### Call Arity: In "e x", the result of "x" is not shared

```in contrast to "e (f x)", where CorePrep will turn it into "let y = f x
in e x". So in
let f = ...
in e (f x)
we know that f is called at most once, but in
let f = ...
in e f
we do not know that.

Previously Call Arity would assume that in "e x", "x" is evaluated at
most once. This rarely would make a difference (the argument "x" is
analized with an incoming arity of 0, so no eta-expansion would be done
anyways), but of course this should still be fixed.

This fixes #11064.

Note the corresponding code dmdTransformThunkDmd in DmdAnal.```
parent 10647d4e
 ... @@ -15,7 +15,7 @@ import BasicTypes ... @@ -15,7 +15,7 @@ import BasicTypes import CoreSyn import CoreSyn import Id import Id import CoreArity ( typeArity ) import CoreArity ( typeArity ) import CoreUtils ( exprIsHNF ) import CoreUtils ( exprIsHNF, exprIsTrivial ) --import Outputable --import Outputable import UnVarGraph import UnVarGraph import Demand import Demand ... @@ -149,10 +149,15 @@ The interesting cases of the analysis: ... @@ -149,10 +149,15 @@ The interesting cases of the analysis: * Case alternatives alt₁,alt₂,...: * Case alternatives alt₁,alt₂,...: Only one can be execuded, so Only one can be execuded, so Return (alt₁ ∪ alt₂ ∪...) Return (alt₁ ∪ alt₂ ∪...) * App e₁ e₂ (and analogously Case scrut alts): * App e₁ e₂ (and analogously Case scrut alts), with non-trivial e₂: We get the results from both sides. Additionally, anything called by e₁ can We get the results from both sides, with the argument evaluted at most once. possibly be called with anything from e₂. Additionally, anything called by e₁ can possibly be called with anything from e₂. Return: C(e₁) ∪ C(e₂) ∪ (fv e₁) × (fv e₂) Return: C(e₁) ∪ C(e₂) ∪ (fv e₁) × (fv e₂) * App e₁ x: As this is already in A-normal form, CorePrep will not separately lambda bind (and hence share) x. So we conservatively assume multiple calls to x here Return: C(e₁) ∪ (fv e₁) × {x} ∪ {(x,x)} * Let v = rhs in body: * Let v = rhs in body: In addition to the results from the subexpressions, add all co-calls from In addition to the results from the subexpressions, add all co-calls from everything that the body calls together with v to everthing that is called everything that the body calls together with v to everthing that is called ... @@ -472,7 +477,12 @@ callArityAnal arity int (App e1 e2) ... @@ -472,7 +477,12 @@ callArityAnal arity int (App e1 e2) where where (ae1, e1') = callArityAnal (arity + 1) int e1 (ae1, e1') = callArityAnal (arity + 1) int e1 (ae2, e2') = callArityAnal 0 int e2 (ae2, e2') = callArityAnal 0 int e2 final_ae = ae1 `both` ae2 -- If the argument is trivial (e.g. a variable), then it will _not_ be -- let-bound in the Core to STG transformation (CorePrep actually), -- so no sharing will happen here, and we have to assume many calls. ae2' | exprIsTrivial e2 = calledMultipleTimes ae2 | otherwise = ae2 final_ae = ae1 `both` ae2' -- Case expression. -- Case expression. callArityAnal arity int (Case scrut bndr ty alts) callArityAnal arity int (Case scrut bndr ty alts) ... ...
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