wiki:Commentary/Compiler/StrictnessAnalysis/KirstenNotes

Version 5 (modified by kirsten, 8 years ago) (diff)

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Linearity

Instead of:

type DmdEnv = VarEnv Demand

we want:

type DmdEnv = Env CoreExpr Demand

We keep track of demands on partial applications.

After calling dmd_anal on the body of a let, which results in demand type dmd_ty with demand env dmd_env, we do the following for each let-bound variable f:

  1. Iterate through all the keys in dmd_env, finding all applications of f to n arguments.
  2. For each i from 1 through n (where n is f's arity), if each of the applications of f to i arguments has usage demand OneOrZero, then it's safe to mark the corresponding lambda-expression as a one-shot lambda.

This might work, but is kind of kludgy.

This may be way too unnecessarily complicated. Can't we just get the same information from the demand on f in the free-var environment of the let body as it is, without changing the environment?

suppose the demand on f is

S1K(S1K(LMX))

if f =

(\ x. (\ y. ...))

then we can mark the outer two lambdas as being one-shot. Right?

Not exactly. Suppose:

let f = \ x. \ y. ... in
  ...(f 1 2)...(f 3 4)...

f will have demand on it:

SMK(SMK(LMX))

because it's called more than once. (But maybe that's okay? If it instead were:

let f = \ x. \ y. ... in
  ...(f 1)...(f 3 4)...

then the demand would be:

SMK(LMX)

and we would know that there was a partial application? Getting confused here.)