diff options
Diffstat (limited to 'compiler/stranal')
| -rw-r--r-- | compiler/stranal/DmdAnal.lhs | 20 |
1 files changed, 14 insertions, 6 deletions
diff --git a/compiler/stranal/DmdAnal.lhs b/compiler/stranal/DmdAnal.lhs index 3294371964..72137c7b4b 100644 --- a/compiler/stranal/DmdAnal.lhs +++ b/compiler/stranal/DmdAnal.lhs @@ -593,7 +593,7 @@ dmdAnalRhs :: TopLevelFlag -- Process the RHS of the binding, add the strictness signature -- to the Id, and augment the environment with the signature as well. dmdAnalRhs top_lvl rec_flag env id rhs - | Just fn <- unpackTrivial rhs -- See Note [Trivial right-hand sides] + | Just fn <- unpackTrivial rhs -- See Note [Demand analysis for trivial right-hand sides] , let fn_str = getStrictness env fn = (fn_str, emptyDmdEnv, set_idStrictness env id fn_str, rhs) @@ -638,7 +638,7 @@ dmdAnalRhs top_lvl rec_flag env id rhs unpackTrivial :: CoreExpr -> Maybe Id -- Returns (Just v) if the arg is really equal to v, modulo -- casts, type applications etc --- See Note [Trivial right-hand sides] +-- See Note [Demand analysis for trivial right-hand sides] unpackTrivial (Var v) = Just v unpackTrivial (Cast e _) = unpackTrivial e unpackTrivial (Lam v e) | isTyVar v = unpackTrivial e @@ -646,15 +646,23 @@ unpackTrivial (App e a) | isTypeArg a = unpackTrivial e unpackTrivial _ = Nothing \end{code} -Note [Trivial right-hand sides] -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Note [Demand analysis for trivial right-hand sides] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider foo = plusInt |> co where plusInt is an arity-2 function with known strictness. Clearly we want plusInt's strictness to propagate to foo! But because it has -no manifest lambdas, it won't do so automatically. So we have a +no manifest lambdas, it won't do so automatically, and indeed 'co' might +have type (Int->Int->Int) ~ T, so we *can't* eta-expand. So we have a special case for right-hand sides that are "trivial", namely variables, -casts, type applications, and the like. +casts, type applications, and the like. + +Note that this can mean that 'foo' has an arity that is smaller than that +indicated by its demand info. e.g. if co :: (Int->Int->Int) ~ T, then +foo's arity will be zero (see Note [exprArity invariant] in CoreArity), +but its demand signature will be that of plusInt. A small example is the +test case of Trac #8963. + Note [Product demands for function body] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |