\begin{code}
module SimplUtils (
-- Rebuilding
- mkLam, mkCase, prepareAlts, bindCaseBndr,
+ mkLam, mkCase, prepareAlts,
-- Inlining,
preInlineUnconditionally, postInlineUnconditionally,
activeInline, activeRule,
- simplEnvForGHCi, simplEnvForRules, simplGentlyForInlineRules,
+ simplEnvForGHCi, simplEnvForRules, updModeForInlineRules,
-- The continuation type
SimplCont(..), DupFlag(..), ArgInfo(..),
contIsDupable, contResultType, contIsTrivial, contArgs, dropArgs,
- countValArgs, countArgs,
+ pushArgs, countValArgs, countArgs, addArgTo,
mkBoringStop, mkRhsStop, mkLazyArgStop, contIsRhsOrArg,
interestingCallContext,
import Name
import Id
import Var ( isCoVar )
-import NewDemand
+import Demand
import SimplMonad
import Type hiding( substTy )
import Coercion ( coercionKind )
| ApplyTo -- C arg
DupFlag
- InExpr SimplEnv -- The argument and its static env
+ InExpr StaticEnv -- The argument and its static env
SimplCont
| Select -- case C of alts
DupFlag
- InId [InAlt] SimplEnv -- The case binder, alts, and subst-env
+ InId [InAlt] StaticEnv -- The case binder, alts, and subst-env
SimplCont
-- The two strict forms have no DupFlag, because we never duplicate them
| StrictBind -- (\x* \xs. e) C
InId [InBndr] -- let x* = [] in e
- InExpr SimplEnv -- is a special case
+ InExpr StaticEnv -- is a special case
SimplCont
- | StrictArg -- e C
- OutExpr -- e; *always* of form (Var v `App1` e1 .. `App` en)
- CallCtxt -- Whether *this* argument position is interesting
- ArgInfo -- Whether the function at the head of e has rules, etc
- SimplCont -- plus strictness flags for *further* args
+ | StrictArg -- f e1 ..en C
+ ArgInfo -- Specifies f, e1..en, Whether f has rules, etc
+ -- plus strictness flags for *further* args
+ CallCtxt -- Whether *this* argument position is interesting
+ SimplCont
data ArgInfo
= ArgInfo {
- ai_rules :: Bool, -- Function has rules (recursively)
- -- => be keener to inline in all args
- ai_strs :: [Bool], -- Strictness of arguments
+ ai_fun :: Id, -- The function
+ ai_args :: [OutExpr], -- ...applied to these args (which are in *reverse* order)
+ ai_rules :: [CoreRule], -- Rules for this function
+
+ ai_encl :: Bool, -- Flag saying whether this function
+ -- or an enclosing one has rules (recursively)
+ -- True => be keener to inline in all args
+
+ ai_strs :: [Bool], -- Strictness of remaining arguments
-- Usually infinite, but if it is finite it guarantees
-- that the function diverges after being given
-- that number of args
- ai_discs :: [Int] -- Discounts for arguments; non-zero => be keener to inline
+ ai_discs :: [Int] -- Discounts for remaining arguments; non-zero => be keener to inline
-- Always infinite
}
+addArgTo :: ArgInfo -> OutExpr -> ArgInfo
+addArgTo ai arg = ai { ai_args = arg : ai_args ai }
+
instance Outputable SimplCont where
ppr (Stop interesting) = ptext (sLit "Stop") <> brackets (ppr interesting)
ppr (ApplyTo dup arg _ cont) = ((ptext (sLit "ApplyTo") <+> ppr dup <+> pprParendExpr arg)
{- $$ nest 2 (pprSimplEnv se) -}) $$ ppr cont
ppr (StrictBind b _ _ _ cont) = (ptext (sLit "StrictBind") <+> ppr b) $$ ppr cont
- ppr (StrictArg f _ _ cont) = (ptext (sLit "StrictArg") <+> ppr f) $$ ppr cont
+ ppr (StrictArg ai _ cont) = (ptext (sLit "StrictArg") <+> ppr (ai_fun ai)) $$ ppr cont
ppr (Select dup bndr alts _ cont) = (ptext (sLit "Select") <+> ppr dup <+> ppr bndr) $$
(nest 4 (ppr alts)) $$ ppr cont
ppr (CoerceIt co cont) = (ptext (sLit "CoerceIt") <+> ppr co) $$ ppr cont
go (Stop {}) ty = ty
go (CoerceIt co cont) _ = go cont (snd (coercionKind co))
go (StrictBind _ bs body se cont) _ = go cont (subst_ty se (exprType (mkLams bs body)))
- go (StrictArg fn _ _ cont) _ = go cont (funResultTy (exprType fn))
+ go (StrictArg ai _ cont) _ = go cont (funResultTy (argInfoResultTy ai))
go (Select _ _ alts se cont) _ = go cont (subst_ty se (coreAltsType alts))
go (ApplyTo _ arg se cont) ty = go cont (apply_to_arg ty arg se)
apply_to_arg ty (Type ty_arg) se = applyTy ty (subst_ty se ty_arg)
apply_to_arg ty _ _ = funResultTy ty
+argInfoResultTy :: ArgInfo -> OutType
+argInfoResultTy (ArgInfo { ai_fun = fun, ai_args = args })
+ = foldr (\arg fn_ty -> applyTypeToArg fn_ty arg) (idType fun) args
+
-------------------
countValArgs :: SimplCont -> Int
countValArgs (ApplyTo _ (Type _) _ cont) = countValArgs cont
go args (ApplyTo _ arg se cont) = go (substExpr se arg : args) cont
go args cont = (reverse args, cont)
+pushArgs :: SimplEnv -> [CoreExpr] -> SimplCont -> SimplCont
+pushArgs _env [] cont = cont
+pushArgs env (arg:args) cont = ApplyTo NoDup arg env (pushArgs env args cont)
+
dropArgs :: Int -> SimplCont -> SimplCont
dropArgs 0 cont = cont
dropArgs n (ApplyTo _ _ _ cont) = dropArgs (n-1) cont
-- motivation to inline. See Note [Cast then apply]
-- in CoreUnfold
- interesting (StrictArg _ cci _ _) = cci
- interesting (StrictBind {}) = BoringCtxt
- interesting (Stop cci) = cci
- interesting (CoerceIt _ cont) = interesting cont
+ interesting (StrictArg _ cci _) = cci
+ interesting (StrictBind {}) = BoringCtxt
+ interesting (Stop cci) = cci
+ interesting (CoerceIt _ cont) = interesting cont
-- If this call is the arg of a strict function, the context
-- is a bit interesting. If we inline here, we may get useful
-- evaluation information to avoid repeated evals: e.g.
mkArgInfo fun rules n_val_args call_cont
| n_val_args < idArity fun -- Note [Unsaturated functions]
- = ArgInfo { ai_rules = False
+ = ArgInfo { ai_fun = fun, ai_args = [], ai_rules = rules
+ , ai_encl = False
, ai_strs = vanilla_stricts
, ai_discs = vanilla_discounts }
| otherwise
- = ArgInfo { ai_rules = interestingArgContext rules call_cont
+ = ArgInfo { ai_fun = fun, ai_args = [], ai_rules = rules
+ , ai_encl = interestingArgContext rules call_cont
, ai_strs = add_type_str (idType fun) arg_stricts
, ai_discs = arg_discounts }
where
vanilla_stricts = repeat False
arg_stricts
- = case splitStrictSig (idNewStrictness fun) of
+ = case splitStrictSig (idStrictness fun) of
(demands, result_info)
| not (demands `lengthExceeds` n_val_args)
-> -- Enough args, use the strictness given.
where
enclosing_fn_has_rules = go call_cont
- go (Select {}) = False
- go (ApplyTo {}) = False
- go (StrictArg _ cci _ _) = interesting cci
- go (StrictBind {}) = False -- ??
- go (CoerceIt _ c) = go c
- go (Stop cci) = interesting cci
+ go (Select {}) = False
+ go (ApplyTo {}) = False
+ go (StrictArg _ cci _) = interesting cci
+ go (StrictBind {}) = False -- ??
+ go (CoerceIt _ c) = go c
+ go (Stop cci) = interesting cci
interesting (ArgCtxt rules) = rules
interesting _ = False
simplEnvForRules = mkSimplEnv allOffSwitchChecker $
SimplGently { sm_rules = True, sm_inline = False }
-simplGentlyForInlineRules :: SimplifierMode
-simplGentlyForInlineRules = SimplGently { sm_rules = True, sm_inline = True }
+updModeForInlineRules :: SimplifierMode -> SimplifierMode
+updModeForInlineRules mode
+ = case mode of
+ SimplGently {} -> mode -- Don't modify mode if we already gentle
+ SimplPhase {} -> SimplGently { sm_rules = True, sm_inline = True }
-- Simplify as much as possible, subject to the usual "gentle" rules
\end{code}
anything, because the byte-code interpreter might get confused about
unboxed tuples and suchlike.
+Note [RULEs enabled in SimplGently]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+RULES are enabled when doing "gentle" simplification. Two reasons:
+
+ * We really want the class-op cancellation to happen:
+ op (df d1 d2) --> $cop3 d1 d2
+ because this breaks the mutual recursion between 'op' and 'df'
+
+ * I wanted the RULE
+ lift String ===> ...
+ to work in Template Haskell when simplifying
+ splices, so we get simpler code for literal strings
+
Note [Simplifying gently inside InlineRules]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We don't do much simplification inside InlineRules (which come from
| otherwise
= case getMode env of
SimplGently { sm_rules = rules_on }
- | rules_on -> Just isEarlyActive
+ | rules_on -> Just isEarlyActive -- Note [RULEs enabled in SimplGently]
| otherwise -> Nothing
- -- Used to be Nothing (no rules in gentle mode)
- -- Main motivation for changing is that I wanted
- -- lift String ===> ...
- -- to work in Template Haskell when simplifying
- -- splices, so we get simpler code for literal strings
SimplPhase n _ -> Just (isActive n)
\end{code}
prepareAlts tries these things:
-1. If several alternatives are identical, merge them into
- a single DEFAULT alternative. I've occasionally seen this
- making a big difference:
-
- case e of =====> case e of
- C _ -> f x D v -> ....v....
- D v -> ....v.... DEFAULT -> f x
- DEFAULT -> f x
-
- The point is that we merge common RHSs, at least for the DEFAULT case.
- [One could do something more elaborate but I've never seen it needed.]
- To avoid an expensive test, we just merge branches equal to the *first*
- alternative; this picks up the common cases
- a) all branches equal
- b) some branches equal to the DEFAULT (which occurs first)
+1. Eliminate alternatives that cannot match, including the
+ DEFAULT alternative.
-2. Case merging:
- case e of b { ==> case e of b {
- p1 -> rhs1 p1 -> rhs1
- ... ...
- pm -> rhsm pm -> rhsm
- _ -> case b of b' { pn -> let b'=b in rhsn
- pn -> rhsn ...
- ... po -> let b'=b in rhso
- po -> rhso _ -> let b'=b in rhsd
- _ -> rhsd
- }
-
- which merges two cases in one case when -- the default alternative of
- the outer case scrutises the same variable as the outer case This
- transformation is called Case Merging. It avoids that the same
- variable is scrutinised multiple times.
+2. If the DEFAULT alternative can match only one possible constructor,
+ then make that constructor explicit.
+ e.g.
+ case e of x { DEFAULT -> rhs }
+ ===>
+ case e of x { (a,b) -> rhs }
+ where the type is a single constructor type. This gives better code
+ when rhs also scrutinises x or e.
+3. Returns a list of the constructors that cannot holds in the
+ DEFAULT alternative (if there is one)
-The case where transformation (1) showed up was like this (lib/std/PrelCError.lhs):
+Here "cannot match" includes knowledge from GADTs
- x | p `is` 1 -> e1
- | p `is` 2 -> e2
- ...etc...
