Preserve demandInfo on lambda binders in the simpifier

In ticket:11731#comment:118707, simonpj writes:

Meanwhile, there is a separate issue in ticket:11731#comment:118677. It seems that for any un-saturated lambda, we nuke (a) the occInfo on the binders (fair enough; the next run of the occurrence analyser will regenerate it), and (b) the demandInfo on the binders. The reasoning is explained in #11778.

But (b) is permanent; it won't be re-generated until the next run of the demand analyser. And, worse, it applies to function definitions

f = \xy . x+y

Here x and y will be marked as strictly demanded, but that info will get stripped after the first run of the simplifier. Try it on

f :: [Int] -> [Int] -> Int
f x y = head x + head y

After the demand analyser we have

f = \ (x_amY [Dmd=<S,1*U>] :: [Int]) (y_amZ [Dmd=<S,1*U>] :: [Int]) -> ...

but after the first run of the simplifier we get

f = \ (x_amY :: [Int]) (y_amZ :: [Int]) -> ...

(f itself still has a strictness signature that says it is strict in both args.)

Does this matter? Well, the main reason is that if f is inlined, we'd like to get a strict let. And now we won't.

Happily I think it is easily fixed. The key thing is that when doing beta-reduction, which effectively does (\x.e) b into let x=b in e, we must kill off x's demand/occurrence info if the lambda is not saturated.

So, idea, in Simplify.hs:

• Give an extra Bool to simplLam indictating "saturated".
• Compute (value) saturation in the simplExprF1 env expr@(Lam {}) cont, before calling simpLam, but do no zapping.
• In simplLam, in the beta-reduction case,

simplLam env (bndr:bndrs) body (ApplyToVal { sc_arg = arg, sc_env = arg_se
, sc_cont = cont })
= do  { tick (BetaReduction bndr)
; simplNonRecE env' (zap_unfolding bndr) (arg, arg_se) (bndrs, body) cont }

do the binder-zapping right there, if "saturated" is not true. (That neatly puts it with the unfolding-zapping code.)

Now the non-beta-reduced lambdas won't be zapped, which is right.

Would you like to try that?

Trac metadata

Trac field	Value
Version	8.1
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture