
+     In general, this is only sound if `y=e1` is `LazyContextFloatable`.
+     (See Note [BindInfo and FloatInfo].)
+     Nothing is won if `x` doesn't become a value
+     (i.e., `let x = let sat = f 14 in g sat in e`),
+     so we return `FloatNone` if there is any float that is
+     `StrictContextFloatable`, and return `FloatAll` otherwise.
+
+To elaborate on (2), consider the case when the floated binding is
+`e1 = divInt# a b`, e.g., not `LazyContextFloatable`:
+  let x = I# (a `divInt#` b)
+  in e
+this ANFises to
+  let x = case a `divInt#` b of r { __DEFAULT -> I# r }
+  in e
+If `x` is used lazily, we may not float `r` further out.
+A float binding `x +# y` is OK, though, and so every ok-for-spec-eval
+binding is `LazyContextFloatable`.
+
+Wrinkles:
+
+ (W1) When the outer binding is a letrec, i.e.,
+        letrec x = case a +# b of r { __DEFAULT -> f y r }
+               y = [x]
+        in e
+      we don't want to float `LazyContextFloatable` bindings such as `r` either
+      and require `TopLvlFloatable` instead.
+      The reason is that we don't track FV of FloatBindings, so we would need
+      to park them in the letrec,
+        letrec r = a +# b -- NB: r`s RHS might scope over x and y
+               x = f y r
+               y = [x]
+        in e
+      and now we have violated Note [Core letrec invariant].
+      So we preempt this case in `wantFloatLocal`, responding `FloatNone` unless
+      all floats are `TopLvlFloatable`.
+-}
+
+data FloatDecision
+  = FloatNone
+  | FloatAll
+
+executeFloatDecision :: FloatDecision -> Floats -> CpeRhs -> UniqSM (Floats, CpeRhs)
+executeFloatDecision dec floats rhs = do
+  let (float,stay) = case dec of
+        _ | isEmptyFloats floats -> (emptyFloats,emptyFloats)
+        FloatNone                -> (emptyFloats, floats)
+        FloatAll                 -> (floats, emptyFloats)
+  -- Wrap `stay` around `rhs`.
+  -- NB: `rhs` might have lambdas, and we can't
+  --     put them inside a wrapBinds, which expects a `CpeBody`.
+  if isEmptyFloats stay -- Fast path where we don't need to call `rhsToBody`
+    then return (float, rhs)
+    else do
+      (floats', body) <- rhsToBody rhs
+      return (float, wrapBinds stay $ wrapBinds floats' body)
+
+wantFloatTop :: Floats -> FloatDecision
+wantFloatTop fs
+  | fs_info fs `floatsAtLeastAsFarAs` TopLvlFloatable = FloatAll
+  | otherwise                                         = FloatNone
+
+wantFloatLocal :: RecFlag -> Demand -> Bool -> Floats -> CpeRhs -> FloatDecision
+-- See Note [wantFloatLocal]
+wantFloatLocal is_rec rhs_dmd rhs_is_unlifted floats rhs
+  |  isEmptyFloats floats -- Well yeah...
+  || isStrUsedDmd rhs_dmd -- Case (1) of Note [wantFloatLocal]
+  || rhs_is_unlifted      -- dito
+  || (fs_info floats `floatsAtLeastAsFarAs` max_float_info && exprIsHNF rhs)
+                          -- Case (2) of Note [wantFloatLocal]
+  = FloatAll
+
+  | otherwise
+  = FloatNone
+  where
+    max_float_info | isRec is_rec = TopLvlFloatable
+                   | otherwise    = LazyContextFloatable
+                    -- See Note [wantFloatLocal], Wrinkle (W1)
+                    -- for 'is_rec'
 
 {-
 ************************************************************************
@@ -2224,26 +2406,28 @@ newVar ty
 
