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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