diff options
Diffstat (limited to 'compiler/GHC/CoreToStg/Prep.hs')
| -rw-r--r-- | compiler/GHC/CoreToStg/Prep.hs | 31 |
1 files changed, 24 insertions, 7 deletions
diff --git a/compiler/GHC/CoreToStg/Prep.hs b/compiler/GHC/CoreToStg/Prep.hs index f8955ae977..ee0aa343a7 100644 --- a/compiler/GHC/CoreToStg/Prep.hs +++ b/compiler/GHC/CoreToStg/Prep.hs @@ -1302,6 +1302,22 @@ tryEtaReducePrep _ _ = Nothing Note [Pin demand info on floats] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We pin demand info on floated lets, so that we can see the one-shot thunks. + +Note [Speculative evaluation] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Since call-by-value is much cheaper than call-by-need, we case-bind arguments +that are either + + 1. Strictly evaluated anyway, according to the StrictSig of the callee, or + 2. ok-for-spec, according to 'exprOkForSpeculation' + +While (1) is a no-brainer and always beneficial, (2) is a bit +more subtle, as the careful haddock for 'exprOkForSpeculation' +points out. Still, by case-binding the argument we don't need +to allocate a thunk for it, whose closure must be retained as +long as the callee might evaluate it. And if it is evaluated on +most code paths anyway, we get to turn the unknown eval in the +callee into a known call at the call site. -} data FloatingBind @@ -1350,19 +1366,20 @@ data OkToSpec mkFloat :: Demand -> Bool -> Id -> CpeRhs -> FloatingBind mkFloat dmd is_unlifted bndr rhs - | is_strict - , not is_hnf = FloatCase rhs bndr DEFAULT [] (exprOkForSpeculation rhs) + | is_strict || ok_for_spec -- See Note [Speculative evaluation] + , not is_hnf = FloatCase rhs bndr DEFAULT [] ok_for_spec -- Don't make a case for a HNF binding, even if it's strict -- Otherwise we get case (\x -> e) of ...! - | is_unlifted = ASSERT2( exprOkForSpeculation rhs, ppr rhs ) + | is_unlifted = ASSERT2( ok_for_spec, ppr rhs ) FloatCase rhs bndr DEFAULT [] True - | is_hnf = FloatLet (NonRec bndr rhs) - | otherwise = FloatLet (NonRec (setIdDemandInfo bndr dmd) rhs) + | is_hnf = FloatLet (NonRec bndr rhs) + | otherwise = FloatLet (NonRec (setIdDemandInfo bndr dmd) rhs) -- See Note [Pin demand info on floats] where - is_hnf = exprIsHNF rhs - is_strict = isStrUsedDmd dmd + is_hnf = exprIsHNF rhs + is_strict = isStrUsedDmd dmd + ok_for_spec = exprOkForSpeculation rhs emptyFloats :: Floats emptyFloats = Floats OkToSpec nilOL |