Revert "Fix binder-swap bug"wip/revert-poly-spec

This reverts commit 41406da55db1915cf1360e3275657f2b1115d530.

This commit introduced core lint errors (see #22357 and #22358)

7 files changed, 106 insertions, 162 deletions

diff --git a/compiler/GHC/Core/Opt/OccurAnal.hs b/compiler/GHC/Core/Opt/OccurAnal.hs
index c60dbfc39d..5e59e149a9 100644
--- a/compiler/GHC/Core/Opt/OccurAnal.hs
+++ b/compiler/GHC/Core/Opt/OccurAnal.hs
@@ -19,7 +19,7 @@ core expression with (hopefully) improved usage information.
 module GHC.Core.Opt.OccurAnal (
     occurAnalysePgm,
     occurAnalyseExpr,
-    zapLambdaBndrs, scrutBinderSwap_maybe
+    zapLambdaBndrs
   ) where
 
 import GHC.Prelude hiding ( head, init, last, tail )
@@ -27,12 +27,11 @@ import GHC.Prelude hiding ( head, init, last, tail )
 import GHC.Core
 import GHC.Core.FVs
 import GHC.Core.Utils   ( exprIsTrivial, isDefaultAlt, isExpandableApp,
-                          mkCastMCo, mkTicks )
+                          stripTicksTopE, mkTicks )
 import GHC.Core.Opt.Arity   ( joinRhsArity, isOneShotBndr )
 import GHC.Core.Coercion
-import GHC.Core.Predicate   ( isDictId )
 import GHC.Core.Type
-import GHC.Core.TyCo.FVs    ( tyCoVarsOfMCo )
+import GHC.Core.TyCo.FVs( tyCoVarsOfMCo )
 
 import GHC.Data.Maybe( isJust, orElse )
 import GHC.Data.Graph.Directed ( SCC(..), Node(..)
@@ -2465,8 +2464,8 @@ data OccEnv
 
            -- See Note [The binder-swap substitution]
            -- If  x :-> (y, co)  is in the env,
-           -- then please replace x by (y |> mco)
-           -- Invariant of course: idType x = exprType (y |> mco)
+           -- then please replace x by (y |> sym mco)
+           -- Invariant of course: idType x = exprType (y |> sym mco)
            , occ_bs_env  :: !(VarEnv (OutId, MCoercion))
            , occ_bs_rng  :: !VarSet   -- Vars free in the range of occ_bs_env
                    -- Domain is Global and Local Ids
@@ -2672,7 +2671,7 @@ The binder-swap is implemented by the occ_bs_env field of OccEnv.
 There are two main pieces:
 
 * Given    case x |> co of b { alts }
-  we add [x :-> (b, sym co)] to the occ_bs_env environment; this is
+  we add [x :-> (b, co)] to the occ_bs_env environment; this is
   done by addBndrSwap.
 
 * Then, at an occurrence of a variable, we look up in the occ_bs_env
@@ -2740,135 +2739,8 @@ Some tricky corners:
 (BS5) We have to apply the occ_bs_env substitution uniformly,
       including to (local) rules and unfoldings.
 
-(BS6) We must be very careful with dictionaries.
-      See Note [Care with binder-swap on dictionaries]
-
-Note [Case of cast]
-~~~~~~~~~~~~~~~~~~~
-Consider        case (x `cast` co) of b { I# ->
-                ... (case (x `cast` co) of {...}) ...
-We'd like to eliminate the inner case.  That is the motivation for
-equation (2) in Note [Binder swap].  When we get to the inner case, we
-inline x, cancel the casts, and away we go.
-
-Note [Care with binder-swap on dictionaries]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-This Note explains why we need isDictId in scrutBinderSwap_maybe.
-Consider this tricky example (#21229, #21470):
-
-  class Sing (b :: Bool) where sing :: Bool
-  instance Sing 'True  where sing = True
-  instance Sing 'False where sing = False
-
-  f :: forall a. Sing a => blah
-
-  h = \ @(a :: Bool) ($dSing :: Sing a)
-      let the_co =  Main.N:Sing[0] <a> :: Sing a ~R# Bool
-      case ($dSing |> the_co) of wild
-        True  -> f @'True (True |> sym the_co)
-        False -> f @a     dSing
-
-Now do a binder-swap on the case-expression:
-
-  h = \ @(a :: Bool) ($dSing :: Sing a)
-      let the_co =  Main.N:Sing[0] <a> :: Sing a ~R# Bool
-      case ($dSing |> the_co) of wild
-        True  -> f @'True (True |> sym the_co)
-        False -> f @a     (wild |> sym the_co)
-
-And now substitute `False` for `wild` (since wild=False in the False branch):
-
-  h = \ @(a :: Bool) ($dSing :: Sing a)
-      let the_co =  Main.N:Sing[0] <a> :: Sing a ~R# Bool
-      case ($dSing |> the_co) of wild
-        True  -> f @'True (True  |> sym the_co)
-        False -> f @a     (False |> sym the_co)
-
-And now we have a problem.  The specialiser will specialise (f @a d)a (for all
-vtypes a and dictionaries d!!) with the dictionary (False |> sym the_co), using
-Note [Specialising polymorphic dictionaries] in GHC.Core.Opt.Specialise.
-
-The real problem is the binder-swap.  It swaps a dictionary variable $dSing
-(of kind Constraint) for a term variable wild (of kind Type).  And that is
-dangerous: a dictionary is a /singleton/ type whereas a general term variable is
-not.  In this particular example, Bool is most certainly not a singleton type!
-
-Conclusion:
-  for a /dictionary variable/ do not perform
-  the clever cast version of the binder-swap
-
-Hence the subtle isDictId in scrutBinderSwap_maybe.
-
-Note [Zap case binders in proxy bindings]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-From the original
-     case x of cb(dead) { p -> ...x... }
-we will get
-     case x of cb(live) { p -> ...cb... }
-
-Core Lint never expects to find an *occurrence* of an Id marked
-as Dead, so we must zap the OccInfo on cb before making the
-binding x = cb.  See #5028.
-
-NB: the OccInfo on /occurrences/ really doesn't matter much; the simplifier
-doesn't use it. So this is only to satisfy the perhaps-over-picky Lint.
-
--}
-
-addBndrSwap :: OutExpr -> Id -> OccEnv -> OccEnv
--- See Note [The binder-swap substitution]
-addBndrSwap scrut case_bndr
-            env@(OccEnv { occ_bs_env = swap_env, occ_bs_rng = rng_vars })
-  | Just (scrut_var, mco) <- scrutBinderSwap_maybe scrut
-  , scrut_var /= case_bndr
-      -- Consider: case x of x { ... }
-      -- Do not add [x :-> x] to occ_bs_env, else lookupBndrSwap will loop
-  = env { occ_bs_env = extendVarEnv swap_env scrut_var (case_bndr', mco)
-        , occ_bs_rng = rng_vars `extendVarSet` case_bndr'
-                       `unionVarSet` tyCoVarsOfMCo mco }
-
-  | otherwise
-  = env
-  where
-    case_bndr' = zapIdOccInfo case_bndr
-                 -- See Note [Zap case binders in proxy bindings]
-
-scrutBinderSwap_maybe :: OutExpr -> Maybe (OutVar, MCoercion)
--- If (scrutBinderSwap_maybe e = Just (v, mco), then
---    v = e |> mco
--- See Note [Case of cast]
--- See Note [Care with binder-swap on dictionaries]
---
--- We use this same function in SpecConstr, and Simplify.Iteration,
--- when something binder-swap-like is happening
-scrutBinderSwap_maybe (Var v)    = Just (v, MRefl)
-scrutBinderSwap_maybe (Cast (Var v) co)
-  | not (isDictId v)             = Just (v, MCo (mkSymCo co))
-        -- Cast: see Note [Case of cast]
-        -- isDictId: see Note [Care with binder-swap on dictionaries]
-        -- The isDictId rejects a Constraint/Constraint binder-swap, perhaps
-        -- over-conservatively. But I have never seen one, so I'm leaving
-        -- the code as simple as possible. Losing the binder-swap in a
-        -- rare case probably has very low impact.
-scrutBinderSwap_maybe (Tick _ e) = scrutBinderSwap_maybe e  -- Drop ticks
-scrutBinderSwap_maybe _          = Nothing
-
-lookupBndrSwap :: OccEnv -> Id -> (CoreExpr, Id)
--- See Note [The binder-swap substitution]
--- Returns an expression of the same type as Id
-lookupBndrSwap env@(OccEnv { occ_bs_env = bs_env })  bndr
-  = case lookupVarEnv bs_env bndr of {
-       Nothing           -> (Var bndr, bndr) ;
-       Just (bndr1, mco) ->
-
-    -- Why do we iterate here?
-    -- See (BS2) in Note [The binder-swap substitution]
-    case lookupBndrSwap env bndr1 of
-      (fun, fun_id) -> (mkCastMCo fun mco, fun_id) }
-
-
-{- Historical note [Proxy let-bindings]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Historical note
+---------------
 We used to do the binder-swap transformation by introducing
 a proxy let-binding, thus;
 