 -- | Like wrapFloats, but only wraps tick floats
 wrapTicks :: Floats -> CoreExpr -> (Floats, CoreExpr)
-wrapTicks (Floats flag floats0) expr =
-    (Floats flag (toOL $ reverse floats1), foldr mkTick expr (reverse ticks1))
-  where (floats1, ticks1) = foldlOL go ([], []) $ floats0
+wrapTicks floats expr
+  | (floats1, ticks1) <- fold_fun go floats
+  = (floats1, foldrOL mkTick expr ticks1)
+  where fold_fun f floats =
+           let (binds, ticks) = foldlOL f (nilOL,nilOL) (fs_binds floats)
+           in (floats { fs_binds = binds }, ticks)
         -- Deeply nested constructors will produce long lists of
         -- redundant source note floats here. We need to eliminate
         -- those early, as relying on mkTick to spot it after the fact
         -- can yield O(n^3) complexity [#11095]
-        go (floats, ticks) (FloatTick t)
+        go (flt_binds, ticks) (FloatTick t)
           = assert (tickishPlace t == PlaceNonLam)
-            (floats, if any (flip tickishContains t) ticks
-                     then ticks else t:ticks)
-        go (floats, ticks) f at FloatString{}
-          = (f:floats, ticks) -- don't need to wrap the tick around the string; nothing to execute.
-        go (floats, ticks) f
-          = (foldr wrap f (reverse ticks):floats, ticks)
-
-        wrap t (FloatLet bind)           = FloatLet (wrapBind t bind)
-        wrap t (FloatCase r b con bs ok) = FloatCase (mkTick t r) b con bs ok
-        wrap _ other                     = pprPanic "wrapTicks: unexpected float!"
-                                             (ppr other)
+            (flt_binds, if any (flip tickishContains t) ticks
+                        then ticks else ticks `snocOL` t)
+        go (flt_binds, ticks) f at UnsafeEqualityCase{}
+          -- unsafe equality case will be erased; don't wrap anything!
+          = (flt_binds `snocOL` f, ticks)
+        go (flt_binds, ticks) f at Float{}
+          = (flt_binds `snocOL` foldrOL wrap f ticks, ticks)
+
+        wrap t (Float bind bound info) = Float (wrapBind t bind) bound info
+        wrap _ f                 = pprPanic "Unexpected FloatingBind" (ppr f)
         wrapBind t (NonRec binder rhs) = NonRec binder (mkTick t rhs)
         wrapBind t (Rec pairs)         = Rec (mapSnd (mkTick t) pairs)
 


=====================================
compiler/GHC/Data/OrdList.hs
=====================================
@@ -16,8 +16,8 @@ module GHC.Data.OrdList (
         OrdList, pattern NilOL, pattern ConsOL, pattern SnocOL,
         nilOL, isNilOL, unitOL, appOL, consOL, snocOL, concatOL, lastOL,
         headOL,
-        mapOL, mapOL', fromOL, toOL, foldrOL, foldlOL, reverseOL, fromOLReverse,
-        strictlyEqOL, strictlyOrdOL
+        mapOL, mapOL', fromOL, toOL, foldrOL, foldlOL,
+        partitionOL, reverseOL, fromOLReverse, strictlyEqOL, strictlyOrdOL
 ) where
 
 import GHC.Prelude
@@ -220,6 +220,25 @@ foldlOL k z (Snoc xs x) = let !z' = (foldlOL k z xs) in k z' x
 foldlOL k z (Two b1 b2) = let !z' = (foldlOL k z b1) in foldlOL k z' b2
 foldlOL k z (Many xs)   = foldl' k z xs
 
+partitionOL :: (a -> Bool) -> OrdList a -> (OrdList a, OrdList a)
+partitionOL _ None = (None,None)
+partitionOL f (One x)
+  | f x       = (One x, None)
+  | otherwise = (None, One x)
+partitionOL f (Two xs ys) = (Two ls1 ls2, Two rs1 rs2)
+  where !(!ls1,!rs1) = partitionOL f xs
+        !(!ls2,!rs2) = partitionOL f ys
+partitionOL f (Cons x xs)
+  | f x       = (Cons x ls, rs)
+  | otherwise = (ls, Cons x rs)
+  where !(!ls,!rs) = partitionOL f xs
+partitionOL f (Snoc xs x)
+  | f x       = (Snoc ls x, rs)
+  | otherwise = (ls, Snoc rs x)
+  where !(!ls,!rs) = partitionOL f xs
+partitionOL f (Many xs) = (toOL ls, toOL rs)
+  where !(!ls,!rs) = NE.partition f xs
+
 toOL :: [a] -> OrdList a
 toOL [] = None
 toOL [x] = One x


=====================================
compiler/GHC/Rename/Env.hs
=====================================
@@ -692,13 +692,14 @@ lookupSubBndrOcc_helper must_have_parent warn_if_deprec parent rdr_name how_lkup
   | otherwise = do
   gre_env <- getGlobalRdrEnv
   let original_gres = lookupGRE gre_env (LookupChildren (rdrNameOcc rdr_name) how_lkup)
-  -- The remaining GREs are things that we *could* export here, note that
-  -- this includes things which have `NoParent`. Those are sorted in
-  -- `checkPatSynParent`.
+      picked_gres = pick_gres original_gres
+  -- The remaining GREs are things that we *could* export here.
+  -- Note that this includes things which have `NoParent`;
+  -- those are sorted in `checkPatSynParent`.
   traceRn "parent" (ppr parent)
   traceRn "lookupExportChild original_gres:" (ppr original_gres)
-  traceRn "lookupExportChild picked_gres:" (ppr (picked_gres original_gres) $$ ppr must_have_parent)
-  case picked_gres original_gres of
+  traceRn "lookupExportChild picked_gres:" (ppr picked_gres $$ ppr must_have_parent)
+  case picked_gres of
     NoOccurrence ->
       noMatchingParentErr original_gres
     UniqueOccurrence g ->
@@ -745,34 +746,36 @@ lookupSubBndrOcc_helper must_have_parent warn_if_deprec parent rdr_name how_lkup
           addNameClashErrRn rdr_name gres
           return (FoundChild (NE.head gres))
 
-        picked_gres :: [GlobalRdrElt] -> DisambigInfo
+        pick_gres :: [GlobalRdrElt] -> DisambigInfo
         -- For Unqual, find GREs that are in scope qualified or unqualified
         -- For Qual,   find GREs that are in scope with that qualification
-        picked_gres gres
+        pick_gres gres
           | isUnqual rdr_name
           = mconcat (map right_parent gres)
           | otherwise
           = mconcat (map right_parent (pickGREs rdr_name gres))
 
         right_parent :: GlobalRdrElt -> DisambigInfo
-        right_parent p
-          = case greParent p of
+        right_parent gre
+          = case greParent gre of
               ParentIs cur_parent
-                 | parent == cur_parent -> DisambiguatedOccurrence p
+                 | parent == cur_parent -> DisambiguatedOccurrence gre
                  | otherwise            -> NoOccurrence
-              NoParent                  -> UniqueOccurrence p
+              NoParent                  -> UniqueOccurrence gre
 {-# INLINEABLE lookupSubBndrOcc_helper #-}
 
--- This domain specific datatype is used to record why we decided it was
+-- | This domain specific datatype is used to record why we decided it was
 -- possible that a GRE could be exported with a parent.
 data DisambigInfo
        = NoOccurrence
-          -- The GRE could never be exported. It has the wrong parent.
+          -- ^ The GRE could not be found, or it has the wrong parent.
        | UniqueOccurrence GlobalRdrElt
-          -- The GRE has no parent. It could be a pattern synonym.
+          -- ^ The GRE has no parent. It could be a pattern synonym.
        | DisambiguatedOccurrence GlobalRdrElt
-          -- The parent of the GRE is the correct parent
+          -- ^ The parent of the GRE is the correct parent.
        | AmbiguousOccurrence (NE.NonEmpty GlobalRdrElt)
+          -- ^ The GRE is ambiguous.
+          --
           -- For example, two normal identifiers with the same name are in
           -- scope. They will both be resolved to "UniqueOccurrence" and the
           -- monoid will combine them to this failing case.
@@ -784,7 +787,7 @@ instance Outputable DisambigInfo where
   ppr (AmbiguousOccurrence gres)    = text "Ambiguous:" <+> ppr gres
 
 instance Semi.Semigroup DisambigInfo where
-  -- This is the key line: We prefer disambiguated occurrences to other
+  -- These are the key lines: we prefer disambiguated occurrences to other
   -- names.
   _ <> DisambiguatedOccurrence g' = DisambiguatedOccurrence g'
   DisambiguatedOccurrence g' <> _ = DisambiguatedOccurrence g'


