Hi Conal, Am Dienstag, den 18.07.2017, 15:35 -0700 schrieb Conal Elliott:
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).
In the code exampleC :: Double -> Double -> Double exampleC = \ t -> let s = sin t in \ x -> x + s the exampleC is the bound things that is eta-expanded. Ok, it is a top- level function, but that does not make a big difference. When eta- expanded, it becomes exampleC :: Double -> Double -> Double exampleC = \ t y -> (let s = sin t in \ x -> x + s) y and from then on, usual simplifications kick in to produce exampleC :: Double -> Double -> Double exampleC = \ t y -> let s = sin t in y + s and eventually exampleC :: Double -> Double -> Double exampleC = \ t y -> y + sin t (I ignore the unboxing of Doubles here).
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.
Hmm. The problem is that GHC’s optimizer uses a cost-model (in this case: calling an anonymous function is worse than recomputing sind) that does not hold in your case. Besides simply turning off some transformations, I am not sure what best you could do to avoid this… Gruß, Joachim -- Joachim Breitner mail@joachim-breitner.de http://www.joachim-breitner.de/