Repeated computations under a lambda

[Joachim Breitner]

Hi,


Am Dienstag, den 18.07.2017, 08:34 -0700 schrieb Conal Elliott:
> I'm seeing what looks like repeated computation under a lambda with
> `-O` and `-O2`. The following definition:
> 
> > exampleC :: Double -> Double -> Double
> > exampleC = \ t -> let s = sin t in \ x -> x + s
> 
> yields this Core:
> 
> > -- RHS size: {terms: 13, types: 6, coercions: 0}
> > exampleC :: Double -> Double -> Double
> > exampleC =
> >   \ (t_afI6 :: Double) (eta_B1 :: Double) ->
> >     case eta_B1 of _ { D# x_aj5c ->
> >     case t_afI6 of _ { D# x1_aj5l ->
> >     D# (+## x_aj5c (sinDouble# x1_aj5l))
> >     }
> >     }

ghc -O -dverbose-core2core shows you that the problem is this phase:

==================== Simplifier ====================
  Max iterations = 4
  SimplMode {Phase = 2 [main],
             inline,
             rules,
             eta-expand,
             case-of-case}

It does not happen with -fno-do-lambda-eta-expansion (but you’d lose in
other parts.)

> I'm concerned because many of my uses of such functions involve
> computations dependent only on `t` (time) but with millions of uses
> (space) per `t`. (I'm working on a GHC Core plugin (compiling to
> categories), with one use generating graphics GPU code.)

Did you measure whether this really is a problem? The benefits of not
dealing with dynamically allocated functions might outweigh the cost of
recalculating sin.

Greetings,
Joachim
-- 
Joachim Breitner
  mail at joachim-breitner.de
  http://www.joachim-breitner.de/
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