@@ -2892,8 +2764,30 @@ But that had two problems:
 
 Doing a substitution (via occ_bs_env) is much better.
 
+Note [Case of cast]
+~~~~~~~~~~~~~~~~~~~
+Consider        case (x `cast` co) of b { I# ->
+                ... (case (x `cast` co) of {...}) ...
+We'd like to eliminate the inner case.  That is the motivation for
+equation (2) in Note [Binder swap].  When we get to the inner case, we
+inline x, cancel the casts, and away we go.
+
+Note [Zap case binders in proxy bindings]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+From the original
+     case x of cb(dead) { p -> ...x... }
+we will get
+     case x of cb(live) { p -> ...cb... }
+
+Core Lint never expects to find an *occurrence* of an Id marked
+as Dead, so we must zap the OccInfo on cb before making the
+binding x = cb.  See #5028.
+
+NB: the OccInfo on /occurrences/ really doesn't matter much; the simplifier
+doesn't use it. So this is only to satisfy the perhaps-over-picky Lint.
+
 Historical Note [no-case-of-case]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 We *used* to suppress the binder-swap in case expressions when
 -fno-case-of-case is on.  Old remarks:
     "This happens in the first simplifier pass,
@@ -2952,8 +2846,53 @@ binder-swap in OccAnal:
 It's fixed by doing the binder-swap in OccAnal because we can do the
 binder-swap unconditionally and still get occurrence analysis
 information right.
+-}
 
+addBndrSwap :: OutExpr -> Id -> OccEnv -> OccEnv
+-- See Note [The binder-swap substitution]
+addBndrSwap scrut case_bndr
+            env@(OccEnv { occ_bs_env = swap_env, occ_bs_rng = rng_vars })
+  | Just (scrut_var, mco) <- get_scrut_var (stripTicksTopE (const True) scrut)
+  , scrut_var /= case_bndr
+      -- Consider: case x of x { ... }
+      -- Do not add [x :-> x] to occ_bs_env, else lookupBndrSwap will loop
+  = env { occ_bs_env = extendVarEnv swap_env scrut_var (case_bndr', mco)
+        , occ_bs_rng = rng_vars `extendVarSet` case_bndr'
+                       `unionVarSet` tyCoVarsOfMCo mco }
+
+  | otherwise
+  = env
+  where
+    get_scrut_var :: OutExpr -> Maybe (OutVar, MCoercion)
+    get_scrut_var (Var v)           = Just (v, MRefl)
+    get_scrut_var (Cast (Var v) co) = Just (v, MCo co) -- See Note [Case of cast]
+    get_scrut_var _                 = Nothing
+
+    case_bndr' = zapIdOccInfo case_bndr
+                 -- See Note [Zap case binders in proxy bindings]
 
