GHC/Core/SimpleOpt.hs

    {-
    (c) The University of Glasgow 2006
    (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
    -}

    {-# LANGUAGE CPP #-}
    {-# LANGUAGE MultiWayIf #-}

    module GHC.Core.SimpleOpt (
            SimpleOpts (..), defaultSimpleOpts,

            -- ** Simple expression optimiser
            simpleOptPgm, simpleOptExpr, simpleOptExprWith,

            -- ** Join points
            joinPointBinding_maybe, joinPointBindings_maybe,

            -- ** Predicates on expressions
            exprIsConApp_maybe, exprIsLiteral_maybe, exprIsLambda_maybe,

        ) where

    #include "HsVersions.h"

    import GHC.Prelude

    import GHC.Core
    import GHC.Core.Opt.Arity
    import GHC.Core.Subst
    import GHC.Core.Utils
    import GHC.Core.FVs
    import GHC.Core.Unfold
    import GHC.Core.Unfold.Make
    import GHC.Core.Make ( FloatBind(..) )
    import GHC.Core.Opt.OccurAnal( occurAnalyseExpr, occurAnalysePgm )
    import GHC.Types.Literal
    import GHC.Types.Id
    import GHC.Types.Id.Info  ( unfoldingInfo, setUnfoldingInfo, setRuleInfo, IdInfo (..) )
    import GHC.Types.Var      ( isNonCoVarId )
    import GHC.Types.Var.Set
    import GHC.Types.Var.Env
    import GHC.Core.DataCon
    import GHC.Types.Demand( etaConvertStrictSig )
    import GHC.Types.Tickish
    import GHC.Core.Coercion.Opt ( optCoercion, OptCoercionOpts (..) )
    import GHC.Core.Type hiding ( substTy, extendTvSubst, extendCvSubst, extendTvSubstList
                                , isInScope, substTyVarBndr, cloneTyVarBndr )
    import GHC.Core.Coercion hiding ( substCo, substCoVarBndr )
    import GHC.Builtin.Types
    import GHC.Builtin.Names
    import GHC.Types.Basic
    import GHC.Unit.Module ( Module )
    import GHC.Utils.Encoding
    import GHC.Utils.Outputable
    import GHC.Utils.Panic
    import GHC.Utils.Misc
    import GHC.Data.Maybe       ( orElse )
    import Data.List (mapAccumL)
    import qualified Data.ByteString as BS

    {-
    ************************************************************************
    *                                                                      *
            The Simple Optimiser
    *                                                                      *
    ************************************************************************

    Note [The simple optimiser]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The simple optimiser is a lightweight, pure (non-monadic) function
    that rapidly does a lot of simple optimisations, including

      - inlining things that occur just once,
          or whose RHS turns out to be trivial
      - beta reduction
      - case of known constructor
      - dead code elimination

    It does NOT do any call-site inlining; it only inlines a function if
    it can do so unconditionally, dropping the binding.  It thereby
    guarantees to leave no un-reduced beta-redexes.

    It is careful to follow the guidance of "Secrets of the GHC inliner",
    and in particular the pre-inline-unconditionally and
    post-inline-unconditionally story, to do effective beta reduction on
    functions called precisely once, without repeatedly optimising the same
    expression.  In fact, the simple optimiser is a good example of this
    little dance in action; the full Simplifier is a lot more complicated.

    -}

    -- | Simple optimiser options
    data SimpleOpts = SimpleOpts
       { so_uf_opts :: !UnfoldingOpts   -- ^ Unfolding options
       , so_co_opts :: !OptCoercionOpts -- ^ Coercion optimiser options
       }

    -- | Default options for the Simple optimiser.
    defaultSimpleOpts :: SimpleOpts
    defaultSimpleOpts = SimpleOpts
       { so_uf_opts = defaultUnfoldingOpts
       , so_co_opts = OptCoercionOpts
          { optCoercionEnabled = False }
       }

    simpleOptExpr :: HasDebugCallStack => SimpleOpts -> CoreExpr -> CoreExpr
    -- See Note [The simple optimiser]
    -- Do simple optimisation on an expression
    -- The optimisation is very straightforward: just
    -- inline non-recursive bindings that are used only once,
    -- or where the RHS is trivial
    --
    -- We also inline bindings that bind a Eq# box: see
    -- See Note [Getting the map/coerce RULE to work].
    --
    -- Also we convert functions to join points where possible (as
    -- the occurrence analyser does most of the work anyway).
    --
    -- The result is NOT guaranteed occurrence-analysed, because
    -- in  (let x = y in ....) we substitute for x; so y's occ-info
    -- may change radically
    --
    -- Note that simpleOptExpr is a pure function that we want to be able to call
    -- from lots of places, including ones that don't have DynFlags (e.g to optimise
    -- unfoldings of statically defined Ids via mkCompulsoryUnfolding). It used to
    -- fetch its options directly from the DynFlags, however, so some callers had to
    -- resort to using unsafeGlobalDynFlags (a global mutable variable containing
    -- the DynFlags). It has been modified to take its own SimpleOpts that may be
    -- created from DynFlags, but not necessarily.

    simpleOptExpr opts expr
      = -- pprTrace "simpleOptExpr" (ppr init_subst $$ ppr expr)
        simpleOptExprWith opts init_subst expr
      where
        init_subst = mkEmptySubst (mkInScopeSet (exprFreeVars expr))
            -- It's potentially important to make a proper in-scope set
            -- Consider  let x = ..y.. in \y. ...x...
            -- Then we should remember to clone y before substituting
            -- for x.  It's very unlikely to occur, because we probably
            -- won't *be* substituting for x if it occurs inside a
            -- lambda.
            --
            -- It's a bit painful to call exprFreeVars, because it makes
            -- three passes instead of two (occ-anal, and go)

    simpleOptExprWith :: HasDebugCallStack => SimpleOpts -> Subst -> InExpr -> OutExpr
    -- See Note [The simple optimiser]
    simpleOptExprWith opts subst expr
      = simple_opt_expr init_env (occurAnalyseExpr expr)
      where
        init_env = (emptyEnv opts) { soe_subst = subst }

    ----------------------
    simpleOptPgm :: SimpleOpts
                 -> Module
                 -> CoreProgram
                 -> [CoreRule]
                 -> (CoreProgram, [CoreRule], CoreProgram)
    -- See Note [The simple optimiser]
    simpleOptPgm opts this_mod binds rules =
        (reverse binds', rules', occ_anald_binds)
      where
        occ_anald_binds  = occurAnalysePgm this_mod
                              (\_ -> True)  {- All unfoldings active -}
                              (\_ -> False) {- No rules active -}
                              rules binds

        (final_env, binds') = foldl' do_one (emptyEnv opts, []) occ_anald_binds
        final_subst = soe_subst final_env

        rules' = substRulesForImportedIds final_subst rules
                 -- We never unconditionally inline into rules,
                 -- hence paying just a substitution

        do_one (env, binds') bind
          = case simple_opt_bind env bind TopLevel of
              (env', Nothing)    -> (env', binds')
              (env', Just bind') -> (env', bind':binds')

