Hi Conal, so if I understand this right, you'd rather not wanted `exampleC` to be eta-expanded (or the binding of `s` to be floated into the lambda)? Or is it that you want CSE to find out that you always supply the same `t` as the first argument and share the partial application and thus the work of computing `s`?
If it's the former: GHC doesn't normally do this, unless it has found out that no sharing (of work done to evaluate `s`) would be lost through eta-expansion. This is the case when `exampleC` is always called with two arguments, so that no binding of `s` is shared, for example. Could you maybe post a complete module/expression representative of all uses of `exampleC`? If it's the latter, I'm afraid I can't really help, but surely someone else can. Cheers, Sebastian On Tue, Jul 18, 2017 at 5:34 PM, Conal Elliott <co...@conal.net> wrote: > 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)) > > } > > } > > The `sinDouble#` here depends only on `t_afI6` (`t`) but still appears > under the binding of `eta_B1` (`x`). > > 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.) > > Does the optimization I expected somehow happen later in the compilation > pipeline? > > Are there Core-manipulating functions in GHC that I can use to make it > happen earlier (via a `BuiltinRule`, i.e., Core-to-Core function)? > > Thanks, > -- Conal > > > _______________________________________________ > ghc-devs mailing list > ghc-devs@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > >
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