+It's a good idea do do this stuff before simplifying the alternatives, to
+avoid simplifying alternatives we know can't happen, and to come up with
+the list of constructors that are handled, to put into the IdInfo of the
+case binder, for use when simplifying the alternatives.
-where @is@ was something like
-
- p `is` n = p /= (-1) && p == n
+Eliminating the default alternative in (1) isn't so obvious, but it can
+happen:
-This gave rise to a horrible sequence of cases
+data Colour = Red | Green | Blue
- case p of
- (-1) -> $j p
- 1 -> e1
- DEFAULT -> $j p
+f x = case x of
+ Red -> ..
+ Green -> ..
+ DEFAULT -> h x
-and similarly in cascade for all the join points!
+h y = case y of
+ Blue -> ..
+ DEFAULT -> [ case y of ... ]
-Note [Dead binders]
-~~~~~~~~~~~~~~~~~~~~
-We do this *here*, looking at un-simplified alternatives, because we
-have to check that r doesn't mention the variables bound by the
-pattern in each alternative, so the binder-info is rather useful.
+If we inline h into f, the default case of the inlined h can't happen.
+If we don't notice this, we may end up filtering out *all* the cases
+of the inner case y, which give us nowhere to go!
\begin{code}
-prepareAlts :: SimplEnv -> OutExpr -> OutId -> [InAlt] -> SimplM ([AltCon], [InAlt])
-prepareAlts env scrut case_bndr' alts
- = do { dflags <- getDOptsSmpl
- ; alts <- combineIdenticalAlts case_bndr' alts
-
- ; let (alts_wo_default, maybe_deflt) = findDefault alts
+prepareAlts :: OutExpr -> OutId -> [InAlt] -> SimplM ([AltCon], [InAlt])
+prepareAlts scrut case_bndr' alts
+ = do { let (alts_wo_default, maybe_deflt) = findDefault alts
alt_cons = [con | (con,_,_) <- alts_wo_default]
imposs_deflt_cons = nub (imposs_cons ++ alt_cons)
-- "imposs_deflt_cons" are handled
-- EITHER by the context,
-- OR by a non-DEFAULT branch in this case expression.
- ; default_alts <- prepareDefault dflags env case_bndr' mb_tc_app
+ ; default_alts <- prepareDefault case_bndr' mb_tc_app
imposs_deflt_cons maybe_deflt
; let trimmed_alts = filterOut impossible_alt alts_wo_default
- merged_alts = mergeAlts trimmed_alts default_alts
+ merged_alts = mergeAlts trimmed_alts default_alts
-- We need the mergeAlts in case the new default_alt
-- has turned into a constructor alternative.
-- The merge keeps the inner DEFAULT at the front, if there is one
impossible_alt _ = False
---------------------------------------------------
--- 1. Merge identical branches
---------------------------------------------------
-combineIdenticalAlts :: OutId -> [InAlt] -> SimplM [InAlt]
-
-combineIdenticalAlts case_bndr ((_con1,bndrs1,rhs1) : con_alts)
- | all isDeadBinder bndrs1, -- Remember the default
- length filtered_alts < length con_alts -- alternative comes first
- -- Also Note [Dead binders]
- = do { tick (AltMerge case_bndr)
- ; return ((DEFAULT, [], rhs1) : filtered_alts) }
- where
- filtered_alts = filter keep con_alts
- keep (_con,bndrs,rhs) = not (all isDeadBinder bndrs && rhs `cheapEqExpr` rhs1)
-
-combineIdenticalAlts _ alts = return alts
-
--------------------------------------------------------------------------
--- Prepare the default alternative
--------------------------------------------------------------------------
-prepareDefault :: DynFlags
- -> SimplEnv
- -> OutId -- Case binder; need just for its type. Note that as an
+prepareDefault :: OutId -- Case binder; need just for its type. Note that as an
-- OutId, it has maximum information; this is important.
