X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2FsimplCore%2FOccurAnal.lhs;h=8cef0fc442808e7e83916db98e9255d2a42493bc;hb=522c1e96173c5573f2cc9b3f428c56a6b5008942;hp=b92239e5e4b26e03393ce550f50f2f5c8ff9b377;hpb=ac7db825a40d6b4e582a9b33969a1b0d5de9b3f6;p=ghc-hetmet.git diff --git a/compiler/simplCore/OccurAnal.lhs b/compiler/simplCore/OccurAnal.lhs index b92239e..8cef0fc 100644 --- a/compiler/simplCore/OccurAnal.lhs +++ b/compiler/simplCore/OccurAnal.lhs @@ -22,7 +22,7 @@ import CoreFVs import CoreUtils ( exprIsTrivial, isDefaultAlt ) import Coercion ( mkSymCoercion ) import Id -import IdInfo +import Name ( localiseName ) import BasicTypes import VarSet @@ -49,13 +49,16 @@ import Data.List Here's the externally-callable interface: \begin{code} -occurAnalysePgm :: [CoreBind] -> [CoreBind] -occurAnalysePgm binds +occurAnalysePgm :: [CoreBind] -> [CoreRule] -> [CoreBind] +occurAnalysePgm binds rules = snd (go initOccEnv binds) where + initial_details = addIdOccs emptyDetails (rulesFreeVars rules) + -- The RULES keep things alive! + go :: OccEnv -> [CoreBind] -> (UsageDetails, [CoreBind]) go _ [] - = (emptyDetails, []) + = (initial_details, []) go env (bind:binds) = (final_usage, bind' ++ binds') where @@ -171,10 +174,10 @@ However things are made quite a bit more complicated by RULES. Remember ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We avoid infinite inlinings by choosing loop breakers, and ensuring that a loop breaker cuts each loop. But what is a - "loop"? In particular, a RULES is like an equation for 'f' that - is *always* inlined if it are applicable. We do *not* disable + "loop"? In particular, a RULE is like an equation for 'f' that + is *always* inlined if it is applicable. We do *not* disable rules for loop-breakers. It's up to whoever makes the rules to - make sure that the rules themselves alwasys terminate. See Note + make sure that the rules themselves always terminate. See Note [Rules for recursive functions] in Simplify.lhs Hence, if @@ -220,13 +223,15 @@ However things are made quite a bit more complicated by RULES. Remember So we must *not* postInlineUnconditionally 'g', even though its RHS turns out to be trivial. (I'm assuming that 'g' is - not choosen as a loop breaker.) + not choosen as a loop breaker.) Why not? Because then we + drop the binding for 'g', which leaves it out of scope in the + RULE! We "solve" this by making g a "weak" or "rules-only" loop breaker, with OccInfo = IAmLoopBreaker True. A normal "strong" loop breaker has IAmLoopBreaker False. So - Inline postInlineUnconditinoally + Inline postInlineUnconditionally IAmLoopBreaker False no no IAmLoopBreaker True yes no other yes yes @@ -237,14 +242,23 @@ However things are made quite a bit more complicated by RULES. Remember * Note [Rule dependency info] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ The VarSet in a SpecInfo is used for dependency analysis in the - occurrence analyser. We must track free vars in *both* lhs and rhs. Why both? - Consider + occurrence analyser. We must track free vars in *both* lhs and rhs. + Hence use of idRuleVars, rather than idRuleRhsVars in addRuleUsage. + Why both? Consider x = y RULE f x = 4 Then if we substitute y for x, we'd better do so in the rule's LHS too, so we'd better ensure the dependency is respected + * Note [Inline rules] + ~~~~~~~~~~~~~~~~~~~ + None of the above stuff about RULES applies to Inline Rules, + stored in a CoreUnfolding. The unfolding, if any, is simplified + at