X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=ghc%2Fcompiler%2Fstranal%2FDmdAnal.lhs;h=e90e413a34d3b32d212fb55e8445389de552ed99;hb=ef1d3a95a6d3cf315a1208743f1265d22e87b878;hp=20b07fbd15808e0262271dc4f2409df4e316ba8b;hpb=973539a893ff512a3e9ac408c1583a080de0abf4;p=ghc-hetmet.git diff --git a/ghc/compiler/stranal/DmdAnal.lhs b/ghc/compiler/stranal/DmdAnal.lhs index 20b07fb..e90e413 100644 --- a/ghc/compiler/stranal/DmdAnal.lhs +++ b/ghc/compiler/stranal/DmdAnal.lhs @@ -22,14 +22,15 @@ import DataCon ( dataConTyCon ) import TyCon ( isProductTyCon, isRecursiveTyCon ) import Id ( Id, idType, idInlinePragma, isDataConId, isGlobalId, idArity, -#ifdef DEBUG +#ifdef OLD_STRICTNESS idDemandInfo, idStrictness, idCprInfo, #endif idNewStrictness, idNewStrictness_maybe, setIdNewStrictness, idNewDemandInfo, + idNewDemandInfo_maybe, setIdNewDemandInfo, idName ) -#ifdef DEBUG +#ifdef OLD_STRICTNESS import IdInfo ( newStrictnessFromOld, newDemand ) #endif import Var ( Var ) @@ -68,12 +69,13 @@ dmdAnalPgm dflags binds = do { showPass dflags "Demand analysis" ; let { binds_plus_dmds = do_prog binds } ; + endPass dflags "Demand analysis" Opt_D_dump_stranal binds_plus_dmds ; -#ifdef DEBUG - -- Only if DEBUG is on, because only then is the old +#ifdef OLD_STRICTNESS + -- Only if OLD_STRICTNESS is on, because only then is the old -- strictness analyser run - let dmd_changes = get_changes binds_plus_dmds ; + let { dmd_changes = get_changes binds_plus_dmds } ; printDump (text "Changes in demands" $$ dmd_changes) ; #endif return binds_plus_dmds @@ -90,7 +92,8 @@ dmdAnalTopBind sigs (NonRec id rhs) ( _, _, (_, rhs1)) = dmdAnalRhs TopLevel sigs (id, rhs) (sigs2, _, (id2, rhs2)) = dmdAnalRhs TopLevel sigs (id, rhs1) -- Do two passes to improve CPR information - -- See the comments with mkSigTy.ignore_cpr_info below + -- See comments with ignore_cpr_info in mk_sig_ty + -- and with extendSigsWithLam in (sigs2, NonRec id2 rhs2) @@ -98,6 +101,7 @@ dmdAnalTopBind sigs (Rec pairs) = let (sigs', _, pairs') = dmdFix TopLevel sigs pairs -- We get two iterations automatically + -- c.f. the NonRec case above in (sigs', Rec pairs') \end{code} @@ -188,7 +192,8 @@ dmdAnal sigs dmd (Lam var body) | Call body_dmd <- dmd -- A call demand: good! = let - (body_ty, body') = dmdAnal sigs body_dmd body + sigs' = extendSigsWithLam sigs var + (body_ty, body') = dmdAnal sigs' body_dmd body (lam_ty, var') = annotateLamIdBndr body_ty var in (lam_ty, Lam var' body') @@ -209,7 +214,7 @@ dmdAnal sigs dmd (Case scrut case_bndr [alt@(DataAlt dc,bndrs,rhs)]) (alt_ty, alt') = dmdAnalAlt sigs_alt dmd alt (alt_ty1, case_bndr') = annotateBndr alt_ty case_bndr (_, bndrs', _) = alt' - case_bndr_sig = StrictSig (mkDmdType emptyVarEnv [] RetCPR) + case_bndr_sig = cprSig -- Inside the alternative, the case binder has the CPR property. -- Meaning that a case on it will successfully cancel. -- Example: @@ -321,7 +326,7 @@ dmdFix top_lvl sigs orig_pairs = loop 1 initial_sigs orig_pairs where bndrs = map fst orig_pairs - initial_sigs = extendSigEnvList sigs [(id, (initial_sig id, top_lvl)) | id <- bndrs] + initial_sigs = extendSigEnvList sigs [(id, (initialSig id, top_lvl)) | id <- bndrs] loop :: Int -> SigEnv -- Already contains the current sigs @@ -358,16 +363,16 @@ dmdFix top_lvl sigs orig_pairs -- old_sig = lookup sigs id -- new_sig = lookup sigs' id + same_sig sigs sigs' var = lookup sigs var == lookup sigs' var + lookup sigs var = case lookupVarEnv sigs var of + Just (sig,_) -> sig + -- Get an initial strictness signature from the Id -- itself. That way we make use of earlier iterations -- of the fixpoint algorithm. (Cunning plan.) -- Note that the cunning plan extends to the DmdEnv too, -- since it is part of the strictness signature - initial_sig id = idNewStrictness_maybe id `orElse` botSig - - same_sig sigs sigs' var = lookup sigs var == lookup sigs' var - lookup sigs var = case lookupVarEnv sigs var of - Just (sig,_) -> sig +initialSig id = idNewStrictness_maybe id `orElse` botSig dmdAnalRhs :: TopLevelFlag -> SigEnv -> (Id, CoreExpr) @@ -401,10 +406,85 @@ mkTopSigTy rhs dmd_ty = snd (mk_sig_ty False False rhs dmd_ty) mkSigTy :: Id -> CoreExpr -> DmdType -> (DmdEnv, StrictSig) mkSigTy id rhs dmd_ty = mk_sig_ty (isNeverActive (idInlinePragma id)) - (isStrictDmd (idNewDemandInfo id)) + ok_to_keep_cpr_info rhs dmd_ty + where + ok_to_keep_cpr_info = case idNewDemandInfo_maybe id of + Nothing -> True -- Is the case the first time round + Just dmd -> isStrictDmd dmd +\end{code} + +The ok_to_keep_cpr_info stuff [CPR-AND-STRICTNESS] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +If the rhs is a thunk, we usually forget the CPR info, because +it is presumably shared (else it would have been inlined, and +so we'd lose sharing if w/w'd it into a function. + +However, if the strictness analyser has figured out (in a previous +iteration) that it's strict, then we DON'T need to forget the CPR info. +Instead we can retain the CPR info and do the thunk-splitting transform +(see WorkWrap.splitThunk). + +This made a big difference to PrelBase.modInt, which had something like + modInt = \ x -> let r = ... -> I# v in + ...body strict in r... +r's RHS isn't a value yet; but modInt returns r in various branches, so +if r doesn't have the CPR property then neither does modInt +Another case I found in practice (in Complex.magnitude), looks like this: + let k = if ... then I# a else I# b + in ... body strict in k .... +(For this example, it doesn't matter whether k is returned as part of +the overall result; but it does matter that k's RHS has the CPR property.) +Left to itself, the simplifier will make a join point thus: + let $j k = ...body strict in k... + if ... then $j (I# a) else $j (I# b) +With thunk-splitting, we get instead + let $j x = let k = I#x in ...body strict in k... + in if ... then $j a else $j b +This is much better; there's a good chance the I# won't get allocated. + +The difficulty with this is that we need the strictness type to +look at the body... but we now need the body to calculate the demand +on the variable, so we can decide whether its strictness type should +have a CPR in it or not. Simple solution: + a) use strictness info from the previous iteration + b) make sure we do at least 2 iterations, by doing a second + round for top-level non-recs. Top level recs will get at + least 2 iterations except for totally-bottom functions + which aren't very interesting anyway. + +NB: strictly_demanded is never true of a top-level Id, or of a recursive Id. + +The Nothing case in ok_to_keep_cpr_info [CPR-AND-STRICTNESS] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Demand info now has a 'Nothing' state, just like strictness info. +The analysis works from 'dangerous' towards a 'safe' state; so we +start with botSig for 'Nothing' strictness infos, and we start with +"yes, it's demanded" for 'Nothing' in the demand info. The +fixpoint iteration will sort it all out. + +We can't start with 'not-demanded' because then consider + f x = let + t = ... I# x + in + if ... then t else I# y else f x' + +In the first iteration we'd have no demand info for x, so assume +not-demanded; then we'd get TopRes for f's CPR info. Next iteration +we'd see that t was demanded, and so give it the CPR property, but +by now f has TopRes, so it will stay TopRes. + +Instead, with the Nothing setting the first time round, we say +'yes t is demanded' the first time. + +However, this does mean that for non-recursive bindings we must +iterate twice to be sure of not getting over-optimistic CPR info, +in the case where t turns out to be not-demanded. This is handled +by dmdAnalTopBind. -mk_sig_ty never_inline strictly_demanded rhs (DmdType fv dmds res) + +\begin{code} +mk_sig_ty