-- Test simpl013 is an example
-> Maybe (TyCon, [Type]) -- Type of scrutinee, decomposed
-> Maybe InExpr -- Rhs
-> SimplM [InAlt] -- Still unsimplified
-- We use a list because it's what mergeAlts expects,
- -- And becuase case-merging can cause many to show up
-
-------- Merge nested cases ----------
-prepareDefault dflags env outer_bndr _bndr_ty imposs_cons (Just deflt_rhs)
- | dopt Opt_CaseMerge dflags
- , Case (Var inner_scrut_var) inner_bndr _ inner_alts <- deflt_rhs
- , DoneId inner_scrut_var' <- substId env inner_scrut_var
- -- Remember, inner_scrut_var is an InId, but outer_bndr is an OutId
- , inner_scrut_var' == outer_bndr
- -- NB: the substId means that if the outer scrutinee was a
- -- variable, and inner scrutinee is the same variable,
- -- then inner_scrut_var' will be outer_bndr
- -- via the magic of simplCaseBinder
- = do { tick (CaseMerge outer_bndr)
-
- ; let munge_rhs rhs = bindCaseBndr inner_bndr (Var outer_bndr) rhs
- ; return [(con, args, munge_rhs rhs) | (con, args, rhs) <- inner_alts,
- not (con `elem` imposs_cons) ]
- -- NB: filter out any imposs_cons. Example:
- -- case x of
- -- A -> e1
- -- DEFAULT -> case x of
- -- A -> e2
- -- B -> e3
- -- When we merge, we must ensure that e1 takes
- -- precedence over e2 as the value for A!
- }
- -- Warning: don't call prepareAlts recursively!
- -- Firstly, there's no point, because inner alts have already had
- -- mkCase applied to them, so they won't have a case in their default
- -- Secondly, if you do, you get an infinite loop, because the bindCaseBndr
- -- in munge_rhs may put a case into the DEFAULT branch!
-
--------- Fill in known constructor -----------
-prepareDefault _ _ case_bndr (Just (tycon, inst_tys)) imposs_cons (Just deflt_rhs)
+prepareDefault case_bndr (Just (tycon, inst_tys)) imposs_cons (Just deflt_rhs)
| -- This branch handles the case where we are
-- scrutinisng an algebraic data type
isAlgTyCon tycon -- It's a data type, tuple, or unboxed tuples.
-- which would be quite legitmate. But it's a really obscure corner, and
-- not worth wasting code on.
, let imposs_data_cons = [con | DataAlt con <- imposs_cons] -- We now know it's a data type
- impossible con = con `elem` imposs_data_cons || dataConCannotMatch inst_tys con
+ impossible con = con `elem` imposs_data_cons || dataConCannotMatch inst_tys con
= case filterOut impossible all_cons of
[] -> return [] -- Eliminate the default alternative
-- altogether if it can't match
_ -> return [(DEFAULT, [], deflt_rhs)]
| debugIsOn, isAlgTyCon tycon, not (isOpenTyCon tycon), null (tyConDataCons tycon)
- -- This can legitimately happen for type families, so don't report that
+ -- Check for no data constructors
+ -- This can legitimately happen for type families, so don't report that
= pprTrace "prepareDefault" (ppr case_bndr <+> ppr tycon)
$ return [(DEFAULT, [], deflt_rhs)]
--------- Catch-all cases -----------
-prepareDefault _dflags _env _case_bndr _bndr_ty _imposs_cons (Just deflt_rhs)
+prepareDefault _case_bndr _bndr_ty _imposs_cons (Just deflt_rhs)
= return [(DEFAULT, [], deflt_rhs)]
-prepareDefault _dflags _env _case_bndr _bndr_ty _imposs_cons Nothing
+prepareDefault _case_bndr _bndr_ty _imposs_cons Nothing
= return [] -- No default branch
\end{code}
-=================================================================================
+%************************************************************************
+%* *
+ mkCase
+%* *
+%************************************************************************
mkCase tries these things
-1. Eliminate the case altogether if possible
+1. Merge Nested Cases
-2. Case-identity:
+ case e of b { ==> case e of b {
+ p1 -> rhs1 p1 -> rhs1
+ ... ...