the same time as the regular RHS of the function, so it should + be treated *exactly* like an extra RHS. + + Example [eftInt] ~~~~~~~~~~~~~~~ Example (from GHC.Enum): @@ -297,9 +311,10 @@ occAnalBind env (Rec pairs) body_usage rec_edges = {-# SCC "occAnalBind.assoc" #-} map make_node pairs make_node (bndr, rhs) - = (ND bndr rhs' rhs_usage rhs_fvs, idUnique bndr, out_edges) + = (ND bndr rhs' all_rhs_usage rhs_fvs, idUnique bndr, out_edges) where (rhs_usage, rhs') = occAnalRhs env bndr rhs + all_rhs_usage = addRuleUsage rhs_usage bndr -- Note [Rules are extra RHSs] rhs_fvs = intersectUFM_C (\b _ -> b) bndr_set rhs_usage out_edges = keysUFM (rhs_fvs `unionVarSet` idRuleVars bndr) -- (a -> b) means a mentions b @@ -322,7 +337,7 @@ occAnalRec (AcyclicSCC (ND bndr rhs rhs_usage _, _, _)) (body_usage, binds) = (body_usage, binds) | otherwise -- It's mentioned in the body - = (body_usage' +++ addRuleUsage rhs_usage bndr, -- Note [Rules are extra RHSs] + = (body_usage' +++ rhs_usage, NonRec tagged_bndr rhs : binds) where (body_usage', tagged_bndr) = tagBinder body_usage bndr @@ -344,8 +359,7 @@ occAnalRec (CyclicSCC nodes) (body_usage, binds) ---------------------------- -- Tag the binders with their occurrence info total_usage = foldl add_usage body_usage nodes - add_usage body_usage (ND bndr _ rhs_usage _, _, _) - = body_usage +++ addRuleUsage rhs_usage bndr + add_usage usage_so_far (ND _ _ rhs_usage _, _, _) = usage_so_far +++ rhs_usage (final_usage, tagged_nodes) = mapAccumL tag_node total_usage nodes tag_node :: UsageDetails -> Node Details -> (UsageDetails, Node Details) @@ -365,10 +379,11 @@ occAnalRec (CyclicSCC nodes) (body_usage, binds) ---------------------------- -- Now reconstruct the cycle - pairs | no_rules = reOrderCycle tagged_nodes - | otherwise = concatMap reOrderRec (stronglyConnCompFromEdgedVerticesR loop_breaker_edges) + pairs | no_rules = reOrderCycle 0 tagged_nodes [] + | otherwise = foldr (reOrderRec 0) [] $ + stronglyConnCompFromEdgedVerticesR loop_breaker_edges - -- See Note [Choosing loop breakers] for looop_breaker_edges + -- See Note [Choosing loop breakers] for loop_breaker_edges loop_breaker_edges = map mk_node tagged_nodes mk_node (details@(ND _ _ _ rhs_fvs), k, _) = (details, k, new_ks) where @@ -398,11 +413,6 @@ occAnalRec (CyclicSCC nodes) (body_usage, binds) where new_fvs = extendFvs env emptyVarSet fvs -idRuleRhsVars :: Id -> VarSet --- Just the variables free on the *rhs* of a rule --- See Note [Choosing loop breakers] -idRuleRhsVars id = foldr (unionVarSet . ruleRhsFreeVars) emptyVarSet (idCoreRules id) - extendFvs :: IdEnv IdSet -> IdSet -> IdSet -> IdSet -- (extendFVs env fvs s) returns (fvs `union` env(s)) extendFvs env fvs id_set @@ -453,98 +463,105 @@ type Node details = (details, Unique, [Unique]) -- The Ints are gotten from the -- which is gotten from the Id. data Details = ND Id -- Binder CoreExpr -- RHS - UsageDetails -- Full usage from RHS (*not* including rules) - IdSet -- Other binders from this Rec group mentioned on RHS - -- (derivable from UsageDetails but cached here) -reOrderRec :: SCC (Node Details) - -> [(Id,CoreExpr)] + UsageDetails -- Full usage from RHS, + -- including *both* RULES *and* InlineRule unfolding + + IdSet -- Other binders *from this Rec group* mentioned in + -- * the RHS + -- * any InlineRule unfolding + -- but *excluding* any RULES + +reOrderRec :: Int -> SCC (Node Details) + -> [(Id,CoreExpr)] -> [(Id,CoreExpr)] -- Sorted into a plausible order. Enough of the