never_inline ok_to_keep_cpr_info rhs (DmdType fv dmds res) | never_inline && not (isBotRes res) -- HACK ALERT -- Don't strictness-analyse NOINLINE things. Why not? Because @@ -475,41 +555,7 @@ mk_sig_ty never_inline strictly_demanded rhs (DmdType fv dmds res) res' = case res of RetCPR | ignore_cpr_info -> TopRes other -> res - ignore_cpr_info = is_thunk && not strictly_demanded - is_thunk = not (exprIsValue rhs) - -- If the rhs is a thunk, we forget the CPR info, because - -- it is presumably shared (else it would have been inlined, and - -- so we'd lose sharing if w/w'd it into a function. - -- - -- Also, if the strictness analyser has figured out (in a previous iteration) - -- that it's strict, the let-to-case transformation will happen, so again - -- it's good. - -- This made a big difference to PrelBase.modInt, which had something like - -- modInt = \ x -> let r = ... -> I# v in - -- ...body strict in r... - -- r's RHS isn't a value yet; but modInt returns r in various branches, so - -- if r doesn't have the CPR property then neither does modInt - -- Another case I found in practice (in Complex.magnitude), looks like this: - -- let k = if ... then I# a else I# b - -- in ... body strict in k .... - -- (For this example, it doesn't matter whether k is returned as part of - -- the overall result.) Left to itself, the simplifier will make a join - -- point thus: - -- let $j k = ...body strict in k... - -- if ... then $j (I# a) else $j (I# b) - -- - -- - -- The difficulty with this is that we need the strictness type to - -- look at the body... but we now need the body to calculate the demand - -- on the variable, so we can decide whether its strictness type should - -- have a CPR in it or not. Simple solution: - -- a) use strictness info from the previous iteration - -- b) make sure we do at least 2 iterations, by doing a second - -- round for top-level non-recs. Top level recs will get at - -- least 2 iterations except for totally-bottom functions - -- which aren't very interesting anyway. - -- - -- NB: strictly_demanded is never true of a top-level Id, or of a recursive Id. + ignore_cpr_info = not (exprIsValue rhs || ok_to_keep_cpr_info) \end{code} The unpack strategy determines whether we'll *really* unpack the argument, @@ -665,6 +711,29 @@ extendSigEnv top_lvl env var sig = extendVarEnv env var (sig, top_lvl) extendSigEnvList = extendVarEnvList +extendSigsWithLam :: SigEnv -> Id -> SigEnv +-- Extend the SigEnv when we meet a lambda binder +-- If the binder is marked demanded with a product demand, then give it a CPR +-- signature, because in the likely event that this is a lambda on a fn defn +-- [we only use this when the lambda is being consumed with a call demand], +-- it'll be w/w'd and so it will be CPR-ish. +-- +-- NOTE: see notes [CPR-AND-STRICTNESS] +-- +-- Also note that we only want to do this for something that +-- definitely has product type, else we may get over-optimistic +-- CPR results (e.g. from \x -> x!). + +extendSigsWithLam sigs id + = case idNewDemandInfo_maybe id of + Nothing -> extendVarEnv sigs id (cprSig, NotTopLevel) + Just (Eval (Prod ds)) -> extendVarEnv sigs id (cprSig, NotTopLevel) + other -> sigs + +cprSig :: StrictSig +cprSig = StrictSig (mkDmdType emptyVarEnv [] RetCPR) + + dmdTransform :: SigEnv -- The strictness environment -> Id -- The function -> Demand -- The demand on the function @@ -894,7 +963,7 @@ lubs = zipWithDmds lub box (Call d) = Call d -- The odd man out. Why? box (Box d) = Box d box (Defer _) = lazyDmd -box Top = lazyDmd -- Box Abs and Box Top +box Top = lazyDmd -- Box Abs and Box Top box Abs = lazyDmd -- are the same box d = Box d -- Bot, Eval @@ -1004,7 +1073,7 @@ boths = zipWithDmds both \begin{code} -#ifdef DEBUG +#ifdef OLD_STRICTNESS get_changes binds = vcat (map get_changes_bind binds) get_changes_bind (Rec pairs) = vcat (map get_changes_pr pairs)