    -- In these functions the substitution maps InVar -> OutExpr

    ----------------------
    type SimpleClo = (SimpleOptEnv, InExpr)

    data SimpleOptEnv
      = SOE { soe_co_opt_opts :: !OptCoercionOpts
                 -- ^ Options for the coercion optimiser

            , soe_uf_opts :: !UnfoldingOpts
                 -- ^ Unfolding options

            , soe_inl   :: IdEnv SimpleClo
                 -- ^ Deals with preInlineUnconditionally; things
                 -- that occur exactly once and are inlined
                 -- without having first been simplified

            , soe_subst :: Subst
                 -- ^ Deals with cloning; includes the InScopeSet
            }

    instance Outputable SimpleOptEnv where
      ppr (SOE { soe_inl = inl, soe_subst = subst })
        = text "SOE {" <+> vcat [ text "soe_inl   =" <+> ppr inl
                                , text "soe_subst =" <+> ppr subst ]
                       <+> text "}"

    emptyEnv :: SimpleOpts -> SimpleOptEnv
    emptyEnv opts = SOE
       { soe_inl         = emptyVarEnv
       , soe_subst       = emptySubst
       , soe_co_opt_opts = so_co_opts opts
       , soe_uf_opts     = so_uf_opts opts
       }

    soeZapSubst :: SimpleOptEnv -> SimpleOptEnv
    soeZapSubst env@(SOE { soe_subst = subst })
      = env { soe_inl = emptyVarEnv, soe_subst = zapSubstEnv subst }

    soeSetInScope :: SimpleOptEnv -> SimpleOptEnv -> SimpleOptEnv
    -- Take in-scope set from env1, and the rest from env2
    soeSetInScope (SOE { soe_subst = subst1 })
                  env2@(SOE { soe_subst = subst2 })
      = env2 { soe_subst = setInScope subst2 (substInScope subst1) }

    ---------------
    simple_opt_clo :: SimpleOptEnv -> SimpleClo -> OutExpr
    simple_opt_clo env (e_env, e)
      = simple_opt_expr (soeSetInScope env e_env) e

    simple_opt_expr :: HasCallStack => SimpleOptEnv -> InExpr -> OutExpr
    simple_opt_expr env expr
      = go expr
      where
        subst        = soe_subst env
        in_scope     = substInScope subst
        in_scope_env = (in_scope, simpleUnfoldingFun)

        ---------------
        go (Var v)
           | Just clo <- lookupVarEnv (soe_inl env) v
           = simple_opt_clo env clo
           | otherwise
           = lookupIdSubst (soe_subst env) v

        go (App e1 e2)      = simple_app env e1 [(env,e2)]
        go (Type ty)        = Type     (substTy subst ty)
        go (Coercion co)    = Coercion (go_co co)
        go (Lit lit)        = Lit lit
        go (Tick tickish e) = mkTick (substTickish subst tickish) (go e)
        go (Cast e co)      = mk_cast (go e) (go_co co)
        go (Let bind body)  = case simple_opt_bind env bind NotTopLevel of
                                 (env', Nothing)   -> simple_opt_expr env' body
                                 (env', Just bind) -> Let bind (simple_opt_expr env' body)

        go lam@(Lam {})     = go_lam env [] lam
        go (Case e b ty as)
           -- See Note [Getting the map/coerce RULE to work]
          | isDeadBinder b
          , Just (_, [], con, _tys, es) <- exprIsConApp_maybe in_scope_env e'
            -- We don't need to be concerned about floats when looking for coerce.
          , Just (Alt altcon bs rhs) <- findAlt (DataAlt con) as
          = case altcon of
              DEFAULT -> go rhs
              _       -> foldr wrapLet (simple_opt_expr env' rhs) mb_prs
                where
                  (env', mb_prs) = mapAccumL (simple_out_bind NotTopLevel) env $
                                   zipEqual "simpleOptExpr" bs es

             -- Note [Getting the map/coerce RULE to work]
          | isDeadBinder b
          , [Alt DEFAULT _ rhs] <- as
          , isCoVarType (varType b)
          , (Var fun, _args) <- collectArgs e
          , fun `hasKey` coercibleSCSelIdKey
             -- without this last check, we get #11230
          = go rhs

          | otherwise
          = Case e' b' (substTy subst ty)
                       (map (go_alt env') as)
          where
            e' = go e
            (env', b') = subst_opt_bndr env b

        ----------------------
        go_co co = optCoercion (soe_co_opt_opts env) (getTCvSubst subst) co

        ----------------------
        go_alt env (Alt con bndrs rhs)
          = Alt con bndrs' (simple_opt_expr env' rhs)
          where
            (env', bndrs') = subst_opt_bndrs env bndrs

        ----------------------
        -- go_lam tries eta reduction
        go_lam env bs' (Lam b e)
           = go_lam env' (b':bs') e
           where
             (env', b') = subst_opt_bndr env b
        go_lam env bs' e
           | Just etad_e <- tryEtaReduce bs e' = etad_e
           | otherwise                         = mkLams bs e'
           where
             bs = reverse bs'
             e' = simple_opt_expr env e

    mk_cast :: CoreExpr -> CoercionR -> CoreExpr
    -- Like GHC.Core.Utils.mkCast, but does a full reflexivity check.
    -- mkCast doesn't do that because the Simplifier does (in simplCast)
    -- But in SimpleOpt it's nice to kill those nested casts (#18112)
    mk_cast (Cast e co1) co2        = mk_cast e (co1 `mkTransCo` co2)
    mk_cast (Tick t e)   co         = Tick t (mk_cast e co)
    mk_cast e co | isReflexiveCo co = e
                 | otherwise        = Cast e co

    ----------------------
    -- simple_app collects arguments for beta reduction
    simple_app :: HasDebugCallStack => SimpleOptEnv -> InExpr -> [SimpleClo] -> CoreExpr

    simple_app env (Var v) as
      | Just (env', e) <- lookupVarEnv (soe_inl env) v
      = simple_app (soeSetInScope env env') e as

      | let unf = idUnfolding v
      , isCompulsoryUnfolding (idUnfolding v)
      , isAlwaysActive (idInlineActivation v)
        -- See Note [Unfold compulsory unfoldings in LHSs]
      = simple_app (soeZapSubst env) (unfoldingTemplate unf) as

      | otherwise
      , let out_fn = lookupIdSubst (soe_subst env) v
      = finish_app env out_fn as

    simple_app env (App e1 e2) as
      = simple_app env e1 ((env, e2) : as)

    simple_app env e@(Lam {}) as@(_:_)
      | (bndrs, body) <- collectBinders e
      , let zapped_bndrs = zapLamBndrs (length as) bndrs
        -- Be careful to zap the lambda binders if necessary
        -- c.f. the Lam caes of simplExprF1 in GHC.Core.Opt.Simplify
        -- Lacking this zap caused #19347, when we had a redex
        --   (\ a b. K a b) e1 e2
        -- where (as it happens) the eta-expanded K is produced by
        -- Note [Linear fields generalization] in GHC.Tc.Gen.Head
      = do_beta env zapped_bndrs body as
      where
        do_beta env (b:bs) body (a:as)
          | (env', mb_pr) <- simple_bind_pair env b Nothing a NotTopLevel
          = wrapLet mb_pr $ do_beta env' bs body as
        do_beta env bs body as = simple_app env (mkLams bs body) as

    simple_app env (Tick t e) as
      -- Okay to do "(Tick t e) x ==> Tick t (e x)"?
      | t `tickishScopesLike` SoftScope
      = mkTick t $ simple_app env e as