Ids have -- IAmALoopBreaker pragmas that there are no loops left. -reOrderRec (AcyclicSCC (ND bndr rhs _ _, _, _)) = [(bndr, rhs)] -reOrderRec (CyclicSCC cycle) = reOrderCycle cycle +reOrderRec _ (AcyclicSCC (ND bndr rhs _ _, _, _)) pairs = (bndr, rhs) : pairs +reOrderRec depth (CyclicSCC cycle) pairs = reOrderCycle depth cycle pairs -reOrderCycle :: [Node Details] -> [(Id,CoreExpr)] -reOrderCycle [] +reOrderCycle :: Int -> [Node Details] -> [(Id,CoreExpr)] -> [(Id,CoreExpr)] +reOrderCycle _ [] _ = panic "reOrderCycle" -reOrderCycle [bind] -- Common case of simple self-recursion - = [(makeLoopBreaker False bndr, rhs)] +reOrderCycle _ [bind] pairs -- Common case of simple self-recursion + = (makeLoopBreaker False bndr, rhs) : pairs where (ND bndr rhs _ _, _, _) = bind -reOrderCycle (bind : binds) +reOrderCycle depth (bind : binds) pairs = -- Choose a loop breaker, mark it no-inline, -- do SCC analysis on the rest, and recursively sort them out - concatMap reOrderRec (stronglyConnCompFromEdgedVerticesR unchosen) ++ - [(makeLoopBreaker False bndr, rhs)] - +-- pprTrace "reOrderCycle" (ppr [b | (ND b _ _ _, _, _) <- bind:binds]) $ + foldr (reOrderRec new_depth) + ([ (makeLoopBreaker False bndr, rhs) + | (ND bndr rhs _ _, _, _) <- chosen_binds] ++ pairs) + (stronglyConnCompFromEdgedVerticesR unchosen) where - (chosen_bind, unchosen) = choose_loop_breaker bind (score bind) [] binds - ND bndr rhs _ _ = chosen_bind + (chosen_binds, unchosen) = choose_loop_breaker [bind] (score bind) [] binds + + approximate_loop_breaker = depth >= 2 + new_depth | approximate_loop_breaker = 0 + | otherwise = depth+1 + -- After two iterations (d=0, d=1) give up + -- and approximate, returning to d=0 -- This loop looks for the bind with the lowest score -- to pick as the loop breaker. The rest accumulate in - choose_loop_breaker (details,_,_) _loop_sc acc [] - = (details, acc) -- Done + choose_loop_breaker loop_binds _loop_sc acc [] + = (loop_binds, acc) -- Done - choose_loop_breaker loop_bind loop_sc acc (bind : binds) + -- If approximate_loop_breaker is True, we pick *all* + -- nodes with lowest score, else just one + -- See Note [Complexity of loop breaking] + choose_loop_breaker loop_binds loop_sc acc (bind : binds) | sc < loop_sc -- Lower score so pick this new one - = choose_loop_breaker bind sc (loop_bind : acc) binds + = choose_loop_breaker [bind] sc (loop_binds ++ acc) binds - | otherwise -- No lower so don't pick it - = choose_loop_breaker loop_bind loop_sc (bind : acc) binds + | approximate_loop_breaker && sc == loop_sc + = choose_loop_breaker (bind : loop_binds) loop_sc acc binds + + | otherwise -- Higher score so don't pick it + = choose_loop_breaker loop_binds loop_sc (bind : acc) binds where sc = score bind score :: Node Details -> Int -- Higher score => less likely to be picked as loop breaker score (ND bndr rhs _ _, _, _) - | workerExists (idWorkerInfo bndr) = 10 - -- Note [Worker inline loop] - - | exprIsTrivial rhs = 5 -- Practically certain to be inlined + | isDFunId bndr = 9 -- Never choose a DFun as a loop breaker + -- Note [DFuns should not be loop breakers] + + | Just (inl_rule_info, _) <- isInlineRule_maybe (idUnfolding bndr) + = case inl_rule_info of + InlWrapper {} -> 10 -- Note [INLINE pragmas] + _other -> 3 -- Data structures are more important than this + -- so that dictionary/method recursion unravels + -- Note that this case hits all InlineRule things, so we + -- never look at 'rhs for InlineRule stuff. That's right, because + -- 'rhs' is irrelevant for inlining things with an InlineRule + + | is_con_app rhs = 5 -- Data types help with cases: Note [Constructor applications] + + | exprIsTrivial rhs = 10 -- Practically certain to be inlined -- Used to have also: && not (isExportedId bndr) -- But I found this sometimes cost an extra iteration when we have -- rec { d = (a,b); a = ...df...; b = ...df...; df = d } -- where df is the exported dictionary. Then df makes a really -- bad choice for loop breaker - | is_con_app rhs = 3 -- Data types help with cases - -- Note [conapp] - + -- If an Id is marked "never inline" then it makes a great loop breaker -- The only reason for not checking that here is that it is rare -- and I've never seen a situation where it makes a difference, -- so it probably isn't worth the time to test on every binder -- | isNeverActive (idInlinePragma bndr) = -10 - | inlineCandidate bndr rhs = 2 -- Likely to be inlined - -- Note [Inline candidates] + | isOneOcc (idOccInfo bndr) = 2 -- Likely to be inlined - | not (neverUnfold (idUnfolding bndr)) = 1 + | canUnfold (idUnfolding bndr) = 1 -- the Id has some kind of unfolding | otherwise = 0 - inlineCandidate :: Id -> CoreExpr -> Bool - inlineCandidate _ (Note InlineMe _) = True - inlineCandidate id _ = isOneOcc (idOccInfo id) - - -- Note [conapp] - -- - -- It's really really important to inline dictionaries. Real - -- example (the Enum Ordering instance from GHC.Base): - -- - -- rec f = \ x -> case d of (p,q,r) -> p x - -- g = \ x -> case d of (p,q,r) -> q x - -- d = (v, f, g) - -- - -- Here, f and g occur just once; but we can't inline them into d. - -- On the other hand we *could* simplify those case expressions if - -- we didn't stupidly choose d as the loop breaker. - -- But we won't because constructor args are marked "Many". - -- Inlining dictionaries is really essential to unravelling - -- the loops in static numeric dictionaries, see GHC.Float. - + -- Checking for a constructor application -- Cheap and cheerful; the simplifer moves casts out of the way -- The lambda case is important to spot x = /\a. C (f a) -- which comes up when C is a dictionary constructor and @@ -553,36 +570,118 @@ reOrderCycle (bind : binds) -- -- However we *also* treat (\x. C p q) as a con-app-like thing, -- Note [Closure conversion] - is_con_app (Var v) = isDataConWorkId v + is_con_app (Var v) = isConLikeId v is_con_app (App f _) = is_con_app f is_con_app (Lam _ e) = is_con_app e is_con_app (Note _ e) = is_con_app e is_con_app _ = False makeLoopBreaker :: Bool -> Id -> Id --- Set the loop-breaker flag --- See Note [Weak loop breakers] +-- Set the loop-breaker flag: see Note [Weak loop breakers] makeLoopBreaker weak bndr = setIdOccInfo bndr (IAmALoopBreaker weak) \end{code} -Note [Worker inline loop] -~~~~~~~~~~~~~~~~~~~~~~~~ -Never choose a wrapper as the loop breaker! Because -wrappers get auto-generated inlinings when importing, and -that can lead to an infinite inlining loop. For example: +Note [Complexity of loop breaking] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +The loop-breaking algorithm knocks out one binder at a time, and +performs a new SCC analysis on the remaining binders. That can +behave very badly in tightly-coupled groups of bindings; in the +worst case it can be (N**2)*log N, because it does a full SCC +on N, then N-1, then N-2 and so on. + +To avoid this, we switch plans after 2 (or whatever) attempts: + Plan A: pick one binder with the lowest score, make it + a loop breaker, and try again + Plan B: pick *all* binders with the lowest score, make them + all loop breakers, and try again +Since there are only a small finite number of scores, this will +terminate in a constant number of iterations, rather than O(N) +iterations. + +You might thing that it's very unlikely, but RULES make it much +more likely. Here's a real example from Trac #1969: + Rec { $dm = \d.