=====================================
compiler/GHC/Rename/Names.hs
=====================================
@@ -1068,13 +1068,17 @@ Notice that T appears *twice*, once as a child and once as a parent. From
 these two exports, respectively, during construction of the imp_occ_env, we begin
 by associating the following two elements with the key T:
 
-  T -> ImpOccItem { imp_item = T, imp_bundled = [C,T]     , imp_is_parent = False }
-  T -> ImpOccItem { imp_item = T, imp_bundled = [T1,T2,T3], imp_is_parent = True  }
+  T -> ImpOccItem { imp_item = gre1, imp_bundled = [C,T]     , imp_is_parent = False }
+  T -> ImpOccItem { imp_item = gre2, imp_bundled = [T1,T2,T3], imp_is_parent = True  }
 
-We combine these (in function 'combine' in 'mkImportOccEnv') by simply discarding
-the first item, to get:
+where `gre1`, `gre2` are two GlobalRdrElts with greName T.
+We combine these (in function 'combine' in 'mkImportOccEnv') by discarding the
+non-parent item, thusly:
 
-  T -> IE_ITem { imp_item = T, imp_bundled = [T1,T2,T3], imp_is_parent = True }
+  T -> IE_ITem { imp_item = gre1 `plusGRE` gre2, imp_bundled = [T1,T2,T3], imp_is_parent = True }
+
+Note the `plusGRE`: this ensures we don't drop parent information;
+see Note [Preserve parent information when combining import OccEnvs].
 
 So the overall imp_occ_env is:
 
@@ -1133,6 +1137,31 @@ Whereas in case (B) we reach the lookup_ie case for IEThingWith,
 which looks up 'S' and then finds the unique 'foo' amongst its children.
 
 See T16745 for a test of this.
+
+Note [Preserve parent information when combining import OccEnvs]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When discarding one ImpOccItem in favour of another, as described in
+Note [Dealing with imports], we must make sure to combine the GREs so that
+we don't lose information.
+
+Consider for example #24084:
+
+  module M1 where { class C a where { type T a } }
+  module M2 ( module M1 ) where { import M1 }
+  module M3 where { import M2 ( C, T ); instance C () where T () = () }
+
+When processing the import list of `M3`, we will have two `Avail`s attached
+to `T`, namely `C(C, T)` and `T(T)`. We combine them in the `combine` function
+of `mkImportOccEnv`; as described in Note [Dealing with imports] we discard
+`C(C, T)` in favour of `T(T)`. However, in doing so, we **must not**
+discard the information want that `C` is the parent of `T`. Indeed,
+losing track of this information can cause errors when importing,
+as we could get an error of the form
+
+  ‘T’ is not a (visible) associated type of class ‘C’
+
+This explains why we use `plusGRE` when combining the two ImpOccItems, even
+though we are discarding one in favour of the other.
 -}
 
 -- | All the 'GlobalRdrElt's associated with an 'AvailInfo'.
@@ -1443,6 +1472,14 @@ data ImpOccItem
         -- ^ Is the import item a parent? See Note [Dealing with imports].
       }
 
+instance Outputable ImpOccItem where
+  ppr (ImpOccItem { imp_item = item, imp_bundled = bundled, imp_is_parent = is_par })
+    = braces $ hsep
+       [ text "ImpOccItem"
+       , if is_par then text "[is_par]" else empty
+       , ppr (greName item) <+> ppr (greParent item)
+       , braces $ text "bundled:" <+> ppr (map greName bundled) ]
+
 -- | Make an 'OccEnv' of all the imports.
 --
 -- Complicated by the fact that associated data types and pattern synonyms
@@ -1474,9 +1511,9 @@ mkImportOccEnv hsc_env decl_spec all_avails =
 