+lookupBndrSwap :: OccEnv -> Id -> (CoreExpr, Id)
+-- See Note [The binder-swap substitution]
+-- Returns an expression of the same type as Id
+lookupBndrSwap env@(OccEnv { occ_bs_env = bs_env })  bndr
+  = case lookupVarEnv bs_env bndr of {
+       Nothing           -> (Var bndr, bndr) ;
+       Just (bndr1, mco) ->
+
+    -- Why do we iterate here?
+    -- See (BS2) in Note [The binder-swap substitution]
+    case lookupBndrSwap env bndr1 of
+      (fun, fun_id) -> (add_cast fun mco, fun_id) }
+
+  where
+    add_cast fun MRefl    = fun
+    add_cast fun (MCo co) = Cast fun (mkSymCo co)
+    -- We must switch that 'co' to 'sym co';
+    -- see the comment with occ_bs_env
+    -- No need to test for isReflCo, because 'co' came from
+    -- a (Cast e co) and hence is unlikely to be Refl
+
+{-
 ************************************************************************
 *                                                                      *
 \subsection[OccurAnal-types]{OccEnv}
diff --git a/compiler/GHC/Core/Opt/SetLevels.hs b/compiler/GHC/Core/Opt/SetLevels.hs
index 5ac447883a..168e0a1dd3 100644
--- a/compiler/GHC/Core/Opt/SetLevels.hs
+++ b/compiler/GHC/Core/Opt/SetLevels.hs
@@ -51,6 +51,17 @@
   The simplifier tries to get rid of occurrences of x, in favour of wild,
   in the hope that there will only be one remaining occurrence of x, namely
   the scrutinee of the case, and we can inline it.
+
+  This can only work if @wild@ is an unrestricted binder. Indeed, even with the
+  extended typing rule (in the linter) for case expressions, if
+       case x of wild % 1 { p -> e}
+  is well-typed, then
+       case x of wild % 1 { p -> e[wild\x] }
+  is only well-typed if @e[wild\x] = e@ (that is, if @wild@ is not used in @e@
+  at all). In which case, it is, of course, pointless to do the substitution
+  anyway. So for a linear binder (and really anything which isn't unrestricted),
+  doing this substitution would either produce ill-typed terms or be the
+  identity.
 -}
 
 module GHC.Core.Opt.SetLevels (
@@ -1591,9 +1602,7 @@ extendCaseBndrEnv :: LevelEnv
                   -> LevelEnv
 extendCaseBndrEnv le@(LE { le_subst = subst, le_env = id_env })
                   case_bndr (Var scrut_var)
-  -- We could use OccurAnal. scrutBinderSwap_maybe here, and perhaps
-  -- get a bit more floating.  But we didn't in the past and it's
-  -- an unforced change, so I'm leaving it.
+    | Many <- varMult case_bndr
   = le { le_subst   = extendSubstWithVar subst case_bndr scrut_var
        , le_env     = add_id id_env (case_bndr, scrut_var) }
 extendCaseBndrEnv env _ _ = env
diff --git a/compiler/GHC/Core/Opt/Simplify/Iteration.hs b/compiler/GHC/Core/Opt/Simplify/Iteration.hs
index 49707fecf4..f3fb5c2f0b 100644
--- a/compiler/GHC/Core/Opt/Simplify/Iteration.hs
+++ b/compiler/GHC/Core/Opt/Simplify/Iteration.hs
@@ -22,7 +22,7 @@ import GHC.Core.Opt.Simplify.Monad
 import GHC.Core.Type hiding ( substTy, substTyVar, extendTvSubst, extendCvSubst )
 import GHC.Core.Opt.Simplify.Env
 import GHC.Core.Opt.Simplify.Utils
-import GHC.Core.Opt.OccurAnal ( occurAnalyseExpr, zapLambdaBndrs, scrutBinderSwap_maybe )
+import GHC.Core.Opt.OccurAnal ( occurAnalyseExpr, zapLambdaBndrs )
 import GHC.Core.Make       ( FloatBind, mkImpossibleExpr, castBottomExpr )
 import qualified GHC.Core.Make
 import GHC.Core.Coercion hiding ( substCo, substCoVar )
@@ -3286,21 +3286,19 @@ zapIdOccInfoAndSetEvald str v =
                          -- see Note [Case alternative occ info]
 
 addAltUnfoldings :: SimplEnv -> Maybe OutExpr -> OutId -> OutExpr -> SimplM SimplEnv
-addAltUnfoldings env mb_scrut case_bndr con_app
+addAltUnfoldings env scrut case_bndr con_app
   = do { let con_app_unf = mk_simple_unf con_app
              env1 = addBinderUnfolding env case_bndr con_app_unf
 
              -- See Note [Add unfolding for scrutinee]
-             env2 | Just scrut <- mb_scrut
-                  , Just (v,mco) <- scrutBinderSwap_maybe scrut
-                  = addBinderUnfolding env1 v $
-                       if isReflMCo mco  -- isReflMCo: avoid calling mk_simple_unf
-                       then con_app_unf  --            twice in the common case
-                       else mk_simple_unf (mkCastMCo con_app mco)
-
+             env2 | Many <- idMult case_bndr = case scrut of
+                      Just (Var v)           -> addBinderUnfolding env1 v con_app_unf
+                      Just (Cast (Var v) co) -> addBinderUnfolding env1 v $
+                                                mk_simple_unf (Cast con_app (mkSymCo co))
+                      _                      -> env1
                   | otherwise = env1
 
-       ; traceSmpl "addAltUnf" (vcat [ppr case_bndr <+> ppr mb_scrut, ppr con_app])
+       ; traceSmpl "addAltUnf" (vcat [ppr case_bndr <+> ppr scrut, ppr con_app])
        ; return env2 }
   where
     -- Force the opts, so that the whole SimplEnv isn't retained
@@ -3363,6 +3361,9 @@ it's also good for case-elimination -- suppose that 'f' was inlined
 and did multi-level case analysis, then we'd solve it in one
 simplifier sweep instead of two.
 
+Exactly the same issue arises in GHC.Core.Opt.SpecConstr;
+see Note [Add scrutinee to ValueEnv too] in GHC.Core.Opt.SpecConstr
+
 HOWEVER, given
   case x of y { Just a -> r1; Nothing -> r2 }
 we do not want to add the unfolding x -> y to 'x', which might seem cool,
@@ -3373,11 +3374,8 @@ piece of information.
 So instead we add the unfolding x -> Just a, and x -> Nothing in the
 respective RHSs.
 
-Since this transformation is tantamount to a binder swap, we use
-GHC.Core.Opt.OccurAnal.scrutBinderSwap_maybe to do the check.
-
-Exactly the same issue arises in GHC.Core.Opt.SpecConstr;
-see Note [Add scrutinee to ValueEnv too] in GHC.Core.Opt.SpecConstr
+Since this transformation is tantamount to a binder swap, the same caveat as in
+Note [Suppressing binder-swaps on linear case] in OccurAnal apply.
 
