diff options
Diffstat (limited to 'compiler/GHC/Core/Opt/DmdAnal.hs')
| -rw-r--r-- | compiler/GHC/Core/Opt/DmdAnal.hs | 69 |
1 files changed, 44 insertions, 25 deletions
diff --git a/compiler/GHC/Core/Opt/DmdAnal.hs b/compiler/GHC/Core/Opt/DmdAnal.hs index c3fd09c6e0..966535897a 100644 --- a/compiler/GHC/Core/Opt/DmdAnal.hs +++ b/compiler/GHC/Core/Opt/DmdAnal.hs @@ -956,16 +956,15 @@ dmdAnalRhsSig top_lvl rec_flag env let_dmd id rhs = -- pprTrace "dmdAnalRhsSig" (ppr id $$ ppr let_dmd $$ ppr rhs_dmds $$ ppr sig $$ ppr lazy_fv) $ (final_env, lazy_fv, final_id, final_rhs) where - rhs_arity = idArity id - -- See Note [Demand signatures are computed for a threshold demand based on idArity] + threshold_arity = thresholdArity id rhs - rhs_dmd = mkCalledOnceDmds rhs_arity body_dmd + rhs_dmd = mkCalledOnceDmds threshold_arity body_dmd body_dmd | isJoinId id -- See Note [Demand analysis for join points] -- See Note [Invariants on join points] invariant 2b, in GHC.Core - -- rhs_arity matches the join arity of the join point + -- threshold_arity matches the join arity of the join point -- See Note [Unboxed demand on function bodies returning small products] = unboxedWhenSmall env rec_flag (resultType_maybe id) let_dmd | otherwise @@ -975,10 +974,10 @@ dmdAnalRhsSig top_lvl rec_flag env let_dmd id rhs WithDmdType rhs_dmd_ty rhs' = dmdAnal env rhs_dmd rhs DmdType rhs_fv rhs_dmds rhs_div = rhs_dmd_ty -- See Note [Boxity for bottoming functions] - (final_rhs_dmds, final_rhs) = finaliseArgBoxities env id rhs_arity rhs' rhs_div + (final_rhs_dmds, final_rhs) = finaliseArgBoxities env id threshold_arity rhs' rhs_div `orElse` (rhs_dmds, rhs') - sig = mkDmdSigForArity rhs_arity (DmdType sig_fv final_rhs_dmds rhs_div) + sig = mkDmdSigForArity threshold_arity (DmdType sig_fv final_rhs_dmds rhs_div) final_id = id `setIdDmdSig` sig !final_env = extendAnalEnv top_lvl env final_id sig @@ -1005,6 +1004,13 @@ dmdAnalRhsSig top_lvl rec_flag env let_dmd id rhs -- See Note [Lazy and unleashable free variables] !(!lazy_fv, !sig_fv) = partitionVarEnv isWeakDmd rhs_fv2 +thresholdArity :: Id -> CoreExpr -> Arity +-- See Note [Demand signatures are computed for a threshold arity based on idArity] +thresholdArity fn rhs + = case isJoinId_maybe fn of + Just join_arity -> count isId $ fst $ collectNBinders join_arity rhs + Nothing -> idArity fn + -- | The result type after applying 'idArity' many arguments. Returns 'Nothing' -- when the type doesn't have exactly 'idArity' many arrows. resultType_maybe :: Id -> Maybe Type @@ -1137,28 +1143,40 @@ meaning one absent argument, returns bottom. That seems odd because there's a \y inside. But it's right because when consumed in a C1(L) context the RHS of the join point is indeed bottom. -Note [Demand signatures are computed for a threshold demand based on idArity] -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -We compute demand signatures assuming idArity incoming arguments to approximate -behavior for when we have a call site with at least that many arguments. idArity -is /at least/ the number of manifest lambdas, but might be higher for PAPs and -trivial RHS (see Note [Demand analysis for trivial right-hand sides]). +Note [Demand signatures are computed for a threshold arity based on idArity] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Given a binding { f = rhs }, we compute a "theshold arity", and do demand +analysis based on a call with that many value arguments. -Because idArity of a function varies independently of its cardinality -properties (cf. Note [idArity varies independently of dmdTypeDepth]), we -implicitly encode the arity for when a demand signature is sound to unleash -in its 'dmdTypeDepth' (cf. Note [Understanding DmdType and DmdSig] in -GHC.Types.Demand). It is unsound to unleash a demand signature when the -incoming number of arguments is less than that. -See Note [What are demand signatures?] in GHC.Types.Demand for more details -on soundness. +The threshold we use is -Why idArity arguments? Because that's a conservative estimate of how many -arguments we must feed a function before it does anything interesting with them. -Also it elegantly subsumes the trivial RHS and PAP case. +* Ordinary bindings: idArity f. + Why idArity arguments? Because that's a conservative estimate of how many + arguments we must feed a function before it does anything interesting with + them. Also it elegantly subsumes the trivial RHS and PAP case. -There might be functions for which we might want to analyse for more incoming -arguments than idArity. Example: + idArity is /at least/ the number of manifest lambdas, but might be higher for + PAPs and trivial RHS (see Note [Demand analysis for trivial right-hand sides]). + +* Join points: the value-binder subset of the JoinArity. This can + be less than the number of visible lambdas; e.g. + join j x = \y. blah + in ...(jump j 2)....(jump j 3).... + We know that j will never be applied to more than 1 arg (its join + arity, and we don't eta-expand join points, so here a threshold + of 1 is the best we can do. + +Note that the idArity of a function varies independently of its cardinality +properties (cf. Note [idArity varies independently of dmdTypeDepth]), so we +implicitly encode the arity for when a demand signature is sound to unleash +in its 'dmdTypeDepth', not in its idArity (cf. Note [Understanding DmdType +and DmdSig] in GHC.Types.Demand). It is unsound to unleash a demand +signature when the incoming number of arguments is less than that. See +GHC.Types.Demand Note [What are demand signatures?] for more details on +soundness. + +Note that there might, in principle, be functions for which we might want to +analyse for more incoming arguments than idArity. Example: f x = if expensive @@ -1175,6 +1193,7 @@ strictness info for `y` (and more precise info on `x`) and possibly CPR information, but * We would no longer be able to unleash the signature at unary call sites + * Performing the worker/wrapper split based on this information would be implicitly eta-expanding `f`, playing fast and loose with divergence and even being unsound in the presence of newtypes, so we refrain from doing so. |