diff options
Diffstat (limited to 'compiler/GHC/Core/Opt/DmdAnal.hs')
| -rw-r--r-- | compiler/GHC/Core/Opt/DmdAnal.hs | 25 |
1 files changed, 18 insertions, 7 deletions
diff --git a/compiler/GHC/Core/Opt/DmdAnal.hs b/compiler/GHC/Core/Opt/DmdAnal.hs index bad3234ca9..5f209701a9 100644 --- a/compiler/GHC/Core/Opt/DmdAnal.hs +++ b/compiler/GHC/Core/Opt/DmdAnal.hs @@ -43,7 +43,8 @@ import GHC.Builtin.PrimOps import GHC.Builtin.Types.Prim ( realWorldStatePrimTy ) import GHC.Types.Unique.Set --- import GHC.Driver.Ppr +import GHC.Utils.Trace +_ = pprTrace -- Tired of commenting out the import all the time {- ************************************************************************ @@ -340,11 +341,12 @@ dmdAnalStar :: AnalEnv -> Demand -- This one takes a *Demand* -> CoreExpr -- Should obey the let/app invariant -> (PlusDmdArg, CoreExpr) -dmdAnalStar env (n :* cd) e - | WithDmdType dmd_ty e' <- dmdAnal env cd e +dmdAnalStar env (n :* sd) e + -- NB: (:*) expands AbsDmd and BotDmd as needed + -- See Note [Analysing with absent demand] + | WithDmdType dmd_ty e' <- dmdAnal env sd e = assertPpr (not (isUnliftedType (exprType e)) || exprOkForSpeculation e) (ppr e) -- The argument 'e' should satisfy the let/app invariant - -- See Note [Analysing with absent demand] in GHC.Types.Demand (toPlusDmdArg $ multDmdType n dmd_ty, e') -- Main Demand Analsysis machinery @@ -427,7 +429,7 @@ dmdAnal' env dmd (Case scrut case_bndr ty [Alt alt bndrs rhs]) WithDmdType alt_ty2 case_bndr_dmd = findBndrDmd env alt_ty1 case_bndr -- Evaluation cardinality on the case binder is irrelevant and a no-op. -- What matters is its nested sub-demand! - (_ :* case_bndr_sd) = case_bndr_dmd + (_ :* case_bndr_sd) = case_bndr_dmd -- Compute demand on the scrutinee -- FORCE the result, otherwise thunks will end up retaining the -- whole DmdEnv @@ -548,7 +550,7 @@ dmdAnalSumAlt env dmd case_bndr (Alt con bndrs rhs) , WithDmdType alt_ty dmds <- findBndrsDmds env rhs_ty bndrs , let (_ :* case_bndr_sd) = findIdDemand alt_ty case_bndr -- See Note [Demand on scrutinee of a product case] - id_dmds = addCaseBndrDmd case_bndr_sd dmds + id_dmds = addCaseBndrDmd case_bndr_sd dmds -- Do not put a thunk into the Alt !new_ids = setBndrsDemandInfo bndrs id_dmds = WithDmdType alt_ty (Alt con new_ids rhs') @@ -557,7 +559,7 @@ dmdAnalSumAlt env dmd case_bndr (Alt con bndrs rhs) Note [Analysing with absent demand] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Suppose we analyse an expression with demand A. The "A" means -"absent", so this expression will never be needed. What should happen? +"absent", so this expression will never be needed. What should happen? There are several wrinkles: * We *do* want to analyse the expression regardless. @@ -566,6 +568,15 @@ There are several wrinkles: But we can post-process the results to ignore all the usage demands coming back. This is done by multDmdType. +* Nevertheless, which sub-demand should we pick for analysis? + Since the demand was absent, any would do. Worker/wrapper will replace + absent bindings with an absent filler anyway, so annotations in the RHS + of an absent binding don't matter much. + Picking 'botSubDmd' would be the most useful, but would also look a bit + misleading in the Core output of DmdAnal, because all nested annotations would + be bottoming. Better pick 'seqSubDmd', so that we annotate many of those + nested bindings with A themselves. + * In a previous incarnation of GHC we needed to be extra careful in the case of an *unlifted type*, because unlifted values are evaluated even if they are not used. Example (see #9254): |