 
 ************************************************************************
diff --git a/compiler/GHC/Core/Opt/SpecConstr.hs b/compiler/GHC/Core/Opt/SpecConstr.hs
index 12d9ceb0fa..0dd84fdf99 100644
--- a/compiler/GHC/Core/Opt/SpecConstr.hs
+++ b/compiler/GHC/Core/Opt/SpecConstr.hs
@@ -35,7 +35,6 @@ import GHC.Core.Unfold
 import GHC.Core.FVs     ( exprsFreeVarsList, exprFreeVars )
 import GHC.Core.Opt.Monad
 import GHC.Core.Opt.WorkWrap.Utils
-import GHC.Core.Opt.OccurAnal( scrutBinderSwap_maybe )
 import GHC.Core.DataCon
 import GHC.Core.Class( classTyVars )
 import GHC.Core.Coercion hiding( substCo )
@@ -1073,8 +1072,8 @@ extendCaseBndrs env scrut case_bndr con alt_bndrs
    = (env2, alt_bndrs')
  where
    live_case_bndr = not (isDeadBinder case_bndr)
-   env1 | Just (v, mco) <- scrutBinderSwap_maybe scrut
-        , isReflMCo mco  = extendValEnv env v cval
+   env1 | Var v <- stripTicksTopE (const True) scrut
+                         = extendValEnv env v cval
         | otherwise      = env  -- See Note [Add scrutinee to ValueEnv too]
    env2 | live_case_bndr = extendValEnv env1 case_bndr cval
         | otherwise      = env1
@@ -1168,10 +1167,6 @@ though the simplifier has systematically replaced uses of 'x' with 'y'
 and 'b' with 'c' in the code.  The use of 'b' in the ValueEnv came
 from outside the case.  See #4908 for the live example.
 
-It's very like the binder-swap story, so we use scrutBinderSwap_maybe
-to identify suitable scrutinees -- but only if there is no cast
-(isReflMCo) because that's all that the ValueEnv allows.
-
 Note [Avoiding exponential blowup]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 The sc_count field of the ScEnv says how many times we are prepared to
diff --git a/compiler/GHC/Core/Subst.hs b/compiler/GHC/Core/Subst.hs
index 9bfca6184e..232f7cb9cf 100644
--- a/compiler/GHC/Core/Subst.hs
+++ b/compiler/GHC/Core/Subst.hs
@@ -41,6 +41,7 @@ import GHC.Core
 import GHC.Core.FVs
 import GHC.Core.Seq
 import GHC.Core.Utils
+import GHC.Core.TyCo.Subst ( substCo )
 
         -- We are defining local versions
 import GHC.Core.Type hiding ( substTy )
diff --git a/testsuite/tests/linters/notes.stdout b/testsuite/tests/linters/notes.stdout
index b50da6b3a2..ac7a453445 100644
--- a/testsuite/tests/linters/notes.stdout
+++ b/testsuite/tests/linters/notes.stdout
@@ -2,6 +2,7 @@ ref    compiler/GHC/Core/Coercion/Axiom.hs:461:2:     Note [RoughMap and rm_empt
 ref    compiler/GHC/Core/Opt/OccurAnal.hs:857:15:     Note [Loop breaking]
 ref    compiler/GHC/Core/Opt/SetLevels.hs:1580:30:     Note [Top level scope]
 ref    compiler/GHC/Core/Opt/Simplify/Iteration.hs:2675:13:     Note [Case binder next]
+ref    compiler/GHC/Core/Opt/Simplify/Iteration.hs:3303:0:     Note [Suppressing binder-swaps on linear case]
 ref    compiler/GHC/Core/Opt/Simplify/Iteration.hs:3854:8:     Note [Lambda-bound unfoldings]
 ref    compiler/GHC/Core/Opt/Simplify/Utils.hs:1257:37:     Note [Gentle mode]
 ref    compiler/GHC/Core/Opt/Specialise.hs:1623:28:     Note [Arity decrease]
diff --git a/testsuite/tests/simplCore/should_compile/all.T b/testsuite/tests/simplCore/should_compile/all.T
index 9cbd330d0b..fc708ef9f0 100644
--- a/testsuite/tests/simplCore/should_compile/all.T
+++ b/testsuite/tests/simplCore/should_compile/all.T
@@ -359,6 +359,7 @@ test('T19586', normal, compile, [''])
 
 test('T19599', normal, compile, ['-O -ddump-rules'])
 test('T19599a', normal, compile, ['-O -ddump-rules'])
+test('T13873',  [expect_broken(21229), grep_errmsg(r'SPEC') ], compile, ['-O -ddump-rules'])
 
 # Look for a specialisation rule for wimwam
 test('T19672', normal, compile, ['-O2 -ddump-rules'])