id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
3697	Method selectors aren't floated out of loops	rl		"Here is a small example:
{{{
foo :: Num a => [a] -> [a]
foo as = filter (/=0) (map (\x -> x-x) as)
}}}
Here is the code that the current HEAD generates for the loop:
{{{
go_smy =
  \ (ds_akN :: [a_aiw]) ->
    case ds_akN of _ {
      [] -> GHC.Types.[] @ a_aiw;
      : y_akS ys_akT ->
        let {
          x_smA :: a_aiw
          x_smA = GHC.Num.- @ a_aiw $dNum_ajk y_akS y_akS } in
        case GHC.Classes./= @ a_aiw lvl_smu x_smA lit_smw of _ {
          GHC.Bool.False -> go_smy ys_akT;
          GHC.Bool.True -> GHC.Types.: @ a_aiw x_smA (go_smy ys_akT)
        }
}}}
Note that the `Eq` dictionary is inspected by `(/=)` in every loop iteration. Floating out `GHC.Classes./= @ a_aiw lvl_smu` (and, perhaps, `GHC.Num.- @ a_aiw $dNum_ajk`) should solve this. In fact, I wonder why that isn't happening already. Do method selectors have a wrong arity?

FWIW, in GHC 6.10 `LiberateCase` moves the method selection out of the loop (not ideal, but it gets the job done) so !NoSlow shows this as a slight performance regression."	bug	new	low	7.6.2	Compiler	6.13				Unknown/Multiple	Unknown/Multiple	Runtime performance bug