     -- See Note [Dealing with imports]
     -- 'combine' may be called for associated data types which appear
-    -- twice in the all_avails. In the example, we combine
-    --    T(T,T1,T2,T3) and C(C,T)  to give   (T, T(T,T1,T2,T3), Just C)
-    -- NB: the AvailTC can have fields as well as data constructors (#12127)
+    -- twice in the all_avails. In the example, we have two Avails for T,
+    -- namely T(T,T1,T2,T3) and C(C,T), and we combine them by dropping the
+    -- latter, in which T is not the parent.
     combine :: ImpOccItem -> ImpOccItem -> ImpOccItem
     combine item1@(ImpOccItem { imp_item = gre1, imp_is_parent = is_parent1 })
             item2@(ImpOccItem { imp_item = gre2, imp_is_parent = is_parent2 })
@@ -1484,11 +1521,13 @@ mkImportOccEnv hsc_env decl_spec all_avails =
       , not (isRecFldGRE gre1 || isRecFldGRE gre2) -- NB: does not force GREInfo.
       , let name1 = greName gre1
             name2 = greName gre2
+            gre = gre1 `plusGRE` gre2
+              -- See Note [Preserve parent information when combining import OccEnvs]
       = assertPpr (name1 == name2)
                   (ppr name1 <+> ppr name2) $
         if is_parent1
-        then item1
-        else item2
+        then item1 { imp_item = gre }
+        else item2 { imp_item = gre }
       -- Discard C(C,T) in favour of T(T, T1, T2, T3).
 
     -- 'combine' may also be called for pattern synonyms which appear both


=====================================
compiler/GHC/Types/Avail.hs
=====================================
@@ -175,7 +175,7 @@ filterAvail keep ie rest =
 -- 'avails' may have several items with the same availName
 -- E.g  import Ix( Ix(..), index )
 -- will give Ix(Ix,index,range) and Ix(index)
--- We want to combine these; addAvail does that
+-- We want to combine these; plusAvail does that
 nubAvails :: [AvailInfo] -> [AvailInfo]
 nubAvails avails = eltsDNameEnv (foldl' add emptyDNameEnv avails)
   where


=====================================
compiler/GHC/Types/Name/Reader.hs
=====================================
@@ -76,6 +76,7 @@ module GHC.Types.Name.Reader (
         -- ** Global 'RdrName' mapping elements: 'GlobalRdrElt', 'Provenance', 'ImportSpec'
         GlobalRdrEltX(..), GlobalRdrElt, IfGlobalRdrElt, FieldGlobalRdrElt,
         greName, greNameSpace, greParent, greInfo,
+        plusGRE, insertGRE,
         forceGlobalRdrEnv, hydrateGlobalRdrEnv,
         isLocalGRE, isImportedGRE, isRecFldGRE,
         fieldGREInfo,
@@ -1165,6 +1166,17 @@ data WhichGREs info where
        }
     -> WhichGREs GREInfo
 
+instance Outputable (WhichGREs info) where
+  ppr SameNameSpace = text "SameNameSpace"
+  ppr (RelevantGREs { includeFieldSelectors = sel
+                    , lookupVariablesForFields = vars
+                    , lookupTyConsAsWell = tcs_too })
+    = braces $ hsep
+       [ text "RelevantGREs"
+       , text (show sel)
+       , if vars then text "[vars]" else empty
+       , if tcs_too then text "[tcs]" else empty ]
+
 -- | Look up as many possibly relevant 'GlobalRdrElt's as possible.
 pattern AllRelevantGREs :: WhichGREs GREInfo
 pattern AllRelevantGREs =
@@ -1199,6 +1211,17 @@ data LookupChild
     -- See Note [childGREPriority].
   }
 
+instance Outputable LookupChild where
+  ppr (LookupChild { wantedParent = par
+                   , lookupDataConFirst = dc
+                   , prioritiseParent = prio_parent })
+    = braces $ hsep
+        [ text "LookupChild"
+        , braces (text "parent:" <+> ppr par)
+        , if dc then text "[dc_first]" else empty
+        , if prio_parent then text "[prio_parent]" else empty
+        ]
+
 -- | After looking up something with the given 'NameSpace', is the resulting
 -- 'GlobalRdrElt' we have obtained relevant, according to the 'RelevantGREs'
 -- specification of which 'NameSpace's are relevant?


