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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