+ pm -> rhsm pm -> rhsm
+ _ -> case b of b' { pn -> let b'=b in rhsn
+ pn -> rhsn ...
+ ... po -> let b'=b in rhso
+ po -> rhso _ -> let b'=b in rhsd
+ _ -> rhsd
+ }
+
+ which merges two cases in one case when -- the default alternative of
+ the outer case scrutises the same variable as the outer case. This
+ transformation is called Case Merging. It avoids that the same
+ variable is scrutinised multiple times.
+
+2. Eliminate Identity Case
case e of ===> e
True -> True;
and similar friends.
+3. Merge identical alternatives.
+ If several alternatives are identical, merge them into
+ a single DEFAULT alternative. I've occasionally seen this
+ making a big difference:
+
+ case e of =====> case e of
+ C _ -> f x D v -> ....v....
+ D v -> ....v.... DEFAULT -> f x
+ DEFAULT -> f x
+
+ The point is that we merge common RHSs, at least for the DEFAULT case.
+ [One could do something more elaborate but I've never seen it needed.]
+ To avoid an expensive test, we just merge branches equal to the *first*
+ alternative; this picks up the common cases
+ a) all branches equal
+ b) some branches equal to the DEFAULT (which occurs first)
+
+The case where Merge Identical Alternatives transformation showed up
+was like this (base/Foreign/C/Err/Error.lhs):
+
+ x | p `is` 1 -> e1
+ | p `is` 2 -> e2
+ ...etc...
+
+where @is@ was something like
+
+ p `is` n = p /= (-1) && p == n
+
+This gave rise to a horrible sequence of cases
+
+ case p of
+ (-1) -> $j p
+ 1 -> e1
+ DEFAULT -> $j p
+
+and similarly in cascade for all the join points!
+
\begin{code}
-mkCase :: OutExpr -> OutId -> [OutAlt] -- Increasing order
- -> SimplM OutExpr
+mkCase, mkCase1, mkCase2
+ :: DynFlags
+ -> OutExpr -> OutId
+ -> [OutAlt] -- Alternatives in standard (increasing) order
+ -> SimplM OutExpr
--------------------------------------------------
--- 2. Identity case
+-- 1. Merge Nested Cases
--------------------------------------------------
-mkCase scrut case_bndr alts -- Identity case
+mkCase dflags scrut outer_bndr ((DEFAULT, _, deflt_rhs) : outer_alts)
+ | dopt Opt_CaseMerge dflags
+ , Case (Var inner_scrut_var) inner_bndr _ inner_alts <- deflt_rhs
+ , inner_scrut_var == outer_bndr
+ = do { tick (CaseMerge outer_bndr)
+
+ ; let wrap_alt (con, args, rhs) = ASSERT( outer_bndr `notElem` args )
+ (con, args, wrap_rhs rhs)
+ -- Simplifier's no-shadowing invariant should ensure
+ -- that outer_bndr is not shadowed by the inner patterns
+ wrap_rhs rhs = Let (NonRec inner_bndr (Var outer_bndr)) rhs
+ -- The let is OK even for unboxed binders,
+
+ wrapped_alts | isDeadBinder inner_bndr = inner_alts
+ | otherwise = map wrap_alt inner_alts
+
+ merged_alts = mergeAlts outer_alts wrapped_alts
+ -- NB: mergeAlts gives priority to the left
+ -- case x of
+ -- A -> e1
+ -- DEFAULT -> case x of
+ -- A -> e2
+ -- B -> e3
+ -- When we merge, we must ensure that e1 takes
+ -- precedence over e2 as the value for A!
+
+ ; mkCase1 dflags scrut outer_bndr merged_alts
+ }
+ -- Warning: don't call mkCase recursively!
+ -- Firstly, there's no point, because inner alts have already had
+ -- mkCase applied to them, so they won't have a case in their default
+ -- Secondly, if you do, you get an infinite loop, because the bindCaseBndr
+ -- in munge_rhs may put a case into the DEFAULT branch!