=====================================
docs/users_guide/conf.py
=====================================
@@ -277,7 +277,6 @@ def setup(app):
     app.add_role('cabal-ref', haddock_role('Cabal'))
     app.add_role('ghc-compact-ref', haddock_role('ghc-compact'))
     app.add_role('ghc-prim-ref', haddock_role('ghc-prim'))
-    app.add_role('parallel-ref', haddock_role('parallel'))
     app.add_role('array-ref', haddock_role('array'))
 
     app.add_object_type('rts-flag', 'rts-flag',


=====================================
docs/users_guide/ghc_config.py.in
=====================================
@@ -23,7 +23,6 @@ lib_versions = {
     'template-haskell': '@LIBRARY_template_haskell_UNIT_ID@',
     'ghc-compact': '@LIBRARY_ghc_compact_UNIT_ID@',
     'ghc': '@LIBRARY_ghc_UNIT_ID@',
-    'parallel': '@LIBRARY_parallel_UNIT_ID@',
     'Cabal': '@LIBRARY_Cabal_UNIT_ID@',
     'array': '@LIBRARY_array_UNIT_ID@',
 }


=====================================
hadrian/src/Base.hs
=====================================
@@ -32,7 +32,7 @@ module Base (
     hadrianPath, configPath, configFile, sourcePath, shakeFilesDir,
     stageBinPath, stageLibPath, templateHscPath,
     buildTargetFile, hostTargetFile, targetTargetFile,
-    ghcBinDeps, ghcLibDeps, haddockDeps,
+    ghcLibDeps, haddockDeps,
     relativePackageDbPath, packageDbPath, packageDbStamp, mingwStamp,
     systemCxxStdLibConf, systemCxxStdLibConfPath
     , PackageDbLoc(..), Inplace(..)
@@ -151,17 +151,12 @@ ghcLibDeps stage iplace = do
         , "llvm-passes"
         , "ghc-interp.js"
         , "settings"
+        , "ghc-usage.txt"
+        , "ghci-usage.txt"
         ]
     cxxStdLib <- systemCxxStdLibConfPath (PackageDbLoc stage iplace)
     return (cxxStdLib : ps)
 
--- | Files the GHC binary depends on.
-ghcBinDeps :: Stage -> Action [FilePath]
-ghcBinDeps stage = mapM (\f -> stageLibPath stage <&> (-/- f))
-    [ "ghc-usage.txt"
-    , "ghci-usage.txt"
-    ]
-
 -- | Files the `haddock` binary depends on
 haddockDeps :: Stage -> Action [FilePath]
 haddockDeps stage = do


=====================================
hadrian/src/Builder.hs
=====================================
@@ -238,17 +238,12 @@ instance H.Builder Builder where
           -- changes (#18001).
           _bootGhcVersion <- setting GhcVersion
           pure []
-        Ghc _ stage -> do
+        Ghc {} -> do
             root <- buildRoot
             touchyPath <- programPath (vanillaContext (Stage0 InTreeLibs) touchy)
             unlitPath  <- builderPath Unlit
 
-            -- GHC from the previous stage is used to build artifacts in the
-            -- current stage. Need the previous stage's GHC deps.
-            ghcdeps <- ghcBinDeps (predStage stage)
-
             return $ [ unlitPath ]
-                  ++ ghcdeps
                   ++ [ touchyPath          | windowsHost ]
                   ++ [ root -/- mingwStamp | windowsHost ]
                      -- proxy for the entire mingw toolchain that