    -- (let x = e in b) a1 .. an  =>  let x = e in (b a1 .. an)
    -- The let might appear there as a result of inlining
    -- e.g.   let f = let x = e in b
    --        in f a1 a2
    --   (#13208)
    -- However, do /not/ do this transformation for join points
    --    See Note [simple_app and join points]
    simple_app env (Let bind body) args
      = case simple_opt_bind env bind NotTopLevel of
          (env', Nothing)   -> simple_app env' body args
          (env', Just bind')
            | isJoinBind bind' -> finish_app env expr' args
            | otherwise        -> Let bind' (simple_app env' body args)
            where
              expr' = Let bind' (simple_opt_expr env' body)

    simple_app env e as
      = finish_app env (simple_opt_expr env e) as

    finish_app :: SimpleOptEnv -> OutExpr -> [SimpleClo] -> OutExpr
    finish_app _ fun []
      = fun
    finish_app env fun (arg:args)
      = finish_app env (App fun (simple_opt_clo env arg)) args

    ----------------------
    simple_opt_bind :: SimpleOptEnv -> InBind -> TopLevelFlag
                    -> (SimpleOptEnv, Maybe OutBind)
    simple_opt_bind env (NonRec b r) top_level
      = (env', case mb_pr of
                Nothing    -> Nothing
                Just (b,r) -> Just (NonRec b r))
      where
        (b', r') = joinPointBinding_maybe b r `orElse` (b, r)
        (env', mb_pr) = simple_bind_pair env b' Nothing (env,r') top_level

    simple_opt_bind env (Rec prs) top_level
      = (env'', res_bind)
      where
        res_bind          = Just (Rec (reverse rev_prs'))
        prs'              = joinPointBindings_maybe prs `orElse` prs
        (env', bndrs')    = subst_opt_bndrs env (map fst prs')
        (env'', rev_prs') = foldl' do_pr (env', []) (prs' `zip` bndrs')
        do_pr (env, prs) ((b,r), b')
           = (env', case mb_pr of
                      Just pr -> pr : prs
                      Nothing -> prs)
           where
             (env', mb_pr) = simple_bind_pair env b (Just b') (env,r) top_level

    ----------------------
    simple_bind_pair :: SimpleOptEnv
                     -> InVar -> Maybe OutVar
                     -> SimpleClo
                     -> TopLevelFlag
                     -> (SimpleOptEnv, Maybe (OutVar, OutExpr))
        -- (simple_bind_pair subst in_var out_rhs)
        --   either extends subst with (in_var -> out_rhs)
        --   or     returns Nothing
    simple_bind_pair env@(SOE { soe_inl = inl_env, soe_subst = subst })
                     in_bndr mb_out_bndr clo@(rhs_env, in_rhs)
                     top_level
      | Type ty <- in_rhs        -- let a::* = TYPE ty in <body>
      , let out_ty = substTy (soe_subst rhs_env) ty
      = ASSERT2( isTyVar in_bndr, ppr in_bndr $$ ppr in_rhs )
        (env { soe_subst = extendTvSubst subst in_bndr out_ty }, Nothing)

      | Coercion co <- in_rhs
      , let out_co = optCoercion (soe_co_opt_opts env) (getTCvSubst (soe_subst rhs_env)) co
      = ASSERT( isCoVar in_bndr )
        (env { soe_subst = extendCvSubst subst in_bndr out_co }, Nothing)

      | ASSERT2( isNonCoVarId in_bndr, ppr in_bndr )
        -- The previous two guards got rid of tyvars and coercions
        -- See Note [Core type and coercion invariant] in GHC.Core
        pre_inline_unconditionally
      = (env { soe_inl = extendVarEnv inl_env in_bndr clo }, Nothing)

      | otherwise
      = simple_out_bind_pair env in_bndr mb_out_bndr out_rhs
                             occ active stable_unf top_level
      where
        stable_unf = isStableUnfolding (idUnfolding in_bndr)
        active     = isAlwaysActive (idInlineActivation in_bndr)
        occ        = idOccInfo in_bndr

        out_rhs | Just join_arity <- isJoinId_maybe in_bndr
                = simple_join_rhs join_arity
                | otherwise
                = simple_opt_clo env clo

        simple_join_rhs join_arity -- See Note [Preserve join-binding arity]
          = mkLams join_bndrs' (simple_opt_expr env_body join_body)
          where
            env0 = soeSetInScope env rhs_env
            (join_bndrs, join_body) = collectNBinders join_arity in_rhs
            (env_body, join_bndrs') = subst_opt_bndrs env0 join_bndrs

        pre_inline_unconditionally :: Bool
        pre_inline_unconditionally
           | isExportedId in_bndr     = False
           | stable_unf               = False
           | not active               = False    -- Note [Inline prag in simplOpt]
           | not (safe_to_inline occ) = False
           | otherwise                = True

            -- Unconditionally safe to inline
        safe_to_inline :: OccInfo -> Bool
        safe_to_inline IAmALoopBreaker{}                  = False
        safe_to_inline IAmDead                            = True
        safe_to_inline OneOcc{ occ_in_lam = NotInsideLam
                             , occ_n_br = 1 }             = True
        safe_to_inline OneOcc{}                           = False
        safe_to_inline ManyOccs{}                         = False

    -------------------
    simple_out_bind :: TopLevelFlag
                    -> SimpleOptEnv
                    -> (InVar, OutExpr)
                    -> (SimpleOptEnv, Maybe (OutVar, OutExpr))
    simple_out_bind top_level env@(SOE { soe_subst = subst }) (in_bndr, out_rhs)
      | Type out_ty <- out_rhs
      = ASSERT2( isTyVar in_bndr, ppr in_bndr $$ ppr out_ty $$ ppr out_rhs )
        (env { soe_subst = extendTvSubst subst in_bndr out_ty }, Nothing)

      | Coercion out_co <- out_rhs
      = ASSERT( isCoVar in_bndr )
        (env { soe_subst = extendCvSubst subst in_bndr out_co }, Nothing)

      | otherwise
      = simple_out_bind_pair env in_bndr Nothing out_rhs
                             (idOccInfo in_bndr) True False top_level

    -------------------
    simple_out_bind_pair :: SimpleOptEnv
                         -> InId -> Maybe OutId -> OutExpr
                         -> OccInfo -> Bool -> Bool -> TopLevelFlag
                         -> (SimpleOptEnv, Maybe (OutVar, OutExpr))
    simple_out_bind_pair env in_bndr mb_out_bndr out_rhs
                         occ_info active stable_unf top_level
      | ASSERT2( isNonCoVarId in_bndr, ppr in_bndr )
        -- Type and coercion bindings are caught earlier
        -- See Note [Core type and coercion invariant]
        post_inline_unconditionally
      = ( env' { soe_subst = extendIdSubst (soe_subst env) in_bndr out_rhs }
        , Nothing)

      | otherwise
      = ( env', Just (out_bndr, out_rhs) )
      where
        (env', bndr1) = case mb_out_bndr of
                          Just out_bndr -> (env, out_bndr)
                          Nothing       -> subst_opt_bndr env in_bndr
        out_bndr = add_info env' in_bndr top_level out_rhs bndr1

        post_inline_unconditionally :: Bool
        post_inline_unconditionally
           | isExportedId in_bndr  = False -- Note [Exported Ids and trivial RHSs]
           | stable_unf            = False -- Note [Stable unfoldings and postInlineUnconditionally]
           | not active            = False --     in GHC.Core.Opt.Simplify.Utils
           | is_loop_breaker       = False -- If it's a loop-breaker of any kind, don't inline
                                           -- because it might be referred to "earlier"
           | exprIsTrivial out_rhs = True
           | coercible_hack        = True
           | otherwise             = False

        is_loop_breaker = isWeakLoopBreaker occ_info

        -- See Note [Getting the map/coerce RULE to work]
        coercible_hack | (Var fun, args) <- collectArgs out_rhs
                       , Just dc <- isDataConWorkId_maybe fun
                       , dc `hasKey` heqDataConKey || dc `hasKey` coercibleDataConKey
                       = all exprIsTrivial args
                       | otherwise
                       = False

    {- Note [Exported Ids and trivial RHSs]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We obviously do not want to unconditionally inline an Id that is exported.
    In GHC.Core.Opt.Simplify.Utils, Note [Top level and postInlineUnconditionally], we
    explain why we don't inline /any/ top-level things unconditionally, even
    trivial ones.  But we do here!  Why?  In the simple optimiser

      * We do no rule rewrites
      * We do no call-site inlining

    Those differences obviate the reasons for not inlining a trivial rhs,
    and increase the benefit for doing so.  So we unconditionally inline trivial
    rhss here.

    Note [Preserve join-binding arity]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Be careful /not/ to eta-reduce the RHS of a join point, lest we lose
    the join-point arity invariant.  #15108 was caused by simplifying
    the RHS with simple_opt_expr, which does eta-reduction.  Solution:
    simplify the RHS of a join point by simplifying under the lambdas
    (which of course should be there).

    Note [simple_app and join points]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In general for let-bindings we can do this:
       (let { x = e } in b) a  ==>  let { x = e } in b a

    But not for join points!  For two reasons:

    - We would need to push the continuation into the RHS:
       (join { j = e } in b) a  ==>  let { j' = e a } in b[j'/j] a
                                          NB ----^^
      and also change the type of j, hence j'.
      That's a bit sophisticated for the very simple optimiser.

    - We might end up with something like
        join { j' = e a } in
        (case blah of        )
        (  True  -> j' void# ) a
        (  False -> blah     )
      and now the call to j' doesn't look like a tail call, and
      Lint may reject.  I say "may" because this is /explicitly/
      allowed in the "Compiling without Continuations" paper
      (Section 3, "Managing \Delta").  But GHC currently does not
      allow this slightly-more-flexible form.  See GHC.Core
      Note [Join points are less general than the paper].

    The simple thing to do is to disable this transformation
    for join points in the simple optimiser

    Note [The Let-Unfoldings Invariant]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    A program has the Let-Unfoldings property iff:

    - For every let-bound variable f, whether top-level or nested, whether
      recursive or not:
      - Both the binding Id of f, and every occurrence Id of f, has an idUnfolding.
      - For non-INLINE things, that unfolding will be f's right hand sids
      - For INLINE things (which have a "stable" unfolding) that unfolding is
        semantically equivalent to f's RHS, but derived from the original RHS of f
        rather that its current RHS.

    Informally, we can say that in a program that has the Let-Unfoldings property,
    all let-bound Id's have an explicit unfolding attached to them.

    Currently, the simplifier guarantees the Let-Unfoldings invariant for anything
    it outputs.

    -}

    ----------------------
    subst_opt_bndrs :: SimpleOptEnv -> [InVar] -> (SimpleOptEnv, [OutVar])
    subst_opt_bndrs env bndrs = mapAccumL subst_opt_bndr env bndrs

    subst_opt_bndr :: SimpleOptEnv -> InVar -> (SimpleOptEnv, OutVar)
    subst_opt_bndr env bndr
      | isTyVar bndr  = (env { soe_subst = subst_tv }, tv')
      | isCoVar bndr  = (env { soe_subst = subst_cv }, cv')
      | otherwise     = subst_opt_id_bndr env bndr
      where
        subst           = soe_subst env
        (subst_tv, tv') = substTyVarBndr subst bndr
        (subst_cv, cv') = substCoVarBndr subst bndr

    subst_opt_id_bndr :: SimpleOptEnv -> InId -> (SimpleOptEnv, OutId)
    -- Nuke all fragile IdInfo, unfolding, and RULES; it gets added back later by
    -- add_info.
    --
    -- Rather like SimplEnv.substIdBndr
    --
    -- It's important to zap fragile OccInfo (which GHC.Core.Subst.substIdBndr
    -- carefully does not do) because simplOptExpr invalidates it

    subst_opt_id_bndr env@(SOE { soe_subst = subst, soe_inl = inl }) old_id
      = (env { soe_subst = new_subst, soe_inl = new_inl }, new_id)
      where
        Subst in_scope id_subst tv_subst cv_subst = subst

        id1    = uniqAway in_scope old_id
        id2    = updateIdTypeAndMult (substTy subst) id1
        new_id = zapFragileIdInfo id2
                 -- Zaps rules, unfolding, and fragile OccInfo
                 -- The unfolding and rules will get added back later, by add_info

        new_in_scope = in_scope `extendInScopeSet` new_id

        no_change = new_id == old_id

            -- Extend the substitution if the unique has changed,
            -- See the notes with substTyVarBndr for the delSubstEnv
        new_id_subst
          | no_change = delVarEnv id_subst old_id
          | otherwise = extendVarEnv id_subst old_id (Var new_id)

        new_subst = Subst new_in_scope new_id_subst tv_subst cv_subst
        new_inl   = delVarEnv inl old_id

    ----------------------
    add_info :: SimpleOptEnv -> InVar -> TopLevelFlag -> OutExpr -> OutVar -> OutVar
    add_info env old_bndr top_level new_rhs new_bndr
     | isTyVar old_bndr = new_bndr
     | otherwise        = lazySetIdInfo new_bndr new_info
     where
       subst    = soe_subst env
       uf_opts  = soe_uf_opts env
       old_info = idInfo old_bndr