\x. op d + {-# RULES forall d. $dm Int d = $s$dm1 + forall d. $dm Bool d = $s$dm2 #-} + + dInt = MkD .... opInt ... + dInt = MkD .... opBool ... + opInt = $dm dInt + opBool = $dm dBool + + $s$dm1 = \x. op dInt + $s$dm2 = \x. op dBool } +The RULES stuff means that we can't choose $dm as a loop breaker +(Note [Choosing loop breakers]), so we must choose at least (say) +opInt *and* opBool, and so on. The number of loop breakders is +linear in the number of instance declarations. + +Note [INLINE pragmas] +~~~~~~~~~~~~~~~~~~~~~ +Avoid choosing a function with an INLINE pramga as the loop breaker! +If such a function is mutually-recursive with a non-INLINE thing, +then the latter should be the loop-breaker. + +Usually this is just a question of optimisation. But a particularly +bad case is wrappers generated by the demand analyser: if you make +then into a loop breaker you may get an infinite inlining loop. For +example: rec { $wfoo x = ....foo x.... {-loop brk-} foo x = ...$wfoo x... } - The interface file sees the unfolding for $wfoo, and sees that foo is strict (and hence it gets an auto-generated wrapper). Result: an infinite inlining in the importing scope. So be a bit careful if you change this. A good example is Tree.repTree in nofib/spectral/minimax. If the repTree wrapper is chosen as the loop -breaker then compiling Game.hs goes into an infinite loop (this -happened when we gave is_con_app a lower score than inline candidates). +breaker then compiling Game.hs goes into an infinite loop. This +happened when we gave is_con_app a lower score than inline candidates: + + Tree.repTree + = __inline_me (/\a. \w w1 w2 -> + case Tree.$wrepTree @ a w w1 w2 of + { (# ww1, ww2 #) -> Branch @ a ww1 ww2 }) + Tree.$wrepTree + = /\a w w1 w2 -> + (# w2_smP, map a (Tree a) (Tree.repTree a w1 w) (w w2) #) + +Here we do *not* want to choose 'repTree' as the loop breaker. + +Note [DFuns should not be loop breakers] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +It's particularly bad to make a DFun into a loop breaker. See +Note [How instance declarations are translated] in TcInstDcls + +We give DFuns a higher score than ordinary CONLIKE things because +if there's a choice we want the DFun to be the non-looop breker. Eg + +rec { sc = /\ a \$dC. $fBWrap (T a) ($fCT @ a $dC) + + $fCT :: forall a_afE. (Roman.C a_afE) => Roman.C (Roman.T a_afE) + {-# DFUN #-} + $fCT = /\a \$dC. MkD (T a) ((sc @ a $dC) |> blah) ($ctoF @ a $dC) + } + +Here 'sc' (the superclass) looks CONLIKE, but we'll never get to it +if we can't unravel the DFun first. + +Note [Constructor applications] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +It's really really important to inline dictionaries. Real +example (the Enum Ordering instance from GHC.Base): + + rec f = \ x -> case d of (p,q,r) -> p x + g = \ x -> case d of (p,q,r) -> q x + d = (v, f, g) + +Here, f and g occur just once; but we can't inline them into d. +On the other hand we *could* simplify those case expressions if +we didn't stupidly choose d as the loop breaker. +But we won't because constructor args are marked "Many". +Inlining dictionaries is really essential to unravelling +the loops in static numeric dictionaries, see GHC.Float. Note [Closure conversion] ~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -624,10 +723,13 @@ occAnalRhs :: OccEnv -- For non-recs the binder is alrady tagged -- with occurrence info -> (UsageDetails, CoreExpr) + -- Returned usage details includes any INLINE rhs occAnalRhs env id rhs - = occAnal ctxt rhs + = (addIdOccs rhs_usage (idUnfoldingVars id), rhs') + -- Include occurrences for the "extra RHS" from a CoreUnfolding where + (rhs_usage, rhs') = occAnal ctxt rhs ctxt | certainly_inline id = env | otherwise = rhsCtxt env -- Note that we generally use an rhsCtxt. This tells the occ anal n @@ -655,12 +757,19 @@ occAnalRhs env id rhs \begin{code} addRuleUsage :: UsageDetails -> Id -> UsageDetails -- Add the usage from RULES in Id to the usage -addRuleUsage usage id - = foldVarSet add usage (idRuleVars id) +addRuleUsage usage id = addIdOccs usage (idRuleVars id) + -- idRuleVars here: see Note [Rule dependency info] + +addIdOccs :: UsageDetails -> VarSet -> UsageDetails +addIdOccs usage id_set = foldVarSet add usage id_set where - add v u = addOneOcc u v NoOccInfo -- Give a non-committal binder info - -- (i.e manyOcc) because many copies - -- of the specialised thing can appear + add v u | isId v = addOneOcc u v NoOccInfo + | otherwise = u + -- Give a non-committal binder info (i.e NoOccInfo) because + -- a) Many copies of the specialised thing can appear + -- b) We don't want to substitute a BIG expression inside a RULE + -- even if that's the only occurrence of the thing + -- (Same goes for INLINE.) \end{code} Expressions @@ -701,11 +810,6 @@ occAnal _ expr@(Lit _) = (emptyDetails, expr) \end{code} \begin{code} -occAnal env (Note InlineMe body) - = case occAnal env body of { (usage, body') -> - (mapVarEnv markMany usage, Note InlineMe body') - } - occAnal env (Note note@(SCC _) body) = case occAnal env body of { (usage, body') -> (mapVarEnv markInsideSCC usage, Note note body') @@ -750,7 +854,9 @@ occAnal env (Lam x body) | isTyVar x occAnal env expr@(Lam _ _) = case occAnal env_body body of { (body_usage, body') -> let - (final_usage, tagged_binders) = tagBinders body_usage binders + (final_usage, tagged_binders) = tagLamBinders body_usage binders' + -- Use binders' to put one-shot info on the lambdas + -- URGH! Sept 99: we don't seem to be able to use binders' here, because -- we get linear-typed things in the resulting program that we can't handle yet. -- (e.g. PrelShow) TODO @@ -774,8 +880,7 @@ occAnal env (Case scrut bndr ty alts) case mapAndUnzip occ_anal_alt alts of { (alts_usage_s, alts') -> let alts_usage = foldr1 combineAltsUsageDetails alts_usage_s - alts_usage' = addCaseBndrUsage alts_usage - (alts_usage1, tagged_bndr) = tagBinder alts_usage' bndr + (alts_usage1, tagged_bndr) = tag_case_bndr alts_usage bndr total_usage = scrut_usage +++ alts_usage1 in total_usage `seq` (total_usage, Case scrut' tagged_bndr ty alts') }} @@ -789,9 +894,10 @@ occAnal env (Case scrut bndr ty alts) -- case x of w { (p,q) -> f w } -- into -- case x of w { (p,q) -> f (p,q) } - addCaseBndrUsage usage = case lookupVarEnv usage bndr of - Nothing -> usage - Just _ -> extendVarEnv usage bndr NoOccInfo + tag_case_bndr usage bndr + = case lookupVarEnv usage bndr of + Nothing -> (usage, setIdOccInfo bndr IAmDead) + Just _ -> (usage `delVarEnv` bndr, setIdOccInfo bndr NoOccInfo) alt_env = mkAltEnv env bndr_swap -- Consider