X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2FsimplCore%2FOccurAnal.lhs;h=ae5c291ef6edb894270342dfd04b748ca7fb2027;hb=9a4ef343a46e823bcf949af8501c13cc8ca98fb1;hp=b92239e5e4b26e03393ce550f50f2f5c8ff9b377;hpb=ac7db825a40d6b4e582a9b33969a1b0d5de9b3f6;p=ghc-hetmet.git diff --git a/compiler/simplCore/OccurAnal.lhs b/compiler/simplCore/OccurAnal.lhs index b92239e..ae5c291 100644 --- a/compiler/simplCore/OccurAnal.lhs +++ b/compiler/simplCore/OccurAnal.lhs @@ -22,6 +22,7 @@ import CoreFVs import CoreUtils ( exprIsTrivial, isDefaultAlt ) import Coercion ( mkSymCoercion ) import Id +import Name ( localiseName ) import IdInfo import BasicTypes @@ -171,8 +172,8 @@ 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 [Rules for recursive functions] in Simplify.lhs @@ -237,8 +238,9 @@ 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 @@ -365,8 +367,9 @@ 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 loop_breaker_edges = map mk_node tagged_nodes @@ -457,42 +460,55 @@ data Details = ND Id -- Binder IdSet -- Other binders from this Rec group mentioned on RHS -- (derivable from UsageDetails but cached here) -reOrderRec :: SCC (Node Details) - -> [(Id,CoreExpr)] +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 @@ -509,7 +525,7 @@ reOrderCycle (bind : binds) -- bad choice for loop breaker | is_con_app rhs = 3 -- Data types help with cases - -- Note [conapp] + -- Note [Constructor applictions] -- 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 @@ -560,22 +576,59 @@ reOrderCycle (bind : binds) 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] +~~~~~~~~~~~~~~~~~~~~~ +Never choose 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. + +A particular 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 @@ -584,6 +637,22 @@ 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). +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] ~~~~~~~~~~~~~~~~~~~~~~~~~ We treat (\x. C p q) as a high-score candidate in the letrec scoring algorithm. @@ -657,10 +726,14 @@ addRuleUsage :: UsageDetails -> Id -> UsageDetails -- Add the usage from RULES in Id to the usage addRuleUsage usage id = foldVarSet add usage (idRuleVars id) + -- idRuleVars here: see Note [Rule dependency info] 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 = addOneOcc u v NoOccInfo + -- Give a non-committal binder info (i.e manyOcc) 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 @@ -841,7 +914,7 @@ 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 -- Hack for build, fold, runST args_stuff | fun_uniq == buildIdKey = appSpecial env 2 [True,True] args @@ -957,6 +1030,16 @@ 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 +1053,67 @@ 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# -> @@ -1010,12 +1154,15 @@ 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