       -- Add back in the rules and unfolding which were
       -- removed by zapFragileIdInfo in subst_opt_id_bndr.
       --
       -- See Note [The Let-Unfoldings Invariant]
       new_info = idInfo new_bndr `setRuleInfo`      new_rules
                                  `setUnfoldingInfo` new_unfolding

       old_rules = ruleInfo old_info
       new_rules = substRuleInfo subst new_bndr old_rules

       old_unfolding = unfoldingInfo old_info
       new_unfolding | isStableUnfolding old_unfolding
                     = substUnfolding subst old_unfolding
                     | otherwise
                     = unfolding_from_rhs

       unfolding_from_rhs = mkUnfolding uf_opts InlineRhs
                                        (isTopLevel top_level)
                                        False -- may be bottom or not
                                        new_rhs

    simpleUnfoldingFun :: IdUnfoldingFun
    simpleUnfoldingFun id
      | isAlwaysActive (idInlineActivation id) = idUnfolding id
      | otherwise                              = noUnfolding

    wrapLet :: Maybe (Id,CoreExpr) -> CoreExpr -> CoreExpr
    wrapLet Nothing      body = body
    wrapLet (Just (b,r)) body = Let (NonRec b r) body

    {-
    Note [Inline prag in simplOpt]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If there's an INLINE/NOINLINE pragma that restricts the phase in
    which the binder can be inlined, we don't inline here; after all,
    we don't know what phase we're in.  Here's an example

      foo :: Int -> Int -> Int
      {-# INLINE foo #-}
      foo m n = inner m
         where
           {-# INLINE [1] inner #-}
           inner m = m+n

      bar :: Int -> Int
      bar n = foo n 1

    When inlining 'foo' in 'bar' we want the let-binding for 'inner'
    to remain visible until Phase 1

    Note [Unfold compulsory unfoldings in LHSs]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When the user writes `RULES map coerce = coerce` as a rule, the rule
    will only ever match if simpleOptExpr replaces coerce by its unfolding
    on the LHS, because that is the core that the rule matching engine
    will find. So do that for everything that has a compulsory
    unfolding. Also see Note [Desugaring coerce as cast] in GHC.HsToCore.

    However, we don't want to inline 'seq', which happens to also have a
    compulsory unfolding, so we only do this unfolding only for things
    that are always-active.  See Note [User-defined RULES for seq] in GHC.Types.Id.Make.

    Note [Getting the map/coerce RULE to work]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    We wish to allow the "map/coerce" RULE to fire:

      {-# RULES "map/coerce" map coerce = coerce #-}

    The naive core produced for this is

      forall a b (dict :: Coercible * a b).
        map @a @b (coerce @a @b @dict) = coerce @[a] @[b] @dict'

      where dict' :: Coercible [a] [b]
            dict' = ...

    This matches literal uses of `map coerce` in code, but that's not what we
    want. We want it to match, say, `map MkAge` (where newtype Age = MkAge Int)
    too. Some of this is addressed by compulsorily unfolding coerce on the LHS,
    yielding

      forall a b (dict :: Coercible * a b).
        map @a @b (\(x :: a) -> case dict of
          MkCoercible (co :: a ~R# b) -> x |> co) = ...

    Getting better. But this isn't exactly what gets produced. This is because
    Coercible essentially has ~R# as a superclass, and superclasses get eagerly
    extracted during solving. So we get this:

      forall a b (dict :: Coercible * a b).
        case Coercible_SCSel @* @a @b dict of
          _ [Dead] -> map @a @b (\(x :: a) -> case dict of
                                   MkCoercible (co :: a ~R# b) -> x |> co) = ...

    Unfortunately, this still abstracts over a Coercible dictionary. We really
    want it to abstract over the ~R# evidence. So, we have Desugar.unfold_coerce,
    which transforms the above to (see also Note [Desugaring coerce as cast] in
    Desugar)

      forall a b (co :: a ~R# b).
        let dict = MkCoercible @* @a @b co in
        case Coercible_SCSel @* @a @b dict of
          _ [Dead] -> map @a @b (\(x :: a) -> case dict of
             MkCoercible (co :: a ~R# b) -> x |> co) = let dict = ... in ...

    Now, we need simpleOptExpr to fix this up. It does so by taking three
    separate actions:
      1. Inline certain non-recursive bindings. The choice whether to inline
         is made in simple_bind_pair. Note the rather specific check for
         MkCoercible in there.

      2. Stripping case expressions like the Coercible_SCSel one.
         See the `Case` case of simple_opt_expr's `go` function.

      3. Look for case expressions that unpack something that was
         just packed and inline them. This is also done in simple_opt_expr's
         `go` function.

    This is all a fair amount of special-purpose hackery, but it's for
    a good cause. And it won't hurt other RULES and such that it comes across.

    ************************************************************************
    *                                                                      *
                    Join points
    *                                                                      *
    ************************************************************************
    -}

    {- Note [Strictness and join points]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Suppose we have

       let f = \x.  if x>200 then e1 else e1

    and we know that f is strict in x.  Then if we subsequently
    discover that f is an arity-2 join point, we'll eta-expand it to

       let f = \x y.  if x>200 then e1 else e1

    and now it's only strict if applied to two arguments.  So we should
    adjust the strictness info.

    A more common case is when

       f = \x. error ".."

    and again its arity increases (#15517)
    -}

    -- | Returns Just (bndr,rhs) if the binding is a join point:
    -- If it's a JoinId, just return it
    -- If it's not yet a JoinId but is always tail-called,
    --    make it into a JoinId and return it.
    -- In the latter case, eta-expand the RHS if necessary, to make the
    -- lambdas explicit, as is required for join points
    --
    -- Precondition: the InBndr has been occurrence-analysed,
    --               so its OccInfo is valid
    joinPointBinding_maybe :: InBndr -> InExpr -> Maybe (InBndr, InExpr)
    joinPointBinding_maybe bndr rhs
      | not (isId bndr)
      = Nothing

      | isJoinId bndr
      = Just (bndr, rhs)

      | AlwaysTailCalled join_arity <- tailCallInfo (idOccInfo bndr)
      , (bndrs, body) <- etaExpandToJoinPoint join_arity rhs
      , let str_sig   = idStrictness bndr
            str_arity = count isId bndrs  -- Strictness demands are for Ids only
            join_bndr = bndr `asJoinId`        join_arity
                             `setIdStrictness` etaConvertStrictSig str_arity str_sig
      = Just (join_bndr, mkLams bndrs body)

      | otherwise
      = Nothing

    joinPointBindings_maybe :: [(InBndr, InExpr)] -> Maybe [(InBndr, InExpr)]
    joinPointBindings_maybe bndrs
      = mapM (uncurry joinPointBinding_maybe) bndrs

    {- *********************************************************************
    *                                                                      *
             exprIsConApp_maybe
    *                                                                      *
    ************************************************************************

    Note [exprIsConApp_maybe]
    ~~~~~~~~~~~~~~~~~~~~~~~~~
    exprIsConApp_maybe is a very important function.  There are two principal
    uses:
      * case e of { .... }
      * cls_op e, where cls_op is a class operation

    In both cases you want to know if e is of form (C e1..en) where C is
    a data constructor.

    However e might not *look* as if

    Note [exprIsConApp_maybe on literal strings]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    See #9400 and #13317.

    Conceptually, a string literal "abc" is just ('a':'b':'c':[]), but in Core
    they are represented as unpackCString# "abc"# by GHC.Core.Make.mkStringExprFS, or
    unpackCStringUtf8# when the literal contains multi-byte UTF8 characters.

