#endif
idNewStrictness, idNewStrictness_maybe,
setIdNewStrictness, idNewDemandInfo,
+ idNewDemandInfo_maybe,
setIdNewDemandInfo, idName
)
#ifdef OLD_STRICTNESS
= do {
showPass dflags "Demand analysis" ;
let { binds_plus_dmds = do_prog binds } ;
+
endPass dflags "Demand analysis"
Opt_D_dump_stranal binds_plus_dmds ;
#ifdef OLD_STRICTNESS
( _, _, (_, 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)
= let
(sigs', _, pairs') = dmdFix TopLevel sigs pairs
-- We get two iterations automatically
+ -- c.f. the NonRec case above
in
(sigs', Rec pairs')
\end{code}
| 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')
(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:
= 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
-- 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)
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
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,
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]
+extendSigsWithLam sigs id
+ = case idNewDemandInfo_maybe id of
+ Nothing -> pprTrace "Yes (bot)" (ppr id) $ extendVarEnv sigs id (cprSig, NotTopLevel)
+ Just (Eval ds) -> pprTrace "Yes" (ppr id) $ extendVarEnv sigs id (cprSig, NotTopLevel)
+ other -> pprTrace "No" (ppr id $$ ppr (idNewDemandInfo id)) $ sigs
+
+cprSig :: StrictSig
+cprSig = StrictSig (mkDmdType emptyVarEnv [] RetCPR)
+
+
dmdTransform :: SigEnv -- The strictness environment
-> Id -- The function
-> Demand -- The demand on the function
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