x = case v of { True -> (p,q); ... } @@ -841,7 +947,8 @@ occAnalApp env (Var fun, args) where fun_uniq = idUnique fun fun_uds = mkOneOcc env fun (valArgCount args > 0) - is_pap = isDataConWorkId fun || valArgCount args < idArity fun + is_pap = isConLikeId fun || valArgCount args < idArity fun + -- See Note [CONLIKE pragma] in BasicTypes -- Hack for build, fold, runST args_stuff | fun_uniq == buildIdKey = appSpecial env 2 [True,True] args @@ -957,6 +1064,16 @@ us to adjust the OccInfo for 'x' and 'b' as we go. The Maybe (Id,CoreExpr) passed to occAnalAlt is the extra let-binding {x=b}; it's Nothing if the binder-swap doesn't happen. +There is a danger though. Consider + let v = x +# y + in case (f v) of w -> ...v...v... +And suppose that (f v) expands to just v. Then we'd like to +use 'w' instead of 'v' in the alternative. But it may be too +late; we may have substituted the (cheap) x+#y for v in the +same simplifier pass that reduced (f v) to v. + +I think this is just too bad. CSE will recover some of it. + Note [Binder swap on GlobalId scrutinees] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When the scrutinee is a GlobalId we must take care in two ways @@ -970,6 +1087,67 @@ When the scrutinee is a GlobalId we must take care in two ways has an External Name. See, for example, SimplEnv Note [Global Ids in the substitution]. +Historical note [no-case-of-case] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +We *used* to suppress the binder-swap in case expressoins when +-fno-case-of-case is on. Old remarks: + "This happens in the first simplifier pass, + and enhances full laziness. Here's the bad case: + f = \ y -> ...(case x of I# v -> ...(case x of ...) ... ) + If we eliminate the inner case, we trap it inside the I# v -> arm, + which might prevent some full laziness happening. I've seen this + in action in spectral/cichelli/Prog.hs: + [(m,n) | m <- [1..max], n <- [1..max]] + Hence the check for NoCaseOfCase." +However, now the full-laziness pass itself reverses the binder-swap, so this +check is no longer necessary. + +Historical note [Suppressing the case binder-swap] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +This old note describes a problem that is also fixed by doing the +binder-swap in OccAnal: + + There is another situation when it might make sense to suppress the + case-expression binde-swap. If we have + + case x of w1 { DEFAULT -> case x of w2 { A -> e1; B -> e2 } + ...other cases .... } + + We'll perform the binder-swap for the outer case, giving + + case x of w1 { DEFAULT -> case w1 of w2 { A -> e1; B -> e2 } + ...other cases .... } + + But there is no point in doing it for the inner case, because w1 can't + be inlined anyway. Furthermore, doing the case-swapping involves + zapping w2's occurrence info (see paragraphs that follow), and that + forces us to bind w2 when doing case merging. So we get + + case x of w1 { A -> let w2 = w1 in e1 + B -> let w2 = w1 in e2 + ...other cases .... } + + This is plain silly in the common case where w2 is dead. + + Even so, I can't see a good way to implement this idea. I tried + not doing the binder-swap if the scrutinee was already evaluated + but that failed big-time: + + data T = MkT !Int + + case v of w { MkT x -> + case x of x1 { I# y1 -> + case x of x2 { I# y2 -> ... + + Notice that because MkT is strict, x is marked "evaluated". But to + eliminate the last case, we must either make sure that x (as well as + x1) has unfolding MkT y1. THe