Thanks very much for this reply, Joachim. I see that `-fno-do-lambda-eta-expansion` with my example prevents moving the computation under the lambda where it gets repeatedly evaluated. I don't understand what this code motion/substitution has to do with eta-expansion. Is it that the `let` expression itself is eta-expanded? The GHC Users Guide describes this flag as "eta-expand let-bindings to increase their arity", which doesn't seem to fit here, since the `let`-bindings are not function-valued (though the `let` expression is).

Thanks also for the suggestion of using `-dverbose-core2core` to see where the unwanted substitution happened.

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.

No, I haven't measured. In this case, I'm compiling Haskell to GLSL for execution on a GPU, where the inner lambda will be over space, which means at least one application per pixel, so the computations moved under the inner lambda will be redundantly computed a few millions of times per frame (and much more with anti-aliasing). Instead, I want to move those calculations to once per frame and stored in quickly accessed video memory. As the space-independent computation gets more complex, I expect the saving to grow.

Thanks again,

-- Conal

On Tue, Jul 18, 2017 at 12:08 PM, Joachim Breitner <mail@joachim-breitner.de> wrote:

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@joachim-breitner.de
http://www.joachim-breitner.de/

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