    For optimizations we want to be able to treat it as a list, so they can be
    decomposed when used in a case-statement. exprIsConApp_maybe detects those
    calls to unpackCString# and returns:

    Just (':', [Char], ['a', unpackCString# "bc"]).

    We need to be careful about UTF8 strings here. ""# contains an encoded ByteString, so
    we call utf8UnconsByteString to correctly deal with the encoding and splitting.

    We must also be careful about
       lvl = "foo"#
       ...(unpackCString# lvl)...
    to ensure that we see through the let-binding for 'lvl'.  Hence the
    (exprIsLiteral_maybe .. arg) in the guard before the call to
    dealWithStringLiteral.

    The tests for this function are in T9400.

    Note [Push coercions in exprIsConApp_maybe]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    In #13025 I found a case where we had
        op (df @t1 @t2)     -- op is a ClassOp
    where
        df = (/\a b. K e1 e2) |> g

    To get this to come out we need to simplify on the fly
       ((/\a b. K e1 e2) |> g) @t1 @t2

    Hence the use of pushCoArgs.

    Note [exprIsConApp_maybe on data constructors with wrappers]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Problem:
    - some data constructors have wrappers
    - these wrappers inline late (see MkId Note [Activation for data constructor wrappers])
    - but we still want case-of-known-constructor to fire early.

    Example:
       data T = MkT !Int
       $WMkT n = case n of n' -> MkT n'   -- Wrapper for MkT
       foo x = case $WMkT e of MkT y -> blah

    Here we want the case-of-known-constructor transformation to fire, giving
       foo x = case e of x' -> let y = x' in blah

    Here's how exprIsConApp_maybe achieves this:

    0.  Start with scrutinee = $WMkT e

    1.  Inline $WMkT on-the-fly.  That's why data-constructor wrappers are marked
        as expandable. (See GHC.Core.Utils.isExpandableApp.) Now we have
          scrutinee = (\n. case n of n' -> MkT n') e

    2.  Beta-reduce the application, generating a floated 'let'.
        See Note [beta-reduction in exprIsConApp_maybe] below.  Now we have
          scrutinee = case n of n' -> MkT n'
          with floats {Let n = e}

    3.  Float the "case x of x' ->" binding out.  Now we have
          scrutinee = MkT n'
          with floats {Let n = e; case n of n' ->}

    And now we have a known-constructor MkT that we can return.

    Notice that both (2) and (3) require exprIsConApp_maybe to gather and return
    a bunch of floats, both let and case bindings.

    Note that this strategy introduces some subtle scenarios where a data-con
    wrapper can be replaced by a data-con worker earlier than we’d like, see
    Note [exprIsConApp_maybe for data-con wrappers: tricky corner].

    Note [beta-reduction in exprIsConApp_maybe]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The unfolding a definition (_e.g._ a let-bound variable or a datacon wrapper) is
    typically a function. For instance, take the wrapper for MkT in Note
    [exprIsConApp_maybe on data constructors with wrappers]:

        $WMkT n = case n of { n' -> T n' }

    If `exprIsConApp_maybe` is trying to analyse `$MkT arg`, upon unfolding of $MkT,
    it will see

       (\n -> case n of { n' -> T n' }) arg

    In order to go progress, `exprIsConApp_maybe` must perform a beta-reduction.

    We don't want to blindly substitute `arg` in the body of the function, because
    it duplicates work. We can (and, in fact, used to) substitute `arg` in the body,
    but only when `arg` is a variable (or something equally work-free).

    But, because of Note [exprIsConApp_maybe on data constructors with wrappers],
    'exprIsConApp_maybe' now returns floats. So, instead, we can beta-reduce
    _always_:

        (\x -> body) arg

    Is transformed into

       let x = arg in body

    Which, effectively, means emitting a float `let x = arg` and recursively
    analysing the body.

    For newtypes, this strategy requires that their wrappers have compulsory unfoldings.
    Suppose we have
       newtype T a b where
         MkT :: a -> T b a   -- Note args swapped

    This defines a worker function MkT, a wrapper function $WMkT, and an axT:
       $WMkT :: forall a b. a -> T b a
       $WMkT = /\b a. \(x:a). MkT a b x    -- A real binding

       MkT :: forall a b. a -> T a b
       MkT = /\a b. \(x:a). x |> (ax a b)  -- A compulsory unfolding

       axiom axT :: a ~R# T a b

    Now we are optimising
       case $WMkT (I# 3) |> sym axT of I# y -> ...
    we clearly want to simplify this. If $WMkT did not have a compulsory
    unfolding, we would end up with
       let a = I#3 in case a of I# y -> ...
    because in general, we do this on-the-fly beta-reduction
       (\x. e) blah  -->  let x = blah in e
    and then float the let.  (Substitution would risk duplicating 'blah'.)

    But if the case-of-known-constructor doesn't actually fire (i.e.
    exprIsConApp_maybe does not return Just) then nothing happens, and nothing
    will happen the next time either.

    See test T16254, which checks the behavior of newtypes.

    Note [Don't float join points]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    exprIsConApp_maybe should succeed on
       let v = e in Just v
    returning [x=e] as one of the [FloatBind].  But it must
    NOT succeed on
       join j x = rhs in Just v
    because join-points can't be gaily floated.  Consider
       case (join j x = rhs in Just) of
         K p q -> blah
    We absolutely must not "simplify" this to
       join j x = rhs
       in blah
    because j's return type is (Maybe t), quite different to blah's.

    You might think this could never happen, because j can't be
    tail-called in the body if the body returns a constructor.  But
    in !3113 we had a /dead/ join point (which is not illegal),
    and its return type was wonky.

    The simple thing is not to float a join point.  The next iteration
    of the simplifier will sort everything out.  And it there is
    a join point, the chances are that the body is not a constructor
    application, so failing faster is good.

    Note [exprIsConApp_maybe for data-con wrappers: tricky corner]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Generally speaking

      * exprIsConApp_maybe honours the inline phase; that is, it does not look
        inside the unfolding for an Id unless its unfolding is active in this phase.
        That phase-sensitivity is expressed in the InScopeEnv (specifically, the
        IdUnfoldingFun component of the InScopeEnv) passed to exprIsConApp_maybe.

      * Data-constructor wrappers are active only in phase 0 (the last phase);
        see Note [Activation for data constructor wrappers] in GHC.Types.Id.Make.

    On the face of it that means that exprIsConApp_maybe won't look inside data
    constructor wrappers until phase 0. But that seems pretty Bad. So we cheat.
    For data con wrappers we unconditionally look inside its unfolding, regardless
    of phase, so that we get case-of-known-constructor to fire in every phase.

    Perhaps unsurprisingly, this cheating can backfire. An example:

        data T = C !A B
        foo p q = let x = C e1 e2 in seq x $ f x
        {-# RULE "wurble" f (C a b) = b #-}

    In Core, the RHS of foo is

        let x = $WC e1 e2 in case x of y { C _ _ -> f x }

    and after doing a binder swap and inlining x, we have:

        case $WC e1 e2 of y { C _ _ -> f y }

    Case-of-known-constructor fires, but now we have to reconstruct a binding for
    `y` (which was dead before the binder swap) on the RHS of the case alternative.
    Naturally, we’ll use the worker:

        case e1 of a { DEFAULT -> let y = C a e2 in f y }

    and after inlining `y`, we have:

        case e1 of a { DEFAULT -> f (C a e2) }

    Now we might hope the "wurble" rule would fire, but alas, it will not: we have
    replaced $WC with C, but the (desugared) rule matches on $WC! We weren’t
    supposed to inline $WC yet for precisely that reason (see Note [Activation for
    data constructor wrappers]), but our cheating in exprIsConApp_maybe came back to
    bite us.