straightforward thing to do is to do + the binder-swap. So this whole note is a no-op. + +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. + Note [Case of cast] ~~~~~~~~~~~~~~~~~~~ Consider case (x `cast` co) of b { I# -> @@ -984,9 +1162,9 @@ Consider case x of y { (a,b) -> f y } We treat 'a', 'b' as dead, because they don't physically occur in the case alternative. (Indeed, a variable is dead iff it doesn't occur in -its scope in the output of OccAnal.) This invariant is It really -helpe to know when binders are unused. See esp the call to -isDeadBinder in Simplify.mkDupableAlt +its scope in the output of OccAnal.) It really helps to know when +binders are unused. See esp the call to isDeadBinder in +Simplify.mkDupableAlt In this example, though, the Simplifier will bring 'a' and 'b' back to life, beause it binds 'y' to (a,b) (imagine got inlined and @@ -1001,7 +1179,7 @@ occAnalAlt :: OccEnv occAnalAlt env case_bndr mb_scrut_var (con, bndrs, rhs) = case occAnal env rhs of { (rhs_usage, rhs') -> let - (alt_usg, tagged_bndrs) = tagBinders rhs_usage bndrs + (alt_usg, tagged_bndrs) = tagLamBinders rhs_usage bndrs bndrs' = tagged_bndrs -- See Note [Binders in case alternatives] in case mb_scrut_var of @@ -1010,12 +1188,15 @@ occAnalAlt env case_bndr mb_scrut_var (con, bndrs, rhs) , not (any shadowing bndrs) -- (b) -> (addOneOcc usg_wo_scrut case_bndr NoOccInfo, -- See Note [Case binder usage] for the NoOccInfo - (con, bndrs', Let (NonRec scrut_var' scrut_rhs) rhs')) + (con, bndrs', Let (NonRec scrut_var2 scrut_rhs) rhs')) where - (usg_wo_scrut, scrut_var') = tagBinder alt_usg (localiseId scrut_var) - -- Note the localiseId; we're making a new binding - -- for it, and it might have an External Name, or + scrut_var1 = mkLocalId (localiseName (idName scrut_var)) (idType scrut_var) + -- Localise the scrut_var before shadowing it; we're making a + -- new binding for it, and it might have an External Name, or -- even be a GlobalId; Note [Binder swap on GlobalId scrutinees] + -- Also we don't want any INLILNE or NOINLINE pragmas! + + (usg_wo_scrut, scrut_var2) = tagBinder alt_usg scrut_var1 shadowing bndr = bndr `elemVarSet` rhs_fvs rhs_fvs = exprFreeVars scrut_rhs @@ -1066,7 +1247,7 @@ type CtxtTy = [Bool] -- the CtxtTy inside applies initOccEnv :: OccEnv -initOccEnv = OccEnv { occ_encl = OccRhs +initOccEnv = OccEnv { occ_encl = OccVanilla , occ_ctxt = [] , occ_scrut_ids = emptyVarSet } @@ -1155,17 +1336,21 @@ v `usedIn` details = isExportedId v || v `localUsedIn` details type IdWithOccInfo = Id -tagBinders :: UsageDetails -- Of scope - -> [Id] -- Binders - -> (UsageDetails, -- Details with binders removed - [IdWithOccInfo]) -- Tagged binders - -tagBinders usage binders - = let - usage' = usage `delVarEnvList` binders - uss = map (setBinderOcc usage) binders - in - usage' `seq` (usage', uss) +tagLamBinders :: UsageDetails -- Of scope + -> [Id] -- Binders + -> (UsageDetails, -- Details with binders removed + [IdWithOccInfo]) -- Tagged binders +-- Used for lambda and case binders +-- It copes with the fact that lambda bindings can have InlineRule +-- unfoldings, used for join points +tagLamBinders usage binders = usage' `seq` (usage', bndrs') + where + (usage', bndrs') = mapAccumR tag_lam usage binders + tag_lam usage bndr = (usage2, setBinderOcc usage bndr) + where + usage1 = usage `delVarEnv` bndr + usage2 | isId bndr = addIdOccs usage1 (idUnfoldingVars bndr) + | otherwise = usage1 tagBinder :: UsageDetails -- Of scope -> Id -- Binders