Optimisation of type class methods / dictionaries

[Thomas Schilling]

Hi,

I was investigating how GHC optimises dictionaries.  So, I'm using the
following variant of the Eq type class:

    class XEq a where
      xeq, xne :: a -> a -> Bool
      xne a b = not (a `xeq` b) }

    instance XEq Bool where
      xeq True  True  = True
      xeq False False = True
      xeq _     _     = False

    instance XEq a => XEq [a] where
      xeq []     []     = True
      xeq (x:xs) (y:ys) = xeq x y && xeq xs ys
      xeq _             _ = False

I then compile this with:

     ghc -c -fforce-recomp -O2 -ddump-simpl -ddump-types <filename.hs>


I now have two questions regarding the output of the compiler.


Question 1: Why are the instance method selectors marked as NOINLINE?

Main.xeq [InlPrag=[NEVER]]
  :: forall a_aeV. Main.XEq a_aeV => a_aeV -> a_aeV -> GHC.Types.Bool



Question 2: The optimised code for the list implementation looks as follows:

Main.$fXEq[]_$cxeq [Occ=LoopBreaker]
  :: forall a_an3.
     Main.XEq a_an3 =>
     [a_an3] -> [a_an3] -> GHC.Types.Bool
[GblId, Arity=3, Caf=NoCafRefs, Str=DmdType LSS]
Main.$fXEq[]_$cxeq =
  \ (@ a_an3)
    ($dXEq_auY :: Main.XEq a_an3)
    (ds_dw8 :: [a_an3])
    (ds1_dw9 :: [a_an3]) ->
    case ds_dw8 of _ {
      [] ->
        case ds1_dw9 of _ {
          [] -> GHC.Types.True;
          : ipv_swB ipv1_swC -> GHC.Types.False
        };
      : x_an4 xs_an5 ->
        case ds1_dw9 of _ {
          [] -> GHC.Types.False;
          : y_an6 ys_an7 ->
            --- AAA
            case Main.xeq @ a_an3 $dXEq_auY x_an4 y_an6 of _ {
              GHC.Types.False -> GHC.Types.False;
              GHC.Types.True ->
                --- BBB
                Main.$fXEq[]_$cxeq @ a_an3 $dXEq_auY xs_an5 ys_an7
            }
        }
    }

If I turn off optimisation there are two calls to "xeq" which correspond
exactly to the two calls from the implementation to either side of "&&".
 In the optimised version, the second "xeq" has been inlined (marker BBB),
but not the first one (marker AAA).  This is despite "xeq" being marked
NOINLINE, by the way.  I would expect the code after AAA to look something
like:

   case $dXEq_auY of _ {
     Main.D:XEq tpl_B2 _ ->
       case tpl_B2 x_an4 y_an6 of _ {
          ... ->

Adding -fdicts-strict to the command line also doesn't seem to convince the
compiler to apply the worker/wrapper transformation and create a loop that
just passes around the implementation of "xeq" instead of the whole
dictionary.

Does anyone know what the issue is here?

Thanks,
  / Thomas

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