    This is rather unfortunate, especially since this can happen inside stable
    unfoldings as well as ordinary code (which really happened, see !3041). But
    there is no obvious solution except to delay case-of-known-constructor on
    data-con wrappers, and that cure would be worse than the disease.

    This Note exists solely to document the problem.
    -}

    data ConCont = CC [CoreExpr] Coercion
                      -- Substitution already applied

    -- | Returns @Just ([b1..bp], dc, [t1..tk], [x1..xn])@ if the argument
    -- expression is a *saturated* constructor application of the form @let b1 in
    -- .. let bp in dc t1..tk x1 .. xn@, where t1..tk are the
    -- *universally-quantified* type args of 'dc'. Floats can also be (and most
    -- likely are) single-alternative case expressions. Why does
    -- 'exprIsConApp_maybe' return floats? We may have to look through lets and
    -- cases to detect that we are in the presence of a data constructor wrapper. In
    -- this case, we need to return the lets and cases that we traversed. See Note
    -- [exprIsConApp_maybe on data constructors with wrappers]. Data constructor wrappers
    -- are unfolded late, but we really want to trigger case-of-known-constructor as
    -- early as possible. See also Note [Activation for data constructor wrappers]
    -- in "GHC.Types.Id.Make".
    --
    -- We also return the incoming InScopeSet, augmented with
    -- the binders from any [FloatBind] that we return
    exprIsConApp_maybe :: HasDebugCallStack
                       => InScopeEnv -> CoreExpr
                       -> Maybe (InScopeSet, [FloatBind], DataCon, [Type], [CoreExpr])
    exprIsConApp_maybe (in_scope, id_unf) expr
      = go (Left in_scope) [] expr (CC [] (mkRepReflCo (exprType expr)))
      where
        go :: Either InScopeSet Subst
                 -- Left in-scope  means "empty substitution"
                 -- Right subst    means "apply this substitution to the CoreExpr"
                 -- NB: in the call (go subst floats expr cont)
                 --     the substitution applies to 'expr', but /not/ to 'floats' or 'cont'
           -> [FloatBind] -> CoreExpr -> ConCont
                 -- Notice that the floats here are in reverse order
           -> Maybe (InScopeSet, [FloatBind], DataCon, [Type], [CoreExpr])
        go subst floats (Tick t expr) cont
           | not (tickishIsCode t) = go subst floats expr cont

        go subst floats (Cast expr co1) (CC args co2)
           | Just (args', m_co1') <- pushCoArgs (subst_co subst co1) args
                -- See Note [Push coercions in exprIsConApp_maybe]
           = case m_co1' of
               MCo co1' -> go subst floats expr (CC args' (co1' `mkTransCo` co2))
               MRefl    -> go subst floats expr (CC args' co2)

        go subst floats (App fun arg) (CC args co)
           = go subst floats fun (CC (subst_expr subst arg : args) co)

        go subst floats (Lam bndr body) (CC (arg:args) co)
           | exprIsTrivial arg          -- Don't duplicate stuff!
           = go (extend subst bndr arg) floats body (CC args co)
           | otherwise
           = let (subst', bndr') = subst_bndr subst bndr
                 float           = FloatLet (NonRec bndr' arg)
             in go subst' (float:floats) body (CC args co)

        go subst floats (Let (NonRec bndr rhs) expr) cont
           | not (isJoinId bndr)
             -- Crucial guard! See Note [Don't float join points]
           = let rhs'            = subst_expr subst rhs
                 (subst', bndr') = subst_bndr subst bndr
                 float           = FloatLet (NonRec bndr' rhs')
             in go subst' (float:floats) expr cont

        go subst floats (Case scrut b _ [Alt con vars expr]) cont
           = let
              scrut'           = subst_expr subst scrut
              (subst', b')     = subst_bndr subst b
              (subst'', vars') = subst_bndrs subst' vars
              float            = FloatCase scrut' b' con vars'
             in
               go subst'' (float:floats) expr cont

        go (Right sub) floats (Var v) cont
           = go (Left (substInScope sub))
                floats
                (lookupIdSubst sub v)
                cont

        go (Left in_scope) floats (Var fun) cont@(CC args co)

            | Just con <- isDataConWorkId_maybe fun
            , count isValArg args == idArity fun
            = succeedWith in_scope floats $
              pushCoDataCon con args co

            -- Look through data constructor wrappers: they inline late (See Note
            -- [Activation for data constructor wrappers]) but we want to do
            -- case-of-known-constructor optimisation eagerly (see Note
            -- [exprIsConApp_maybe on data constructors with wrappers]).
            | isDataConWrapId fun
            , let rhs = uf_tmpl (realIdUnfolding fun)
            = go (Left in_scope) floats rhs cont

            -- Look through dictionary functions; see Note [Unfolding DFuns]
            | DFunUnfolding { df_bndrs = bndrs, df_con = con, df_args = dfun_args } <- unfolding
            , bndrs `equalLength` args    -- See Note [DFun arity check]
            , let in_scope' = extend_in_scope (exprsFreeVars dfun_args)
                  subst = mkOpenSubst in_scope' (bndrs `zip` args)
                  -- We extend the in-scope set here to silence warnings from
                  -- substExpr when it finds not-in-scope Ids in dfun_args.
                  -- simplOptExpr initialises the in-scope set with exprFreeVars,
                  -- but that doesn't account for DFun unfoldings
            = succeedWith in_scope floats $
              pushCoDataCon con (map (substExpr subst) dfun_args) co

            -- Look through unfoldings, but only arity-zero one;
            -- if arity > 0 we are effectively inlining a function call,
            -- and that is the business of callSiteInline.
            -- In practice, without this test, most of the "hits" were
            -- CPR'd workers getting inlined back into their wrappers,
            | idArity fun == 0
            , Just rhs <- expandUnfolding_maybe unfolding
            , let in_scope' = extend_in_scope (exprFreeVars rhs)
            = go (Left in_scope') floats rhs cont

            -- See Note [exprIsConApp_maybe on literal strings]
            | (fun `hasKey` unpackCStringIdKey) ||
              (fun `hasKey` unpackCStringUtf8IdKey)
            , [arg]              <- args
            , Just (LitString str) <- exprIsLiteral_maybe (in_scope, id_unf) arg
            = succeedWith in_scope floats $
              dealWithStringLiteral fun str co
            where
              unfolding = id_unf fun
              extend_in_scope unf_fvs
                | isLocalId fun = in_scope `extendInScopeSetSet` unf_fvs
                | otherwise     = in_scope
                -- A GlobalId has no (LocalId) free variables; and the
                -- in-scope set tracks only LocalIds

        go _ _ _ _ = Nothing

        succeedWith :: InScopeSet -> [FloatBind]
                    -> Maybe (DataCon, [Type], [CoreExpr])
                    -> Maybe (InScopeSet, [FloatBind], DataCon, [Type], [CoreExpr])
        succeedWith in_scope rev_floats x
          = do { (con, tys, args) <- x
               ; let floats = reverse rev_floats
               ; return (in_scope, floats, con, tys, args) }

