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 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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