+
+mkCase dflags scrut bndr alts = mkCase1 dflags scrut bndr alts
+
+--------------------------------------------------
+-- 2. Eliminate Identity Case
+--------------------------------------------------
+
+mkCase1 _dflags scrut case_bndr alts -- Identity case
| all identity_alt alts
- = do tick (CaseIdentity case_bndr)
- return (re_cast scrut)
+ = do { tick (CaseIdentity case_bndr)
+ ; return (re_cast scrut) }
where
identity_alt (con, args, rhs) = check_eq con args (de_cast rhs)
(_,_,Cast _ co) -> Cast scrut co
_ -> scrut
+--------------------------------------------------
+-- 3. Merge Identical Alternatives
+--------------------------------------------------
+mkCase1 dflags scrut case_bndr ((_con1,bndrs1,rhs1) : con_alts)
+ | all isDeadBinder bndrs1 -- Remember the default
+ , length filtered_alts < length con_alts -- alternative comes first
+ -- Also Note [Dead binders]
+ = do { tick (AltMerge case_bndr)
+ ; mkCase2 dflags scrut case_bndr alts' }
+ where
+ alts' = (DEFAULT, [], rhs1) : filtered_alts
+ filtered_alts = filter keep con_alts
+ keep (_con,bndrs,rhs) = not (all isDeadBinder bndrs && rhs `cheapEqExpr` rhs1)
+mkCase1 dflags scrut bndr alts = mkCase2 dflags scrut bndr alts
--------------------------------------------------
-- Catch-all
--------------------------------------------------
-mkCase scrut bndr alts = return (Case scrut bndr (coreAltsType alts) alts)
+mkCase2 _dflags scrut bndr alts
+ = return (Case scrut bndr (coreAltsType alts) alts)
\end{code}
-
-When adding auxiliary bindings for the case binder, it's worth checking if
-its dead, because it often is, and occasionally these mkCase transformations
-cascade rather nicely.
-
-\begin{code}
-bindCaseBndr :: Id -> CoreExpr -> CoreExpr -> CoreExpr
-bindCaseBndr bndr rhs body
- | isDeadBinder bndr = body
- | otherwise = bindNonRec bndr rhs body
-\end{code}
+Note [Dead binders]
+~~~~~~~~~~~~~~~~~~~~
+Note that dead-ness is maintained by the simplifier, so that it is
+accurate after simplification as well as before.
+
+
+Note [Cascading case merge]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Case merging should cascade in one sweep, because it
+happens bottom-up
+
+ case e of a {
+ DEFAULT -> case a of b
+ DEFAULT -> case b of c {
+ DEFAULT -> e
+ A -> ea
+ B -> eb
+ C -> ec
+==>
+ case e of a {
+ DEFAULT -> case a of b
+ DEFAULT -> let c = b in e
+ A -> let c = b in ea
+ B -> eb
+ C -> ec
+==>
+ case e of a {
+ DEFAULT -> let b = a in let c = b in e
+ A -> let b = a in let c = b in ea
+ B -> let b = a in eb
+ C -> ec
+
+
+However here's a tricky case that we still don't catch, and I don't
+see how to catch it in one pass:
+
+ case x of c1 { I# a1 ->
+ case a1 of c2 ->
+ 0 -> ...
+ DEFAULT -> case x of c3 { I# a2 ->
+ case a2 of ...
+
+After occurrence analysis (and its binder-swap) we get this
+
+ case x of c1 { I# a1 ->
+ let x = c1 in -- Binder-swap addition
+ case a1 of c2 ->
+ 0 -> ...
+ DEFAULT -> case x of c3 { I# a2 ->
+ case a2 of ...
+
+When we simplify the inner case x, we'll see that
+x=c1=I# a1. So we'll bind a2 to a1, and get
+
+ case x of c1 { I# a1 ->
+ case a1 of c2 ->
+ 0 -> ...
+ DEFAULT -> case a1 of ...
+
+This is corect, but we can't do a case merge in this sweep
+because c2 /= a1. Reason: the binding c1=I# a1 went inwards
+without getting changed to c1=I# c2.
+
+I don't think this is worth fixing, even if I knew how. It'll
+all come out in the next pass anyway.
+
+
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