diff options
Diffstat (limited to 'compiler/GHC/Types/Demand.hs')
| -rw-r--r-- | compiler/GHC/Types/Demand.hs | 61 |
1 files changed, 56 insertions, 5 deletions
diff --git a/compiler/GHC/Types/Demand.hs b/compiler/GHC/Types/Demand.hs index 80c725bbfc..65f3239a9e 100644 --- a/compiler/GHC/Types/Demand.hs +++ b/compiler/GHC/Types/Demand.hs @@ -607,13 +607,17 @@ multCard (Card a) (Card b) -- isn't any evaluation at all. If you don't care, simply use '(:*)'. data Demand = BotDmd - -- ^ A bottoming demand, produced by a diverging function, hence there is no + -- ^ A bottoming demand, produced by a diverging function ('C_10'), hence there is no -- 'SubDemand' that describes how it was evaluated. + | AbsDmd -- ^ An absent demand: Evaluated exactly 0 times ('C_00'), hence there is no -- 'SubDemand' that describes how it was evaluated. + | D !CardNonAbs !SubDemand -- ^ Don't use this internal data constructor; use '(:*)' instead. + -- Since BotDmd deals with 'C_10' and AbsDmd deals with 'C_00', the + -- cardinality component is CardNonAbs deriving Eq -- | Only meant to be used in the pattern synonym below! @@ -1561,10 +1565,12 @@ isTopDmdType :: DmdType -> Bool isTopDmdType (DmdType env args div) = div == topDiv && null args && isEmptyVarEnv env +{- Unused -- | The demand type of an unspecified expression that is guaranteed to -- throw a (precise or imprecise) exception or diverge. exnDmdType :: DmdType exnDmdType = DmdType emptyDmdEnv [] exnDiv +-} dmdTypeDepth :: DmdType -> Arity dmdTypeDepth = length . dt_args @@ -1626,21 +1632,66 @@ findIdDemand (DmdType fv _ res) id -- exception, we act as if there is an additional control flow path that is -- taken if e throws a precise exception. The demand type of this control flow -- path --- * is lazy and absent ('topDmd') in all free variables and arguments +-- * is lazy and absent ('topDmd') and boxed in all free variables and arguments -- * has 'exnDiv' 'Divergence' result +-- See Note [Precise exceptions and strictness analysis] +-- -- So we can simply take a variant of 'nopDmdType', 'exnDmdType'. -- Why not 'nopDmdType'? Because then the result of 'e' can never be 'exnDiv'! -- That means failure to drop dead-ends, see #18086. --- See Note [Precise exceptions and strictness analysis] deferAfterPreciseException :: DmdType -> DmdType -deferAfterPreciseException = lubDmdType exnDmdType +-- deferAfterPreciseException = lubDmdType exnDmdType +deferAfterPreciseException (DmdType fvs ds r) + = DmdType (mapVarEnv defer fvs) + (map defer ds) + (r `lubDivergence` ExnOrDiv) + where + defer :: Demand -> Demand + defer AbsDmd = AbsDmd + defer BotDmd = AbsDmd + defer (D n sd) = lubCard n C_00 :* lubSubDmd sd (Poly Boxed C_00) + + -- Roughly: defer d = d `lubDmd` D C_00 (Poly Boxed C_00) + -- It is very important that we `lub` with `Boxed`; see + -- Note [deferAfterPreciseException] + -- But that formulation fails the assert in :*, + -- because (D C_00 (Poly Boxed C_00)) is not a legal demand + -- So we write defer out more explicitly here -- | See 'keepAliveDmdEnv'. keepAliveDmdType :: DmdType -> VarSet -> DmdType keepAliveDmdType (DmdType fvs ds res) vars = DmdType (fvs `keepAliveDmdEnv` vars) ds res -{- +{- Note [deferAfterPreciseException] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +The big picture is in Note [Precise exceptions and strictness analysis] +The idea is that we want to treat + case <I/O operation> of (# s', r #) -> rhs + +as if it was + case <I/O operation> of + Just (# s', r #) -> rhs + Nothing -> error + +That is, the I/O operation might throw an exception, so that 'rhs' never +gets reached. For example, we don't want to be strict in the strict free +variables of 'rhs'. + +So roughly speaking: + deferAfterPreciseException = lubDmdType (DmdType emptyDmdEnv [] exnDiv) + +But that doesn't work quite right for boxity becasuse + case <I/O operation> of + (# s', r) -> f x + +uses `x` *boxed*. If we `lub` it with `(DmdType emptyDmdEnv [] exnDiv)` +we'll get an *unboxed* demand on `x`, which led to #20746. There is +a fuller example in that ticket. + +TL;DR: deferAfterPreciseException is very careful to preserve boxity +in its argument. + Note [Demand type Divergence] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In contrast to DmdSigs, DmdTypes are elicited under a specific incoming demand. |