=====================================
hadrian/src/Rules/Program.hs
=====================================
@@ -85,8 +85,6 @@ buildProgram bin ctx@(Context{..}) rs = do
     need [template]
   -- Custom dependencies: this should be modeled better in the
   -- Cabal file somehow.
-  when (package == ghc) $ do
-    need =<< ghcBinDeps stage
   when (package == haddock) $ do
     -- Haddock has a resource folder
     need =<< haddockDeps stage


=====================================
hadrian/src/Settings/Packages.hs
=====================================
@@ -302,13 +302,11 @@ rtsPackageArgs = package rts ? do
     let ghcArgs = mconcat
           [ arg "-Irts"
           , arg $ "-I" ++ path
-          , notM targetSupportsSMP           ? arg "-DNOSMP"
           , way `elem` [debug, debugDynamic] ? pure [ "-DTICKY_TICKY"
                                                     , "-optc-DTICKY_TICKY"]
           , Profiling `wayUnit` way          ? arg "-DPROFILING"
           , Threaded  `wayUnit` way          ? arg "-DTHREADED_RTS"
-          , notM targetSupportsSMP           ? pure [ "-DNOSMP"
-                                                    , "-optc-DNOSMP" ]
+          , notM targetSupportsSMP           ? arg "-optc-DNOSMP"
           ]
 
     let cArgs = mconcat
@@ -326,6 +324,8 @@ rtsPackageArgs = package rts ? do
           , arg "-Irts"
           , arg $ "-I" ++ path
 
+          , notM targetSupportsSMP           ? arg "-DNOSMP"
+
           , Debug     `wayUnit` way          ? pure [ "-DDEBUG"
                                                     , "-fno-omit-frame-pointer"
                                                     , "-g3"


=====================================
testsuite/tests/rename/should_compile/T24084.hs
=====================================
@@ -0,0 +1,11 @@
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE NoImplicitPrelude #-}
+
+module T24084 where
+
+import T24084_B (Foo, Bar)
+
+data X
+
+instance Foo X where
+  type Bar X = X


=====================================
testsuite/tests/rename/should_compile/T24084_A.hs
=====================================
@@ -0,0 +1,8 @@
+{-# LANGUAGE TypeFamilies #-}
+
+{-# LANGUAGE NoImplicitPrelude #-}
+
+module T24084_A (Foo (..)) where
+
+class Foo a where
+  type Bar a


=====================================
testsuite/tests/rename/should_compile/T24084_B.hs
=====================================
@@ -0,0 +1,7 @@
+
+{-# LANGUAGE NoImplicitPrelude #-}
+
+module T24084_B (module T24084_A) where
+
+import T24084_A
+


=====================================
testsuite/tests/rename/should_compile/all.T
=====================================
@@ -216,6 +216,7 @@ test('T23510b', normal, compile, [''])
 test('T23512b', normal, compile, [''])
 test('T23664', normal, compile, [''])
 test('T24037', normal, compile, [''])
+test('T24084', [extra_files(['T24084_A.hs', 'T24084_B.hs'])], multimod_compile, ['T24084', '-v0'])
 test('ExportWarnings1', extra_files(['ExportWarnings_base.hs', 'ExportWarnings_aux.hs']), multimod_compile, ['ExportWarnings1', '-v0 -Wno-duplicate-exports -Wx-custom'])
 test('ExportWarnings2', extra_files(['ExportWarnings_base.hs', 'ExportWarnings_aux.hs', 'ExportWarnings_aux2.hs']), multimod_compile, ['ExportWarnings2', '-v0 -Wno-duplicate-exports -Wx-custom'])
 test('ExportWarnings3', extra_files(['ExportWarnings_base.hs', 'ExportWarnings_aux.hs']), multimod_compile, ['ExportWarnings3', '-v0 -Wno-duplicate-exports -Wx-custom'])



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/39751402c6a48f3a4024e5c9748ec8b3604d55d5...cc5c6853fd3f322236cc1daa02ddcf3391965447

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/39751402c6a48f3a4024e5c9748ec8b3604d55d5...cc5c6853fd3f322236cc1daa02ddcf3391965447
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