        ----------------------------
        -- Operations on the (Either InScopeSet GHC.Core.Subst)
        -- The Left case is wildly dominant
        subst_co (Left {}) co = co
        subst_co (Right s) co = GHC.Core.Subst.substCo s co

        subst_expr (Left {}) e = e
        subst_expr (Right s) e = substExpr s e

        subst_bndr msubst bndr
          = (Right subst', bndr')
          where
            (subst', bndr') = substBndr subst bndr
            subst = case msubst of
                      Left in_scope -> mkEmptySubst in_scope
                      Right subst   -> subst

        subst_bndrs subst bs = mapAccumL subst_bndr subst bs

        extend (Left in_scope) v e = Right (extendSubst (mkEmptySubst in_scope) v e)
        extend (Right s)       v e = Right (extendSubst s v e)

    -- See Note [exprIsConApp_maybe on literal strings]
    dealWithStringLiteral :: Var -> BS.ByteString -> Coercion
                          -> Maybe (DataCon, [Type], [CoreExpr])

    -- This is not possible with user-supplied empty literals, GHC.Core.Make.mkStringExprFS
    -- turns those into [] automatically, but just in case something else in GHC
    -- generates a string literal directly.
    dealWithStringLiteral fun str co =
      case utf8UnconsByteString str of
        Nothing -> pushCoDataCon nilDataCon [Type charTy] co
        Just (char, charTail) ->
          let char_expr = mkConApp charDataCon [mkCharLit char]
              -- In singleton strings, just add [] instead of unpackCstring# ""#.
              rest = if BS.null charTail
                       then mkConApp nilDataCon [Type charTy]
                       else App (Var fun)
                                (Lit (LitString charTail))

          in pushCoDataCon consDataCon [Type charTy, char_expr, rest] co

    {-
    Note [Unfolding DFuns]
    ~~~~~~~~~~~~~~~~~~~~~~
    DFuns look like

      df :: forall a b. (Eq a, Eq b) -> Eq (a,b)
      df a b d_a d_b = MkEqD (a,b) ($c1 a b d_a d_b)
                                   ($c2 a b d_a d_b)

    So to split it up we just need to apply the ops $c1, $c2 etc
    to the very same args as the dfun.  It takes a little more work
    to compute the type arguments to the dictionary constructor.

    Note [DFun arity check]
    ~~~~~~~~~~~~~~~~~~~~~~~
    Here we check that the total number of supplied arguments (including
    type args) matches what the dfun is expecting.  This may be *less*
    than the ordinary arity of the dfun: see Note [DFun unfoldings] in GHC.Core
    -}

    exprIsLiteral_maybe :: InScopeEnv -> CoreExpr -> Maybe Literal
    -- Same deal as exprIsConApp_maybe, but much simpler
    -- Nevertheless we do need to look through unfoldings for
    -- Integer and string literals, which are vigorously hoisted to top level
    -- and not subsequently inlined
    exprIsLiteral_maybe env@(_, id_unf) e
      = case e of
          Lit l     -> Just l
          Tick _ e' -> exprIsLiteral_maybe env e' -- dubious?
          Var v
             | Just rhs <- expandUnfolding_maybe (id_unf v)
             , Just l   <- exprIsLiteral_maybe env rhs
             -> Just l
          Var v
             | Just rhs <- expandUnfolding_maybe (id_unf v)
             , Just b <- matchBignum env rhs
             -> Just b
          e
             | Just b <- matchBignum env e
             -> Just b

             | otherwise
             -> Nothing
      where
        matchBignum env e
             | Just (_env,_fb,dc,_tys,[arg]) <- exprIsConApp_maybe env e
             , Just (LitNumber _ i) <- exprIsLiteral_maybe env arg
             = if
                | dc == naturalNSDataCon -> Just (mkLitNatural i)
                | dc == integerISDataCon -> Just (mkLitInteger i)
                | otherwise              -> Nothing
             | otherwise
             = Nothing

    {-
    Note [exprIsLambda_maybe]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    exprIsLambda_maybe will, given an expression `e`, try to turn it into the form
    `Lam v e'` (returned as `Just (v,e')`). Besides using lambdas, it looks through
    casts (using the Push rule), and it unfolds function calls if the unfolding
    has a greater arity than arguments are present.

    Currently, it is used in GHC.Core.Rules.match, and is required to make
    "map coerce = coerce" match.
    -}

    exprIsLambda_maybe :: HasDebugCallStack
                       => InScopeEnv -> CoreExpr
                       -> Maybe (Var, CoreExpr,[CoreTickish])
        -- See Note [exprIsLambda_maybe]

    -- The simple case: It is a lambda already
    exprIsLambda_maybe _ (Lam x e)
        = Just (x, e, [])

    -- Still straightforward: Ticks that we can float out of the way
    exprIsLambda_maybe (in_scope_set, id_unf) (Tick t e)
        | tickishFloatable t
        , Just (x, e, ts) <- exprIsLambda_maybe (in_scope_set, id_unf) e
        = Just (x, e, t:ts)

    -- Also possible: A casted lambda. Push the coercion inside
    exprIsLambda_maybe (in_scope_set, id_unf) (Cast casted_e co)
        | Just (x, e,ts) <- exprIsLambda_maybe (in_scope_set, id_unf) casted_e
        -- Only do value lambdas.
        -- this implies that x is not in scope in gamma (makes this code simpler)
        , not (isTyVar x) && not (isCoVar x)
        , ASSERT( not $ x `elemVarSet` tyCoVarsOfCo co) True
        , Just (x',e') <- pushCoercionIntoLambda in_scope_set x e co
        , let res = Just (x',e',ts)
        = --pprTrace "exprIsLambda_maybe:Cast" (vcat [ppr casted_e,ppr co,ppr res)])
          res

    -- Another attempt: See if we find a partial unfolding
    exprIsLambda_maybe (in_scope_set, id_unf) e
        | (Var f, as, ts) <- collectArgsTicks tickishFloatable e
        , idArity f > count isValArg as
        -- Make sure there is hope to get a lambda
        , Just rhs <- expandUnfolding_maybe (id_unf f)
        -- Optimize, for beta-reduction
        , let e' = simpleOptExprWith defaultSimpleOpts (mkEmptySubst in_scope_set) (rhs `mkApps` as)
        -- Recurse, because of possible casts
        , Just (x', e'', ts') <- exprIsLambda_maybe (in_scope_set, id_unf) e'
        , let res = Just (x', e'', ts++ts')
        = -- pprTrace "exprIsLambda_maybe:Unfold" (vcat [ppr e, ppr (x',e'')])
          res

    exprIsLambda_maybe _ _e
        = -- pprTrace "exprIsLambda_maybe:Fail" (vcat [ppr _e])
          Nothing